Hello all. Im having some driveability issues. My vette when at idle runs rough (sometimes like it wants to stall) and it stumbles at acceleration. This morning I checked for vacuum leaks and none appeared to be present. I sprayed some carburetor cleaner around the carburetor gasket and the idle did not change, I checked all hoses, no leaks. I sprayed carburetor cleaner into the primary venturi and the idle smoothed out for a second. I then took my Mighty Vac vacuum gauge to get a vacuum reading. The needle bounces between 13 and 18 inches. When I put a load on the engine, the needle holds steady between 20-22 inches. According to the gauge, it is one point from the red zone (late ignition timing/green zone (normal engine). My initial timing at 10 degrees (vacuum advanced plugged). I run on manifold vacuum. The vette in question is a 1979. My engine idle speed is 900rpm in park and drops to 700 in drive (auto trans) Im looking to get it to run smoother and not stumble at acceleration.



Here are some engine specs if they may help with these issues:

350cid +.060
L82 forged pistons
Lunati Cam 414/442 Lift, 278/288 Duration@.050. (as I recall, misplaced cam card)
Edelbrock RPM heads 64cc
Edelbrock Performer 650cfm carb (to be replaced by 750 Mighty Demon still in box)
HEI ignition

Thanks all

 

I'm sure your cam numbers are off. If you really had that much duration it would not idle below 1200 rpm. Probably more in the 218 to 225 @ .050 range judging by the lift numbers. Those duration numbers you list are huge race cams.

 

Yeah im sure you're right. Im remember writing down the cam specs and taping it to the glovebox door. But that was in 2000. I removed it for some dumb reason. 4 years ago, a speed shop found the specs for my cam and said it was a good street cam but I did not write it down. I hope I don't have burnt valves.

 

time for a carb rebuild?

 

Could it be possible you don't have enough timing advance (10*) for that cam?

Try to get your hands on a dial-back timing light, & set your timing at 36* total vacuum unplugged & see how it runs then. Forget initial, unless you know exactly your mechanical & vacuum advance it's not a very good way to set it.

 

never set a motor by the initial

plug the vac. adv. and run the motor up to 3500 rpm and look at the total. Your max should be 38 or less. I'm unfamiliar with the Edl chamber design.

Then you need to adjust your idle screws for smoothest idle and highest vacuum

278/288 is a common advertised duration. Like the Comp Cams ex284.....

It's .050 number is generally about 70-80 degrees less with h-flat cams and only 40-50 less with roller cams

 

very very slowly close the choke and see if it smooths out.
if it does and vac then steady, u have confirmed a carb problem. (dirt in idle circuit, etc)
.
gkull- so i've been doing it wrong 30+ years, setting BEST base, Best curve & Best total?

 

You just want the total to be correct. You can only change the initial by recurving the amount of total mechanical advance

 

That's the idea, isn't it? I'll call it super tuning, since most guys won't dare do the stuff i always do.
My dist had 40 mech;
By that method, it would be set at 38 total and have Negative base timing. That, while it seems to be a popular method, is a poor method by my experience. Car will run like crap. Then guys get a 383 to get more torque!
But, if the dizzy is FIRST set up by an expert, curve & max mech, it can be quickly put in your way and work great.