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1941 Roadmaster coupe


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 Peter, it depends on how the ports on the intake are. Mine had a mod long before I had it that allows me to put a vacuum gauge on it. On my engine is there a fitting on the area in between the two carbs that goes to the fuel pump (I think), which is where I'm able to get a gauge on. The other possibility, and one has to be careful, is to get a line on the end that goes into the fuel pump vacuum assist, and see how the manifold vacuum reads.

 Valve adjustment is tricky, I've found as it has to be at full operating temp, and with the cover off, and the engine off, the rockers cool very quickly, which tightens the clearances, due to the aluminum stands. I've done it with the engine running, which is messy, of course, and more hazardous to you.

 I'll tell you that it is tricky to get these engines set up so that you have a nice smooth idle. I've always been a pretty good setup guy, but this always challenges me. A air flow meter will help. I did it by feel for ages, and when I got the flow meter, it was much easier to get both carbs drawing close to the same. Sometimes tough to get them exact to each other, but close is usually good.

 The other thing that I've had issues with is the butterfly which is positioned under the rear carb. This makes the rear work like the vacuum secondary on a four barrel carb. When air flow is high enough, it opens the allow for greater airflow through the rear carb, but on mine, at least, it tends to stick in one position or the other. Also, unlike the heat riser valves, this won't mess you up, it just affects performance. If it is stuck open, you will have rough idle, if the carbs have been balanced with it closed. One and quarter to one and a half turns out on the mixture screws is a good starting point.

 Also, what about the heat riser, is it stuck?

 Also, as Ben said, how does it drive?

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All good advice. I use 0.017 go, 0.018 no go on the valves, this is what the Shop Manual suggests when doing the old run-it-and-then-pull-off-the-valve-cover-real-quick-before-it-cools routine. On the vac gauge, the manifold connection should be tapped for 1/8 MIP unless someone has messed with it. It will probably have an elbow which goes to the vacuum steel line. If you run into problems PM me, I think you are only a few miles away.

 

Cheers, Dave

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Thanks for the comments. Having trouble locating a good (or any) vacuum port. Would hooking up a tach essentially do the same thing? I could monitor RPMs while making adjustments. 

And is this thread getting too long?? Maybe I should start another one now that the subject has changed. 

Oh, and while I don't name my cars,  folks are pressuring me to name it Jessica Rabbit. Must be the wide whites..

Peter

41buick5.jpg

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8 hours ago, valk said:

And is this thread getting too long?? Maybe I should start another one now that the subject has changed. 

 

I don't think any thread on here gets "too long."  We're all interested in whatever you want to post.  But you might consider starting a thread in the "Me and My Buick" forum.  That way you can have a place to post any and all developments about your Buick going forward.

 

https://forums.aaca.org/forum/58-me-and-my-buick/

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I am committed to getting this car running as well as it looks. Been fiddling with the idle mix screws and I am in need of some detective work.  

After cleaning the plugs, I adjusted all 4 idle mix screws (2 carbs) 1 and 1/8 turns out. Ran it a few minutes, gunned it a little (not hard),  and idle was a little rough, some body shake, and so was coming off idle. Ran better at higher RPM. Pulled the plugs and found a wide spectrum of differences. The 4 plugs nearest the rear were really wet, almost as if the cylinders were not firing. Of course they were because I would have noticed if half the engine wasn't firing. Of the remaining 4 plugs, the 2 nearest the rear were sooty and the 2 most toward the front were actually good. So I'm figuring the rear carb is way too rich and is messing up not only the 4 rear cylinders, but also the 2 nearest them, and the main carb is set pretty good. Does the rear carb have a choke that could be stuck closed? I haven't checked that yet. Lucky for me I have fellow member Dave Stovall coming over tomorrow to take a look. 

Am I on the right track?

Peter

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You are definitely on the right track to have Dave coming over! 😄  As far as your questions, I'm not super knowledgeable about this topic, but unless you mean that you REALLY gunned it and actually drove it at high RPM, I don't think the rear carb would have had any effect on what you observed.  My understanding is that the rear carb is mechanically linked to only kick in at around 75% throttle or above.  You can observe this by just tweaking the throttle linkage when you are under the hood.  The rear carb doesn't begin to open until you've moved the linkage quite a bit.  Also, the manifold is set up so the both carbs supply fuel to all eight cylinders.

Edited by neil morse (see edit history)
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Thanks Neil. I've learned both carbs work at idle but the rear doesn't do anything else until you jump on it. The rear carb also has no choke (I looked) so I think I just have the rear carb set too rich. I'll make a correction and check the plugs again. Regrettably, I don't have a shop manual yet but I do have portions of it. 

Peter

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Actually, I just plotted that out and that's a fancy way of saying the front carb feeds the front 4 cylinders and the rear carb feeds the rear 4 so I should be good. I'm just a bit puzzled why there was so much difference between the 2 carbs when they have identical settings. 

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I have a shop manual, and could scan a few pages for you, if it would help. Also, I have a Stromberg carb manual, if they are Stromberg's. Carter's were used as well, but most I've seen are have Strombergs.

 There is that butterfly under rear carb, you can see the counter weight on the side. That might need freeing up.

 But Dave might have all of this info.

 Let me know if there is anything that I do to help with.

 Keith

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Dave and I gave it the once over and we were able to determine a few things.  The compression, timing, spark strength and firing order were all good but we could not eliminate all the fluctuation on the vacuum gauge by tweeking the idle speed and idle mix screws. The front 4 cylinders appear to be firing correctly but the rear 4 are still running very rich, even when the mix screws are leaned out.  We concluded that the rear carb may be the cause but noted a bad carb usually results in a lean condition as opposed to a rich one(?)  So the jury is still out...any ideas? 

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The front carb is the only one with a choke or idle circuit. It should be offline at idle and light throttle openings. Vacuum holds the butterfly under the rear carb closed, it opens at higher throttle openings as the rear carb comes online. If you have it adding fuel at idle, that would explain why the rear cylinders are running rich. It should be offline at idle. When the engine is off, move the throttle linkage through its range of travel and you will see that the front carb is already pretty far open before the rear carb starts to open. It's probably better to think of the rear carb as mechanical secondaries on a traditional 4-barrel carb. I don't think there's anything wrong with your car (other than the missing weight, as Dave mentions), you're just looking at the rear carb the wrong way for tuning purposes. It is usually offline at idle and under normal driving.

 

As far as getting the idle buttery smooth, it probably won't happen. Strombergs are simpler than Carters for tuning purposes, but they don't seem to idle as smoothly. However, I have yet to see a big series '41 Buick with dual carbs idle perfectly smooth. These were high-performance engines and I suspect that they never really idled perfectly. I spent a lot of time tuning and tweaking mine and figured that it should idle imperceptibly, but I was never able to achieve it. I talked to Doug Seybold about it and he said that he can sometimes make it perfect, but it's rare and he believes it has to do with the cam, not the tuning. I have not had actual cams in my hands to do measurements, but there are some indications that the dual carb cams are slightly different and therefore do not idle as smoothly as the single carb cams that came before and after. I think "pretty good" is as good as you'll get, but I also think that's part of the deal with a '41 or '42 large series dual carb car. I've come to accept that it's an acceptable trade-off for the rather extraordinary performance advantage. These are still pretty fast cars!

 

As long as it idles relatively smoothly and pulls cleanly at all speeds without any stumbles or stutters, that's a win. You'll be thrilled with how hard it pulls!

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Thanks Matt. Maybe I am asking too much. I'm still a bit fogged regarding the idle circuits, though, in that the 2 sources I have ('41 shop manual Dave lent me and a booklette titled "Your 1941 Buick Fireball Eight; How to know it better...so it will serve you best!") state the idle systems of both carbs are functioning at idle and partial throttle up to 15MPH or so.  It notes, " There is sufficient clearance around the flies of damper valves to permit idle system of rear carburator to function with damper valve closed".  And I don't know of a way to shut down the rear idle system without turning the idle screws all the way in.  What am I missing??  In any case, something is making the 4 rear cylinders too rich and I have to turn the idle screws in almost all the way to get a decent plug reading. Ugh...

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 I hate to disagree with Matt, as I have great respect for him, but the info I have indicates that the rear carb has an idle circuit. I did not realize that when I first got mine on the road, and had a terrible idle, till I realized that a previous owner had blocked the fuel off to it with a small screw jammed into a passage. I guess to try to save fuel. Then I had a more acceptable idle. This tends to confirm my belief that the rear carb aids in idle. I have had consistent trouble with the butterfly which is under the base of the rear sticking in one position, or the other. When its' closed you don't get the extra performance of the rear carb, but honestly, its' like icing on the cake, the car still performs good without it working. But if its' stuck open, then more air is fed through the rear one, and that will upset the idle quality. I am seriously considering taking it off and machining it for small ball bearings during the winter, in the hope that it will work more reliably.

 I can't agree MORE with Matt on this, it is tough to get that silky smooth idle that I have in other Buicks with the '41's. I have mine set so that it idles decently, and it goes great when I put my foot in it. Driving dymanics is great as well, and when one considers the age of these cars.

 It is possible that there is some internal issue with the rear one that is causing the problem, but hopefully it can be solved with tuning.

 Keith

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Well I understand the confusion. And Matt brings up another pertinent variable, the cam, which on mine was recently reground with what I'm told was a slight "performance" grind, which might contribute to a lumpy idle. Something ain't normal, though, as the vacuum gauge shows. Something I haven't explored thoroughly is a vacuum leak  but not sure how to go about tracking it down if it exists. 

Peter

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 The cam grind certainly could contribute to a lumpy idle, depending on the specs. Though with the some of the plugs showing rich running, it tends to show a fuel issue.

 Got to tell you a bit more about my carbs. A few years ago I had my Strombergs rebuilt by a reputable place, and it ran fabulous for some time, I could go up hills on the highway at 70, with lots to spare, then last spring it gave my troubles on high speed running. Though through all of this the rear plugs tended to run rich, a bit sooty from no5 on back with 7 &8 the worst, no matter how I tried to set it up. Nice idle with those though. So I went back to a set of Carters that I'd had. Late last year I took the Strombergs to a local guy, who came highly recommended, and finally got them back only a few weeks ago. With about 300+ miles of driving on it, I pulled all the plugs late last week, and they were all looking just about right, so it looks like the setup is about right. Hopefully it will keep on running well. The only other issue I have with these, is that I have trouble getting the idle low enough, with matching airflow.

 I bought an airflow meter off of eb.y a few years ago, and I find that it really helps with trying to balance air draw from the front carb to the rear.

 Vacuum leaks can be tough to find, one way that was recommended to me, which I've used, is too use acetylene or a propane torch to feed raw gas to various suspect areas when running, and see if the idle changes. It could be a simple as a bad gasket somewhere.

 Don't know if I missed something in a past post about the vacuum gauge reading you got. I'll can read back through your thread, and see I can find the info.

 Have you had the car out on the road?

 Keith

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I have an air flow meter that I use to balance the SU's on my TR3. I can make up an adapter for the larger carb. Good suggestion.

 

After some mixture tuning, his vac gauge at idle shows readings close to 20 but periodically dropping 2 or 3 inches. Definitely missing or very weak combustion. Cylinder balance test showed very little RPM drop on rear 4 cylinders. This matches with what his plugs are showing. He is going to see what the damper position is. It looks right now like the rear carb is dumping in way too much fuel. Carbs are Carter WCDs by the way.

 

Cheers, Dave

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14 hours ago, valk said:

Well I understand the confusion. And Matt brings up another pertinent variable, the cam, which on mine was recently reground with what I'm told was a slight "performance" grind, which might contribute to a lumpy idle. Something ain't normal, though, as the vacuum gauge shows. Something I haven't explored thoroughly is a vacuum leak  but not sure how to go about tracking it down if it exists. 

Peter

Usually you can hear a vac leak but not all the time. I use wd-40 to locate a vac leak. with engine idling, spray wd, one place at a time, intake to head, base of carbs. Engine will speed up when detected. Your rear carb should not be putting any fuel in the system until kicked in progressively. Makes me think flooding over(not stopping the flo of gas) trash in needle/seat or too high float level.

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Yea, I think we are all zeroing in on the rear carb. Thanks for all the vacuum leak detection suggestions. A rebuild kit is on the way courtesy of the guy I bought the car from (is that cool or what?) and I'll check the float and needle valve in the meantime.  But here's a mystery;  both "heat control valves" -  which are located between the carbs and exhaust manifolds and have a wound spring on one end and a counter weight on the other -  are stuck solid. I would think I should be able to at least budge these with my hands and makes me think they were set up this way since both are the same. 

 

And thanks all for hanging in there for me even though this is getting somewhat tedious. 

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 Number one, we have seen far more tedious than this,  so don't feel bad.   Number two, I would wager that the heat riser valves on most cars from this era are stuck closed because people don't need them since they only drive in warm weather, and they are a big hassle to keep working correctly. They are stuck closed on my '41.

Edited by neil morse (see edit history)
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Congratulations Peter  !!!!

So glad she's found a good home.

It's just too bad Colorado has lost another beautiful Buick.

We don't have that many to spare.

 

Mike in Colorado

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Man, not much to these rear carbs, is there?  No real obvious problem detected other than it's got a gritty residue all in it and it stinks to high heaven of stale gas. The crud in this one is telling me to clean the main one too but it won't be this simple. 

carb.JPG

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Peter, the heat control,  or heat riser valves were stuck solid on mine. I sandblasted them clean, soaked them all kinds of penetrating solvents, and as much heat from the torch and hammering as I dared. This went on for about two weeks, but all to no avail. They were stuck in a mid open position, and bent them to fully open. My bet is that they hadn't been lubed since they left the factory.

Hope the cleaning and rebuild goes well.

Keith

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I think Peter is OK for now on the exhaust bypass. Both weights were in the correct 2 o'clock position.

 

The 2 longer tubes in your pic are the low speed jets. They are spec'd to a 65 drill. This is at the lower pinched part. You also have the bypass and economizer ports to check in the carb body, the spec should be on the sheets I sent you.

 

So what position is the damper valve in, does it move freely?

 

Cheers, Dave

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13 hours ago, valk said:

Yea, I think we are all zeroing in on the rear carb. Thanks for all the vacuum leak detection suggestions. A rebuild kit is on the way courtesy of the guy I bought the car from (is that cool or what?) and I'll check the float and needle valve in the meantime.  But here's a mystery;  both "heat control valves" -  which are located between the carbs and exhaust manifolds and have a wound spring on one end and a counter weight on the other -  are stuck solid. I would think I should be able to at least budge these with my hands and makes me think they were set up this way since both are the same. 

 

And thanks all for hanging in there for me even though this is getting somewhat tedious. 

Both flappers were froze up on my `41 248 dual carb engine ex manifolds, I also tried the soaking and heat, wouldn`t budge. So taking a closer look at the flappers both were a little offset(larger gap on one side), I then set it up in my press and applied pressure to the shaft(oppisite end of the gap)to move the shaft sideways, both came right loose, little stiff at first, worked them manually with some spray lube and both were free moving as they should.

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14 hours ago, valk said:

Man, not much to these rear carbs, is there?  No real obvious problem detected other than it's got a gritty residue all in it and it stinks to high heaven of stale gas. The crud in this one is telling me to clean the main one too but it won't be this simple. 

carb.JPG

You might consider cleaning gas tank also..

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When I pulled the manifolds on my '40 LTD (single carb) the damper was so stuck open, there was no hope of freeing it up, so I fabricated a plate from 14 gage sheet metal and brazed it to what was left of the shaft in the closed position and never looked back.

Pont35cpe is right in that pulling the gas tank is a must.

Then put a BIG clear fuel filter back at the tank, so you can check for trash.

 

Mike in Colorado

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I regret to report no progress on my '41 Buick woes. Rear 4 plugs still wet with fuel and now it smokes worse than ever. I'm afraid the fuel will wash the oil off the  cylinder walls and ruin the rings so I'm chicken to drive it.  Best looking car with a shitty engine I've ever seen. Anybody want to buy a Roadmaster coupe? 

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Sorry boys.  After a belt or 2 of Woodford Reserve, I am braced to face whatever lies ahead. Damn the torpedos....I will plow ahead and get this thing back on the road. 

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I feel your pain, Peter. My 1935 Lincoln has been moments away from a sledgehammer more than once.

 

Fortunately, this is just a case of carburetor adjustments and that's a very curable condition. I think my next step might be disconnecting and blocking the fuel line at the rear carburetor and seeing how it runs.

 

After that, I'd check the linkage. The rear carb really shouldn't be doing much until you have the throttle pretty far open. Is your linkage working properly? There should be two separate rods from the accelerator pedal assembly and the one to the rear carb should not open it until the pedal is about 3/4 of the way through its travel. And I still don't believe there is an idle circuit in the rear carb. Yes, some fuel might get pulled in just as a function of vacuum, but it is not designed to feed fuel at idle. The disassembled carburetor verifies this. 

 

The flapper, meaning the little one under the carb, might be stuck. I believe it is expressly designed for this very situation--it should be closed at high vacuum levels and open at low vacuum levels (that's what the weight is for). If it's not working correctly or is open at idle, of course the rear carb is going to dump fuel. 

 

I have a fairly comprehensive training book that I've been meaning to scan. It has very detailed carburetor adjustment procedures. I'll try to get on that job in the next few days and post it so we can all use it. But in the meantime, try running it without any fuel going to the rear carb. Yes, it might lean out a bit at big throttle openings, but I think that'll be OK while you test. That will at least narrow down the culprit to the rear carb.

 

Keep us posted.

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