The Stovebolt Site Forums The Shop Area The HiPo Shop 350 SBC Bad Backfire Thru exhaust

Fired it up after sitting for a while and this sucker was backfiring awful, really on left side. Has a 4 bbl Edelbrock. Jerry once told me that water can accumulate under the dist cap. I checked, no water. No points. Double checked the timing, it was a little advanced, about 16 deg with no vac., set it back to 10 deg with vac can on dist unplugged and vac gauge on carb vac inlet. Also double checked TDC on no 1. All square. Still backfiring like crazy. Compression test, all 170 psi +/- dry. Wet all about 20 higher. Plugs are gassy, way too rich. Now to idle air adj. Start, still backfire badly. Adjust idle screw to 650 rpm, lean out right side idle mix screw, when rpm increases by 40, adjust idle screw back to 650. Repeat right and left side until idles at 650 and when lean out idle mix reduces rpm. Now backfire stops. Tighten dist hook vac can back up go for a drive, no backfire. About 30 deg total advance at 2000 rpm. I forgot to pull plugs to see if they are still wet and running rich. What do you guys think was causing the backfire?

Crossed up spark plug wires- - - - -probably #5 and #7 wires swapped.
Jerry

Possible, but don't think so Jerry, Only reason is I've got little stickers on both ends of plug wires to keep 'em straight. 1-8-4-3-6-5-7-2 up top. I'll double check when I pull those plugs today. She's running fine now though.

Possible you had a stuck intake valve that freed up while you messed with the adjustments.

Just think about your girlfriend to remember the firing order- - - - -18- - - too young- - - -72 - - - -too old- - - -between 43 and 65- - - -just right! Good grief- - - -what am I saying? I'm 72 and a few of the gals who came to our recent class of 63 reunion still look pretty good!
LOL!
Jerry

Allen,

If adjusting the idle mixture screws solved the problem isn't it clear that there was an idle mixture problem that is now corrected, or am I missing something?

Does Texas gas contain ethanol? California gas does and it causes a variety of problems when it evaporates in a carburetor that is on an engine that hasn't run for a while. For example one of my chain saws that hasn't been run for a while wouldn't start or run. Apparently I hadn't run it out of gas before I stored it. There is a tiny filter screen in the carburetor that had a clear film over it that totally plugged the mesh preventing gas flow. I saw that problem described on a arborist site and opened up the carburetor to check that screen. I've heard of similar carburetor problems related to ethanol gas from off road motorcycle guys who didn't drain their carburetors before parking the bikes for long storage.

Ray, it was running perfect when I parked it about 3 mo's ago. It has happened before, but cleared up a lot quicker before. This one wasn't going away. It was running rich this time, but I'm not sure by the time I got to the carb and it was good and warm, either it got all the bad gas out of the system or as Martin says, a sticky valve freed up. It started up instantly this morning, so appears we're good, Plugs looked good. Was hoping some of you guys had experienced something similar. Yes we do have ethanol in our gas, but never had a problem like this. My 235 and the old tractors sit for longer often and fire right up with the stabilized gas.

Next time it happens, I'll just pull the plug wires off that left side to try and spot the offending valve. Thanks Martin.

Your as young as you feel Jerry. Keep on truckin'.

Maybe you should start it more often?

That reminds me of the story of the fella that told his Doctor, it hurts when I lift my arm, and the Doc's solution was "well don't lift your arm".

Run a heavy dose of Sea Foam in the gas- - - -it stabilizes it between infrequent startups, and also keeps the valve stems clean and free-moving.
Jerry