I have a Holley 670cfm street avenger carb and a full msd ignition setup. I am having issues fine tuning my engine. i have the timing set at about 8 degrees, it was higher but i changed to lighter springs in the distribtor and lowered the initial timing. i cant seem to get rid of the bog then kick up when the secondaries open i also am feeling a lot of stumbling at steady cruising speeds. Any suggestions on adjustments to my primaries, accelerator pump, etc...?

 

stumbling on cruise is often ignition too far advanced.
do you have vacuum advance picking up at manifold or ported vacuum.
maybe change the springs back to heavier ones (medim).
So you have 8 initial and 34 all in at ~3000 (with vacuum disconnected and plugged)

what do your plugs look like? lean/rich/hot/cold?

 

Stumbles and bogs are often from either too much fuel or not enough. What do your plugs look like? What is your AFR?

 

First of all you need more than 8 initial for timing. With maybe 34-36 total. Have you done anything to jetting, the accelerator pump or the secondary spring?

 

Actually, with GT40X heads, 8* is probably about good, with maybe 30-32 total. Those are fast burn heads, so you don't need to run as much timing as you would with a normal chamber.

 

stumbling on cruise is often ignition too far advanced.
do you have vacuum advance picking up at manifold or ported vacuum.

My vacuum advance picks up from the manifold. i did have the initial timing set at about 12-14 and it ran great but still had a flat spot before the secondaries kicked in and it idled high and i couldnt adjust mit very well when it was that advanced. keep in mind i also have the timing advanced on the gear drive. i set it at the 2 degree advance mark.

 

You should probably be using ported vacuum source for the advance according to the books. You can actually use either and see what works best for you. As I mentioned before, you need to know if you are having a rich or lean issue when this stumble occurs. What do your plugs look like?

My guess is it sounds like if there is a delay stumble between opening the throttle and when your vacuum secondaries roll in, then you are running into a lean bog. This would be addressed with the power valve and accelerator pump, cam, and nozzles. Those two devices form the intermediate or transition circuit. When you stumble at cruise, is it when you hit a little bit of a load? You probably need to install a manifold vacuum gauge and give us some info on what that is doing when this all occurs. My guess at this point is too little fuel at times.

 

there's quite a few people who run it from manifold vacuum for broader spectrum. but if the vac is not adjusted to this you'll end up too far advanced. i had a jitter (annoying) when driving at idle in gear. retarted to 8 degrees, changed to ported vacuum->gone.
I'll play around with the manifold vacuum again for advance buti have better result with ported at the moment. So maybe it helps you as well. give it a try sure

 

Sorry, i had a little bit of a brain fart. For some reason when vacuum advance was mentioned i got vacuum secondary in my mind. ive been messing with the carb a lot. i dont run a vacuum advance, its a mechanical advanceits a MSD probillet distributor that runs off the MSD 6AL box with a mechanical advance.