Corvair Forum

Once is a rare while I get a brief dropout when pulling off from a light with my 66.
Runs great under all other conditions.

Yesterday after getting off the interstate and stopping at a friend's work parking lot I left it idling while we talked.
In about five minutes it stopped dead. No signs, burps, farts or noises. Running 93 octane in it and fresh gas.

I left it alone for about 5 more minutes while we talked and it started and ran fine on the way home.

Seems to be only when I am at low RPMs after extended heated runs.

OK experts weigh in with thoughts, suggestions, opposing viewpoints etc.

Thanks,

Mike

Please click on the following link for a post that may be helpful...

viewtopic.php?f=55&t=12217&p=83540&hilit=vapor#p83540

I had a 56 Buick that on warm summer nights in Nebraska while waiting in line or in traffic would vapor lock. An old timer told me to put clothespins on my fuel line. I did, and never had a problem again, supposedly because they act as heat sinks. I don't know if it solved the problem or not, but since it helped when I bought my next 56 Buick and on 51 Plymouth I put clothespins. If nothing else it makes a great conversation starter at the car shows and drive-in nights.

Had similar problem, usually after parking on a hot day. Car would sit, then crank and crank with no fuel in carburetors. Found the problem was the fuel lines run from the front of the car, under the body and close to the hot parking area. As the car sits for a while, the fuel absorbs the heat, and then becomes a vapor in the metal fuel lines. The mechanical pump will not move vapor very well.

Solved the problem by installing an electric fuel pump near the tank. The fuel pump is able to "push" the fuel up to the carburetors, rather than, as the mechanical pump does, hopes for atmospheric pressure to overcome the vapor.

No longer have the vapor lock problem.

If it only happens when pulling away from a light then isn't vapor lock. Vapor lock will occur on hot days after the vehicle has run and sat for a little bit. The liquid fuel in the lines will turn to vapor.
I had the stumble when pulling away from a light. It was the carbs needed to by synchronized.

notched wrote:If it only happens when pulling away from a light then isn't vapor lock. Vapor lock will occur on hot days after the vehicle has run and sat for a little bit. The liquid fuel in the lines will turn to vapor.
I had the stumble when pulling away from a light. It was the carbs needed to by synchronized.

Would that also cause it to stop running after idling for a while?
Seems to only do it when the weather is warm and the car has been driven for 20-30 minutes never when the weather is cool.

Thanks

Mike



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MinVairMike wrote:Would that also cause it to stop running after idling for a while?
Seems to only do it when the weather is warm and the car has been driven for 20-30 minutes never when the weather is cool.

I would suspect heat buildup in the carburetors, as excessive heat is transferred from the aluminum cylinder head intake manifold to the aluminum carburetor housing. When you are driving the fuel in the float bowls and in the carburetor fuel passages is continually in motion, since more fuel is used by the engine. But when the car is stationary and idling for long periods, less fuel is used so the static fuel begins to heat, and possibly boil or vaporize. This would be more-pronounced on warm days, since more heat would accumulate and could be transferred to the carburetor bodies. The problem was noted by GM engineers, and addressed as described in the material shown below. To correct this problem, some internal changes were made in the Rochester carburetors, such as the main well aluminum inserts mentioned.

Also, plastic insulators are normally installed between the carburetor mount and the carburetor, to isolate the carburetor from the heat present in the aluminum cylinder heads. You should check for the presence of these insulators under each carburetor. They are sometimes omitted when the carburetors are removed and reinstalled. The aging insulators can crack and break into pieces when removed. Carburetor rebuild kits may include a thin gasket for each carburetor base, but they seldom include new insulators. The mechanic may choose to install a single gasket under each carburetor — thinking that a gasket will suffice. IT IS NOT ENOUGH. The recommendation is two gaskets — one on either side of a thick plastic insulator. Without the insulator, heat may accumulate in each carburetor and can cause the problem you describe.

http://www.corvair.com/user-cgi/catalog ... IN&page=56

The carburetor insulator is clearly shown as a part of the assembly in the attached 1965 Corvair Assembly Manual.

1965 Corvair Assembly Manual - CARBURETOR & FUEL SYSTEM.pdf

1965 Corvair Assembly Manual - CARBURETOR & FUEL SYSTEM

(3.92 MiB) Downloaded 40 times

Also, on page 9–13 of the attached section of the 1961 Corvair Shop Manual, the very first step in the instructions for installation of the carburetors on the engine reads as follows…

Installation on Engine

1. Install insulator block in place, install carburetor on intake manifold studs…

1961 Chevrolet Corvair Shop Manual - Section 9 - Fuel & Exhaust Systems.pdf

1961 Chevrolet Corvair Shop Manual - Section 9 - Fuel & Exhaust Systems

(1.31 MiB) Downloaded 41 times

This would seem to indicate that the insulator blocks have been in use under the carburetors since the very first model years. It is recommended that a carburetor gasket also be installed above and below the insulator to ensure a proper seal and to avoid vacuum leaks. The insulators themselves can develop cracks over time, and can also be damaged when they are removed from the intake manifold. The insulator should be carefully inspected to ensure that no small cracks have formed that could produce a vacuum leak at the base of the carburetor.



The issue with heat buildup in the Rochester carburetor is also mentioned on page 6 of the attached Delco Rochester Carburetor Service Manual, as follows…

MAIN WELL INSERTS

During very hot operation, the fuel in the main fuel well tends to boil and this produces vapor bubbles. The bubbles tend to interfere with fuel flowing into the main well tube which upsets good metering. To help reduce the effect of heat and remove the fuel vapor bubbles, an aluminum main well insert is used. This resembles the main well tube and completely surrounds it. This reduces the amount of vapor getting into the main well tube, helps dissipate heat and improves hot operation of the system.

DELCO ROCHESTER - Models H, HV Carburetor Service Manual.pdf

DELCO ROCHESTER - Models H, HV Carburetor Service Manual

(1.79 MiB) Downloaded 37 times

So it would seem that heat transfer from the air-cooled Corvair heads via the integrated aluminum intake manifolds to the carburetor mounting pads and on to the carburetor bodies was a well-known problem to Chevrolet engineers, and as such it was addressed in several ways — using insulator blocks between the intake manifold and the carburetors, and internally in the design of the Rochester H carburetors using an aluminum main well insert.

Have your verified spark AND fuel (or lack of) immediately after it stalls? Will it restart immediately? Do you have a good accelerator pump shot on both primary carburetors? Verify that you have a good pump shot when the engine is cool. Next time it does it while driving see if you still have a good pump shot.
Was this a problem that recently started occurring or has it been doing it for a long time?
Was there any repairs or work performed to the engine recently that it could be tied to?

notched wrote: ↑Sat Mar 11, 2017 8:20 am Have your verified spark AND fuel (or lack of) immediately after it stalls? Will it restart immediately? Do you have a good accelerator pump shot on both primary carburetors? Verify that you have a good pump shot when the engine is cool. Next time it does it while driving see if you still have a good pump shot.
Was this a problem that recently started occurring or has it been doing it for a long time?
Was there any repairs or work performed to the engine recently that it could be tied to?

Very good questions and some things to check out. Here goes:

Starts back immediately, doesn't die entirely just drops out.
Need to check the pump shots when cold
Been doing it since I got it in October just infrequently
No repairs recently that I know of, did sit for over a year before I bought it

Thanks,

Mike

Mike,
Typically with vapor lock you would have a situation where the engine will not restart when hot.
You say it "drops out". Do you mean it starts idling low and possibly rough? If so, it could be a plugged idle jet which can cause that. It can happen to one carburetor and cause the engine to run on one bank.
The pump shot is important and the 2 discharge holes on each carb are small and plug easily. A lot of guys will use a small gauge wire to clear the holes.
Since you are not sure of the history I would pop the tops off the carbs and check the float adjustment and make sure the float drop is correct.
I would check and make sure the carburetors are synchronized properly.
If it still has points I would inspect the point gap and check the dwell.
I went through all of this recently with my 69 when I first bought it.

notched wrote: ↑Mon Mar 13, 2017 9:09 am Mike,
Typically with vapor lock you would have a situation where the engine will not restart when hot.
You say it "drops out". Do you mean it starts idling low and possibly rough? If so, it could be a plugged idle jet which can cause that. It can happen to one carburetor and cause the engine to run on one bank.
The pump shot is important and the 2 discharge holes on each carb are small and plug easily. A lot of guys will use a small gauge wire to clear the holes.
Since you are not sure of the history I would pop the tops off the carbs and check the float adjustment and make sure the float drop is correct.
I would check and make sure the carburetors are synchronized properly.
If it still has points I would inspect the point gap and check the dwell.
I went through all of this recently with my 69 when I first bought it.

Great ideas and a weekend in the garage coming up soon to check these out!

Thanks,

Mike

OK folks thanks first for all the ideas. I have about an hour in the garage this afternoon to look into all the great suggestions and report back. Here is what I found out in no particular order.

Both chokes set correctly cold with the engine off, right one was open a bit more than the left.
Vacuum pump confirmed both choke pulloffs worked, right one may have a slight vacuum leak but it was very slow on the gauge.
Started engine, a little grumpy cold as always.
Both choke pulloffs were completely closed.
Chokes opened as the engine warmed up.
When fully warm I checked the accel pumps on both primaries
With a good snap of the main pedal rod had plenty of gas spray and nice pattern out of each.
Even manually gave each secondary a snap of their individual rods, engine bogged as you can imagine but both gave a nice squirt of gas and engine RPM picked up after the bogging.

Now for the interesting part, and maybe I'm going down a dirt road here but stay with me.....
I was checking the vacuum advance hose to see if it was on and my hand touched the noise suppression capacitor/condenser bolt to the coil. Wowsy was it hot. Made me wonder so I touched the coil. Could barely stand to touch it too hot for sure.

Now what could it be? We under the coil I felt a hot blast of air. Seems there is a shroud opening about 1" by 2" under the coil and it was getting a direct hot blast of air from under the turkey roaster shroud.

Seems to my mind that coils don't like to run too hot. Is there supposed to be a plug in that opening? Maybe I'm overheating the coil.
To my mind a coil getting too hot could cause ignition dropout under certain conditions and definitely the stopping while idling for a while after a long interstate blast.

What say the aircooled experts?

Thanks,

Mike

On the late models there were 2 different right side damper door assemblies. One has the slot under the coil and the other does not. My 66 turbo had it. So did the 3 140's I have here. The 140 in my 69 convertible has it as well. I have found ones without the slot and they are typically on 95 and 110 engines.
Looking through the GM parts book they list the part numbers but do not specify which engines and why they would have them. No, they are not supposed to have a cover over them.
Have you verified if you lose spark or fuel when it stalls?

Have not verified which happens first. Before the stalling while idling for a while it was a dropout when starting off from a light when hot and running for quite a while. Hard to troubleshoot that one in the turn lane or at the redlight, LOL.

Thanks,

Mike

My 180 killed a coil once, same exact symptoms you have. Moved the coil off the hot engine away from the heat, never happened again.

Well another evening in the garage and help from my friend Bob. Attached are two photos, one of mine built in April 66 with the hole under the coil open. Another of Bob's built in Feb 66 with a cover plate bolted over the hole under the coil.

Now to find either the pn, a photo or sketch of that plate so I can make/buy one.
Any one see one of these before?

Thanks,

Mike