Thread: Timing Questions

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Hey guys, I'm running a 355 cu inch sbc with 9.7 comp ratio, 76cc ported stock heads, Holley Street Dominator intake with 750 Holley double pump mech secondary on top, Comp Magnum 280h running the show. The engine needs to idle at 1000/1100 to be happy. My total advanced timing being 36* makes my timing at idle 14*. None of these are problems until I shut her down. Then she runs on and/or backfires through the carb. I have leaned out the idle to the point that it doesn't idle right. Any suggestions? Changing weights in the dist?
Thanks Bud

What Denny suggests will cure the symptom you seem to have. IMO the disease may well be an improperly working idle circuit, usually due to improper set-up. This is one of the most common afflictions of the aftermarket scene. You might try some researching in here and maybe go through the set-up again per Holley's manual.

Kitz

Your timming is fine you may have too hot of spark plugs heat range or there some hot spot on your combustion chamber of your heads cause one to try to run after the key is off.

Hey guys, thanks for the feedback. Without the benefit of an air/fuel meter I'm only guessing at my idle setting, but eliminating the timing side of this will hopefully make troubleshooting easier.
Thanks for being here for guys like me.

Bud

If at all possible pick up a book such as this one
Go Flyers

Due to the size of your cam, it sounds like the butterflys are open beyond the range of the idling circuits. You can drill a small hole in the front half of the primary butterflys ( I use 3/16" or less ). This will allow you to close the butterflies, but still get air. I found this tip in "How to Hot Rod a SBC".

I used this trick on a roller cammed BBC and a SBC. Worked like a charm and improved idle quality.

Are you running a manual trans or automatic? Did you use a vacuum guage to adjust the idle? Have you checked for a vacuum leak in the distributor vacuum cannister? You shouldn't have to drill holes in the throttle plates with that cam. I run a Comp Extreme Energy XE274H which isn't that much more cam on a 355/Turbo 350 and I don't have any of those problems. It's probably something simple.

By the way, what spark plugs are you running and what's the gap?

Wow! Thanks guys, AC delco r44t gapped @ 35. Automatic 350 w/2400 stall. Negative on the vacuum gauge to set idle. Checked for vacuum leaks around carb base but not distributor. Have a Holley book but not that one. Booya on Flyers! I set my idle with the idle screw, closed my air bleed adjusting screws completely. So far no run on/backfire. Doesn't seem right though.
Bud

Last writer asked about dumping fuel. You need a vacuum guage, just something cheap to help you adjust the idle screws. They're cheap. If that Holley is a double pumper, you have two(2) power valves that open when the vacuum drops. They are probably 6.5's if nobody else has messed up this carb for you already. All street Holleys seem to come with 6.5's. You need to know your vacuum at idle in drive. If it drops below 6.5, you are very rich and dumping fuel. You need to be 1-1.5 below your idle vacuum. If you read 6, you need a 5 or 4.5. This is one reason why vacuum secondaries are recommended for automatic street cars. It took me a long time to figure this out for myself because nobody could give me an answer. I picked up a free Holley catalog at a World of Wheels show and before that I probably had five other books on Holleys. I now run a 750 vacuum vacuum secondary 3310 and it runs great. That doesn't mean you should ditch the double pumper. It just means you have to buy twice as many power valves. It helps to know your fuel pressure but if you know that your pump is rated less than 7-8 psi, than you should be OK. Holleys can take up to that before the needles are overcome by too much pressure. You can regulate it, but why add all that extra plumbing if you just change to the right pump to begin with. Here's my setup. Chevy 355, Turbo 350, 10" converter, Holley 3310 750cfm, Comp XE274H, Carter street fuel pump rated 6 1/2 psi. Idle vacuum in drive around 6-6.5. I run a 5hg power valve. Runs great. Alot of fun. Hope this helps.

The 280H is supposed to have some what of a lopey Idle???? In bigger cubic inches it will loose some of it but for a 355 it should have a rough idle???? Maybe your trying to tune it out??? You will definitely need a vacuum guage to tune also. You will also need a tach to set rpm. First off I would say your rpm is to high.

Get the book and read it , it will give you all the info to tune your Holley. Dont worry its not a big book. After reading it and using it for refrence it will be a breeze to tune your Holly. Dont hesitate to ask questions.

Again, you guys make me proud to be a member of the Club Hot Rod. I'll try to address all of the questions.
DennyW, do not know how to answer your questions about tools/gauges. I am running a mallory pressure regulator set at 7psi. I agree that the idle screws should not be bottomed in.
Selmorides, I have a vaccum gauge and will check the power valve size soon. Good to know what my vaccuum should be, very helpful. I got away from vaccuum secondaries years ago when I had diaphram problems.
Big Truck Driver, I know the 280H has a lumpy idle. I am not trying to eleminate it. Just make it more streetable. Using Sun tach wired to the distributor for rpm readings.
DennyW, There is no doubt that I have limited knowledge, but don't we all?
Thanks for all of this very helpful info. I'll gather some information and get back to you guys. And Big Truck Driver don't worry I always seem to have questions.
Bud

Again, you guys make me proud to be a member of the Club Hot Rod. I'll try to address all of the questions.
DennyW, do not know how to answer your questions about tools/gauges. I am running a mallory pressure regulator set at 7psi. I agree that the idle screws should not be bottomed in.
Selmorides, I have a vaccum gauge and will check the power valve size soon. Good to know what my vaccuum should be, very helpful. I got away from vaccuum secondaries years ago when I had diaphram problems.
Big Truck Driver, I know the 280H has a lumpy idle. I am not trying to eleminate it. Just make it more streetable. Using Sun tach wired to the distributor for rpm readings.
DennyW, There is no doubt that I have limited knowledge, but don't we all?
Thanks for all of this very helpful info. I'll gather some information and get back to you guys. And Big Truck Driver don't worry I always seem to have questions.
Bud

Again, you guys make me proud to be a member of the Club Hot Rod. I'll try to address all of the questions.
DennyW, do not know how to answer your questions about tools/gauges. I am running a mallory pressure regulator set at 7psi. I agree that the idle screws should not be bottomed in.
Selmorides, I have a vaccum gauge and will check the power valve size soon. Good to know what my vaccuum should be, very helpful. I got away from vaccuum secondaries years ago when I had diaphram problems.
Big Truck Driver, I know the 280H has a lumpy idle. I am not trying to eleminate it. Just make it more streetable. Using Sun tach wired to the distributor for rpm readings.
DennyW, There is no doubt that I have limited knowledge, but don't we all?
Thanks for all of this very helpful info. I'll gather some information and get back to you guys. And Big Truck Driver don't worry I always seem to have questions.
Bud