C3 (69) Blues (Solved)

[Rob M.]

It seems like we have solved the C3 Blues mistery. At the end it was the carb. gasket causing the problems. It was a thin type which resulted in not blocking the exhaust bypass port effectively causing hot air to blow into the carb. Using a thicker (and therefore more stiffer) type the hot air is blocked resulting in normal behaviour of the fuel system.

Thanks all for helping me out (especially Jack Humphrey for putting me on the right track and Sander for helping me out with all the pratical details) with all your suggestions and tips.

I wouldn't know how to maintain and drive these cars without you all!!!

greetings from windmill, tulip, cheese and wooden shoe country,
Rob.

[Jack H.]

And the land of Heiniken!!! *NM*

[Joe L.]

Re: C3 (69) Blues (Solved)

Rob-----

If you have an active heat groove in the manifold (i.e. not plugged at both ends), then you MUST use the stainless steel shield between the carburetor and the gasket REGARDLESS of what gasket you use. The heat will degrade ANY gasket, regardless of thickness.

My recommendation is to ALWAYS plug off the heat groove in the manifold by plugging the holes at both ends of the groove. 1969 was the last year that this idiotic system was used.

[David B.]

Re: C3 (69) Blues (Solved)

Joe,

Do you have a recommendation regarding how to do the blocking?

Thanks,
Dave

[Jack H.]

Re: C3 (69) Blues (Solved)

Most engine rebuild gasket kits include two sets of intake manifold cross-over block-offs. One fully blocks off the cross-over and the other severely restricts it with only a small hole in the center of the block-off plug. I'll bet your local auto parts shop can get the plugs for you...

[Mark #28455]

block the holes under the carb

If your car is street driven in cold weather or if the exhaust heat riser valve is intact, you should NOT block the crossover passage in the intake manifold. The passage to block is the small groove directly under the carb that has 2 about 3/8" holes feeding it. Usually this is done with the intake off the engine - just drill and tap the holes for about a 1/4" pipe plug. Then if later desired, the plugs can be remove.

Mark

[Joe L.]

Re: C3 (69) Blues (Solved)

Dave-----

The easiest way is to use small, cup-shaped plugs (just like those used to plug the oil gallery openings at the front of small blocks). These are simply driven in with a large punch. If I recall correctly, 3/8" is the size.

If the manifold is off the car, you can drill and tap the 2 holes for pipe plugs. I use allen head pipe plugs. I think that the size is 1/4" NPT. Very little overbore is necessary to tap the holes for whatever the correct size is.

[Joe L.]

Re: block the holes under the carb

Mark-----

I'm not recommending the blocking of the exhaust cross-over passage through the manifold. I never recommend blocking that under any circumstances for a street car. This is primarily due to the complete loss of divorced choke operation if that is done. I'm referring to blocking of the carburetor "heat slot" used on many pre-1970 Chevrolet engines, including most 1969 engines. This can be blocked without causing any other problems and without regard to whether the car has an operational heat riser, or not. After 1969, no Chevrolet engines had the "carb heat groove".

[David B.]

Re: C3 (69) Blues (Solved)

Joe,

As a matter of fact, I have some SBC oil galley plugs left over from previous rebuilds. Sounds good - Thanks.

Dave

[Mark #28455]

Agree, I think Jack's post blocked it. *NM*

[John H.]

Re: C3 (69) Blues (Solved)

Here's the "hot-slot" in question, with the feed holes blocked; on a small-block, 7/16" cup plugs will usually do the job. If not, drill and tap and use allen-head plugs as mentioned above.