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Back FIRE through the carb

J

JBauer4363

Member
Been getting my car back together to go in to get my front end alignment check but after sitting for the last 6 months it was running rough. When I give it any gas it boggs down and backfires. Adjusted the idle screws w/vac. gauge, checked/adjusted timing and idle, left the shaker off and started to drive it and wow several backfires w/big flames on minor accel. Didnt get a block. I am certainly not very good at this stuff but I had it running good last fall (I just dont remember how). Heres the specs:

351C Holley 600 4bb
Auto
Mild cam
Timing 10 to 12deg. btdc-have used both runs about the same. Even tried 6 deg like service manual says still hesitated.
Pertronix 2 - Flamethrower coil.
Idle 950 to 980
Vac 16Lbs bounces between 15-16 at idle. If i rev it up it vac goes up and doesnt bounce around at all.
Idle screws 1 1/2 turns out (turning 1/2 turn more either way doesnt seem to affect vac.)

anything about these numbers look off? What would cause a hesitation/backfire like mentioned? Gonna tackle it again Sunday but if anyone has any tips I would be appreciated. Thanks

JBauer4363
 
Installed new tank last fall. Had about a 1/8 of a tank that sat since Oct. Used alot of that messing with it just idling in the last couple days. Never thought of that! Went and bought 4 gallons in a can tonight and put it in after I quit messing with it. Never had a problem before with old gas. What should I do about that?

Thanks
JBauer4363
 
Gas from last October should be no problem, especially if you fill it up with new gas. My boat sits from August/September to May/June every year. I try to put it away with as little fuel as possible, then fill it up when I get it back out in May/June. I've never had a start or carb problem with it.

My first guess is that you have a blown power valve, if this is an older Holley. The older versions are notorious for blowing the power valve on a back fire. The newer versions have a check valve to prevent blowouts in the event of a backfire.
 
Correct me if I'm wrong... I think backfire is caused by the carburetor running lean, or too much timing. I'd look for dirt in the inlet filters and change any swollen gaskets in the carb.
 
"hivewax" said:
Correct me if I'm wrong... I think backfire is caused by the carburetor running lean, or too much timing. I'd look for dirt in the inlet filters and change any swollen gaskets in the carb.
Click to expand...

+1

Also, a non-closing valve will cause a backfire, but if nothing's really been done to the engine, this is probably a non issue.

What is your initial timing set to, and what is your total timing? With a timing light, is the timing curve smooth as you open up the throttle?

I would also check the float levels to make sure the fuel level is not low in the bowls causing a lean condition.
 
"johnpro" said:
What is your initial timing set to, and what is your total timing? With a timing light, is the timing curve smooth as you open up the throttle?

I would also check the float levels to make sure the fuel level is not low in the bowls causing a lean condition.
Click to expand...

I dont know check the float level so Ill start with the timing. Ill check and post the results.

Thanks
 
"JBauer4363" said:
I dont know check the float level so Ill start with the timing. Ill check and post the results.

Thanks
Click to expand...

With the engine running, simply remove the plugs in the side of the float bowl. The fuel level should be RIGHT AT the bottom of the plug hole. If it's low, you'll easily create a lean condition. If it's high, it can easily lead to flooding, but that's not your problem. I would only do one float bowl at a time, as you can dump fuel on the engine quickly and easily when the plugs are out. You can use clear plugs, in the holes, but I find them to not be of much use.

If the fuel levels are low/high, you simply adjust the floats with the adjusters on the top of the float bowls, to raise/lower the float, thereby adjusting the level (and amount) of fuel in the bowls.
 
If it was running good, and you haven't changed anything then its BAD GAS.

This happened with an engine I just sold about a month ago. I had old gas in it, and tired to start it, when kapow, pop back through the carb and headers.

I played with the timing, and everything, even tried new gas. It finally smoothed out, and I got it sold.

Last week I finished installation of a Tremec TKO tranny into my 65 fastback. The gas has been in there since Sept last year.

Same thing pop's through the carb, and dieing. Put some new gas in it and after a bit of acting up it finally smooths out. Also, if you have a MSD 6 AL ignition make sure you slow recharge the battery as the engine may try to start and even run just a bit, but it will behave like bad gas unless the battery is fully charged.
 
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