140 miss at constant speed only

[sh011956@aol.com]

65 Corsa with 66 160.

I have been chasing this miss for a year. I have what feels like a slight misfire when running at a constant speed. Does not happen on acceleration.

So far I've replaced the sparkplugs, wires, cap, points, point plate, coil and condensers.
with no change. Carbs have been rebuilt and electric fuel pump added with no change. Valves have been adjusted a couple of times.

This may be the telling point:
Compression check
#1 115 #3 120 #5 115
#2 95 #4 120 #6 100

Could low compression on LH bank be the problem?

If I pull the head, what should I expect to find? Burned valves? Bad valve guides?

The drivetrain is coming out in a couple of weeks to address a clutch issue.

Any input would be appreciated.

Steve

[chris]

Maybe it's the vacuum advance acting up? It's free to test at least! If you're pulling the drivetrain anyway, taking that head and checking is probably a good idea, and it's definitely not out of the question that it could be the problem.

[cnicol]

I've seen this before. The ones I've fixed were caused by poor contact at the points-plate grounding spring. Vacuum advance only occurs at cruise and when it advances, the points plate moves and the grounding spring loses contact between its moving and fixed element.

[Corventure Dave]

Many years ago I had a problem like that with my 140. Holding a constant speed or going down a slightly down hill grade.
For me, I solved it by re-synking the secondaries and also reducing the secondary carb jet size.
Craig has good advice also.

Corventure Dave

[66vairguy]

I've seen this before. The ones I've fixed were caused by poor contact at the points-plate grounding spring. Vacuum advance only occurs at cruise and when it advances, the points plate moves and the grounding spring loses contact between its moving and fixed element.

Good comment. A common fix is to add a ground wire between the moving and fixed plate. NOTE: I've also seen this issue with the Pertronix plates. The Pertronix "kit" should have a wire to add between the plates (goes under screws) and OFTEN it is missing. For awhile Pertronix shipped items loose in a box and the wire would fall out of the box when it was moved prior to shipping.

Also note that excessive vacuum advance will cause "surging" at constant highway speeds that feels like the engine is missing. The ignition book gives the advance vs. vacuum curve to check it. A simple test is to unplug the vacuum advance hose (and plug it) at the distributor vacuum advance can. Go driving and if the "missing" goes away it is either too much vacuum advance or a poor ground due to the plate moving as Craig said.

Since you said the car runs fine under full throttle up to higher RPM I suspect the distributor bearings are fine, but they should be checked.

As others mentioned it could be a lean fuel condition or a vacuum leak ---- BUT one thing at a time!!! The ignition is easiest to check first.

[sh011956@aol.com]

Thanks for the input, guys. I have more work to do. I would really like to solve this before I pull the engine.

[RexJohnson]

Make sure that the secondaries aren't opening to soon or you can have a lean condition.

[American Mel]

Do NOT PULL YOUR ENGINE!
Not yet, anyway.
Due to the air-cooled nature of our favored little vehicles, doing a compression test does not really give you a true sense of engine condition UNLESS the numbers come back WAY LOW.
Because our cylinders can change shape during heat cycles, It is not uncommon for the reading to vary between compression checks.
Take that same car down the highway on a good half hour run, and do another compression check, and I bet your numbers will ALL be different.
Every heat cycle shifts things around a bit.
Do it the next day, and they will be different AGAIN.
.
If it runs good, doesn't burn an excessive amount of oil, and doesn't look like Vector Control going down the highway, keep driving that thing!
Listen to Rex and Craig, they have your most likely remedies.

[bbodie52]

The secondary carburetors do not have much spring pressure closing the throttle butterfly valves. Only the small springs on the secondary accelerator pump mechanism apply any pressure to close those secondary throttles.

THEORY: Because the secondary carburetors see little use, the throttle mechanism can get dirty or "gummed up" from a lack of activity. If one of the throttles fails to close completely, it could create what amounts to a vacuum leak in one of the secondary carburetors — leaving one of the secondary throttle butterfly valves opened slightly, which may admit some air but not draw any fuel through the venturi to mix with that air. This might result in a lean fuel/air mixture in the intake manifold on one or both sides of the engine, which might produce the misfire you have noticed. A misfire is usually thought to be caused by the ignition system, from a faulty spark plug, a bad plug wire, a fault in the distributor cap, etc. But the fault you have noticed may not be ignition system-related at all! It could be mechanical — as with the valve train, or a fault with the carburetion. In fact, the misfire may be present all the time, but not as noticable when the engine is working hard under hard acceleration.

You might try disconnecting the secondaries, spray cleaning the secondary carburetor throttle mechanism, and then manually closing the secondary throttles while leaving them disconnected. When you are certain that the secondary throttles are fully closed, see if the misfire you noticed before while cruising at a steady speed is still present with the secondaries fully closed.

I hope this theory is helpful as you search for the cause of your engine misfire.

If you suspect a faulty valve seal in a cylinder, a cylinder leakdown test may reveal a mechanical seal problem. The cylinder leakdown test can help to diagnose internal engine faults, like bad rings, valves, and leaking head gaskets before you decide to remove a cylinder head or two.

https://www.fcpeuro.com/blog/how-to-do- ... xwQAvD_BwE

https://www.bestproductsreviews.com/cyl ... zkQAvD_BwE

[vairmech]

I just had a customer having a similar issue, constant throttle 2000-2500 rpm. The engine ran fine under acceleration and we did everything you did so far looking for the issue. Compression numbers were a little off just like yours. The customer got it in his head to rebuild the engine so he pulled it and took it apart. Of course he expected me to put it back together! Someone had been in the engine before so it was somewhat warranted. BTW, he is running the TSP distributor with an MSD box!

So now we have a new engine and I fir the engine up with the old carbs and guess what????? Do I really need to say it? the engine did not run smooth even on the stand! I got totally different bases and the only thing I used from those rebuilt carbs was the throttle shaft, (not even the butterfly), even different clusters and the hard parts. So I fire it up again and it was better but not cured so I changed the clusters again with different ones and Wa La, the engine now runs smooth as silk at the 2000 rpm.

So it took me 3 different clusters to find a good pair. I was able to isolate which cluster it was by blocking off the bleed hole above the idle jet. One side made the engine almost die and the other side made the engine smooth out!

[jimbrandberg]

I like that closing off the idle bleed hole idea for testing purposes. It made me glad I came around today.
Jim Brandberg
Isanti, MN
CorvairRepair.com

[sh011956@aol.com]

The vacuum advance unit is new as I replaced it some time ago while I was chasing this problem. I verified that it is a 140 distributor, and the point plate has a ground wire.

Yesterday afternoon disconnected the vacuum advance on the distributor and went for a test drive, including 70+ mph on the interstate. No more surging.

So...Leave it unplugged?

Hook it back up and retard the timing a little?

Or try an electronic distributor? Anyone have experience with the California Corvair electronic distributors?

[66vairguy]

Some great comments so far. I just noticed you may have a 66 engine in your 65. Oh Boy! Just my experience, but the 66 carburetors have an issue with corrosion. The 66 model primaries had a revised idle circuit with a small vertical slot above the mixture needle hole in the throat of the carburetor. This allowed for a leaner, but more even idle mixture and it worked well UNTIL the internal passages from the cluster jet to the idle port become corroded or full of debris. NOTE this results in a poor idle, but should not affect cruising on the highway AFAIK! As Vairmech noted these old carburetors can be an issue.

That said you solved the issue by eliminating the vacuum advance at cruise on the highway. I have a Harbor Freight hand vacuum pump like a MityVac, BUT the Harbor Freight unit works much better (surprise). I use it to control the vacuum canister while I have someone hold a timing light. I get the vacuum reading and how much it advances. The maximum advance is 20-25 at 15 in. HG. 7=11 at 10 in HG. There should be no timing advance at 5 in HG or less.

NOW -- Before you buy an electronic distributor make sure, as other said, the secondary carburetors are closed fully. There should be no or only a slight air flow at idle and make sure they don't open until about 2/3rds gas pedal depression (do not open at highway speeds of less than 70MPH). Also check the harmonic balancer for slippage. There is a cut mark between the inner and outer part that should line up. Sooner or later the harmonic balancer rubber fails and the outer part (with timing mark) slips and that will cause basic timing adjustment to be incorrect.

The Corvair stock DELCO distributor is a good unit and usually only one bushing wears out. I've rebuilt a few and they work fine HOWEVER---- I sometimes find parts installed incorrectly or the wrong parts installed. Folks can post information about distributor numbers and parts.

Good luck as you have some trouble shooting work to do. Just don't start "throwing parts at it" trying to fix it. This can result in more problems. Always good to know someone who has a spare part you can swap in to try and locate issues.

[RexJohnson]

When I was fighting my lean condition from my secondaries opening too soon unhooking the vacuum advance would make it run better. Unless the points plate is loose and changing dwell as the vacuum advance moves then unplugging it is just covering up what the problem truly is.

[sh011956@aol.com]

I will check my secondaries thoroughly. Troubleshooting may slow down as we are leaving for Helen, Ga this week.
When I am confident that this issue is resolved, the drive train is coming out to address a throwout bearing issue.

I'll be driving my 66 Monza to Helen. It runs great!

[66vairguy]

When I was fighting my lean condition from my secondaries opening too soon unhooking the vacuum advance would make it run better. Unless the points plate is loose and changing dwell as the vacuum advance moves then unplugging it is just covering up what the problem truly is.

Agree, although I occasionally find a bad vacuum advance unit causing a problem. Point is -- Don't make up your mind until you check everything!

I did not notice anyone bring up the rubber couplings at the vacuum balance tube to the carburetor pads. I must admit to forgetting about these and vacuum leaks will cause odd issues. The rubber coupling gets HOT and deteriorates and sooner or later they must be replaced.