| Re: [GMCnet] Timing the Onan - Ken B. [message #83414 is a reply to message #83396] |
Thu, 06 May 2010 21:50   |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
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Many people crank on the points adjustment screw until the generator runs smoothly or runs to their satisfaction. They think they are changing the only the dwell or the point gap. They are indeed doing that but they are also changing the timing. Dan's (with the points running it) was already set quite a bit advanced. So we left it that way when installing the Pertronix unit.
We turned the flywheel over and found the timing mark in the little window. Then we stuck a piece of white double faced tape on the outside of the flywheel at that same point so we could more easily see the mark in the bright sunlight. After that it is simple math to figure out how far around the circumference of the flywheel to go to accomplish one, or two, or three, or more degrees advance from that marked base point. You can also just measure the circumference and divide it by 360 to accomplish the same thing.
If you want to be more scientific / accurate about it, you need to determine which flywheel timing mark you have. I believe, If I remember correctly, they made a 24, a 22, and a 20 degree marked flywheel in the years that engine was used in GMC installed generators. I believe mine was marked at 24 degrees. The value was stamped right in the flywheel on mine. Onan also used that same engine in other things. We have a gasoline Onan powered welder at the airport that uses that same engine. I believe it was marked at 25 BTDC.
I was introduced to a retired Onan engineer around the time that I did mine. This was 5 or 6 years ago. I mentioned to him what we had done with the Pertronix unit. He told me Onan looked at Electronic ignition in the 70's but never got away from the mechanical push rod type of trigger which was what was really needed to be replaced resolve the varying advance problem.
He told me that our Onan would run perfectly at 30 degrees advance at 1800 RPM under full load as long as the timing was stable and not bouncing all over the place. He further stated that they initially retarded the advance due to the poor mechanics of the point push rod system. After a few Onans came back under warranty with detonation damage, they reduced the advance spec. by 2 degrees. That still did not completely resolve the problem and a few more came back with detonation damage, so they reduced it a couple more degrees. They kept doing this until no more damaged returns were received. I think he said over several years they settled on 20 BTDC. It was a band-aid fix rather than a real fix of the mechanical push rod points system problem.
With all of that information, to be safe I decided to settle on some advance figure of less than 30 degrees for mine with the Pertronix installed.
Keep in mind that the Onan is a fixed advance system. It does not retard for starting or for high loads so going too far advanced can cause difficult cranking for starting and detonation for high load applications.
On air cooled airplane engine we have a similar system at similar RPMs but we also have retard for starting. Even with that system the advance is only set for 25 BTDC for all running conditions.
With all of this information I suggest that the maximum you want to go is probably around 27 degrees BTDC.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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