The Stovebolt Site Forums The Shop Area Engine Shop Backfire

Thanks Otto. I don`t drive the one but I crank it every day. Just don`t drive it down the road.
Right? Its not legal. No tag or lights. I`ll try the 87 octane in spring. I have less than a month till I have to gear up for hunting season.
That truck will be drained of fuel and put on skates to sit over the winter. Moved over into a corner till spring.
The other two ,I`ll drive till they start putting salt on the roads. Then its Good Made Chevrolet time.

My Carter YF has over 30,000 miles on the last rebuild running only 87 octane unleaded with ethanol.

Just to be complete, you might consider checking the vacuum advance to make sure the diaphragm has not ruptured. Make up a little stub fitting out of a piece of brake line, attach a small rubber hose. Disconnect the vacuum line from the advance and install your testing tube. Suck on the hose or use a mitivac if you have one. If you are easily breathing through, the diaphragm is toast and likely the root cause, or a contributing cause, of the backfiring. If the diaphragm is good, reconnect the vacuum line, start the motor and set the timing.

You set the timing by using the screw on the clamp, not the octane selector bolt. Ideally, the selector should be set to 0 (hash mark on the distributor pad and the 0 mark on the selector plate lined up). Set the motor to anywhere between tdc at the bb to 5 degrees btdc. You should certainly be able to run 87 octane.

The other issue that might be contributing to the problem, a malfunctioning mechanical advance in the distributor. It is not all that uncommon when I am rebuilding a distributor for a 235 to find the advance springs missing, or one broken off, or jammed in the weights, or just rusty, gummed up weights that are just not working.

Thanks Mike

To add, the engine will fire right up. All that I need to do is
pull the choke 1/4 way and turn the key. I`ve not cranked the thing in 5 or 6 days and
just like that. Its just waiting to crank cold. Thats a good sign...? Nothing could be real wrong with it.
My 327`s won`t crank that easy.

I added 5 gallons of 87 octane to the 91 and the back firing stopped. Here is a video of the whole shooting match.
www.youtube.com/watch?v=-fRDTgqoGCM

Good for you!
You ain't the first guy to run into the "too much octane for too little compression engine" scenario.
Guys who try to run 87 octane in a high compression engine, with a performance cam have just as much trouble with how it runs, but their problems can be catastrophic.