Inliners International Forums General Discussion Engines Rough Idle

Hey guys. I have a freshly rebuilt '67 Chevy 250 that has a miss in cylinders 3 and 4 at idle. Under acceleration and higher RPM cruising, it runs like a top. Everything on the ignition side is new along with a fresh carb. Compression is above minimum spec and all cylinders are within a couple PSI of each other. Manifold vacuum is 18" at idle.

My guess is that I have a vacuum leak in the middle port of the intake. I have tried spraying carb cleaner all around the port without success, but I can't reach the underside because the exhaust port is in the way. This thing is driving me nuts. Any help would be appreciated.

Thanks.


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Have you double checked your valve adjustment?This could cause a skip.

Readjusting the valves was my last ditch effort to fix to problem. No go.

I did, however, mill the manifolds during the engine rebuild to seemingly get a better seal to the head. Maybe that caused the problem. I suppose I should pull the manifolds off again and measure the flatness of the head mating surface.

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[This message has been edited by L6Ragtop (edited 08-04-2003).]

Is your PCV valve plumbed to a pipe that inserts into the 3-4 intake runner? If so, then check your PCV valve and hose to make sure there are no cracks anywhere that could draw air. Small vacuum leaks from either of these could make for a misfire. Once the engine gets up to speed, the leak becomes too small to affect the air-fuel ratio.

There is also a possibility the PCV valve is not the correct application (could have been packaged wrong) and is drawing in too much air. Double-check it. They usually have the part number stamped into the side of the metal body. A valve for a big block chevy looks almost identical to a late L6 valve and will pass too much air.

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David
newbie #4153

If you have power brakes make sure you don't have a leak around the brake booster.I had one that leaked once and it took me forever to find the problem.Pinch the vaccum hose off that goes to the booster and if the engine smooths out bingo.Mine was leaking at the seal where the brake master cyl bolts to the booster.If this isn't your problem check that you have the right source of vaccum going to your distributor advance.In most situations it should be ventura vaccum.......I think.........anyway it shouldn't pull any at idle but should as you rev the engine.I had mine hooked up wrong once and when I found the problem I had to reset my base idle and ignition timing.Good luck and if I can think of any more possibilities I will post them.

Thanks guys for all the suggestions. I don't have power brakes, so that's an easy one.

I have taken the PCV valve and hose off the intake and plugged the nipple. While this did change the idle speed, the miss was still there.

The vacuum running to the timing advance canister is ported and is working correctly.

Since the miss is in two adjacent cylinders, could the problem be associated with the head or head gasket, eventhough I have good compression? I'm still betting it is a vacuum leak that I can't find.

Thanks for all the help!

If your description of symptoms is complete and accurate, then you probably do have a vacuum leak somewhere that hasn't been found yet.

Did you have the manifolds milled while bolted together or separately? The best way is while they are bolted together, but some shops don't like to do it that way.

If they were separated when faced, then you may have to take them off the head, and put a new gasket in between them, and only BARELY snug the bolt/nuts. Install the combo onto the head WITHOUT using a gasket. Snug up the retainers against the head and then tighten the mating bolt/nuts between the manifolds. That way you are sure to have a straight, flat surface for the head-to-intake gasket to seal against. Disassemble it and put a new intake/exhaust gasket on, and do a final reassembly.

If you did this when assembling it the last time, or they were never separated for facing, then all I can say is to keep on trying and looking for the leak.

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David
newbie #4153

How do you know for sure you have a miss on #3and #4?Did you do whats called a power balance test?This is where you pull the plug wires one at a time with the engine running and the rpms will decrease and you can hear the engine go down....they all should sound the same when you do it.If one or more cylinders don't pull the engine down then theres a problem with that cyl.A good example is a cyl. with a badly burnt valve will not change at all when you pull the wire off for it isn't providing anything to the running engine and will have very low compression if any.I realize you probably already know this but am making sure......
Take care...........Gerald

I had the same problem eairler this year and it indeed was an intake manifold gasket leak. You mentioned you used carb cleaner to check for the leak. Many of the new product are not flammable. make sure you read the label and use something (WD 40 type product) that says " Highly flammable" If engine speed picks up when you spray the joint you have found the leak. I know Gumout brand carb cleaner is not flammable. Read the can. That is how I found my leak.
Gael

Thanks guys.

I'll keep plugging along.

Dan

An unlit propane torch is a very good way to do this. It won't harm paint or pool anywhere dangerous. Just keep the gas flow low.

Using an old spark plug boot to adapt a length of vacuum hose to that propane bottle will give you the ability to reach under/behind things and pin-point that leake better/easier.

Joe

I think I have finally got it. Turned out to be a couple of things. First, the PCV valve was indeed the wrong one (thanks "just a six"), even though the auto parts store assured me it was correct. Secondly, the throttle shaft of the "reman " carb was leaking baddly on the thottle linkage end. I had my original carb's throttle shaft rebushed and rebuild by a quality local shop and Bingo! Now the 250 idles so smoothly I can't even hear the darn thing running.

Thanks for all the suggestions guys!