Hi Guys...I need help. I am running a 396, 10.5:1, 375HP cam, Holley 3310 professionally rebuilt, MSD Ready to Run off ported vacuum, headers. Initial timing 12 degrees at 900 rpm, 9" manifold vacuum. Total timing w/o vacuum 38 degrees at 3500 rpm, 19" manifold vacuum. Runs lopey but fine at idle, rough between 1200 and about 2500 rpm. When I hook up the vacuum advance it backfires in the exhaust at any constant rpm above idle. I can rev it through the range but not hold steady rpm with vacuum advance hooked up. I can advance initial timing to as much as 19 degrees (maybe more) but this makes the backfiring worse when I hook up the vacuum advance. The carb has the stock #65 power valve. I've set the rotor on number 1 at 20 degrees advance to make sure I'm not firing another plug when the advance comes in. Vacuum advance is 4 degrees at 8", 8 at 9", 10 at 10" up to 16 degrees. MSD says this is correct and to keep advancing initial timing. I'm stumped....can you give me some advice on what to look for next?

Thanks much,
Sam

Any particular reason you're running ported vacuum? If you run full manifold vacuum with an advance can that's fully-deployed by 2" Hg. less than you pull at normal idle, your issue should disappear. Leave your initial where it is (12* with vac advance disconnected and plugged), and manifold-connected vacuum advance will add another 15*-16* for total idle timing of 27*-29*, and your total timing will stay the same as it is now (vacuum advance has nothing to do with total timing, which is measured with the vac advance disconnected - it's only initial + centrifugal).

If you like, e-mail me and I'll send you my "Timing And Vacuum Advance 101" paper. :)

:beers:

Thanks....I'm on port at MSD's advice. I already have your paper...really good. I'll switch to manifold, as you suggest, and see. FYI, manifold is about 1 1/2" greater vacuum than port up the rpm curve with no load...that's 2-4 degrees advance on my distributor. Can you tell me why I'm getting the backfire? MSD says either cross fire (I've double checked the rotor alignment) or not enough advance. But, as I add initial advance the problem is worse when I add vacuum. If I go to manifold, I'll be adding even more vacuum at a given rpm. I really don't understand what's going on to know which way to tune. Thanks for your help...Sam

John Z....I went back and re-read your paper. You say "When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean." Here is what I get at steady rpm. I'm not measuring the transition vacuum as the throttle opens.
VACUUM
RPM PORT MANIFOLD
850 7" 9"
1200 11" 12"
1500 13" 15"
2000 17" 18"
2500 17.5" 18.5"
3000 18.5" 19"
3500 18" 19.5"
4000 19.5"

I can rev through the range, as the vacuum must be dropping and retarding the advance as the throttle opens. But, I can't hold a steady rpm with vacuum engaged without backfires but I can without it. I don't have an exhaust analyzer but I can't believe I'm so lean as to cause this and if I were lean, adding vacuum advance would be in the right direction. I can smell the hydrocarbons and get a puff of black smoke at times. Is it possible I am so rich to cause this...wouldn't I be pinging under this condition? Can I be confusing pinging with backfires at no load? Am I thinking about this right? I appreciate your help....Sam

Hi Guys...I need help. I am running a 396, 10.5:1, 375HP cam, Holley 3310 professionally rebuilt, MSD Ready to Run off ported vacuum, headers. Initial timing 12 degrees at 900 rpm, 9" manifold vacuum. Total timing w/o vacuum 38 degrees at 3500 rpm, 19" manifold vacuum. Runs lopey but fine at idle, rough between 1200 and about 2500 rpm. When I hook up the vacuum advance it backfires in the exhaust at any constant rpm above idle. I can rev it through the range but not hold steady rpm with vacuum advance hooked up. I can advance initial timing to as much as 19 degrees (maybe more) but this makes the backfiring worse when I hook up the vacuum advance. The carb has the stock #65 power valve. I've set the rotor on number 1 at 20 degrees advance to make sure I'm not firing another plug when the advance comes in. Vacuum advance is 4 degrees at 8", 8 at 9", 10 at 10" up to 16 degrees. MSD says this is correct and to keep advancing initial timing. I'm stumped....can you give me some advice on what to look for next?

Thanks much,
Sam

I would install a vacuum can that is all in at idle.

John Z....I went back and re-read your paper. You say "When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean." Here is what I get at steady rpm. I'm not measuring the transition vacuum as the throttle opens.
VACUUM
RPM PORT MANIFOLD
850 7" 9"
1200 11" 12"
1500 13" 15"
2000 17" 18"
2500 17.5" 18.5"
3000 18.5" 19"
3500 18" 19.5"
4000 19.5"

I can rev through the range, as the vacuum must be dropping and retarding the advance as the throttle opens. But, I can't hold a steady rpm with vacuum engaged without backfires but I can without it. I don't have an exhaust analyzer but I can't believe I'm so lean as to cause this and if I were lean, adding vacuum advance would be in the right direction. I can smell the hydrocarbons and get a puff of black smoke at times. Is it possible I am so rich to cause this...wouldn't I be pinging under this condition? Can I be confusing pinging with backfires at no load? Am I thinking about this right? I appreciate your help....Sam

From this info it looks like you dont have vac. advance at all. Try hooking to manifold and see how ot runs. It could be your rich also with the black smoke, when do you get it on decell or accell? Low vac engines have a hard time staying in idle circuts on the carb. Is it original cam?

Thanks to all.... Pulled the plugs...I am definetly rich...get the smoke on accel. I have the idle jets out 1 turn based on max manifold vac. Vacuum is not very responsive to the idle jets. The cam is an after market...I don't have the card...the builder says a 375HP replacement. The car is running rectangular port heads. I'll work on carburation and timing from the manifold today and let you know what I learn.

Thanks,
Sam

Your manifold vacuum should be dropping off as rpm increases. You shouldn't have 19.5in. at 3500 rpms. Something doesn't make sense.

Vacuum is unrelated to engine rpm - it's a function of load on the engine. The only vacuum reading that means anything is at idle. If it doesn't respond to changes in the idle mixture screws, the primary throttle plate is open too far, and it's idling on the transfer slot; back the idle SPEED screw down and go back to work on the mixture screws. :thumbsup:

Your manifold vacuum should be dropping off as rpm increases. You shouldn't have 19.5in. at 3500 rpms. Something doesn't make sense.
Assuming this is all at WOT.3500 rpm in 1st at 25% throttle angle will generate high manifold vacuum.I agree with phel69.

Vacuum is unrelated to engine rpm - it's a function of load on the engine. The only vacuum reading that means anything is at idle. If it doesn't respond to changes in the idle mixture screws, the primary throttle plate is open too far, and it's idling on the transfer slot; back the idle SPEED screw down and go back to work on the mixture screws. :thumbsup:
Vacuum at wide open throttle at max rpm indicates a restriction in the intake tract.It applys to both ends.Cracking the secondarys can also allow the primarys to be rolled back and cover the transfer slots.I settled down my idle issues by drilling .060 holes in all butterflys allowing me to return to 4 corner idle circuits and not exposing any transfer slots.

i have a 91 camaro rs with a 305, it is running really rich, alot of smoke and unburnt fuel, pops through the exhaust, any advise? mike