Spits through carb on real hard acceleration

[Rab563]

I just rebuilt my engine and now when I give it real hard acceleration from idle it pops through the carb. Engine is a 350 with a mild cam, MSD 6al and Dist. I also freshened up the carb with the gaskets and spitter. I did not change jetting. I checked to make sure that I wasn't 180 out on the Dist. I even tried a different carb. Could I be off a tooth ont the timing gear? Could there be something wrong with the MSD dist or 6AL box? Any help is greatly appreciated.

Thanks,

RAB563

 

[Rodney Gold]

I would check first for vaccum leaks
The next step is to change the shooter in the carb if it's a Holley - go up a size till it doesnt backfire thru the carb
Then check your timing and the timing curve.

 

[c31980]

recheck your timing spitting mostley indecate your dist . not set to #1

 

[navy2kcoupe]

Maybe a broken or weak intake valve spring............
Andy

 

[Hib Halverson]

Does the problem occur as the throttle is opening or does it occur after it's full open and only at high rpm?

 

[Rodney Gold]

In the initial post he says its on hard accleration from idle .. a lean misfire?

 

[Rab563]

I have checked that I am not 180 out on the distributer. If let the engine warm up at idle and then go to WOT as fast as I can it will spit fire throught carb. I have tried two different intakes.
I thought it might be a lean condition so I put a road race carb on which should have plenty of spitter volume and it still backfires through the carb.

I did check vacuum and it appears to low at around 9 but stable. Should be higher at 15 to 20 correct? If I had a bad valve spring or the time gear was off a tooth shouldn't the vacuum gauge fluxurate?

Sorry for slow reply I have been on vacation.

Thanks,

Ron

 

[Rodney Gold]

A vacuum figure of 9 sounds very low - unless you have a pretty radical cam- sounds like a vacuum leak somewere to me?? At any rate , at 9" , its pretty likely your power valve is not correct if you are using a holley - it needs to be a 4.5 or below , not the usual 6.5.
Could be a valve train related issue tho , but if it was , the car most likely wouldnt perform well right thru its rev range not just on hard acceleration.

 

[black_81_vette]

How did you adjust the valves? sounds like there too tight.Also this is rare but ive seen it on some heads if the springs were changed the springs were coil binding at full lift. mostly this would be a problem with heads that have exhaust valve rotators on them,the reason is the spring is too tall combined with the rotator and it binds.And that too will cause a backfire through the carb also.

 

[twoseater]

I had that problem once (backfiring through carb). It was a worn cam lobe. It was really apparent when I took off the valve covers while the engine was running and there was no movement.

 

[Rab563]

A vacuum figure of 9 sounds very low - unless you have a pretty radical cam- sounds like a vacuum leak somewere to me?? At any rate , at 9" , its pretty likely your power valve is not correct if you are using a holley - it needs to be a 4.5 or below , not the usual 6.5.
Could be a valve train related issue tho , but if it was , the car most likely wouldnt perform well right thru its rev range not just on hard acceleration.

What do you mean that it should be 4.5 or below? Are you talking about the vacuum or the size of the spitter?

 

[73shark]

He's referring to the power valve opening vacuum. Should be somewhat below your idle vacuum so it's closed then. There's a rule of thumb about how much below but can't remember it. If someone else does come up with it, will look it up.

 

[Rodney Gold]

The rule of thumb is 1/2 manifold vacuum at idle.
Normally they are 6.5 inches which is fine for normal engines , but cammed motors with low vac of 9 inches will require a smaller value than 6.5

 

[rgtrough]

Hey Rab, alot of info being thrown at you. First find that vacuum leak. Spray wd40 around carb base intake to head surface. Even with mild mods you should have 15+". Next what is your all in timing? Start at 35 degrees. Plug vacuum advance from manifold with small bolt/golf tee check with timing light. Then go to carb. If Holley it can be too much or too little fuel. Get their accel pump cams and start changing them out. You'll find it.

 

[vett'n 2]

if its not the timing off or a tight valve does it have good plugs not those accel junk ,good wires,cap.i change an enigine once that ran when i pulled it out went and put it in the other car ran like crap back firing it was those good ol accel plugs

 

[vett'n 2]

oh and how long did it sit the gas in mine is pretty darn old and it like to spit fire unless i fill the carb with fresh gas also had a 74 that would get water in the gas tank