How do you fine tune a distributer?

DeWayne Utrup · Jul 17, 2014

I have been playing with a dyno. Last night I put my 650 gas tractor on and was very short on power. Mechanically the engine is in great shape. The fuel pump and carburetor were recently rebuilt. My manual calls for 9.5d advance at 400 rpm and 20d at 800. At idle (380 rpm) the tractor starts and runs very sweet at about 20d advance but then at high idle (1650 rpm) it is running about 40d advance. While it was under load I changed the timing to 30d advance and had a 12 hp increase but when I put it back down to idle it ran rough. I am not planning to pull this tractor but I always like to have them running at factory specs and it is fun learning from you folks. Thanks in advance.

notjustair · Jul 17, 2014

Whether this is right or wrong - here's my thinking.

I was taught (when I was busting knuckles on old cars) that total advance is the one thing your really need to be concerned with. That has been my approach with old tractors as well. Most times they are doing the most work at WOT and they have mechanical advance so I set them at WOT for their greatest advance. From there I adjust the carb so that it operates well at other throttle levels. Granted, they sound like they are running so well with more advance, but they don't last long like that.

David G · Jul 17, 2014

You will get more power with the correct advance, and 30 degrees is right at the limit. The reason the idle was rough is that the curve starts early.

I would crank the total to about 25 and see how it runs. I am finding that I can run about 22 on my high compression engine.

pete 23 · Jul 17, 2014

Your manual is quoting distributor rpm and not engine. Therefore the 20 degree advance at 800 rpm would be equivalent to 40 degree advance at 1600 rpm. That 40 degree advance was for the original cast iron piston with standard bore. If you have installed a thin wall sleeve and aluminum pistons the degrees drop to 30 degrees at full advance at engine. If you have a high alt piston or fire crater the advance continues to drop to close to 22 degrees. Over advance causes detonation and lack of power with damage to pistons. If you adjust a 40 degree advance distributor to have 30 degrees at full throttle you of course will be late at low speed and not run freely. You can take the distributor apart and fill in the hole in plate that controls maximum advance and then play with the springs by curling the ends to get close to what ever advance gives you the best performance. It takes trial and error. You can get different distributor shafts and correct springs for that shaft also if you want to go that route. I have no idea what pistons you have so can't say what advance you actually need.

Earl-IL · Jul 17, 2014

The dist. advance weights are worn out. If you have over 30 deg total advance and it runs best at 25 total. You will be too retard at idle 5 deg. The springs could be bad also, but usually only effects coming advanced too quick. I had a Deere 2cyl that had 55 deg. of range it ran terrible.

How do you fine tune a distributer?

DeWayne Utrup

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notjustair

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David G

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pete 23

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Earl-IL

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