Guest 66chev Posted July 3, 2015 Share Posted July 3, 2015 have a 66 Chevy with 250 single barrel carb having troubles. At idle I can unplug wires from1 or 6 and doesn't change engine at all. Vacuum reading fluctuates at idle from about 12 to 18 quickly. When I give it some throttle will steady out around 20. Has great compression all cylinders, timing good, new plugs and wires, new distributor, cylinder leak down test all cylinders are within 5% of each other. Any thoughts would be appreciated.I know the vacuum reading sounds a lot like a burnt valve or a bad valve seal but if either of those were true would I still be getting good leak down numbers. Also there is no smoke at start up Link to comment Share on other sites More sharing options...
NTX5467 Posted July 3, 2015 Share Posted July 3, 2015 What do the spark plug insulators look like? Like the other 4 or unique to just the end plugs? It also sounds like the port you're using for vacuum readings is a "ported vacuum" port rather than seeing direct intake manifold vacuum. Possibly even the distributor vacuum advance supply port . . . due to the fact that you crack the throttle slighty and the vacuum increases and such. Might also need to adjust the carb mixture strictly to "best idle/highest intake manifold specs. Please advise. NTX5467 1 Link to comment Share on other sites More sharing options...
Rusty_OToole Posted July 3, 2015 Share Posted July 3, 2015 Are you sure the plug wires are on the correct plugs? Are the spark plugs firing? Your plug test indicates #1 and #6 are not firing or are firing at the wrong time. Link to comment Share on other sites More sharing options...
Guest 66chev Posted July 3, 2015 Share Posted July 3, 2015 Checked multiple times and wires are on the correct plugs, wires are new as well. Plugs are firing really good. I'm wondering if for some reason it's not getting fuel to those cylinders because plugs are not wet and do not smell like fuel. The pot I'm checking vacuum at is on top of intake where the pcv hose plugs up. Link to comment Share on other sites More sharing options...
padgett Posted July 3, 2015 Share Posted July 3, 2015 That single barrel carb is about the simplest there is an unless someone stuffed shop towels in the manifold ends it is hard to have 1 and 6 very different from the others. That is also pre-computer (in fact pre-emissions) so no 'puter correcting the idle. About the only thing left (and unlikely) is something keeping the intake or exhaust valve shut (bent pushrod, broken rocker, etc). Could have good compression and fire & no power but those should make noise. Link to comment Share on other sites More sharing options...
Rusty_OToole Posted July 3, 2015 Share Posted July 3, 2015 (edited) But he says all cylinders have good compression and it passed a leakdown test. Maybe it is haunted. Seriously, if an engine has fuel, compression and spark, at the right time, it has to run. It has no choice. There is something you are not telling us, something you may not even know about - but "think" is OK. Edited July 3, 2015 by Rusty_OToole (see edit history) Link to comment Share on other sites More sharing options...
Guest 66chev Posted July 3, 2015 Share Posted July 3, 2015 (edited) Ok trying to think of what else to tell you, we are talking about an engine with less than 20,000 miles on it. Brand new short block and reworked head, new valves, pushrods, lifters. Even measured the lift on the cam at each rocker all were with in spec, at idle it sounds pretty smooth but when under a load power is just not there and then there is the whole plug issue with unplugging the wires and it not effecting the engine. I have been all over this engine with an old chitons manual checking everything I can think of and it all seems to be checking ok. It's about to drive me crazy. Checked the tension on all the valve springs, checked each on for cracks. Let it run with valve cover off and all valves are working up and down like they are supposed to . That's just it no strange noises no rattles no smoke no fluid losses. To me it acts like a vacuum issue somewhere but where is the big question if it is it has to be in some odd place that would not be obvious. Again I really do appreciate you guys making suggestions and I am checking each possibility to see if I have missed something Edited July 3, 2015 by 66chev (see edit history) Link to comment Share on other sites More sharing options...
vermontboy Posted July 4, 2015 Share Posted July 4, 2015 Have you checked for an intake manifold leak by spraying carb cleaner along the intake manifold while the engine is idling ??? 1 Link to comment Share on other sites More sharing options...
Guest 66chev Posted July 4, 2015 Share Posted July 4, 2015 Yeah tried that one to even took fan belt off so fan wouldn't blow the carb cleaner away Link to comment Share on other sites More sharing options...
TexasJohn55 Posted July 4, 2015 Share Posted July 4, 2015 Intake leaks at 1 and 6, that would explain the pulsing intake vacuum readings at idle and the dry plugs. May be worth pulling the manifold. Link to comment Share on other sites More sharing options...
Fleek Posted July 4, 2015 Share Posted July 4, 2015 If it has been doing this ever since the rebuild, then I would think something may have slipped when installing the intake gasket blocking the flow or some foreign object in the manifold is blocking it. Link to comment Share on other sites More sharing options...
Rusty_OToole Posted July 4, 2015 Share Posted July 4, 2015 If it is an intake gasket leak, squirting WD40 around the joints when it is running will tell the tale (engine will speed up). My first guess was #1 and #6 spark plug wires crossed but OP says he has checked this multiple times. Link to comment Share on other sites More sharing options...
NTX5467 Posted July 4, 2015 Share Posted July 4, 2015 The "low power" issue MIGHT be reversed leads to the ignition coil, or even a weak ignition coil itself (not very likely, from my experiences). In the older service manuals, there was a test for this, using a lead pencil point, to see which direction the spark flared from the lead, when it was put between the spark plug and the plug wire end, as it fired. You mentioned the "short block", is it a reman unit (from another vehicle) or was the original motor rebuilt? Reason I'm curious is that in some cases, the keyway in the crank can not be completely-accurately located, which can retard the timing as the timing mark on the pulley/balancer/hub will be accurate for if the keyway was cut accurately, but will follow the keyway whichever way it's not correct, which can affect how the base timing is set, PLUS the cam timing too. Now, I'm also curious as to if the pistons in the short block are correct for the model year. I suspect all of the inline 6s had the same compression ratio for many years, but it's the combination of piston AND cylinder head that make compression ratio. Although the Rochester 1bbl is very simple, IF the power enrichment mixture is not there, "no power under load", plus backfiring. NTX5467 Link to comment Share on other sites More sharing options...
TexasJohn55 Posted July 4, 2015 Share Posted July 4, 2015 The sixes of that era were 194, 230 and 250. Hard to tell apart. The full size in '66 introduced the 250, '67 in smaller cars. If I recall correctly, the cylinder head design was later changed to what they call "integral intake" which was different from earlier years which had the intake and exhaust sandwiched and bolted together, the '66 would have been that earlier style. If they are not assembled correctly, you could have problems sealing up everything. Link to comment Share on other sites More sharing options...
Guest 66chev Posted July 4, 2015 Share Posted July 4, 2015 (edited) The original block was cracked so I ordered a new short block from a machine shop out of Texas that does a lot of automotive work for dirt track racers. I thought about the key bring off but if I align the zero mark on the timing then both valves on the #1 cylinder are closed. I actually thought the harmonic balancer may have shifted inside of itself but doesn't seem to be the case.The intake exhaust is not integral. When I first put the engine in the truck it ran really good for quite a while then this problem began to slowly develop and has gotten to where it is now. So it's not something that happened all of the sudden. When I say no power under load what I mean is goong down the highway 3rd gear about 45-50 mph is all it will do.Coil is new in fact I just switched it over to one of those Petronix ignitions that takes the place of the points. Edited July 4, 2015 by 66chev (see edit history) Link to comment Share on other sites More sharing options...
John348 Posted July 4, 2015 Share Posted July 4, 2015 (edited) When you replaced the head did you separate the intake and exhaust manifold from each other? I recall I had a similar problem a long time ago a friend of mine had when he replaced the gasket for the heat riser pocket between the manifolds. As I recall the manifolds get tightened to the head with the three or four bolts holding the two manifolds together snug but not tight. This allows the manifold bolts and retainers to pull in the manifolds evenly to the head. Then go back and tighten the bolts that hold both manifolds together. Considering that both 1 and 6 are the furthest away from center,,,,, just a thought considering you have seem to have tried everything, it might be something restricting them from pulling in tight. Also maybe plug off every vacuum line at the intake source to see if it may be a secondary leak. Good luck Edited July 4, 2015 by John348 (see edit history) Link to comment Share on other sites More sharing options...
Guest 66chev Posted July 4, 2015 Share Posted July 4, 2015 John thanks for the thought actually I bolted the intake exhaust togather tight then put them on the head I will take that all loose check to see if any leaks around the gasket then put all back on. Again thanks for the suggestion man I hope it is something that simple will keep you posted Link to comment Share on other sites More sharing options...
John348 Posted July 4, 2015 Share Posted July 4, 2015 I remember reading somewhere how this can screw things up even cause a hairline crack in the intake manifold in the heat riser pocket which you can not detect.Good luck again Link to comment Share on other sites More sharing options...
Beltfed Posted July 4, 2015 Share Posted July 4, 2015 You might take this problem over to a forum called "Stovebolt". I use to run the same exact engine and those guys gave me a lot of help as they are experts on that engine. Scroll down to the shop area and you will see a section called 'the engine shop'. Post there and a guy called 'Hotrod Lincon' or others will come to your aid. Link to comment Share on other sites More sharing options...
John348 Posted July 5, 2015 Share Posted July 5, 2015 Not that those guys won't offer any help but these are the post stovebolt era six cylinders Link to comment Share on other sites More sharing options...
Beltfed Posted July 5, 2015 Share Posted July 5, 2015 Correct, but still an inline Chevy six cylinder which they are very familiar with and also it's big brother the 292. Link to comment Share on other sites More sharing options...
Guest 66chev Posted July 7, 2015 Share Posted July 7, 2015 john348 I loosened all the bolts on the manifolds then re torqued the manifold to head bolts starting at the center and working out to the ends. Then I tightened the 3 that bolt the two togather. I really think it helped because once the engine warmed up it took gas really well. Before it would hesitate when I would rev the engine. Vacuum readings didn't really change not sure what's going on there. Tomorrow I'm going to take it down the road a little and see how it runs. Link to comment Share on other sites More sharing options...
padgett Posted July 7, 2015 Share Posted July 7, 2015 153624 Link to comment Share on other sites More sharing options...
John348 Posted July 18, 2015 Share Posted July 18, 2015 So how is it running? Link to comment Share on other sites More sharing options...
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