| Re: [GMCnet] Timing the Onan - Ken B. [message #83420 is a reply to message #83414] |
Thu, 06 May 2010 22:19   |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma:
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Senior Member |
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Ken,
Thanks! Now it all makes sense!
Regards,
Rob Mueller
Sydney, Australia
AUS '75 Avion-The Blue Streak TZE365V100428
USA '75 Avion-Double Trouble TZE365V100426
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org
[mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of Ken Burton
Sent: Friday, 7 May 2010 12:50 PM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] Timing the Onan - Ken B.
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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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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