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dvs1

Onan Carb Cleaning

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This topic usually comes up often so I thought I would do a little DIY on how to clean an Onan generator carb and possibly save yourself $200-$300 on a new carb. This should fix 90% of all carb related issues like no start, over rev and shut down, surging, and maybe some other problems... My gen is an Onan 5500 Marquis Gold 5.5HGJAB-1038C with 925 hrs. I have a 2005 Forest River that I have had for 6 years and it seems to be about every 2 ish years and I need to really clean out the carb. I use stabil, but I do let it sit for months at a time without running. This is the same carb that I have cleaned out 3 times now and I haven't had to replace anything on it.

Take off the air filter housing 2 10mm bolts and 2 10mm nuts that also hold the carb together.

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Disconnect the solenoid connector on the bottom of the carb, disconnect the two leads to the choke, unhook the spring that is on the rod that attaches to the carb throttle and the big arm with a heavier spring, the rod with the little blue bushing. Leave the rod alone until you start to pull the carb off. The rod will slip out of the plastic bushings easier once the carb is moved out a little. Don't remove the bigger spring on the generator arm unless you mark what tooth its adjusted too. The gen is on spring mounts for vibration, you need to pull down on the generator so you can clear the diaphragm past the roof of the gen cover/housing when sliding the carb off of the two mounting studs.

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There is another white plastic bushing for the throttle rod at the end of my finger. Don't lose these. The throttle lever here is usually sticky and the cause of most of the the problems. You can start to see the throttle blade and how gummed up everything is.

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Now remove the throttle plate. The two scews are loctited in so make sure you have a good screw driver and get a good bite on the screws. They strip really easy. If you strip it, its a PITA to get some vise grips on the head to remove. If you strip it out and then successfully remove the screw replace it with a hex head/allen head screw. (I don't know the size, I haven't stripped mine out yet.)

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Now you can remove the shaft and start cleaning the throttle bore.

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Make sure to clean out both sides of the shaft bore also. This area will really hold up the throttle movement if its not free of the gummed up varnish.

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Next take off the cover on top of the carb. There is a gasket under there, I have never torn it but be carefull. Then remove the long jet and clean.

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You should be able to see daylight through it. If not, the best way I have found to clean out clogged jets is to find some 10-12 guage wire that has a fairly coarse copper winding but small enough to fit in the jet. Push one strand of the copper through the jet and clear the blockage.

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Remove the solenoid at the bottom of the carb bowl with a 15mm wrench. The bowl will pull straight off. Don't twist and try to unscrew it, there is an oring that you might damage.

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The oring is actually square and must be seated down in the groove nicely or you will pinch it and probably have leaks.

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There is also a spacer/washer that is the same aluminum color as the carb that needs to be in place before putting the bowl back on. I moved it to the side in this pic. Don't lose it.

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Reassembly is in reverse. The carb bowl does have a copper crush washer. I've reused it 3 times without leaks. The throttle blade does have a small knife edge to it so it really only wants to sit in the bore and on the shaft one way. Start the screws and only tighten after your sure the throttle blade moves freely.

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Reinstall the carb back on to the gen. Remember to pull down on the gen so the diaphragm clears the cover. There is a paper gasket in between the intake and the carb. Again, I have pulled mine off and on 3 times without having to replace any gaskets. Remember to install the throttle rod back into the plastic bushings and hook up the spring. You should be able to move the arm I have my finger on and in turn operate the throttle very easy and smoothly.

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Reinstall the air filter housing and remember to connect the little rubber crank case breather tube to the back of it. There is a oringed gasket that will seal the air filter housing to the carb. Don't lose it.

2 10mm bolts for the housing, 2 10mm nuts to tighten down the carb. Doesn't have to hold the world together... just tight. Reinstall air filter and latch the cover back on. Prime the system and check for leaks BEFORE trying to start it.

NOTE: Originally these carbs came with a secondary fuel filter that attached right at the carb fuel inlet. This has been known to leak badly and possibly cause a fire. You can see in some of the pics there is a long hex fitting with a barb/nipple end attached right to the carb inlet. That is the replacement for the secondary fuel filter.

If there are no leaks, fire it up!! Hopefully, you just saved yourself the cost of a new carb. Like I said, I have had to do this 3 times over the course of 6 years. Everytime it works like a champ and I have smooth running gen for a couple of years.

Edited by dvs1

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Also, I have found that Sea Foam Deep Creep works pretty good at dissolving the gummed up varnish crap.

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They should make this a sticky. Very informative.

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I agree....sticky would be nice! Great post btw!

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Great write up!

I'll be using it on my genny soon

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Sticky it is. Great job dvs1...!

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I wonder if an ultra sonic cleaner would do the job too? I have access to one, could I drop the whole carb in and let it sit?

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nicely done oldtimer,

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Very nice write up.

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:thumbup: Thanks, hope it helps. Here is the part number for the Onan fuel filter adapter that replaces the fuel filter right at the carb AO29S253. Highly recommend putting this adapter in there. When the filter leaks its scary how much fuel pours all over everything.

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I wonder if an ultra sonic cleaner would do the job too? I have access to one, could I drop the whole carb in and let it sit?

Probably, but I think you would still want to break down the carb a little to drop into a tank. I don't think the choke, diaphragm, the paper gasket up top, or the solenoid would like it.

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I read the whole right up and will try it since I have a code 14 which is over reving. 

I used the genny 3 days straight and approx 15 hours each day. On the 3rd day at night it shut off.  After about 20 mins I walked to the rv and started the genny with no problems. About 30 mins later it shut off.

After this shut down it starts but as soon as I let go of the start button it shuts down and give me the error/ service code.

Could I have a dirty carb although it was used for 3 days straight without a problem?

 

 

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1 hour ago, sinverguenza said:

I read the whole right up and will try it since I have a code 14 which is over reving. 

I used the genny 3 days straight and approx 15 hours each day. On the 3rd day at night it shut off.  After about 20 mins I walked to the rv and started the genny with no problems. About 30 mins later it shut off.

After this shut down it starts but as soon as I let go of the start button it shuts down and give me the error/ service code.

Could I have a dirty carb although it was used for 3 days straight without a problem?

 

 

Normally the the 20 min timeout is always the fuel pump. Been their.

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Carb should be fine.. this seems like your fuel pump went bad. 

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So the over reving can be caused by the fuel pump.

 

I will look into that. Thanks. 

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I have cleaned mine several times with great results. I never understood why people would buy a replacement carb without cleaning the one they have. 

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Ya that is just maintained properly as soon as I got an issue it's the norm oil and filter, air cleaner and carb cleaning. Clears up most issues.

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Ok. I did the oil and filter change before leaving on my trip Thanksgiving. Genny worked flawless until Saturday. 

Will clean carb and go from there. 

Thanks everyone for your help.

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My new to me toy hauler had sat for about a year without having the genny ran. Got it running but died shortly after. Pulled off the carb and it was dark red with varnish. Followed the instructions word for word. Came out looking perfect. The issue I am having into now is that it will idle smooth for 20 seconds then do three-five drops in idle(almost stalls) then goes back to idling smooth. It just keeps repeating this process. Tapped on the idle valve and there was no change. Tank had about 5 gallons of gas (over a year old) and I have added about 10 gallons of fresh fuel to the tank. I have ran it now for about an hour and a half with no improvement. The trailer is 43Ft, so could it be that the new fuel has not made it to the carb? Thank you for reading and any help would be appreciated.

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Any help on the above? I was thinking of adding another 5 gallons of fresh fuel and also pulling off the solenoid and cleaning it again. 

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16 minutes ago, CarWashGuru said:

Any help on the above? I was thinking of adding another 5 gallons of fresh fuel and also pulling off the solenoid and cleaning it again. 

I had mine shutting down due to over voltage issues (what the code said).  I pulled the fuel line off the genny, mixed a can of Seafoam into 2 gallons of gas, then ran a fuel hose from the genny over to the gas can and ran it for an hour with both A/C's on.  Been working good for two years now.  

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A friend is going thru a similar thing. Runs for about 20 minutes and then quits. Let it set for a while and try it again. Runs a few minutes and quits. We think its the fuel pump so it will get changed soon. Some posts above mention this.

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Does it surge right before it shuts off?

I had that happen and thought it was the carb.  It ended up being the fuel pump.

You can test it by pulling the fuel line off the carb and push the "PRIME" switch/button down.  If you don't get fuel...case closed.  You can get fuel pumps A LOT cheaper online through eBay or Amazon...like over $100 cheaper.

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It runs just fine, then hunts (drops idle 3-4 times) then idles super smooth. I ran it for over 40 minutes straight and just did the same thing. I am going to pull the bowl and solenoid off and clean them again and see if that helps. I am also going to pull the fuel line off the carb and try to get a good amount of the old fuel out of the line.

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Sounds like the fuel pump might be going out to me, but could be a passage or jet still clogged up.  Pull the fuel line and try to pump into a bucket and see if the pump is good.  If pump seems strong, try another round of cleaning again.  The solenoid is a fuel shut off.  Usually when they go bad its done and no more fuel.  Never heard of one intermittently cutting out only enough to drop idle and then come back to life.    

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