Backfires

[dwleo]

Hi guys,
Almost finished my T-bucket.
One problem I have is the engine. Its a 350 sbc and when I start it, it fires immediately and runs strong for about 30 - 45 seconds then starts to misfire and backfire through the pipes and ends with (sometimes) a carb backfire then dies. It will startup right away and repeat itself.
I am running a slightly mild cam and a fuel slayer 4 barrel. Changing the timing seems to make it worse so I am leaning towards incorrect valve lash.
Is this a reasonable assumption?
Thanks.

- Daryl

[Bill Cleveland]

Might be too high a setting for the carb float(s), causing unburned fuel to collect in the exhaust & backfires result.

[dwleo]

Through the two sight glasses, the level is at 1/2 way - where it is supposed to be.

[Rolla Hessel]

You aren't 180 out on your distributor are you?? If not for running for 30 seconds before acting up that would be my first check. Make sure #1 is TDC on compression stroke and check to see if your rotor is on # 1 plug wire at cap. Just sounds like a timing / firing issue to me.

[Bill Cleveland]

I'm still thinking fuel mixture - too rich at idle. Air bleeds adjusted properly? Primary jets too big? How do spark plugs look?

[Indycars]

Did you build the engine? How did you set the valve lash? I don't really think this
is your problem, but it does need to be correct of course.

What do you know about the carburetor...... is it new? If not, then was it running
correctly before? Even new carburetors can have dirt inside. Remove and disassemble,
spray carb cleaner thru passages then blow out all passages with compressed air.

Have you put a timing light on it to verify it's right? Is this distributor a known good one?

You need to work thru it logically, testing and verifying that each system is correct.
Let us know what you find. Good luck!!!

[dwleo]

Everything is new, new, new. Except, of course, the bare block. Yes did all myself.
I agree about checking and verifying. So far all seems correct that's why I was wondering about the valve lash.

[Orville Duncan]

Sounds to me like the valves may have been set with the Hyd. lifters dry. Try just loosening the valves up just a little. See if that helps - if so - reset the valves with the hyd. lifters pumped up. Good Luck.

[dwleo]

I think it's best to give the engine details first then some history

Intake: ProComp 22000
Heads: Aluminium
Cam: Elgin e921p
Lift: intake .450 Exhaust .460
Duration @.050 224/224 lobe center 114
Lifters: Hydraulic
Carb: Quickfuel Slayer 600
Primary Main Jet: 68
Secondary Jet: 74
Idle Air Bleed: 70 / 39
High Speed Bleed: 31 / 31
Pri Nozzle: 31
Sec Nozzle: X
Needle & Seat: 100
Power Valve: 6.5

I assembled this engine and had it running smoothly at 2000 rpm for 20 mins or so to break in the cam as required about 6 months ago. I then changed the oil and then worked on other things. I had reason to pull the dizzy and relash the valves as a looseness occurred. Nothing has been right since. I would crank the engine until the marks would show on the damper and both valves would be closed and the piston is at the top of the cylinder. I set the distributer to hit plug #1 at that point. When I finally got the setup almost right, the engine started right up albeit now with the aforementioned backfiring that would happen after about 20 - 45 seconds of running. The engine would either die or I would turn it off. Hit the gas once and crank it and it would start. But again for the limited time then backfire would start. Over the past two days I have checked the valve lash and readjusted the mixture screws to 1 full turn out. I regapped the plugs (disgustingly fuel dirty with soot) to .040 and rechecked the vacuum lines. I hooked up a vacuum gauge a short while ago to the full vacuum line on the carb bottom and started it up. I kept it running (through the backfires) by upping the rpm's a bit but the vacuum showed nothing. I popped off the cap for the PVC line (don't have one) and the gauge read 6 immediately and drop to zero when I put my finger on it.
So, because it was running ok during break-in period and prior to when I pulled it, the distributer is naturally suspect however that would not explain the backfiring. There is a fuel smell to the exhaust, plugs are sooty and it almost seems (to me) that the carb is getting overcome with fuel and is flooding into the cylinders leading to misfiring and backfires. If I play with the timing setting, it doesn't get better but does get worse.
I'm flummoxed.

[Bill Cleveland]

I assembled this engine and had it running smoothly at 2000 rpm for 20 mins or so . . . . I regapped the plugs (disgustingly fuel dirty with soot) to .040 and rechecked the vacuum lines.

This tells me two things: I believe the backfiring is caused by an overly rich mixture (dirty plugs show this) allowing fuel to 'puddle' in the plenum after a few seconds and causing the backfiring. Also, I believe you may have blown the power valve in one of the backfires, and this would contribute to the fuel mixture problem. Just my 2 cents!