The Stovebolt Site Forums The Shop Area Engine Shop ignition (timing) questions - 55 235.

My truck has a 1955 235 engine. I believe that carb, distributor, and all innards are original. I seem to be past all the "water in oil" concerns and now trying to get a smooth running engine but have some basic questions. The engine is currently running (relatively) smoothly at 900 to 1000 RPM (I will address some carb issues separately). With the engine running, the timing light spots a point on the fly wheel that is to the right of the "bb" mark. Does this mean that the timing is advanced or retarded? How many arc-inches from the "BB" mark should I be at this RPM (do the centrifugal weights inside the distributor have an effect at this RPM)? And in which direction?

Thanks, Bob

Try to get the idle speed down to 500-600 RPM before checking the timing with a light. You're probably getting a little centrifugal advance at 1000. It would also be possible to get some vacuum on the distributor with the throttle plate open far enough to idle at that speed. Ideally, the BB is a good starting point for initial timing. It's not set in stone, particularly since the gasoline quality in the 1950's and what's available today can be very different, and ignition timing needs to be tweaked to the octane rating of the fuel that's available.

If the BB is to the left of the pointer, the timing is advanced a little. Turn the distributor housing clockwise to align the pointer and the ball.
Jerry

If you set your timing at 900 to 1000 rpm on the ball chances are your overall ignition timing is incorrect and most likely retarded

At 375 distributor rpm (750 engine rpm) you generally have about 0-2 degrees of mechanical advance coming in (assuming you are using a 1112403 distributor and a mechanical advance that is working properly). At 700 distributor rpm (1400 engine) you are looking at about 4-6 degrees mechanical advance. 1350 (2700 engine rpm) you should be at 9-11 degree of mechanical, and 1750 (3500 engine rpm) you are all in at 12-14 degrees. So at 900 engine rpm you are already 2 degrees advanced and maybe even a little more, so like sstock noted above setting the timing on the bb probably has you retarded a little.

But the mechanical advance is just part of the equation. The vacuum advance may or may not play a part here. The vacuum advance begins to start coming in when the diaphragm on the vacuum advance assembly sees about 4-6 inches of vacuum. You end up with a total of about 7.5 distributor degrees (15 engine degrees) when the vacuum reaches 7.5-10 inches of vacuum. The vacuum line is attached to the carb and the carb's vacuum port is actually located above the throttle plate, so its on the atmosphere side of the throttle blade as opposed to the engine side. When the blade moves and starts to open on acceleration, the vacuum port sees vacuum as air flows past the throttle blade and vacuum port. The vacuum level will increase as the throttle blade opens, then will decrease as the throttle gets closer to wide open. If you have the throttle blade cracked open far enough at idle the port may be seeing enough vacuum for the vacuum advance to come in a bit. So it could be that you have some mechanical advance and some vacuum advance coming in at your 900 degrees of idle which, again, probably has your timing a little retarded.

So, yes, as previously suggested, you should try to get your idle down to 5-6 hundred rpm. If you can, then the bb will get you at tdc. If not, use the above as a guide as to how much advance you may already have at 900 and make your timing adjustments accordingly. You might also consider removing and plugging the vacuum advance line when you are setting your initial timing. Once you have your initial timing set. you can connect the vacuum line, then start to add a little timing to see what your motor likes (if it pings you went to far). Every motor is a little different in what it likes timing wise, and gas and elevation above or below sea level also play a part so tinker away!

Thanks, this helps. But - altitude! How does this affect things? I am at 5000 feet.
Bob

At altitude such as yours, one can advance the timing slightly, and can also burn slightly lower octane fuel, which probably is not available for our vehicles. I just advance the timing until I get a slight ping under high loading, low rpm operation. One can retard slightly from that point if it might be a problem.

Ed