Been reading for years on and off without much to really dive into. I am now at wit's end and this is a hail Mary. I started wrenching in 1978 back when we used to tune up Brontosaurs. My first venture was to set the valve lash on a solid roller. Built many things through the years and have experience from headlights to tail lights including building and maintaining an Offy for awhile. I got out of this in 2010 and was drug back into it in 2017. Got into 60s Lincolns for the cruise aspect instead of the traditional hot rod genre. Brought 3 462s back from slumber after 11. 30 and 32 years. Then had the chance to buy my wife a 68 Vette factory air small block automatic convertible. (birth year car and the car her dad promised her before he wrapped it around a pole) First car I have EVER bought complete. I hate this thing. I should have drug home a pile of elephant dung and built it. I don't fault the car, it is a victim of whatever has been done to it for the last 57 years. It is a 350 block with 350 horse 327 heads on it. Quadrajunk carb. The converter is a bit small for whatever cam is in it as it has a solid thump when put into D or R. I have replaced the brakes, radiator, converted to a 1 wire alternator, converted to HEI, new balancer (old one had spun), of course tuned it up, replaced the starter 3 times and now have a gear reduction mini starter on it along with a new flywheel. Ran fine until I fixed it. Out driving one night she pulled into traffic and it lost a cylinder. Limped it home and number 8 intake pushrod had broken. Evaluation showed the valve was mushroomed. New rocker and pushrod sat at an angle but the car ran fine. It bothered me that we were waiting for it to happen again and I was not happy with the oil flow. I started looking into replacing 1 valve since everything else is fine. Found out about valve caps. Dremel'ed the end of the valve and the cap fits nice and snug. Rocker sits straight. Talked to a friend who has been at Comp forever and he told me he had a new type of lifters that he felt would be fine on an old cam. (I have the box if anyone is interested) Pulled the intake which is where I think the source of angst is but replaced 16 lifters. Put engine back together. Adjusted rockers first by hand and then by sound. Loosen until tapping then 1/2 turn. Cranks easy and idles good but I have 5 inches of vacuum at idle and NO power. Thought maybe valves are a little too tight - backed them all off 1/4 turn. No change. Compression test has everything close enough to 175 to say they are all at 175. To be safe I have pulled the wires off the car and re-wired it. New plugs (yes, I gapped them). Tested every prong on the distributor. Tested every wire. Sprayed cans of carb cleaner over everything with no change in idle. Made sure all vacuum ports on the Quadrajet except the vacuum advance were capped. The only other thing is that I had adjusted the carb before (vacuum gauge) and had nice clean plugs. Now I have sooty plugs and she is running rich which makes no sense if I have little or no vacuum but here we are. I have had 3 friends who are also old and lifetime wrenchers look at it and we all stand there and stare at it. Only thing IMHO left to do is R&R the intake but other than no vacuum I don't find a smoking gun that it is the problem... I fixed it into a new problem!
Not that I know of/have noticed. I did do a new heat shield/gasket when I did the intake but nothing out of the ordinary on the old one/visual inspection.
Low vacuum is typically caused by Retarded timing Vacuum leaks (you have checked and plugs sooty = not likely) Big cam Sooty plugs are typically caused by Too much fuel (carb problems) Poor ignition (you have checked) Poor combustion (possibly big cam) Ran fine until I fixed it. Ouch, I know the feeling! But you also say it's problems are due to the stuff done before. Own up to the fact that you are now the caretaker. Bring it back to stock or better now. 3 starters sounds like cheap replacement stuff. Do it right or do it over and over. I would check that cam, try to figure out what you are working with. Since it ate a part, inspect the valvetrain fully. I'd also make sure that the new balancer shows accurate TDC. You should be able to tune by ear, but it's nice to have a number. You pulled the intake, so the dist. did come out. Compression at 175 indicates a pretty high compression, especially if it has a big cam bleeding off at cranking RPM. After that, I'd either swap a 'known good' carb or go through the q-jet. They are pretty good carbs if right, although your naming indicates a distain. Just to cover all bases, is the exhaust in good shape? Blockage would mess things up but the symptoms would be no power with higher RPM. Also, is this the OE dist with the correct curve, or another replacement?
Polymer Quadrajet bowl floats can and do absorb fuel and sink. Then it dumps gas in the engine, making it run poorly and foul plugs. Worth checking.
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