Re: Cold Running Motor…Revisited

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johnyrude200


Replies: 782
Topics: 186
#12767

Well, here’s what I found. The test tank water was up to a balmy 50° today, and the motor was able to get up to about 98°. Correction to my earlier posts…last night I was barely up to 72° even at sustained medium throttle.

I was able to lean the carb out 1/4 turn (so, about 1 & 1/2 turns out), and of course adjust the idle setting down to correspond. Last night I could not do this, the motor would spit terribly or just plain stop running. I did note both last night and today that when I leaned it out to 1 turn (as the manual describes), it did help it run smoother, but would spit and stall almost immediately at the low range (anything above a 1/3rd throttle seemed OK, but still had misfires at 3/4 range).

The misfire at mid range was no longer present today, but it was blubbering like hell all the way through, loading up with fuel/oil. At 3/4, after about 10 seconds, it would clear out and smooth, but there was a whole bunch of smoke and unburned fuel being thrown away. It was struggling back and forth between smooth and rich pretty much at all ranges except WOT, and I don’t run them in the test tank with test props at WOT not for more than 2-4 seconds due to overtaching (easily reach 7000RPM in a hurry = bad). Service manual recommends 4900 RPM for calibration.

My conclusion is that the ignition system is/was fine, and that it is just running cold and rich. I guess I’ll chalk this one up to yet another one of these ’68-76 18-20-25’s that behaves this way. The misfiring is what cued me to reach out to everyone – just more experience for a rookie I guess.

Don and Frank, I will take one of these motors and do the conversion to check out the difference. I’ve been going through a batch of these in the past couple of months and have a bunch more waiting in the wings, so I might as well learn about it and if it makes them run better make it part of the routine. Don I did check the thermostat seal and it was in good shape.