SN-95

Took the cat to the track for the first time last night. The night before it was running good, very smooth. Only thing we did was change the oil before we took it to the track. So we get there, pull it off the trailer, and it's got a little stumble/misfire/hesitation at part throttle again. I was still going to run it because last time it did this is was fine at WOT. As soon as I hit it at the line, it popped and shot flames out of the carb 3-4 times. Timing curve is very similar to what alot of people are running on here, checked rockers, checked compression, im lost. Would a bad lifter cause this?

SN-95

Car* lol

melsie68

iTrader: (8)

What is your combo and what ignition are you running?

SN-95

lq4, victor junior, holley 750, 234/238, patriot springs, and 1 7/8 dynatechs. Using the 6010 box. Ive tried the pills, and my own curve and the problem hasnt changed. It starts up just fine, but then stumbles and backfires through the carb and exhaust. I through a timing light lead over the plug wire to show when it was sparking, and a few of the cylinders seem to cut out completely when i hit the gas. I called msd and they dont know what it would be. I have changed cam and crank sensors, checked rockers, pushrods, timing gear alignment. Also made sure we had a good ground and constant 12v. Im lost...

melsie68

iTrader: (8)

I don't mean to sound insulting at all so please don't take it that way...just some basic questions to understand what is happening... Can you tell if the carb is loading up on you? Will it idle at all? That may cause a backfire through the exhaust and will certainly foul plugs causing misfires.

If the engine won't rev up at all without backfiring through the exhaust it could be timing or too much fuel making it a carb issue. The pop through the carb absolutely sounds like a timing issue to me. Have you tested all the coils to make sure they are okay? What coils are they?

Any chance the reluctor teeth on the crank could have been damaged or bent on the rebuild?

A bad lifter could either cause a loss of power in the cylinder if it doesn't achieve full lift or a complete misfire if it doesn't even open the valve (destroyed lifter). It shouldn't cause a backfire of any type unless it is hanging the valve open which in that case your pushrod length is too long depressing the lifter plunger and also causing the rocker to open the valve while the lifter is on the base circle of the cam. But it doesn't really sound like a lifter is your issue.

Does MSD offer any advice or indication as to what your issue may be? Maybe your harness has the coil leads switched firing the cylinders out of time? That would be if you had the wrong plug to the coils on the other side of the engine. Not sure it would run at all in that case, but it sure would sound like crap if it did.

SN-95

Any advice is great at this point.
The carb was loading up at one time, but fixed that by adjusting the floats. Now it will idle, but it sticks around 1400 rpms, even with the curb idle screw backed out all the way. I've tried 3 sets of coil packs, all do the same thing. I never thought about the push rods being to long. They are stock pushrods, and the cam is nothing to crazy so I thought they would be fine. I talked to msd and they were really no help, I was pretty disappointed. I installed a relay to insure steady voltage to the 6010 tonight, no help. I don't know if the car would crank if the reluctor wheel was bad, not sure though. It cranks up almost instantly every time, even cold

rennat_2006

iTrader: (10)

Backfire out of the carb is either timing too far advanced or the motor is going lean. Since you didnt say anything about adjusting or messing with the timing im going to assume that it is going lean.

Take the carb off and flip it over, Look at the baseplate of the carb and you will see a little flat head screw on the end of the secondary shaft that will open/shut the secondaries. Set this to where the transfer slots(slots cut into the throat of the carb) are just squared off(showing as much up and down as they are wide). This will cause the high idle and screw up your idle mixture settings if too much of the transfer slots are opened up.

Check the adjustment on the accelerating pump linkages also, You want them adjusted to where your not getting any pump shot at idle but as soon as that throttle is even touched the pump shot starts.

SN-95

Thanks, I'll def check all of that as soon as I get off. I'll let you know how it goes

allblowdup

lso check for vacum leaks in the system. Do you have pwr brakes? The booster can fail and give you a huge vacum leak that you won't hear and creat a lean condition.

SN-95

Manual brakes. We actually hooked a smoke machine up to the system to check for leaks. The only leaks were from around the throttle shafts, and somebody said that was normal. Seems like it could be an issue though

Pop N Wood

iTrader: (1)

My car was having intermittented back fires through the carb. Turns out according to the MSD web site a disconnected cam sensor has a "50-50 chance" of coming up right. That is what was happening to mine. The sensor plug had come loose, so half the time the car would come up right and the other half it would be 180 out. You can try it yourself, the car will run with the sensor completely disconnected.

The bad thing is there was no other symptoms or diagnositcs. I reseatted the plug and all is good. May not be your problem but something to keep in mind.

1 FMF

iTrader: (1)

a bad lifter could cause the problem you describe, but i think it is unlikely. And if it were a bad lifter such as a collapsed lifter you would hear it. It would more likely be improperly adjusted rocker arms or pushrod lengths resulting in incorrect lifter preload, but if it were this then you would have the problem all the time. It would not run great one day, then not the next.
My vote would be it's the carb, most likely **** in the transition circuit in the carb.

SN-95

I have actually tried 2 cam sensors, and 2 crank sensors, and made sure they were seated properly because that was the only thing that I could find online that anybdy had had similar problems with.

I tried another carb, and got it to idle better, but still backfiring