Stumble (miss) at Idle. Please help.

[Frank D.]

Here's the facts: L75, M35, C60, N40, Carter AFB. Engine professionally rebuilt in 2011; transmission and rear end rebuilt in 2010; carb rebuilt 2011; owned since 2005 doing an extensive frame-on restoration in 2009-2010. Only non-stock engine item installed is the Lectric Limited Breakerless-SE electronic ignition conversion in 2008. Plugs and wires are brand new, distributor cap and coil replaced in 2010. On a Sun engine analyzer the car appears to be running without any problems. The distributor moves smoothly if you put suction to the advance hose. Replaced all the vacuum hoses looking for a leak as well. When we bench-tested the engine in 2010 it ran strong with no problems, but once installed in the car the stumble has always been there.

Two seasoned auto shop owners and I can't seem to solve the problem. The car is tuned properly, dwell, timing and idle speeds are all correct but at idle (575 rpm) and in neutral (1000 rpm) there is an engine "stumble" (miss?) and immediate drop of 100 rpm. The car shutters badly when this happens. You really don't notice the problem when driving, but the car does seem to hesitate slightly when accelerating. The left side exhaust seems to be "choppy" as the engine runs while the right exhaust note seems to be smoother.

Any ideas?

Thank you in advance for your suggestions.

[Thomas H.]

Re: Stumble (miss) at Idle. Please help.

Frank,

Can you set the engine analyzer up to monitor the ignition at each cylinder (or at least the left bank) and watch what happens when the stumble occurs? My initial thoughts are an intermittant ignition dropout and you may be able to capture the affected cylinder using the analyzer.

Do you have the ignition shielding on?

Tom

[Duke W.]

Re: Stumble (miss) at Idle. Please help.

A few questions:

1. Are you absolutely sure it has an OE replacement camshaft? (Actually the '65 OE 300HP cam is long discontinued and was replaced by the '67 300 HP cam, but they have identical idle behavior.)

2. Check the AMA specs or shop manual, but the idle speed should be 450-500 in Drive. With A/C it might need to be a little higher so the compressor doesn't stall the engine when it switches on. Could this problem - just be the A/C
compressor switching on?

3. What is manifold vacuum idling in Drive? State vacuum and engine speed that it was obtained

4. What is the ID number on the VAC?

Duke

[Timothy B.]

Re: Stumble (miss) at Idle. Please help.

To add to the above, if the car idles in neutral at 1000 then approx 600 in drive the 400rpm drop tells me it's lean.

The stumble and no load miss may be a lean miss, how much inital timing do you have and how far out are the A/F emulsion screws, they should be approx the same turns out. If you have too much idle timing the engine will run fine very lean but when it's put under load it will not have any power. IMO, that's why the idle drops 400rpm when placed in drive.

[Wayne W.]

Re: Stumble (miss) at Idle. Please help.

The most common miss at idle and off idle stumble is a lean 5-8 runner caused by a vacuum leak. Close off all of those hoses and check for that leak.

I agree with the others, lean, vacuum leak, high timing, bad accelerator pump.

[Frank D.]

Re: Stumble (miss) at Idle. Please help.

Frank,

Can you set the engine analyzer up to monitor the ignition at each cylinder (or at least the left bank) and watch what happens when the stumble occurs? My initial thoughts are an intermittant ignition dropout and you may be able to capture the affected cylinder using the analyzer.

Do you have the ignition shielding on?

Tom

TOM: When we used the analyzer, each cylinder was isolated and nothing unusual showed on on the screen with the engine running. All the shielding is off the car to make it easy to get to everything.
Thanks,
Frank

[David H.]

Re: Stumble (miss) at Idle. Please help.

.... When we bench-tested the engine in 2010 it ran strong with no problems, but once installed in the car the stumble has always been there. ....

When the engine was bench tested (no apparent problems)were mufflers installed? e.g. was there any exhaust back pressure during the bench test?

The thought is: a possible crack in the intake manifold allowing exhaust gas from the heat riser to bleed into the intake thus leaning out a cylinder. A bench test with no mufflers (minimal exhaust back pressure) might not indicate a problem, whereas with once mufflers were installed, the increased back pressure might be enough to force exhaust gas through a crack into the intake (leaning out a cylinder).

[Frank D.]

Re: Stumble (miss) at Idle. Please help.

Duke:

Question #1. It does have a replacement camshaft (neglected to mention that change, sorry). We ordered it after much discussion with the guys at Comp Cams. It is their model 12-206-2 for Chevrolet engines. As my car is a cruiser not a racer, we choose the mild camshaft with mid-range torque. The specs are very close to the original camshaft and are listed on the Compcams.com website.

Question #2. Idle speed is a little high (575 rpm) to compensate for the air conditioning to avoid stalling when you come to a stoplight in "drive". When you say, "Could this problem - just be the A/C compressor switching on?" are you referring to me pulling out the cable switch in the dash or the compressor cycling with the a/c off? I have an STV Valve update kit that converts the factory valve to a cycling clutch system, if that could possibly affect anything. Having the a/c running does not stop the stumbling, it just reduces the rpms of the engine slightly.

Question #3. I'll check this later this week when I take it back to the garage.

Question #4. The ID number on the vac is B20. That's the vacuum advance you suggested I use back in July of 2010 in answer to my question about advances in a car with automatic and air.

Frank

[Thomas H.]

Re: Stumble (miss) at Idle. Please help.

TOM: When we used the analyzer, each cylinder was isolated and nothing unusual showed on on the screen with the engine running. All the shielding is off the car to make it easy to get to everything.
Thanks,
Frank

Frank,

Since you have ruled out a fault in the ignition system by doing this, I'd be looking into the mixture and then vacuum leak possibilities. Maybe make a run at the valve adjustment also.

Is the side that seems to have weaker exhaust the side that has the heat riser?

Tom

[Duke W.]

Re: Stumble (miss) at Idle. Please help.

Duke:

Question #1. It does have a replacement camshaft (neglected to mention that change, sorry). We ordered it after much discussion with the guys at Comp Cams. It is their model 12-206-2 for Chevrolet engines. As my car is a cruiser not a racer, we choose the mild camshaft with mid-range torque. The specs are very close to the original camshaft and are listed on the Compcams.com website.

Question #2. Idle speed is a little high (575 rpm) to compensate for the air conditioning to avoid stalling when you come to a stoplight in "drive". When you say, "Could this problem - just be the A/C compressor switching on?" are you referring to me pulling out the cable switch in the dash or the compressor cycling with the a/c off? I have an STV Valve update kit that converts the factory valve to a cycling clutch system, if that could possibly affect anything. Having the a/c running does not stop the stumbling, it just reduces the rpms of the engine slightly.

Question #3. I'll check this later this week when I take it back to the garage.

Question #4. The ID number on the vac is B20. That's the vacuum advance you suggested I use back in July of 2010 in answer to my question about advances in a car with automatic and air.

Frank

First, I would not discount an intermittent glitch in the ignition system. Your description sounds like one or more cylinders could be cutting out intermittently, and this could be ignition related. Just because parts are "new" does not mean they work. Manufacturing defects occur, even with name brand parts. Date-coded repro plug wires are known to be poor quality beyond visual apprearance, and electronic points converstions have been known to cause intermittent problems. One test is to put points back in and see if the problem goes away. Intermittent misfires are more likely to be ignition than fuel-system related.

I wish I had a dime for every problem on a complicated system where the component and subsystem engineers said all tests had been passed, but it still didn't work, and it usually took some intense brain storming to come up with a likey cause to check out or a different test procedure that caught what the original tests couldn't detect.

The other thing to do is with the engine idling in Drive, wiggle and tug on the plug wires, particularly at the terminal ends. Do the same with the primary circuit at the ballast and body connector, and don't overlook the ignition switch. If you have a dwell meter installed while testing, the miss occurs, and the dwell meter needle deflects significantly, even for a fraction of a second, there is likely an intermittent problem in the primary circuit.

You must not have talked to me about the cam, as I certainly would not have recommended what you installed. Increasing overlap with Powerglide often leads to tuning issues, and your installed cam has considerably more overlap than the OE cam. The excess overlap also kills low end torque, which is not a good idea on the two-speed PG with an 1800-2000 stall converter. The additional low rev exhaust gas dilution usually requires a richer mixture.

Go through the idle speed/mixture adjustment at your choosen idle speed in Drive. If the problem continues richen the mixture a quarter turn in two increments to see if this helps.

Also take a manifold vacuum reading idling in Drive. If it's less than 13" you need a B28 VAC.

Notwithstanding the above, I would not assume that the ignition system is okay. Intermittent electrical glitches can be tough to find, but they usually get more frequent, so eventually you are able to duplicate the problem while testing, but you can loose half your hair in the meantime.

Here's another car war story. In a nearby thread I talked about how using resistor plugs in place of the OE non-resistor plugs on my '88 MBZ 190E 2.6 increased idle misfires and emissions. A similar rough idle and emissions problem occured later even with proper non-resistor plugs. I removed all the plug wires and tested them while twisting and bending them. They all checked out. In desparation I replaced the cap and rotor with new OE Bosch parts that cost over a hundred bucks, and the problem went away.

The cap has 2K RFI resistors in each terminal tower. I checked the old cap and they were okay until I put some load on the cap towers. One then went open, and that was the problem. Normal thermal and mechanical stress caused an intermittent open, which was only barely noticeable at idle. That one cost me a several hundred head hairs in addition to the exorbitant price of the parts.

Duke

[Frank D.]

Re: Stumble (miss) at Idle. Please help.

The most common miss at idle and off idle stumble is a lean 5-8 runner caused by a vacuum leak. Close off all of those hoses and check for that leak.

I agree with the others, lean, vacuum leak, high timing, bad accelerator pump.

Wayne: Can you explain what you mean by, "...a lean 5-8 runner" please? When checking for a vacuum leak, I have a/c, power brakes, auto transmission vac line with modulator, etc., etc. Would I be best served to plug all the vacuum lines and re-connect them one at a time? Also, can you suggest a best, proper sequence to checking for vacuum leaks?
Frank

[Frank D.]

Re: Stumble (miss) at Idle. Please help.

David:
We did have the full pipes and the mufflers installed when we did the bench test. One of the previous owners from the seventies DID mention that he once repaired a crack in the intake manifold, but we couldn't find any evidence of the repair when we examined it when rebuilding the engine. Thanks for your interest in solving my problem.

Frank

[Wayne W.]

Re: Stumble (miss) at Idle. Please help.

The intake runner that supplies #5-#8 is the runner that the vacuum devices pull from. Check all of those hoses for vacuum leaks.

[Frank D.]

Re: Stumble (miss) at Idle. Please help.

Guys:
I will work to check out every suggestion you each have made. With the mention of vacuum leaks causing the car to run lean, there is one other problem I have always had with this car, even after having the automatic transmission and rear-end rebuilt: the car shifts very hard when moving the shift lever from park to reverse, and from park to drive. Would this be another indicator as to which area could be the one causing my problem?

Please keep your ideas coming,
Frank

[Frank D.]

Re: Stumble (miss) at Idle. Please help.

The intake runner that supplies #5-#8 is the runner that the vacuum devices pull from. Check all of those hoses for vacuum leaks.

Wayne:
When I checked all the spark plugs after driving a few hundred miles, only plug #7 was fouled (very black but not gassy-smelling). We were thinking that was the cylinder that was missing as it is on the left side of the engine, the side that has the "choppy" exhaust.
Frank