Help with engine tuning problems

[Jeff C.]

Hi,
Newly rebuilt 1962 340HP Duntov solid lifter cam. 100 miles on engine. I can't seem to figure out and solve the following problems. Cold engine starts and runs fine. Fast idle is good , normal idle OK (newer AFB with electric choke). Drives well with no stumble or hesitation. After engine has set for 20 min is hard to start and runs very poorly. Driving is OK at 3000 RPM but at lower RPM's stumbles, hesitates and generaly runs like .... Engine has correct heads, flat top pistons and a 340HP vac adv dist with correct vac can. Has 14" vac at idle. Reset valve lash after 90 miles (18 & 24). Engine was rebuilt by shop with a very good reputation.
Any sugestions as to what to do will be greatly appreciated.
Jeff

[Jeff C.]

Corection valve lash 12 & 18 *NM*

[Duke W.]

Re: Help with engine tuning problems

"Correct" vacuum advance control is NONE, since the '62 340 HP was not so equipped originally.

So if you have replaced the original no vacuum advance dual point distributor with a vacuum advance single point type what is the ID number or measured characterisitics of the installed VAC? Is the vacuum source for the VAC ported or full time?

Is the heat riser valve free and functioning properly? Do the inlet manifold gaskets have open or blocked head riser passages?

Provide more details on this "newer" AFB, like manufacturer and model number. Is is jetted specifically for your application or is it one of those "one size fits" all types.

Duke

[Clem Z.]

does this problem clear itself up after

driving? if it does you could have a fuel boiling problem caused by heat from the heat riser crossover. does it emit black exhaust smoke when it starts after the hard starting hot start?

[Clem Z.]

also check to see if the electric choke is

closing in this time period because it should not and needs to be reset.

[Jeff C.]

Re: Help with engine tuning problems

I realize the original distributor was a dual point mecanical advance distributor. It had been replaced with a 1110891 vac advance distributor which was for the 250/300 HP engine. I dissassembled it to check the shaft bearings and installed light springs and the vac can you recommended for the cam a napa VC1810. The vac is full time. The heat riser is functioning properly and the intake gaskets are open. The newer AFB is not a performer carb but an actual AFB model 9500 (500 CFM) carb the one with the 5 1/2 air horn and electric choke. These were not used on corvettes but on other GM engines and who knows what else. The choke is functioning properly. I am ingnorant about the jetting that was in these carburators. I rebuilt the carb my self but these are pretty simple carbs. The carb is on a 2101 performer manifold. BTW I had a performer 1406 600 CFM carb on the engine and it ran the same. I have the inital timing at 12 degrees and seem to be getting all of vac adv at idle and 20 + out of the mechanical. The engine is not over heating. I am at loss to understand the difference in the engine in the fisrt few min after it is started incuding diving it and after is run for a while or restarted. any help will be much appreciated.
Jeff Cheney

[Duke W.]

Re: Help with engine tuning problems

Ignition sounds okay. Since that carb was used on smaller displacement engines, I might question the off-idle fuel calibration, which relies on manifold vacuum, so it might be lean, but if the poor performance only happens after hot soak then I would suspect some sort of vapor lock/fuel percolation issue.

One experiment to try is wire the heat riser valve opens and determine if that mitigates the problem. Run a hot soak experiment with the hood open and closed and see if that makes a difference. Maybe even use a house fan to blow air over the engine as one set of test conditions.

Fuel line routing too close to the exhaust manifolds could also be an issue, especially if they are different than OE. Fuel pump is always suspect.

It's also possible that heat soak is opening up a vacuum leak somewhere that is causing lean operation.

Duke

[Duke W.]

Re: Help with engine tuning problems

One thing I forgot. Remove the air clearer cover before shutdown. Then after shutdown observe the main venturi discharge nozzle for fuel. If any dribbles out the float level is incorrect, or the floats could be leaky. They're easy to check by shaking them and listening for liquid sloshing around.

Duke

[Chuck R.]

Re: Do you have a thermostatic space

between the carb abd the intake? You may be boiling your fuel if you don't.

Chuck

[Jeff C.]

Thanks everyone!! *NM*