carburation

[John Thomas]

I would appreciate a discussion on carburation. I have replaced my 327/350 L79 motor with a 350 cu in motor set up to look and sound like my original. The motor is bored 30 over to clean up the cylinder walls. The compression is 11:1 and I have a L79 profile cam. I want to drive my 67 coupe without damaging the original motor that has over 100,000 miles. I will probably re-install it some day. I would like the 350 and the engine compartment to look as original as possible, so I am trying to use the original intake, carb, and air cleaner. The problem is I can't get any response to the idle mixture screws. The carb is set per 327/350 specs. I can screw them all the way in and the motor keeps on running. It idles a little fast but is controlable with the idle speed adjust screw. The motor responds well but seems to surge at constant speed partucularly at or below 35 mph.

Now the questions. Should the carb be set per 350/370 specs? What are they? Is the carb too small? Will it ever be able to function properly? If not what should I replace it with (350/370)? Could the 327/350 vacuum advance be the problem? Should I replace it with a 350/370 vacuum advance? What is it?

As you can see I have a lot of questions and perhaps someone more informed than I am could offer some insight.

Thanks
John

[Joe C.]

Re: carburation

John:

Sounds like you have a significant vacuum leak somewhere. Assuming you assembled everything correctly, there may be a compatibility issue somewhere--leave that to someone better informed than myself.
The carb will work fine with the added displacement, and the L79 vacuum can will work OK also, as it is designed for your camshaft. The first place I would go, would be to look for a vacuum leak.

Joe

[G B.]

Internal leaks

I assume that the original carb you're using is a Holley 3810. Like all its Holley relatives, this model has a tendency to develop a warped main body over time. This warpage causes internal fuel leaks which can keep the engine running even when you shut both idle mixture screws.

Average carb rebuilders install new gaskets and then return your carb for you to test. Good carb rebuilders always machine Holley main body gasket surfaces to make certain they are flat and then hot test your carb on an engine before returning it. I suggest you find a good Holley rebuilder.

[Mike McKown]

Re: Internal leaks

Jerry's advice would be good to follow. I guess I've been lucky. I've not experienced this problem even though many apparently have.

You are getting too much fuel in there, not too much air. If you haven't already, check the float levels before you pull it off the engine.

Your carburetor should work fine for that engine if it is in good shape. No alterations should be required.

Mike

[Gene M.]

Re: carburation

John,
Assuming you have a non leaking carb and every thing is flat, (not warped) the setting is not right. If you have the throttle plates open past the transition slots you will not get any response from the idle screws. Fix this, getting the throttle plates so they just cover the transition slots before you assemble the carb on the engine.