Sorry for the long post I can't seem to concentrate thoughts into small numbers of words...
I have just swapped a 396 BBC into my '73, and I'm having a bit of trouble. The engine is a 1969 truck 396 with semi open chamber oval port heads, it's been rebuilt and bored .30 over with domed pistons (I think just over 9.5:1) and an RV cam. I'm running an Edelbrock Torker II intake manifold with a Holley 650 VS carb (marine in fact), a stock HEI from I don't know what, and 2" primary headers. The carb only appears to have a full manifold vacuum source, so that's where I plugged the advance line into. Once I got the thing running, I advanced the distributor (with the advance hooked up) with the idle RPM coming up untill it leveled off, and then backed it off (retarded) until it just started to come back down. I've only driven the thing a little bit, and it has BOOKOO TORQUE, but when I give it the first little bit of throttle, like when the light turns green and I try to smoothly drive away, it has a real hesitation and sometimes quits. My first thought was that the accelerator pumps may not be working, so I pulled the air cleaner off and looked in and they squirt just fine, my next thought was timing. I tried to advance it a little more but it didn't crank well as if the intake charge was keeping the piston from coming up, so I retarded it a bit and the hesitation didn't go away.
I'm going to borrow my neighbor's timing light tomorrow, and I'm wondering what is a good initial setting for the distributor? Should I disconnect and plug the vacuum advance because I don't have ported vacuum? What else should I look into? I have no tuning experience but I know some of you out there do so all help will be greatly appreciated!

 

Without ported vacuum you are getting FULL vacuum advance at idle, then as the rpm's come up your timing is retarding. I'm no expert, but isn't this exactly opposite of what vacuum advance is supposed to do??
I think if you disconnect the vacuum advance and re-adjust your timing per Lar's paper you will have much better acceleration.

 

Kevin 73 is right on the money- I don't think you can beat the info & advice from Lars. I'll just add a couple of quick thoughts. When you checked your accelerator pumps did you note when they start to squirt? They should be adjusted to squirt right when you start to open the throttle. If they activate late it can cause a stumble- I found this out 1st hand on a new carb I bought.

Also, I think Lars would tell you that the optimal ignition curve can vary depending on alot of variables, one of which is cylinder pressure. If your engine is healthy it may have fairly high cylinder pressure w/ your mild RV cam. His guidelines will definitely get you close but you may have to make some adjustments to get it just right.

Lastly, if you get the igniton dialed in & still have the problem it might be your intake. The Torker2 makes very good power but can sometimes cause problems with idle & off idle transitions.

This assumes that your carb is in good health. Best of luck. :cheers:

 

Thanks guys! Where do I find Lars' paper? It sounds like just what I need.

 

Another thing that could cause stumble is too small of a squirter for the accelerator pump. A 650 is border line too small for a 396 with those mods. Check the plugs to see if they look lean; you may need to jet up a little. And see if you have a #25 squirter; if it's smaller than that it might not be getting enough pump shot.

 

Go to the Tools > Tech Tips section. There are many useful papers in there. Lars' papers are always top notch. ;)