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Onan b48 slow rpm


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#1 garjess OFFLINE  

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Posted June 12, 2016 - 09:27 AM

I recently got an onan b48 engine, basically a barn find.  I don't know any history of the motor.  I did the usual things: plugs, plug wires, coil, points, condenser, carb rebuild, compression check 120-125psi both cylinders.   I've got it running good but its slow. It cranks right up and idles at 1300 and will rev up to about 2800 smoothly but gets bumpy faster than that.  The engine plate says 18hp at 3600 rpm but I can only get it to run at about 3100 and as soon as it has a load on it, it slows to 2400 rpm.  The linkages seem to be tight and free, and the throttle linkage is pulled against the stop.  Any suggestions? Shouldn't the governor keep it close to top speed unless overloaded?

Gary



#2 GWest OFFLINE  

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Posted June 12, 2016 - 10:08 AM

There is a spring between the manual throttle and the governor arm.

The manual throttle tries to increase rpm by adding tension to the spring.

The governor tries to slow the rpm down by increasing it's tension related to rpm.

The rpm you end up with is the balance of those two forces.

Maximum rpm is governed by limiting the travel of the manual throttle.

 

If there are numerous holes for the governor spring they allow you to adjust the governor sensitivity.

 

Garry


Edited by GWest, June 12, 2016 - 10:09 AM.


#3 garjess OFFLINE  

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Posted June 13, 2016 - 09:19 AM

Is there something that could be wrong with the governor internally to make it slow the motor too much?  I've tried changing which hole the governor arm is in, and moved the spring to different holes. Those changes  made it either run slower, or over rev out of control.  



#4 toppop52 ONLINE  

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Posted June 13, 2016 - 12:44 PM

Does it miss fire, or just hit a wall?

#5 garjess OFFLINE  

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Posted June 13, 2016 - 02:10 PM

It runs smoothly up to about 2800 then starts spitting up to about 3100.  If its running at 2800 and I put any load on it, it drops to 22-2400.



#6 GWest OFFLINE  

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Posted June 14, 2016 - 03:55 AM

It runs smoothly up to about 2800 then starts spitting up to about 3100.  If its running at 2800 and I put any load on it, it drops to 22-2400.

Did you do the adjustment that matches the carb to the governor?

 

Garry



#7 garjess OFFLINE  

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Posted June 14, 2016 - 06:26 AM

I don't know about matching the carb to the governor.  How is that done?  I just adjusted the carb by screwing the main jet adjustment screw in and out and finding the happy middle point.  Thats what the instructions in the rebuilt kit said to do, it didn't say anything about matching to the governor.

 

Gary



#8 GWest OFFLINE  

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Posted June 14, 2016 - 09:29 AM

There is a good description in this manual

http://www.wheelhors...-ipl-a-1427pdf/

 

This is all done without the engine running. The idea is to get the wide open throttle position of the carb the same as the wide open throttle position of the governor arm.

 

This just controls the engine speed. It won't cause the engine to sputter but when that does happen the governor will try to compensate for the loss of rpm and that is likely compounding the problem.

 

Garry



#9 toppop52 ONLINE  

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Posted June 14, 2016 - 09:41 AM

I would check fuel flow, if normal, then check every part of the ignition, points, plugs, coil, wires, condenser or whatever setup you have and make sure you have a strong 12v source to power the ignition.



#10 garjess OFFLINE  

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Posted June 28, 2016 - 01:55 PM

After talking to the previous owner (sort of) I'm not sure I have the correct carb.  The motor is a b48m 18hp and I have a DD.11 carb.  Does anybody know if this is the correct carb?

 20160317_200442 (1).jpg

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