I have a 350 with a magnum 280cam, sportsman II heads, RPM intake, 9.25:1 CR, and a 750cfm Speed Demon.

The motor is an 18,000 mile GM goodwrench crate motor and I just swapped the cam, heads, and intake. I'm using the same demon carb that I used on my last motor. But this new motor has the same problems as the other motor did.


I get a backfire through the carb, sometimes it stalls out and other times it keeps running. It doesn't happen all the time, but when it does happen it’s when I'm just starting out from an idle. The backfire happens at random times and it doesn't matter if the motor is hot or cold.

I also have a bog at WOT and the motor doesn't pull like it should. The motor runs fine when cruising around, but feels slow when I punch it.

My last motor: 350 w/ XE268 cam. I had the same type of backfiring and bog but it happened less and the bog wasn't as bad. That motor had other issues: broken piston rings and bad cam. I figured that those problems were causing the motor to run poorly. But now I have a completely different motor and the same problem.


What should I do to solve the problem? Has anyone here experienced random backfire through their carb?

The timing is correct and carb is jetted with 74/81 jets.

 

Richen it up. Backfire through the carb is a lean condition. Then, make sure you're running at least 18 degrees initial timing and 36 total.

 

Should I try going to 76/83 jets?

I have to pull the plugs and see what they look like. if they are white that means lean?

 

A backfire (lean condition) on acceleration calls for a step up in pump nozzle size.

 

Yes, if they're white or very light, you're lean. If I were you, I'd bump it even more just so you get a noticable change in setup - this will tell you if you're on the right track. Go 78/85 and see how the car reacts. If the backfire and hesitation disappear, you can do some fine-tuning and lean it to the point where you're on the border. The worst that will happen if you go too big on the jets is that you'll puff some black smoke on acceleration - then you'll know for sure you're rich and not lean...

 

I just checked my notes on how the carb was set up. I set the throttle plates at .020” gap, changed to 74/81 jets, and have a 6.5 power valve. The mixture screws are set to ¾ of a turn out. The floats are set to slightly higher then the first line.

The timing is set using Lars weights and springs. w/ 36* approx 3500rpm.



This weekend I will try richen the carb up a bit and see what happens.

 

My MSD caused this today...backfiring thru carb....but it would not run/start afterwards...(it had died on me earlier when stopping..twice)...then it just crapped out..

 

Over the weekend, I pulled the carb off and swapped jet sizes a few times.

I first swapped to 78/85 Jets. I set the throttle blades at .020" and had the 4-idle mixture screws out 1 turn. I also had the vac. for the headlights blocked off, just for the testing. With this jetting the car started up and idled by itself right away. I don't have a choke and with the other jets, 74/81, it took like 10min to start idling on its own.
The idle was high, ~1150 rpm, but I decided to test drive it before I adjusted the 4 mixture screws. There was a huge difference in performance of the car. I went to WOT from 15mph in 1st and there was a very slight hesitation but then it pulled hard. I smacked 2nd gear at 5,000rpm and the rear tires broke loose and the car fishtailed for a second . The carb used to bog so much it wouldn't even churp the tires half the time. The car also ran better just driving around town normal. I started out from a stop sign in 2nd gear without any problem. I also went up a hill in 4th gear, pulling from 1,000rpm. The carb used to stumble badly and I would have to downshift in these situations.

Despite the increased performance and better drivability of the car the carb seems too rich. I got black smoke under hard acceleration and the exhaust had a heavy gas odor. I played around with the idle-mixture screws a little bit. I turned them all in a 1/2 turn and the idle increased to 1350 rpm. Then I brought them all out a 1/2 of turn, this dropped the idle to 850rpm but the idle was rough and it was way too rich. I could see a gas vapors coming out of the exhaust at an idle.

I next tried 77/84 jets. I set the idle-mixture screws 1.25 turns out. At that setting the idle was 1050rpm. I dropped the idle to 850-900 by just slightly backing off on the 2-throttle blade screws.

This is where I left it for now. The car stills runs good at normal cruising speeds and WOT. But there is still a slight hesitation in the low end throttle response. If I hammer it from 2,000-2,500rpm in 1st and 2nd gear there is a slight hesitation. But If I hammer it from 3,000 rpm it rips. I also still get a heavy gas smell from the exhaust.

Next weekend I will try 76/83 jets and see what happens.

Are there any tuning tricks to get better low-end response? If the carb is set up correctly, should there be a heavy gas odor?

 

sorry for the long post, i just wanted to get all the details in.

I almost forgot to mention that it doesn't backfire through the carb anymore.

 

Bump TTT