drhach Posted June 26 Share Posted June 26 Don't rule out pinion angle. Sagging springs or a bent control arm can move things just enough out of whack to make a difference. It's certainly worth checking if this doesn't solve it. I think Spicer calls for no more than 3 degrees difference. That isn't much. 1 Link to comment Share on other sites More sharing options...
Lahti35 Posted June 28 Author Share Posted June 28 Driveline shop called, the shaft was balanced. They said it was perfect, no need to do anything to it... The plot thickens! 1 Link to comment Share on other sites More sharing options...
EmTee Posted June 28 Share Posted June 28 How's the pinion bearing? I'd probably swap tires/wheels around next. If the vibration is felt in the seat, swap front and rear tires to see whether any vibration appears at the steering wheel. 1 Link to comment Share on other sites More sharing options...
Lahti35 Posted June 28 Author Share Posted June 28 40 minutes ago, EmTee said: How's the pinion bearing? Felt good, no movement in the bearings. Just some slop like normal on an old vehicle when turning before it engages the ring gear. I didn't see anything bad with the any wheel bearings when the wheels were off last year. When I put the drive shaft back in I'll take it for a spin, get it up to 55 and put it in neutral and see if it still happens. On or off the gas didn't seem to make a difference, speed did. It's something you feel in the seat and floor and it's annoying as all get out! Tire shop could have screwed up with balancing or maybe a tire defect? It's experiment time. The only really weird thing about it is that when I first start the truck and go down the road it's much less but gets worse as things warm up... 5 miles down the road and it's pulsating a lot, whereas the first mile is nearly smooth as can be at the same speeds. Link to comment Share on other sites More sharing options...
Laughing Coyote Posted June 28 Share Posted June 28 (edited) Torque converter maybe??? or is it a standard??? Edited June 28 by Laughing Coyote (see edit history) 1 Link to comment Share on other sites More sharing options...
Lahti35 Posted June 28 Author Share Posted June 28 38 minutes ago, Laughing Coyote said: Torque converter maybe??? or is it a standard??? I'm going to warm it up this weekend and leave it in park while revving it up... I don't think it does it standing still but I haven't ruled it out 100%. The torque converter is a new name brand unit not the original so it is a possiblity. Time to make a list and start checking things off one at a time... 2 Link to comment Share on other sites More sharing options...
Lahti35 Posted June 30 Author Share Posted June 30 I spent the afternoon chasing vibrations after re-installing the driveshaft... First I took it for a 10 mile run to get stuff warmed up good. Once warm I started playing with speed, dropping it into neutral then back into gear, accelerating slowly then fast. I tried to do everything I could think of to pinpoint the exact spot the vibration showed up but found it kinda moved on me... I would feel it in the steering wheel, then it would migrate to the floor, then the whole truck seemed to shake but it always happened between 40-55mph. Back at home I put the rear end up on jack stands so I could run it up to 55 without any road stuff confusing the issue. I found it was pretty severe at the same 40-55mph, even worse than on the road driving. Weird! At least I eliminated the front tires at that point since they weren't moving. I then tried it all over again with the rear tires dismounted, still a bad vibration... Since I head the rear wheels off I adjusted the brakes and even pulled an axle to check the bearings but found no smoking gun. I even checked the differential for gear lube to make sure I had indeed put some in and found it full. There was some dried mud on the inside of the rims and around some of the wheel lugs so I cleaned all that off and revved it up again with no change. The last thing I did with the rear axle was to swap the front tires to the rear and found no change there either. At this point I'm really scratching my head, nothing seemed to make any difference so I hopped in the cab and just sort of played with the engine RPM in park and found a fair bit of vibration there. Not the pulsating vibration when driving on the road but it did kind of behave the same, just not as severe. I'd been playing with the idle screws since this engine has been running and they were always a little funky and seemed to be too far out for my taste to get the idle smooth (2 1/2 turns). The stock setting starts at 1 1/2 turns so again I dropped it back down to that and watched it idle. It ran ok but the rpm wandered some and when I pulled individual plug wires sometimes the rpm would drop and sometimes it wouldn't. All the plug wires were sending the juice after doing some testing so I pulled a spark plug next and found it white. I'm talking like super white, almost new looking which screamed a lean running condition. The engine has not been overheating in the truck. I verified timing and it was still where I set it at the factory 6 degrees so that wasn't the issue. I ended up pulling all the plugs and they all looked the same, white with a somewhat discolored base electrode. I backed out the idle adjustment again, this time to 3 turns and it ran much better... I also started thinking about the choke and how when I start out driving the vibration is not there, it's only after a few miles or so it starts and that's about the time I start backing the choke down. I pulled out the factory manual and verified the carb type and jet size at sea level and that all matched up but I had installed a different camshaft when I rebuilt the engine. I didn't put a super cam in there, nothing crazy but it was bigger than stock and may have created my own problem with that choice. With today's findings I have a theory that the engine is just not getting enough fuel. It's much happier with the idle jets backed way out but that's no substitute for main jets that are less restrictive. My next move is to buy a few sets of main jets 2 and 4 sizes over and try them out. It's worth a shot and pretty cheap, not much is cheap in this hobby so if it does fix it it'll be a pretty sweet deal. 5 Link to comment Share on other sites More sharing options...
Laughing Coyote Posted June 30 Share Posted June 30 On my 2007 2500HD Silverado I was getting a bad hop in the rear of the truck a while ago and it came in at 50 and was bad at 53. Felt like riding a bucking bronco. I had tires that were 3/4 of tread wear gone so I thought it was because I needed new tires. I put brand new tires on and it was still there even after messing with tire pressures. Even pulling a loaded dump trailer it was still there. I changed the rear shocks thinking they were bad since they were the originals. No change, then changed the center bearing, no change. The U joints are tight and no slop. I just recently changed the torque converter and it has since been gone. I know you probably don't want to hear that and the job sucks to do. I'm sure you will get it figured out. Hopefully it's the jets. 1 Link to comment Share on other sites More sharing options...
Laughing Coyote Posted July 1 Share Posted July 1 Also, I would check for any worn bushings on the rear suspension if you already haven't. Leaf springs, trailing arms, etc. 1 Link to comment Share on other sites More sharing options...
EmTee Posted July 1 Share Posted July 1 Did you try holding 2nd gear to see whether the vibration occurs at 30~35 mph, or stays above 40? If the vibration tracks engine RPM, then lean misfire could be a factor. LC's speculation about the torque converter is also a possibility. An easy visual check would be to remove the inspection cover and rotate the converter one complete turn, looking for evidence of a missing balance weight. A longshot, but a spot-welded weight could go missing and lead to an imbalance. Along the same line of thinking, did you mark the position of the torque converter relative to the flexplate when the engine was removed for the rebuild? If not, consider rotating the converter 180° relative to the flexplate to see whether anything changes. (That assumes the bolt holes are evenly spaced.) Link to comment Share on other sites More sharing options...
Lahti35 Posted July 1 Author Share Posted July 1 (edited) 14 hours ago, EmTee said: Did you try holding 2nd gear to see whether the vibration occurs at 30~35 mph, or stays above 40? If the vibration tracks engine RPM, then lean misfire could be a factor. LC's speculation about the torque converter is also a possibility. An easy visual check would be to remove the inspection cover and rotate the converter one complete turn, looking for evidence of a missing balance weight. A longshot, but a spot-welded weight could go missing and lead to an imbalance. Along the same line of thinking, did you mark the position of the torque converter relative to the flexplate when the engine was removed for the rebuild? If not, consider rotating the converter 180° relative to the flexplate to see whether anything changes. (That assumes the bolt holes are evenly spaced.) I didn't do any 2nd gear testing, that would have been a good idea but I didn't think of it. Something to try for sure! The TC is a new unit as is the flexplate. The flexplate is a stock replacement while the TC is a Hughes unit with a low stall rating for better MPG. The original TC had a scored shaft so it was replaced. I know I put the flexplate on correctly as it can only go one way for the bolts to line up. I'm going to try the jets first as the fatter fuel mixture had a lot less vibration at lower RPM. I did not take it for a spin after reading the plugs just play it safe. My new jett should be here later this week and I'll do some trials again. Edited July 2 by Lahti35 (see edit history) 2 Link to comment Share on other sites More sharing options...
Lahti35 Posted July 6 Author Share Posted July 6 I didn't get a chance to drive it today as I was working on the 1920 Dodge also but I did get the new jets swapped in... I do recall now intending to replace the main jets in the carb when I rebuilt it as they were kinda chewed up but it looks like I forgot to... Stock size in the manual is 55F (.055"). Someone some time ago swapped in a 54F (.054") for the left jet so there's another potential issue. I picked up a set each of .059" and .057" main jets and went straight to the .059" set. I also verified the float level and it's still set where it is recommended so the fuel level wasn't low in the carb. I'll take it for a spin this weekend and see if there has been any effect... 6 Link to comment Share on other sites More sharing options...
Lahti35 Posted July 7 Author Share Posted July 7 So I won and lost on the jet swap. The truck starts much better and revs like a demon now, it's much happier with the bigger jets, a win! Whether I stay with the 59 jets or drop it back some will depend on what the plugs look like. The vibration is still there... It does it in all gears stopped or driving when warm... Next steps are to do a compression check and then maybe disconnect the torque converter and run it to see if I can pinpoint it to the engine or transmission. Man this is frustrating but what can you do? When I had the rotating assembly balanced they had the flex plate and damper. The shop is know for quality work, never had an issue there. I'll open up the bellhousing cover and see what if anything is going on in there. Old cars! 4 Link to comment Share on other sites More sharing options...
EmTee Posted July 8 Share Posted July 8 2 hours ago, Lahti35 said: Next steps are to do a compression check and then maybe disconnect the torque converter and run it to see if I can pinpoint it to the engine or transmission. Have you tried removing the accessory belts? I feel your frustration; hang in there and keep eliminating items from the list of possibilities. 1 Link to comment Share on other sites More sharing options...
Lahti35 Posted July 8 Author Share Posted July 8 3 hours ago, EmTee said: Have you tried removing the accessory belts? I feel your frustration; hang in there and keep eliminating items from the list of possibilities. Not yet but it's on the list. Definitely worth trying. I was reading last night about a guy who had a water pump that ended up being the root cause of a vibration he was going nuts trying to find. Another I read about found it was the fan clutch... 2 Link to comment Share on other sites More sharing options...
Bloo Posted July 8 Share Posted July 8 (edited) Inspect your fan closely. The fan clutch vibration is common. If the fan clutch feels wrong, or is seized, and/or you see a little flake of plastic sticking out at a goofy angle around the shaft at the center, the fan is probably not running true. Also look the fan itself over real good for loose rivets or cracks. When a fan comes apart it does a lot of damage. Edited July 8 by Bloo (see edit history) 2 Link to comment Share on other sites More sharing options...
EmTee Posted July 8 Share Posted July 8 (edited) I am suspicious of the fan and/or clutch also... Maybe use a timing light to look at the fan with the engine running. If you have one with an adjustable advance/retard you could vary that to see different orientations. Look for a fore-aft wobble at the tips. Edited July 8 by EmTee (see edit history) 1 Link to comment Share on other sites More sharing options...
Lahti35 Posted July 9 Author Share Posted July 9 OK try this one out... The weekend before last when I was under the hood with it running I pulled the spark plug boots one at a time to see if there was a drop in RPM like you would expect. When I pulled the #4 cylinder plug wire by the boot I got a a few nasty shocks while holding it, definitely plenty of spark! However the RPM didn't drop like you would expect with the wire disconnected... I was thinking about it at work today and when I got home I pulled that spark plug again to check it now that it's run a bit with the fatter jets installed. When pulled before the new jets it was bright white with a few black specs, a sure sign of the plug overheating. Looking at it again tonight it had some nice tan toasting going on so it's much happier now. I've ordered a new set of plugs to replace these, the same Autolite 45 copper electrode type. I closely examined the #4 spark plug wire and found no cracks or splits. I didn't have a used one long enough to swap so I thought I'd switch the spark plug with the one next to it in cylinder #3 and see if the shocking continued. I reinstalled the plugs and checked for contact to the block with my multimeter and they had no issues there. Now this is where it gets funky... With the truck running I pulled the #4 plug wire off by the boot again and there was a noticeable drop in RPM and no shocking me unlike last time. I moved over and pulled the #3 wire off by the boot and got a nice steady stream of shocks! I actually saw a spark travel out from between the wire and boot and go up an inch into my finger. Also interesting was that I was hearing a light cracking/snapping in the same pulsating manner as the vibrations when the engine is running, it was very slight but there. Anyone ever run into something like this? I'll of course try it again with the new plugs in when they arrive in a few days but i've never had a plug wire zap me when disconnected based on moving a spark plug around. Something has got be to going on with the plug as the problem moved with it when I put it in the other cylinder. If I didn't know better I would have said it was the plug wire but when the plug is moved the old wire no longer misbehaves and the shocking moves to the other wire. Weird! 3 Link to comment Share on other sites More sharing options...
Laughing Coyote Posted July 9 Share Posted July 9 Well I don't have an answer for the issue. Bad internal in the wire??? At least it sounds like you got the carb dialed in nicely. I'm sure you will get it figured out. 1 Link to comment Share on other sites More sharing options...
Bloo Posted July 9 Share Posted July 9 Sounds like the plug is bad, and not conducting in the center electrode. There is a resistor in there, or at least there is in any plugs you can still buy for 390. An extra gap to jump or extremely high resistance causes the spark voltage to be high. Since the symptom moved with the plug, I blame the plug. Maybe it was mismanufactured. Get another plug. 5 Link to comment Share on other sites More sharing options...
Lahti35 Posted July 12 Author Share Posted July 12 Well, no love in the garage tonight! The new spark plugs didn't change anything and I still got zapped by the #3 wire again, lol. I'm going to mist some water on the engine at night and see if I can find any sparkies. I pulled the belt off the fan and alternator also and no difference either. So I'll do a compression check and mist the engine to check for sparking wires this weekend and maybe disconnect the torque converter If I have time to burn... 2 Link to comment Share on other sites More sharing options...
Bloo Posted July 12 Share Posted July 12 (edited) Do I take it to mean #3 zaps you WORSE than the others? Or is is just zapping you because that is where you are working? If it is the former, something has to have gone high resistance, probably the #3 wire. EDIT: Is this your intake manifold? If so, you have a vacuum port on #3. Plug it, readjust carb if necessary and see if your life improves. Edited July 12 by Bloo (see edit history) 2 Link to comment Share on other sites More sharing options...
Lahti35 Posted July 12 Author Share Posted July 12 1 hour ago, Bloo said: Do I take it to mean #3 zaps you WORSE than the others? Or is is just zapping you because that is where you are working? If it is the former, something has to have gone high resistance, probably the #3 wire. EDIT: Is this your intake manifold? If so, you have a vacuum port on #3. Plug it, readjust carb if necessary and see if your life improves. #3 is the only one that zaps me, but it was only #4 that was doing it before I swapped spark plugs between cylinders 3&4. #4 does not zap me anymore now, the plug wires were not swapped. I'm going to run a resistance test on those plug wires and see if anything odd pops up. I tested the resistance on the plugs and there were OK. That is my intake manifold. The vacuum port by #3 is for the brake booster. I like your style, that would be good to eliminate or confirm as an issue... More testing to come! 2 Link to comment Share on other sites More sharing options...
EmTee Posted July 12 Share Posted July 12 (edited) What does that vacuum port feed? (OK, I see you identified the brake booster.) Edited July 12 by EmTee (see edit history) Link to comment Share on other sites More sharing options...
Lahti35 Posted July 13 Author Share Posted July 13 I keep hunting and pecking around the ignition system, my spidey sense keeps bringing me back. I decided to pull the distributor and check things over again. When setting the timing with the light the flashes didn't seem as consistent as they could be and the timing seemed to wander a bit back and forth. I kind of put this down to a vintage timing light but I thought it was worth another look. I set the engine to TDC firing on #1 and the TDC mark on the damper lines up, no issue there. I then pulled the distributor and checked the endplay with was over 2x the spec. I also found some play in the shaft side to side. When I rebuilt this 2 years ago I did not pull the shaft, just cleaned it up and put new points, condenser, vacuum advance, and a cap. More of a refresh than a rebuild you could say... I pulled the shaft today and while the shaft seemed ok the bushing at the top was worn. The shaft must be some hard stuff as it came in at .427 all around and the bushing is steel. I pulled the old bushing in preparation for the new oilite bushing by threading it and winding it out with a spacer and bolt. The new bushing should be here this week. I was a little concerned when I pulled the shaft that the length inside the distributor tail was dry. It's got a spiral cut into it to keep oil from creeping up the shaft but I would think it should get some spray from inside the crankcase to keep it lubed. I'll have to look into it, the outside of the distributor tail was thoroughly soaked in oil but dry as a bone inside. In other news some guy in a corvette crashed into my ditch, slid 30 yards down it until he hit the culvert then flipped 360 degrees over my driveway and landed in the same direction he was traveling on the other side. Pretty gory as he wasn't wearing a seatbelt but he survived! The road and yard looked like somebody lit of an explosive in the ditch, mud everywhere. Always something going on over here... 7 Link to comment Share on other sites More sharing options...
Laughing Coyote Posted July 13 Share Posted July 13 Hope you're on to something with the distributor. Too bad for the Vette, I bet it was a wild ride. 1 Link to comment Share on other sites More sharing options...
Lahti35 Posted July 13 Author Share Posted July 13 1 hour ago, Laughing Coyote said: Hope you're on to something with the distributor. Me too, I thought it was worth a try before I start disconnecting torque converters. 1 Link to comment Share on other sites More sharing options...
Jeff Perkins / Mn Posted July 14 Share Posted July 14 Thumbs up for your latest post….thumbs down for the Corvette. Wonder what the BA of the driver was……. 1 Link to comment Share on other sites More sharing options...
Bloo Posted July 14 Share Posted July 14 It is probably unrelated to the problem you are currently having, but the one thing that does commonly wear out in those distributors is the breaker plate. There are several types (including aftermarket). Most pivot, and the pivot must not be sloppy. The vacuum advance is constantly moving and dwell variations with a bad plate are massive, ignition can intermittently stop firing, etc. Insect it closely. Replacement parts vary from excellent to absolute crap. Also make sure the ground wire is in there. 3 Link to comment Share on other sites More sharing options...
Laughing Coyote Posted July 14 Share Posted July 14 3 minutes ago, Bloo said: Also make sure the ground wire is in there. Good point Bloo. Also there should be a ground wire from the back of the intake manifold to the firewall. My Mercury has one. 1 Link to comment Share on other sites More sharing options...
Lahti35 Posted July 14 Author Share Posted July 14 3 hours ago, Bloo said: It is probably unrelated to the problem you are currently having, but the one thing that does commonly wear out in those distributors is the breaker plate. There are several types (including aftermarket). Most pivot, and the pivot must not be sloppy. The vacuum advance is constantly moving and dwell variations with a bad plate are massive, ignition can intermittently stop firing, etc. Insect it closely. Replacement parts vary from excellent to absolute crap. Also make sure the ground wire is in there. I did replace both the vacuum canister and the breaker plate back when I did it originally, they needed it. Also put a brand new ground wire in there too so i'm covered there. 2 hours ago, Laughing Coyote said: Good point Bloo. Also there should be a ground wire from the back of the intake manifold to the firewall. My Mercury has one. Yup, put a new one of those on too, all set there. 6 Link to comment Share on other sites More sharing options...
Lahti35 Posted July 14 Author Share Posted July 14 3 hours ago, Jeff Perkins / Mn said: Thumbs up for your latest post….thumbs down for the Corvette. Wonder what the BA of the driver was……. Not sure but this is the second white corvette incident I've seen in my area... Last one was a guy at an intersection who was revving, made a right turn and gunned it only to have his hard top fly off and land in the street. He had to turn around and do the walk of shame to go get his now scuffed up top out of the thorofare while everybody watched. Wish I'd had a dash cam for that one! 1 Link to comment Share on other sites More sharing options...
EmTee Posted July 15 Share Posted July 15 Good find on the distributor end/side play. I'm interested to see whether that clears-up the issue with #3 (or is it #4?). 1 Link to comment Share on other sites More sharing options...
Lahti35 Posted July 15 Author Share Posted July 15 2 minutes ago, EmTee said: Good find on the distributor end/side play. I'm interested to see whether that clears-up the issue with #3 (or is it #4?). That's the funny thing... It was #4 but when I switched the plugs between 3&4 and #3 was then zapping me. I did check the gap and resistance on that plug but it was no more/less than the others. It's got new plugs in there now. I'll see how it runs with the new distributor bushing. If spark was inconsistent on #1 according to the timing light I would assume it's that way on all, resistance was ok on #1 plug wire so that's not an issue. I was looking at endplay specs and though my distributor is out of spec it shouldn't be bouncing up and down as the cam gear rotation draws it downward. The distributor gear itself sits on a machined surface in the block that sets it's depth. I'm tempted to leave the endplay alone and just do the bushing to fix the side play first, try it and see what I get. Fixing endplay would involve shims or redrilling the shaft to move the stop collar up some, neither of which I'm keen on. 3 Link to comment Share on other sites More sharing options...
Bloo Posted July 15 Share Posted July 15 That extra endplay might not be wear(?). If I remember correctly, they all have quite a bit. I suspect they were never as tight as the book says. Side play matters, but even though it's technically possible to lose a cylinder that way, it would have to be really bad. Apart from crappy breaker plates, those distributors just work. On 7/13/2024 at 3:04 PM, Lahti35 said: When setting the timing with the light the flashes didn't seem as consistent as they could be That might be normal. It's funny how that works. If the light is a digital dialback, it's gonna be a little unstable no matter what. If it's a plain light that just flashes, maybe not, but you can't depend on it. 1 Link to comment Share on other sites More sharing options...
Lahti35 Posted July 15 Author Share Posted July 15 (edited) 24 minutes ago, Bloo said: That extra endplay might not be wear(?). If I remember correctly, they all have quite a bit. I suspect they were never as tight as the book says. Side play matters, but even though it's technically possible to lose a cylinder that way, it would have to be really bad. Apart from crappy breaker plates, those distributors just work. That might be normal. It's funny how that works. If the light is a digital dialback, it's gonna be a little unstable no matter what. If it's a plain light that just flashes, maybe not, but you can't depend on it. My timing light is just a plain old craftsman from the 70's, no dial back... For my sake I'm hoping this is the issue! Plus the fact that the idle is changing some and the timing marks on the damper are moving... You can hear it with your ear as the engine speeds up and slows down at a darn consistent rate, RRRRRRrrrrrrrRRRRRRrrrrrr... Edited July 15 by Lahti35 (see edit history) Link to comment Share on other sites More sharing options...
Bloo Posted July 15 Share Posted July 15 (edited) I still think you have a vacuum leak at that manifold port because it explains just about everything. 50 minutes ago, Lahti35 said: You can hear it with your ear as the engine speeds up and slows down at a darn consistent rate, RRRRRRrrrrrrrRRRRRRrrrrrr... That sounds like a condition we called in my line of work "idling on the curve". If the idle speed is higher than the beginning of the mechanical advance, it will advance. The rpm goes up until the idle jets no longer supply enough air/fuel, then it lean misses, the idle falls, and it repeats. If the throttle is also too far open, it will uncover the distributor port, and then the vacuum advance, which shouldn't have vacuum at idle, will aggravate the condition. Idling on the curve isn't the ONLY thing that causes surging like that but it's a good bet. Is your idle speed too high? You might do well to get a dialback, or at least some timing tape if it proves you need to recurve the distributor, and you might if this is what is wrong and the engine isn't stock. Ideally the mechanical curve should start 50-100 RPM above idle speed. It advances fast for a little bit, then probably around 2000-2500 rpm the second spring with the big loop comes into play and the advance goes much slower the rest of the way to 5000-ish RPM. Initial timing plus mechanical is going to be about 35 crank degrees total. ALL of that needs to be determined experimentally for the individual engine except the part about the mechanical curve beginning slightly above idle. This is set up for best full throttle power ONLY with no pinging, with NO VACUUM ADVANCE (temporarily disconnected). You pretty much need experiment to get this about perfect if you go down this path, there is no "rule of thumb". Vacuum advance is additional for part throttle operation. It shouldn't cause pinging when you hook it up, and it goes to ported vacuum on the carb (if there's no Ford vacuum valve weirdness). Many vacuum cans floating around are for smog era cars and they have too much advance, an engine with EGR needs about 5(?) degrees more vacuum advance than one without. It's too much to ignore. Or if the engine is still stock or real close, you can go with the values in the book. That's way easier. One thing the factories sometimes did, especially on cars with Automatics, was make a lazy mechanical curve, and then add a little more vacuum advance. Sometimes trucks are even lazier, to make absolutely sure it won't ping when heavily loaded and floored. In that program, they rely on the vacuum advance, which is connected to ported vacuum, to give a sudden advance kick when you take off, without any help from the mechanical curve. The mechanical curve might start as much as 300-400 rpm above idle. Advantage? Probably less warranty claims, because when the mechanical curve starts to get even lazier because it's gummed up, the owner doesn't notice. Disadvantage? Less full throttle power. The owner probably doesn't notice that either. Whatever you do, the mechanical curve can't start at a lower RPM than the idle speed. Edited July 15 by Bloo (see edit history) 2 Link to comment Share on other sites More sharing options...
Lahti35 Posted July 15 Author Share Posted July 15 5 minutes ago, Bloo said: I still think you have a vacuum leak at that manifold port because it explains just about everything. That sounds like a condition we called in my line of work "idling on the curve". If the idle speed is higher than the beginning of the mechanical advance, it will advance. The rpm goes up until the idle jets no longer supply enough air/fuel, then it lean misses, the idle falls, and it repeats. If the throttle is also too far open, it will uncover the distributor port, and then the vacuum advance, which shouldn't have vacuum at idle, will aggravate the condition. Idling on the curve isn't the ONLY thing that causes surging like that but it's a good bet. I can see why you'd go with that, it's not off the table but it's not really a surge so much as it just toggles between two RPM's. It not continually increasing rpms, then falling off, then increasing again. It's like a switch is being flipped between two fairly close, distinct, RPM's. Driving is ok other than the vibration, the idle is a bit rougher than I'd like. The idle was set at the factory 650 spec for an auto trans and the dwell was set also to factory spec. When I had the smaller jets in you could feel it needed more gas at higher RMP. Now with the larger jets in there the idle screws are happy at 1 1/2 turns out and the power at higher RPM is much better. The plugs are showing a tan tone on the porcelain now, no one cylinder appears to have a plug that is different from the rest. When I get the distributor back in I'll start from scratch and do the timing/dwell and set it with a vacuum gauge again and see where it gets me. I do recall pulling the brake booster line off late last year and there is a noticeable drop in RPM and rough running that goes away when you plug it back in. I will however block it off when I set the timing again just to be sure. A vacuum leak is certainly possible but my vacuum gauge shows no fluctuations other than the idle moving slightly then back slowly, it's steady just on either side of 19. 2 Link to comment Share on other sites More sharing options...
Laughing Coyote Posted July 15 Share Posted July 15 Coil maybe? Can you swap one out to check? Just throwing it out there. My Vette would surge up and down at idle when I had the manifold vacuum hooked up to the headlight actuator hose. After pulling the hose off and plugging the manifold port then the carb flattened out and runs smooth. The headlight actuators must have an internal vacuum leak. 1 Link to comment Share on other sites More sharing options...
Laughing Coyote Posted July 15 Share Posted July 15 From my 1961 Mercury repair manual. For 352 and 390. 2 Link to comment Share on other sites More sharing options...
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