4kw onan acting up

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chuck7

Member
Joined
Dec 18, 2006
Posts
19
I have a 86 bounder with a 4 kw onan. The gen appears to be original by all the dirt and oil down on the cylinders but when I bought the MH the last owner said it was rebuilt and the hour meter only showed 90 hrs on it but the gen ran great up until about 6 months ago when it wouldn't start so I pulled the carb and found some small pieces of black rubber that was fuel line so I replaced the line and put it all back together I also replaced the fuel pump because the filter was missing. At some point someone had removed the filter. It would start but surged for a good 10-15 seconds before it would smooth out. Now when I start it all it does is surges and will not smooth out at all. I adjusted the carb by what the book said. I'm at a loss. Am i missing something?
 
My guess is that the carb is dirty.  You need to take it out again and clean it .  I have had the same problem with lawn mower engines.  In small gas engines it is called "hunting".
 
Yeah, you need to "rebuild" the carb. Rebuild, however, usually just means a through cleaning of the tiny orifices inside, which get clogged with dirt in the fuel or varnish from evaporated fuel. The black specs of rubber were just the part you can see with the naked eye - there will be a film of finer stuff coating everything inside the carb.
 
I would add that you and Gary talked about black specs of rubber they can get in the internal circuits of the carb and plug them up. I would add an inline fuel filter to help prevent re contamination in the future after you do all the work to get it running right.
Bill
 
I also recommend removing the carb and carefully cleaning. Be sure to install a good fuel filter.

Check the float level. If it's a solid float, I would just replace it while you have the carb off. If it's a hollow, copper float, shake it to see if it has liquid in it. If not....OK.... If it feels like something is in the float, replace it. These engines are very sensitive to float levels. Also install a new needle & seat (some needles have a black rubber tip  ;) ). Be sure to install a new base gasket. If it has a ceramic spacer between the carb & manifold, check it carefully for cracks and level or just replace it.

As far as previous owner's stating that the genset has been rebuilt...... I'd be skeptical.
 
The rubber pieces came from the fuel line between the in line fuel filter and the fuel pump on the genset so I replaced all of the line all the way to the ridged line on the frame. If I would have known there was no filter in the pump I wouldn't be going thru this. when I pulled the carb I made sure to note the adjustments on the idle screw and and mixture screw also I made sure to clean the carb before I put it back together. I was thinking that maybe the choke adjustment could be off or sticking or the governor is out of adjustment and yes I am very skeptical about the genset being rebuilt.
 
chuck7 said:
The rubber pieces came from the fuel line between the in line fuel filter and the fuel pump on the genset so I replaced all of the line all the way to the ridged line on the frame. If I would have known there was no filter in the pump I wouldn't be going thru this. when I pulled the carb I made sure to note the adjustments on the idle screw and and mixture screw also I made sure to clean the carb before I put it back together. I was thinking that maybe the choke adjustment could be off or sticking or the governor is out of adjustment and yes I am very skeptical about the genset being rebuilt.
Does it surge under load? If it does, it's not the idle or air mixture screw (air mixture is only effected at idle). My guess would be float level and/or needle & seat leaking (or water in the bowl).
 
Wavery said:
Does it surge under load? If it does, it's not the idle or air mixture screw (air mixture is only effected at idle). My guess would be float level and/or needle & seat leaking (or water in the bowl).

When it started to act up it would surge when I started but smooth out right away and would run smooth under a load. no water in the bowl or in the fuel system. I always kept fuel stabilizer in the tank if it sat more then a few months also.
 
SeilerBird said:
Chuck - Were you running the gen at least once a month before the trouble started? They don't handle being ignored too well.

I would start it every month and let it run for 10 minutes or so and would run the roof a/c for a few minutes as well. My guess is the carb needs a rebuild. I'm going to check to see if the choke is sticking closed or not opening all the way also.
 
chuck7 said:
When it started to act up it would surge when I started but smooth out right away and would run smooth under a load. no water in the bowl or in the fuel system. I always kept fuel stabilizer in the tank if it sat more then a few months also.
OK.... If it surges at idle but smooths out under load, you either have a vacuum leak or your idle mixture crew needs to be adjusted until it smooths out.
 
A helpful hint is to take some pictures with your digital camera before and while you are taking the carb apart.  It really comes in handy when you are trying to remember where the different linkages are attached.
 
When you had carb apart, did you use the tube and spray carb cleaner into every orifice you found? Be generous with the carb cleaner. It has to both dissolve and clear out debris.

Looking for vacuum leaks.
With the gennie running, spray carb cleaner directly on intake joints. Use the tube to prevent overspray entering carb and giving false reaction. If the engine speed changes, up or down, or the hunting steadies up, even a little, there is a vacuum leak on or near the joint you just sprayed. Near the joint may be a vacuum line or dashpot.

I don't know if aircooled Onans are much different than old VW's. Both are horizontally opposed, aircooled engines. On VW's there is a lot of heat traveling up the manifold. It cooks paper/fiber gaskets, makes them brittle. I coat paper/fiber type intake gaskets for aircooled VW's with wheel bearing grease, both sides. Keeps them soft and helps seal too, they don't stick and pull apart as much and are reusable more often than not.

Bill
 
driftless shifter said:
When you had carb apart, did you use the tube and spray carb cleaner into every orifice you found? Be generous with the carb cleaner. It has to both dissolve and clear out debris.

Looking for vacuum leaks.
With the gennie running, spray carb cleaner directly on intake joints. Use the tube to prevent overspray entering carb and giving false reaction. If the engine speed changes, up or down, or the hunting steadies up, even a little, there is a vacuum leak on or near the joint you just sprayed. Near the joint may be a vacuum line or dashpot.

I don't know if aircooled Onans are much different than old VW's. Both are horizontally opposed, aircooled engines. On VW's there is a lot of heat traveling up the manifold. It cooks paper/fiber gaskets, makes them brittle. I coat paper/fiber type intake gaskets for aircooled VW's with wheel bearing grease, both sides. Keeps them soft and helps seal too, they don't stick and pull apart as much and are reusable more often than not.

Bill
Good tips..... I would use WD40 for testing for vacuum leaks though. It's a little safer...................... Never re-use and intake gasket. It's best to buy the factory intake gasket.
 
driftless shifter said:
I've had great success reusing greased paper/fiber gaskets on ac VW's for years.

Bill
Just like anything else........ you may get lucky.... may not....... for best results, I recommend new intake gaskets every time (what I do may differ from what I recommend). For sure, if I am having issues, no way would I use the old gasket......... but that's just me. ;)
 
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