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.
~ BD. "You won't find me in an old folks home" Pioneer Florida Cracker 1958 Apache Stepside 1958 Apache 1/2 ton Panel 1959 Apache 1/2 ton Panel
My Carter YF has over 30,000 miles on the last rebuild running only 87 octane unleaded with ethanol.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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.
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.
~ BD. "You won't find me in an old folks home" Pioneer Florida Cracker 1958 Apache Stepside 1958 Apache 1/2 ton Panel 1959 Apache 1/2 ton Panel
And higher octane fuel will often cause the fuel level in the bowl to be too high, causing the engine to run rich.
A hundred years ago, Stromberg specified float adjustment settings if using higher octane than for which the engine was designed. Generally, Stromberg would specify an "initial" setting based on the octane, and a final fuel level setting. Generally the initial setting would be close, but the level setting was the one desired.
Those old low compression engines were much more forgiving on settings than modern ones. If the settings were worth changing on 6.0 compression engines, certainly worth checking on more modern engines.
A good "rule of thumb" is the needed octane number should be about 10 times the compression ratio. If I remember right (and if I didn't, someone will point it out ), higher compression 235's were 8.0 to 1, whereas the lower compression 7.5 to 1.
Thus 75~80 octane should be about right.
EDIT: the post by Otto above is correct, if real gasoline:
87 octane has 117,600 BTU/gallon 91 octane has 117,600 BTU/gallon 93 octane has 117,600 BTU/gallon and (if you can still find it) Sunoco 260 has 117,600 BTU/gallon
Jon
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
~ BD. "You won't find me in an old folks home" Pioneer Florida Cracker 1958 Apache Stepside 1958 Apache 1/2 ton Panel 1959 Apache 1/2 ton Panel
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.
1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission
), higher compression 235's were 8.0 to 1, whereas the lower compression 7.5 to 1.