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  #1  
Old 02-18-2010, 08:22 PM
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cub149 cub149 is offline
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Default setting timing on Onan B48G

I have searched and searched and can't find any info on setting the timing on the Onan B48G engine. I looked at the service manual but all it tells as far as timing goes is the setting. I can't find any markings on the flywheel. Where are the markings? I do see two long lines with hash marks in the middle on the casing, are these the marks to set the flywheel marks to? Also, which line do I use? The manual says 21 degrees, which line would that be? I have included a picture of the marks in question. Am I in the right place to begin with?
Which wire is considered the no. 1 wire? I was thinking the longest on, the wire to the plug on the left side.

And yes I know the engine is nasty, that is next on the list. I put new points in and I am still getting a rapid backfire at times during driving but not at idle. It's not like a big loud backfire but almost a machine gun type backfiring and as far as I can tell it is coming from the exhaust.

Thanks for any help
Chip
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Old 02-20-2010, 09:54 AM
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cub149 cub149 is offline
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Anybody want to take a shot at this?
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Old 02-20-2010, 10:02 AM
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R Bedell R Bedell is offline
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Be patient.....there isn't as many members with Onans as opposed to the Kohlers.

I am sure a member with an Onan will come along soon.

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Old 02-21-2010, 10:59 AM
ol'George ol'George is offline
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I will try to help here, but it is a general answer that would apply to most any engine.
If you have the points set about .020 with a feeler gauge @ top dead center it should be close enough to run properly.
If it is missing/backfiring at higher speed it is possible it is lean on the mixture.
You might try going richer say 1/4 turn and see what it does.
Usually, counter clockwise turn is richer.( make a note where it is now) so you can return to that setting if no improvement.
It can be a lot of things, but this would be a simple starting place.
sometimes a little thing like a bad plug or wire other times it can be more complex. let us know what you find.
I believe that John Deere used this engine or simular, and a little reading over on their site might give some help is no one here has an answer for you.
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Old 02-21-2010, 12:48 PM
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cub149 cub149 is offline
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Thanks for the help, I will give the carb the adjustment you suggest. I already thought about the wires being the culprit, they look very old but I did turn off all the lights in the shop and it was pitch black dark and I couldn't see any spark from the wires, not sure if that is the best test but thought I would try it. I have heard of spraying a mist of water on the wires to help them spark, ever heard of that?
The reason for wanting to set the timing is I have read about so many people doing it and how much it helped.
I went to set the timing on my 149 and it was dead on. Anyway, I borrowed a timing light and am just trying to use it while I can.
I'll let you know what happens but it will be a couple of days before I can get back to it.
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Old 02-21-2010, 02:00 PM
murphycc
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I was gonna answer that question a while back but didn't want to seem disagreeable.

I'll go with the carb setting as well. Onan's are also known for vacume leaks. Also check your head gaskets.

You are setting the points gap at TDC and that's all you are doing. The fact that it runs fine at idle and not at FT is not an indicator of a points gap problem. You don't have an 'timing advance' on this so the timing will not change through the RPM range, so there's really no timing to be set beyond closed at TDC or a few degrees before TDC. Timing is determined at the cam and nothing is really adjustable anyway.

Check the other things and you can make up your own new wires for a couple of bucks, not a big deal.

Scott
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Old 02-21-2010, 02:06 PM
murphycc
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If it's not a carb setting or carb problem (fuel starvation at high RPM)-

Then look at the carb to intake gasket. Also look at the intake to head gasket, make sure bolts are tight.

My gut says head gaskets....take a look.

Plugs may foul also, replace with Autolites.

Scott
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Old 02-21-2010, 02:37 PM
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cub149 cub149 is offline
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Quote:
Originally Posted by murphycc View Post
I was gonna answer that question a while back but didn't want to seem disagreeable.



Scott
Not sure what you meant by that Scott, but I don't take anything that way. I am looking to learn and I can take whatever you through at me. If I don't like it I will let you know. I may not always understand but all I ask is be patient. If I am doing something that is a waist of time, then tell me. Don't let me waist my time, it's precious these days. If it isn't the problem then tell me.
Did you see my other post about the points plunger sticking? If not you should look at it also, see my other post on Onan and you will find it.

You and Ol'George have given me some good starting points and I will get to it now that I have somewhere to start. Thanks and I will post what I find in a few days.
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Old 02-21-2010, 03:37 PM
murphycc
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Quote:
Originally Posted by cub149 View Post
Not sure what you meant by that Scott, but I don't take anything that way. I am looking to learn and I can take whatever you through at me. If I don't like it I will let you know. I may not always understand but all I ask is be patient. If I am doing something that is a waist of time, then tell me. Don't let me waist my time, it's precious these days. If it isn't the problem then tell me.
Did you see my other post about the points plunger sticking? If not you should look at it also, see my other post on Onan and you will find it.

You and Ol'George have given me some good starting points and I will get to it now that I have somewhere to start. Thanks and I will post what I find in a few days.

Nothing, absolutely nothing. I have a history of arguing.


Plunger? Rod that pushes the points and rides the cam? Remove it and stick one end in a drill and spin it while using some 800 grit wet-n-dry paper in the other hand and cleanit up, then chuck the other end and clean it up. Try to get a polished finish, see if that helps.



Scott
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Old 02-21-2010, 03:42 PM
murphycc
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If the guide it rides in is mucked up try a long Qtip with some acetone or laquer thinner on it and see if you can clean it up too.


Another thing I'll do is take a 1/4 sheet of that 800 grit and roll it up real tight so it just fits into the rod guide and stroke it up and down in the guide to clean up any burrs.


Scott
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