Alright, i need some help, because im about ready to burn the T/A to the ground. Just swapped onto the 350 a set of vortec heads, an xtreme energy 262h cam, and an air gap intake. Bolt everything on, and get the motor back in the car, hook everything up, and ive got an interesting problem.

The motor starts and runs fine at idle, but hammer on the gas from idle, and it backfires out the carb. Originally thought it was carb that was causing it, but i swapped the carb i had with an edelbrock i had laying around, and its still doing it.

I have re-checked the plug wires, and they are in the correct firing order. Swapped in a new cap and rotor and ignition module, and its still doing it. The mechanical advance is set to start advancing at 600 rpms and have full advance by 2200, and im running 8 degrees initial advance.

whats weird is that if you take the rpms above 3000 and go to WOT, it doesnt backfire out the carb. But if you do it off idle, it does. I seriously need some help here, because im at my wits end!

 

I had a similar problem with my 79 Z28, 350 cid and eddy 750cfm - lean , lean and lean :/ those edelbrock carbs sucks

 

sound like a cross between timing and carb tunning. Did you say your inital timing is set at 8*?

 

Backfiring out the carb is bad. Usually blows the the power valve and it will have to be replaced.

 

i dont think its running lean, i really think its in ignition. the first carb i had on it was a holley 650 vaccum secondaries with 31s for the accelerator pump and 71 jets. which considering i live at about 4800 feet above sea level and it was at least 85 degrees outside, it shouldnt be running lean...

 

oh yeah, just discovered that it is occasionally backfiring out of the exhaust at idle.... freaking thing.... this is some serious

 

yeah, its running 8 degrees initial and checked the power valve, and its fine....

 

I don't get the idea of people threatening to harm their car...I say do it, torch that sucker and show it who's boss!!


All joking aside, a couple of things come to mind:

1.) Did you degree the cam during installation? If so, how is it installed...straight up, 4deg advanced, etc.?

2.) Carb accelerator pump tuning...bring the pump shot on earlier and harder. With Holley carbs it's a little crescent-shaped eccentric screwed onto the primary throttle shaft, you can buy kits w/ various lobe shapes, not sure about the others.

 

well i had the gas can out, and was getting ready to do it, but at the last seconds she cast her "im waaay to pretty to do that" spell...

the cam is degreed straight up. which i think means that from the factory its four degrees advanced (think i read that in the little booklet that came with the cam, but not sure)

well... just to update, we tried another HEI, and it did the same stuff, so we know for sure now that its not ignition.

As far as the carb goes, pulled the plugs and they are way too dark for it to be running lean. but i don't know if i can really trust the plugs with it doing the stuff its doing.

We're going to try doing a compression test in all the cylinders, to make sure that none of the valves are sticking open... but can't do that until my dad brings home a different tester, the one we have wont fit in some of the holes due to the headers being in the way...

So, if the compression checks out, we'll try to screw with the accelerator pump.

Thanks for your help everybody!! its greatly appreciated! Gold stars for all the suggestions!

 

Try bumping the timming to 10-12*, and thats with the vaccum advance clamped off. Also are the floats adjusted right?

I would check for a vaccum leak also. Check all vaccum lines and and all fittings on the intake. Make sure you dont have any leaks around the center intake bolt holes.

 

I was having the exact same problem when i swapped mine from TBI to carb, im running edelbrock 1406 carb. I set my timing to 18 degrees base (36 total) and tuned the idle mixture screws on the carb and i fixed everything.

 

we did try setting the timing up higher (don't know how far up, didnt have the timing light hooked up) and it was still doing it. I initially had it set at 10 and backed it off to 8 (and we always set the timing with the advance blocked off). Also, we previously checked for vaccum leaks, and it didnt have any... also checked the float levels, and they were fine. sorry guys, forgot to mentions that stuff. thanks for all the suggestions, keep em coming!!

 

Well, just to update, the compression test checked out ok... and so did the cylinder leakdown test. so now we are back to square one. My dad did get the car to stop backfiring out the carb sitting in the garage, but it still is running like crap when driving it. we are going to try another carb on the car some time soon. The only other idea we have is that maybe the cam is messed up from the company.... hope not...

 

I think it's lean too.

-Tom

 

Well, sorry to dig up this old thread, but i just wanted all of you to know that the car wasnt running lean, and it wasnt ignition. Turns out the cam is degreed 6 degrees retarded. And we didnt do that, so apparently comp sucks @$$. but yeah, the plan is to now degree the cam back to where it is supposed to be, and run the car like that.

 

Thanks for the update. It's always nice to hear some feed back.

I think Comp sucks too.

-Tom

 

Seems like me and my dad have been having some serious problems with aftermarket companies... The intake from powerhouse performance that wasnt milled correctly, the pertronix hei distributor that just fell apart, the cam thats degreed incorrectly, the mallory rev limiter box thing that took a dump (after the first one that never worked straight out of the box). WHAT HAS HAPPENED TO QUALITY CONTROL?????????????

On the other hand, we are going to degree the cam correctly tonight, and see if the car will run right like that. so ill keep you updated...

 

How did you determine that? Piston stop and a degree wheel?

 

yep thats exactly how. I don't know of any other way of doing it.

 

Well if you'd tried to use the balancer mark or something I wouldn't have been surprised it was 6 degrees out. So your peak lift didn't happen where you expected it to? On every lobe?

 

yeah centerline is supposed to be at 106 and its at 112. we only checked the number one though. we are going to just set the degree to zero... then see if it runs right. if it doesnt.. im going to send the cam back to comp and demand compensation.

 

Why do you think the problem is with the cam and not the timing set then?

 

well, the timing set is brand new.... a double roller set. and we checked and rechecked the degreeing three times... so it seems to me that the only thing it can be is that we either installed it wrong (which considering we both checked it several times before the cover went on it isnt very likely) or the cam is off...

 

sorry, i think i understand what you were saying now after i reread it. its possible that the cam gear was punched wrong or something. one way or the other we are going to fix it tonight though... (hopefully)

 

well, just for an update, we tried and tried and tried to degree the cam tonight, and kept coming up with these crazy numbers. So we head inside, planning on trying something else tomorrow. thats when my dad realized what we did wrong. All night long we were trying to do the intake centerline on the #1 exhaust.....

So after this humbling realization... we decided we should try again tomorrow on the correct lobe, and see if it helps a little bit.

I feel sufficiently retarded for not noticing it earlier, but i guess thats what happens when you get all worked up and try to do things quickly. so hopefully, tomorrow we will fix the car! (unless tonight's trend of thinking continues, in which case i will probably be convinced that my car is in fact a row boat)

 

Well, this is the final update!! Finally got the car running!! After degreeing the cam, it was still doing the same problems... so we started over again. Put a new coil in the car, as well as new wires and new plugs. Then, out of the blue my dad put a vacuum gauge on the port vacuum, and the car was only pulling 7 inches of vacuum at idle. Then, while talking to a fellow racer (who has been at it much longer than me, and who owns a speed shop here) he mentioned that on one of his motors he had an intake that was leaking into the lifter valley, thereby causing a massive vacuum leak, and that it was causing the same symptoms...

so while i was at super Chevy today, my dad pulled the intake and resealed it, making sure that it was sealing pefectly. Now the car is running great under normal driving conditions!! it has a bit of a stumble at WOT, above about 3500 rpms, but i think its most likely just tuning...

so apparently all my ranting about the cam was all for nought....

 

Glad you got it figured out, and even more glad that you posted what the results were. You might think it was stupid, but human error is always a factor in why a car wont work after you wrench on it... it's nice to see that some people are willing to use the correct tools and find out, in a logical, step by step order what has gone wrong.


I know there are some guys out there who would rather do everything else EXCEPT pull the cam just because its a pain in the butt.


Good job, and happy tuning