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Old 12-14-2011, 10:24 PM
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Vacuum readings, unstable idle

Hi everyone, got an issue thats really starting to bug me. Been to a couple different mechanics, and still no joy....

Engine L31, RPM manifold, 600 holley

Basically, my idle feels like it is surging, and occasionally feels like it will stall.

I have been all over it looking for vacuum leaks, tried propane and smoke and also had the mech do it. Can't find anything.

I have replaced the plugs, leads, PCV and brake booster hoses. New air cleaner and air filter. Set float levels, and adjusted idle mixtures many times. Mechanic said the dist cap is ok.

Still doing it. I am about to throw the bloody thing out :-(

Seems fine while driving, everything feels ok from just above idle up to full noise, just when you are sitting with it in gear, and to a lesser extent, when idling in neutral.

Open to any suggestions for something I might have missed, or something i should recheck?

Vacuum gauge reading is about 20 in neutral but it flickers rapidly but only over about 1". It never steadies out. It occasionaly drops a bit slower maybe 1.5" then back up, and its when it does this it seems the motor surges, or stumbles.

Any ideas or suggestions would be welcome. Motor has only done about 30k kms so I am hoping its not major like valves. Please help?!!!

Note I don't really trust mechanics anymore, he conclusively said it was plugs, changed them no joy. Then he said conclusively, leads, changed no joy. Then he crashed the car into a BMW.

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Old 12-14-2011, 11:08 PM
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sounds like a sticking valve
http://www.classicinlines.com/Vacuum.asp

look on this website and there is a break down on how to read a vacuum gauge which will explain everything pretty well i think
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Old 12-14-2011, 11:45 PM
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Quote:
Originally Posted by holden350
Hi everyone, got an issue thats really starting to bug me. Been to a couple different mechanics, and still no joy....

Engine L31, RPM manifold, 600 holley

Basically, my idle feels like it is surging, and occasionally feels like it will stall.

I have been all over it looking for vacuum leaks, tried propane and smoke and also had the mech do it. Can't find anything.

I have replaced the plugs, leads, PCV and brake booster hoses. New air cleaner and air filter. Set float levels, and adjusted idle mixtures many times. Mechanic said the dist cap is ok.

Still doing it. I am about to throw the bloody thing out :-(

Seems fine while driving, everything feels ok from just above idle up to full noise, just when you are sitting with it in gear, and to a lesser extent, when idling in neutral.

Open to any suggestions for something I might have missed, or something i should recheck?

Vacuum gauge reading is about 20 in neutral but it flickers rapidly but only over about 1". It never steadies out. It occasionaly drops a bit slower maybe 1.5" then back up, and its when it does this it seems the motor surges, or stumbles.

Any ideas or suggestions would be welcome. Motor has only done about 30k kms so I am hoping its not major like valves. Please help?!!!

Note I don't really trust mechanics anymore, he conclusively said it was plugs, changed them no joy. Then he said conclusively, leads, changed no joy. Then he crashed the car into a BMW.
If you have altered the mechanical advance rate by installing lighter springs, you may be right at the point where the advance is trying to come in. One heaver spring replacing one of the lighter springs would help if this is the case.

If you're using a vacuum advance it might also be right at the point where it wants to come in if you're using ported vacuum. Even at idle if the throttle blades are open enough there can be some vacuum present at a ported vacuum port. Hooking the vacuum advance to a manifold vacuum source would help this. You'll need to lower the idle speed after switching the vacuum source.
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Old 12-15-2011, 07:30 PM
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Hi guys thanks for the replies i shot out and checked out the suggestions straight away.

Cobalt, tried changing one of the springs, put on a really heavy one but didn't seem to fix it. I should have thought of that and checked it first, but it took your reminder to rule that out so thanks. With regard to the vacuum, it had no vac advance when I got it, one of the first things i did was hook it straight up to full manifold. Everyone on this site helped me out with my last little 327 and I am a firm believer in vac advance to full manifold vac. So thanks for the suggestions. I have an adjustable can, but it doesn't seem to change no matter the setting, so I am fairly sure its not vac can related (could be wrong still )

U&C thanks for that link, best one on vacuum tuning I have seen, and I have done a lot of searches!

Anyway, given most other options have been explored, and the closest vac reading was the one for sticky valves, I wanted to try the seafoam to perhaps free it up. But of course we dont have seafoam in NZ, bugger! Closest thing I could find was Rislone Engine Treatment (I read good and bad reviews on this before I chucked it in, but it was a last resort type of thing). And it seems to have made a little difference. There is still a bit of a shake in the vacuum gauge but not to the same extent as before. I have only done about 40 miles, I assume it keeps working as you drive so will need to give it a bit longer til I know if its fixed the problem.

So the ongoing saga continues, just wanted to drop in to say thanks for your replies, I will post back if the Rislone works, or if I need more suggestions.

Many thanks
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Old 12-15-2011, 08:24 PM
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something you could try IV NEVER done it but some people like it is marvels mystery oil supposedly you can put it in the gas tank and in with the engine oil as an additive, i dont believe in putting a band-aid on an open wound. buuuut if it is just sticking that may solve your problem, also another idea is with the engine running and all warmed up choke wide open dribble water in the carb im talking drops not splashes lol and that will basically steam clean the combustion chambers and cylinder heads, i think cobalt might know more about this than i do. iv seen the topic ,mentioned else where, me id just pull the heads clean everything by hand and reassemble but to each his own , good luck
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