QuestionQUESTION: I have a 1967 Corvette, 4 speed, that I recently added sidepipes. The car ran and runs great, BUT with the pipes it will backfire (thru the pipes not the carb) when it is up to temp and I let off the gas. The higher the RPMs, the more likely it will backfire when I let off the gas and allow the engine to slow the car. The car didn't have the original engine in it when I bought it, so I put a 2005 ZZ4 crate motor with a MSG electrionic ignition system in. It never backfired with the normal ehxaust. Any tips? Thanks!
ANSWER: Wow I got a Vette question YAHOO!
A properly tuned exhaust on a motor allows fuel to be burned at it's highest output..not only free flowing output but a bit of restriction is needed in sync with the output "wave"
Restriction allows the burning mixture to sit in the chamber long enough to be completely burned ,scavenged to soon popping sound is made, ( and loss of hp )that is what happens on quick deceleration..you could say that you are running to "rich" and get the noise but what you need to know is---is all the fuel being burned on acceleration?
Um Anyway You may not be burning all your fuel on accel.
Try playing with your mixture screws or you may have to rejet. Or up your plug gap a notch?
SMI carbs out of Cali does great job custom building quadjets<---- all I'll ever run.
Good luck... I need to go comb my hair now after pulling the 61 in from little ride the honey and I took.
Mike
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QUESTION: Thanks for the quick response! I played with the mixture, but they seem to be right on. I have a Holley 4160 (750), and don't want to replace it. It has treated me well, so far. The carb instructions did mention rejetting lower with high flow exhaust applications. Anyway...Should I just drop the jet size one number at a time until the popping stops or is there a more scientific way to approach this?
ANSWER: Thats why I love quadjets so much! But anyway you can try dropping a size at a time. Are you running headers out to tubes? or manifolds? a balance pipe would help but its custom fab like I had to do on my old one.
Question for you..how do your plugs look? and is there any back pressure at all in your side tubes? or they open?
Bad thing is they look really cool but I never did get rid of my pooping er i mean poping I just dinked with it enough to live with and most others will say the same thing:( but hell it sure sounds mean dont it muhahaha
keep me posted
mike
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QUESTION: Mike,
First answers to your questions- I have headers to pipes & my pipes are open. My plugs are tanish grey (just like my grandfather taught me!) There are no buildups of unburned fuel, etc. I set my carb mixture with a vacuum gauge, so I am pretty sure it is correct. The open pipes probably mean that I won't get rid of all the popping. Correct? That is fine, I would just like to stop most of it. I don't want to sound like a machine gun everytime I brake with the clutch! My concern about dropping one size jet at a time is - Where do I stop? Eventually I am going to be doing more harm than good. Is three sizes too much? I don't know.
You have a 61, huh? NICE!!!
Answer
Brian, follow up on 5/18
Any luck on fixing? let me know how its going
Mike
I would say drop one jet size and see where your at
plugs sound good so your not running rich its just not scavenging all that it needs to
What kind of headers? See most people will say when you change over to tubes you now will run rich ( them 2 stroke guys all think alike) but I was taught through trial n error that it was the fuel not getting burned and being forced out (crappy headers as well)(I made that mistake as well) without the scavenging. They do sell inserts in jegs catalog
prolly easier to just call me at..810-845-0350 and I could give you a few ideas as to what Ive done in the past kinda hard to play email tag sometimes.
lemme know
Mike
