Onan 2.8: new carb installed, now runs too fast and overvoltage

[Andy Waddell]

Hi there. Have an Onan 2.8 KV series Spec K in our 2004 Coachmen Freedom 258DB. When we got it last fall the generator was surging and hunting. I disconnected the main fuel line and plugged it then ran a separate feed of a strong Seafoam and fuel mixture and that seemed to do the trick and cleaned it enough to give smooth running. I drained the carb bowl via the bowl screw on the bottom and stored the vehicle for winter.

This spring I got the vehicle out of storage and brought it home. Gen started and ran fine, but seemed to occasionally want to hunt a bit again. I figured what the heck, I have no idea what this thing has been through before we got it so I'll go ahead and pop for a brand new Onan carb for it for peace of mind. (Onan says their carbs aren't rebuildable, they must be replaced).

Got the carb a couple days ago and installed it. (Should say now that I was a Service Engineer for General Motors Corporate for 29 years, so let's just say I'm REAALLY comfortable working with engines, and have rebuilt hundreds of carbs and engines, etc. I know my way around a wrench and engine).

Installing the carb was a piece of cake, nothing to it. A little fiddly since there was not a lot of room, but figured out how to safely get the old one out and the new one installed no problem. Started her up, and it ran nice and smoothly but seemed to be running a lot faster than the old one did. No way to really tach it to tell for sure, but it most certainly was running quite a bit faster than the old-carb engine. Decided to measure at one of the outlets (had the gen plugged into the coach), and saw 165 volts!! Yeah, she was running a bit fast, huh?!! Shut her down, then connected my surge protector between the gen and coach since it has a voltage display. Read 141 volts. Tried adjusting both the engine speed and governor sensitivity screws (of course counting my turns so I could put them back to original), and got no significant change. Fiddled with the carb and linkage and everything seemed fine, but decided to pull the carb and check the linkages and carb one more time to be really sure there was no binding or differences between the old carb and new. Nope, nothing different. Compared the carbs side by side very closely and there's no difference. Carbs are also stamped with the same part number.

Put it back together, same thing. Starts and runs nicely, but way too fast. Surge protector reads 141 volts. Called and spoke with the tech at the place I bought the OEM Onan carb from; he's been doing gens for 25 years and has never heard of this problem. We talk a little and agree that there's nothing that a carb could do to make this happen short of differences in the position of the butterfly links or something, but no...that's not it. The carbs are identical in that regard.

I can manually pull down the engine speed by regulating the governor with my finger/pulling back the linkage, so it can be done/the linkage isn't bound up. Governor internal mechanism itself is working; when the engine starts and build a little speed you can see the governor arm moving in response to the pressure buildup inside. Have attached a couple photos of the linkage (yep, I pulled the whole damn generator from the vehicle so I could get the lid off and get a good look at everything. A lot of work but no result).

So, there I am. Both me and the tech at the generator shop have no idea where to go next with this. Any ideas? How could installing a new IDENTICAL carb in design and function cause a "runaway" engine speed? Hoping someone out there has seen this before and can help me out! Thanks.

[okcnewbie]

With it being so tight is there any way you bent the linkage or didn't get it back on right?
I have the 4K model and have worked on it a few times and by no means an expert. The guys over at Smokstak have helped me.
I imagine you have the manual already. Also, if you don't have the Kill-a-watt meter from harbor freight you're going to need that to adjust it right on.

[Andy Waddell]

Originally Posted by okcnewbie

With it being so tight is there any way you bent the linkage or didn't get it back on right?
I have the 4K model and have worked on it a few times and by no means an expert. The guys over at Smokstak have helped me.
I imagine you have the manual already. Also, if you don't have the Kill-a-watt meter from harbor freight you're going to need that to adjust it right on.

OKCNEWBIE, thanks for the reply. No, no possibility of bending the linkage. I was super careful of that because I knew that was indeed the only thing I could do to screw up on removal and reinstall. I was very delicate with the removal paying attention to not putting any loads on the linkage. They're definitely not bent or misaligned.

I do have a manual. Thanks for the Kill-A-Watt reference, but the problem is that the adjustments don't do anything...the governor is "overdriving" the system so much that the adjustment screws don't come into play. So "adjusting it right on" isn't a possibility at this point.

In the first photo the speed screw is on the left; even when that is backed out all the way to the point of almost falling out it's still running too fast. The one on the right is the "sensitivity" screw; that one you adjust as to how aggressively the governor will pour on the coals when the genny is hit with a load (like turning on the AC). The manuals say that one should only be adjusted a couple of turns at most. It didn't make any difference so I also put that back where it was.

In the second photo you can see the linkages properly installed in the carb. I don't have photos of the other end, but I confirmed the choke (one on the left) was fully off/the choke fully open when the engine was warmed up. The one on the right moves freely by hand but is pulled heavily by the governor when the engine is running. That's the one that controls the throttle function; I can slow the gen down by by pulling the little triangular portion of the arm toward the bottom of the photo. As soon as I let go, though, the governor pulls 'er back up to "way too much throttle" position.

[pchuckie]

Just a guess here, what's the chances the previous owner or a tech had an issue with governor control arm and loosened the governor output shaft and moved the governor control arm. The governor arm on its shaft is one thing I would never move. I'm guessig maybe what ever issue they had before you they may have moved the arm on the shaft and now the slack in the governor is not taken up.

Can you feel the governor trying to make any speed adjustments?

[Andy Waddell]

Originally Posted by pchuckie

Just a guess here, what's the chances the previous owner or a tech had an issue with governor control arm and loosened the governor output shaft and moved the governor control arm. The governor arm on its shaft is one thing I would never move. I'm guessig maybe what ever issue they had before you they may have moved the arm on the shaft and now the slack in the governor is not taken up.

Can you feel the governor trying to make any speed adjustments?

Gen ran OK for a few hours a number of times after doing the Seafoam treatment, so I'm confident it can/will run properly with the governor settings it has/had. Also, when I had the gen out of the RV I looked very carefully at the governor shaft for witness marks indicating slippage or prior position: there were none.

PS: The gen had only 22 hours on it when we got the RV, so it's "new"...nobody has fiddled with it.

[okcnewbie]

Originally Posted by Andy Waddell

Gen ran OK for a few hours a number of times after doing the Seafoam treatment, so I'm confident it can/will run properly with the governor settings it has/had. Also, when I had the gen out of the RV I looked very carefully at the governor shaft for witness marks indicating slippage or prior position: there were none.

PS: The gen had only 22 hours on it when we got the RV, so it's "new"...nobody has fiddled with it.

I guess it has to be the carb then? Based on what you're saying; ran fine, changed carb doesn't run fine.

Low hours on an old genset isn't a good thing IMHO. Slip rings can be really rusty, etc, etc.

[Andy Waddell]

Originally Posted by okcnewbie

I guess it has to be the carb then? Based on what you're saying; ran fine, changed carb doesn't run fine.

Low hours on an old genset isn't a good thing IMHO. Slip rings can be really rusty, etc, etc.

I'm with you on the "new carb = problem there" even though a carb shouldn't do it, but I hate to spend another 180 bucks to have TWO carbs that result in a gen running too fast. I'm gonna talk with the place I bought the carb and ask if they'd be willing to sell me one at cost to see if a different new one cures the situation.

As far as low hours on the gen, the RV was stored indoors for its entire life before us, and we've used the gen for, oh, four hours or so. I would run it under load when the RV was in the driveway to exercise it, and also we used it a few times at rest stops to run the microwave to make lunch, etc. It seems to produce power just fine. I think it's probably OK.

[okcnewbie]

I'm in a similar boat; Our RV came with a Onan 4Ky that wasn't working. It ended up being the rotor (bummer). I bought a used 4ky for 400 and it didn't work under load. The voltage reg. was the problem. Made one good generator out of two. Ran fine last year. Even ran it 8 hrs once while we were down town sightseeing. Now it runs for a couple minutes than sputters out. I know it could be filter/pump but while sitting the carb is probably getting worse. Since we can't rebuild these I'm probably gonna have two problems now.

Good luck with yours....keep us informed!

[F4Gary]

Did you try putting the old carb back on to see if it goes back to normal operation?

[Andy Waddell]

Originally Posted by F4Gary

Did you try putting the old carb back on to see if it goes back to normal operation?

That's a part of the story I left out for clarity's sake. The old carb was running OK, but was not inspiring confidence. It still hemmed and hawed sometimes when running and I just wasn't 100% confident that it was OK. So, this spring when I got the RV out of storage, I decided to clean the carb and see if it ran better.

Now, like I said Onan says you can't rebuild their carbs, you have to buy new. But as I'd said, I've rebuilt and worked on literally 100 carbs or more during my life, including "non-rebuildable" ones. So I figured I'll take off the old one and clean it, and if something's bad inside or it otherwise is bad after the cleaning, I'll buy a new one. Essentially I decided to give the old one one more shot before I flat-out replaced it. So I removed it and disassembled it as much as was feasible (often non-rebuildable means there are metal caps swaged over adjustment screws, etc., so you really can't get at them without a lot of semi-destructive effort. Other times the caps are easy to remove, if you've done them before. So the point is I had a decent shot at just cleaning it)

So, I sprayed carb cleaner and compressed air through the passages, removed the couple jets they'd let me get at and ran a tag wire through the orifices to confirm cleanliness/passage, etc. Did general cleaning with no "heroic" disassembly.

Put it back on and it wouldn't start, was running WAY too rich for some reason. Oh well, I gave it a shot and was prepared to order a new one anyway, so ordered the new one.

So the direct answer to your question is: yes, I put the old one back on after the new one showed problems, but the old one wouldn't run at all so that was out as a potential diagnostic step.

[Scatterbrain]

Send it back for a new one. As a tech I've seen more than my fair share of brand new parts that were D.O.A. Distributors that were keyed 180* out, carbs with sealed vacuum advance ports or stuck power valves. MAF sensors so far out of spec they'd foul the plugs in minutes. Fuel pumps that run but don't build pressure, etc, etc. It sucks, but what are you gonna do?

[twinboat]

Put the carbs side by side.

If the throttle plate is closed on both and the governor link hole is in the same place, there is nothing wrong with the carb. The only thing that could be different is the index of the throttle plate shaft and lever that is pressed on

Can you close the throttle until the adjusting screw in your picture hits the stop tab ?

You should be able to move the link, with your finger, to the idle position where the adjustment screw hits the tab.

You don't show the other end of the throttle link. Are there different holes to place it in ?

Are there any bends in the rod that can be bent slightly to shorten the rod ?

[Scatterbrain]

The jet needle could be wrong. . . . ..

[twinboat]

Originally Posted by Scatterbrain

The jet needle could be wrong. . . . ..

That would affect the air fuel ratio. Lean would have no power, rich would produce black smoke.

The throttle plate controls the air flow. The air flow draws the fuel thru the venturi.