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Old 10-27-2010, 05:57 PM
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carb backfire problems

Iv posted a few messages revolving around the same thing. I wasnt very clear in them so I wanted to give it another shot. Motor is a 400sbc, holley 750dp,holley black fuel pump, fuel pressure set around 6 1/2 psi. Total timing is around 38*. I have recently replaced the intake gasket and put a new power valve in the carb because I was told a backfire from the carb could blow mine.

My problem is, the car idles fine, and usually from around 2700 rpms and up it revs fine. I can be sitting in neutral though and stab it from an idle, and it backfires and shoots a flame out of the carb. Same thing if im rolling between an idle and 2700 rpms. Also when im driving it and hit it anywhere about 2700, sometimes it blows the tires off, sometimes it just doesnt seem to have as much power. I noticed when setting the timing the timing mark on the balancer seems to jump around a little and when reved occasionally will jump up a good little bit. The balancer is fairly new.

Hope this was a little easier to understand this time and im hoping I can get the answer to my problems. Thanks

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Old 10-27-2010, 06:02 PM
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What distributer set-up do you have in it now ? How's the float levels ?



Cole
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Old 10-27-2010, 06:04 PM
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Take the cap off the distributor and make sure rotor is pointing at where you have number one cylinder on the cap. Of course thats after you find tdc on number one cylinder
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Old 10-27-2010, 06:10 PM
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Its an HEI distributor. It has a red cap and a polished shaft like an msd but i believe its an engine works. I have an msd coil in it though. But it is new, I was having the same problem with my old distributor, I took the msd coil out of the old one though and put it in the new one, could the coil cause this?
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Old 10-27-2010, 08:13 PM
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Backfire through the carb will not kill the powervalve on any holley newer than the 1990's, they have a ball bearing to protect it.

Backfire is usually a lean condition.

Float levels are important (as mentioned), idle mixture screws after adjusted try turning out another 1/2 turn richer on each side. Whats your vacuum reading at idle, that sets the proper power valve you need but a wrong one usually gives bad idle and not much else. The random timing might just be because your rpm is all over the place and your dizzy is advancing. Are you using vac advance at all? How correct is your timing curve? Its possible the rotor in the dizzy is not 100% lined up with the #1 and your spark is jumping to another cylinder (bad rotor phase).
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Old 10-28-2010, 04:14 PM
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The vac advance is plugged. You say the rotor may not be lined up properly with #1...i dont understand this because its an hei..so when i sat the timing it was lined up with #1 but then advancing or retarding it makes it not lined up perfectly anymore does it not? I was told by a guy at a local speed shop to try going to a lighter spring and a heavier weight in the dizzy.
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Old 10-28-2010, 04:34 PM
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backfire

If all else sems to check out make sure your accel pump shot is enough, it will back fire off idle and when stabbed at lower RPMs.
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Old 10-28-2010, 04:36 PM
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Accelerator pump out of adjustment, wrong accelerator pump cam, something plugged, float levels to low, to small of a shooter.

I would first check the pump arm to be sure there is no play at idle. It should start to move as soon as you move the throttle and at wot you should be able to move the arm at least .015 off the cam.

There are several different pump cams for Holleys and most have 2 positions they can be installed in. You may need a more aggressive pump cam. You may want to take a peek at the pump diaphram to make sure it is in tact and the pump well doesn't have any crap in it. A lees common issue is the squirter is to small for the required pump shot. I had a similar issue to the one you describe and I went up 2 sizes on the squirter and installed a different cam and the issue went away.

All this being said you need to look at the basics first. The float level is critical as well as the idle mixture.
There are a lot of other carburetor related things that can cause your issue but they are less common.
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Old 10-28-2010, 05:20 PM
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I forgot to mention in response to bubba...im around 9psi of vacuum at 1100 rpms at idle. Iv checked the float levels. Before I changed the intake gasket it was running ok so I dont feel anything on the carb is to big/small. Could it be the distributor curve? I almost want to say I wasnt having this problem until I started to mess with that, but its been so long I cant say for sure. Should I try swapping out for a different coil just to eliminate that as a cause?

One other thing is with my electric fan off its idling around 1500 but when i turn it on it drops down to around 1100. The belt is loose so im guessing thats the problem and has nothing to do with my current problem.
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Old 10-28-2010, 06:36 PM
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Tightening the belt didnt fix the problem but it seemed to fix itself. I did notice something else tonight though. I was in 4th at around 35mph so i was in fairly low rpms, i was going thru a yellow light and when i gave it more gas it stumbled before it picked back up.
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Old 10-28-2010, 06:57 PM
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Quote:
Originally Posted by blue400
I forgot to mention in response to bubba...im around 9psi of vacuum at 1100 rpms at idle. Iv checked the float levels. Before I changed the intake gasket it was running ok so I dont feel anything on the carb is to big/small. Could it be the distributor curve? I almost want to say I wasnt having this problem until I started to mess with that, but its been so long I cant say for sure. Should I try swapping out for a different coil just to eliminate that as a cause?

One other thing is with my electric fan off its idling around 1500 but when i turn it on it drops down to around 1100. The belt is loose so im guessing thats the problem and has nothing to do with my current problem.
9psi would need a 4.5 power valve, anything larger will dump fuel randomly at idle. Stock is 6.5 which is not good for you.

Regarding the rotor at #1: I didnt explain clearly. Google ROTOR PHASING. The vacuum advance mechanism (even if not used) affects "where" your spark occurs. Ideally it should be perfect with the #1 on the cap but in reality it might be 1/4" before or 1/4" after (meaning its off by a few degrees). This is common and ok. However, sometimes you're off by 20' and the spark is almost between the #1 and a different cylinder....so the spark randomly jumps to the wrong plug because of bad phasing. Its a pain to check but the easiest way is to buy a cheap see through hei dizzy cap (summit jegs etc) and point the timing light at the cap....you'll see exactly where the rotor is firing.

Since you mentioned the coil you might want to go back to that. It could simply be a misfire. All suggestions are just opinions based on past experience, doesnt mean you have anything more than a simple misfire.
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Old 10-28-2010, 06:58 PM
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Quote:
Originally Posted by blue400
Tightening the belt didnt fix the problem but it seemed to fix itself. I did notice something else tonight though. I was in 4th at around 35mph so i was in fairly low rpms, i was going thru a yellow light and when i gave it more gas it stumbled before it picked back up.
Sounds like a text book accelerator pump or accelerator pump adjustment (as TBucket suggeste). Very common misadjustment problem. Based on your low idle vacuum Id almost bet you need a different accelerator pump cam but thats trial an error (kits are $15-$20 on average, takes 3 minutes to swap em out with carb on engine)
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Old 10-28-2010, 07:20 PM
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I really appreciate the help so far. Iv tried both 6 and 4.5 power valves. Iv also had 2 different distributors so i dont know if the rotor phasing is it, but i will give it a try and its a good idea. I believe my lowish vacuum is because of the cam i have ( http://www.summitracing.com/parts/SUM-1108/ )

How do i go about adjusting the accelerator pump?
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Old 10-28-2010, 07:34 PM
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This definitely sounds like an lean condition. You need to probably go up in squirter size. To adjust the accelarator pump I usually when the carb is closed adjust the spring tension so the bolt is just touching the arm for the accelarator pump. Holley has a spec for a feeler gauge size which I can't remember, but I have always used thee above method with great results. If the motor is built, I have found the squirter size usually to be the problem, or the spring/screw for the adjustment to be to compressed.
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Old 10-28-2010, 07:35 PM
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Ok this right here might help a lot. I went out and hooked my vacuum advance back up...and originally when i flat footed it from idle it backfired out the carb every time..now that its hooked up...i flat footed it 10 times and it backfired twice. Maybe i just need to advance my timing a little more at idle..what do ya think? Then again that could have just been luck. Anyways, I still would like to know how to adjust the accelerator pump though. Thanks
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