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Another Onan Conundrum

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Just when I thought the Onan BF engine on my Sears SS16 was running good it started to go through this automatic RPM oscillation. It's more pronounced at lower RPMs. Then, adding to the confusion, the problem goes away when a load is placed on the engine like a rotary mower or going up an incline and reappears after the load is removed. Any ideas on what's goin' on?

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HubbardRA
It sounds like the fuel/air mixture is not adjusted right. Is that carb adjustable? If not, then you may have something constricting the main jet. This type of surging usually indicates a lean fuel condition.

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Guest
Rodney - If you're thinking fuel/air mixture, why does the oscillation only happen under a no load condition? Doesn't make sense to me.

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D-17_Dave
quote:
Originally posted by Beeser
Rodney - If you're thinking fuel/air mixture, why does the oscillation only happen under a no load condition? Doesn't make sense to me.
Less govn. means the butterfly in the carb. is in a more closed position. This means the fuel is pulled partially from the idle jet more than the main jet. Normally on these small engines a lean idle mixture will cause the govn. chase as the lean mixture starts to starve the engine rpm's start to fall. When this happens the govn. opens the butterfly enough to force more fuel out of the main jet. When a decent fuel flow is sustained the govn. now finds the buterfly to far open giveing it more rpm's and closes the buterfly. Then the lean condition reappears starting the cycle again. When a load is applied the govn. compensates w/ a more open position and the problem disapates. Answer, if it's a carb problem, rithcen the idle a little and it should dissapear. I find once the main jet is set idle the engine and ritchen the idle untill you notice rpm's fall and the ritch lope starts. Then slowly turn it back in untill it just smoothes out. Thereby running it as ritch as possible without the lope. Haveing to readjust a carb. after it has been repaired and run a few hrs isn't uncommon.

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firefoxz1
I agree with Dave 100% and he explained quite well. Most manuals say to adjust the idle mixture first then the running/fast mixture. I do this but then go back to the idle and try to adjust it so there's no flutter/hesitation when moving the throttle from idle to WOT. Hope this helps.

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HubbardRA
Only thing that I disagree with is that usually when I have had that type of surge, it has been the main jet that was out of adjustment. A load on the motor prevents the engine from responding quickly and the governor can then keep of with the oscillations. Try richening the main jet about 1/8 to 1/4 turn. Usually this is turning the screw counterclockwise. I also had a similar problem on a Tecumseh engine that couldn't be adjusted out. There ended up being a tiny spiral spring around the throttle shaft that held a small load on the throttle to keep it from moving suddenly. This spring had broken. That one nearly drove me crazy trying to figure out the problem. Traded that engine to a friend. He found the broken spring when he decided to rebuild the carb.

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dhardin
I will very a guess that its the governor. If it is a BF that was a generator set up (genset) its got a centrifugal type governor, controlling the high and low speed. And I don't know of any thing but to changed out completely. On my Onan generator if you was to try to rev up the rpm much past 1800 (I think) the governor will physically pull the carb back. To slow or at a load much under 1800 rpms the governor will open the carb for more gas. Its set up for generation consistent 120v. Now for you guy that will say hay wait a sec. Typically you need 3600 rpm's to generate 120V, but the reason these are great generator is it has a double winding. I say this but have never put my tack on it but it works so well and smooth and dose not sound like it 3600 RPMs, I have worked on a guys small 8hp Colman generator and it runs I thought real fast I did put my tack on it and it was 3600. Or the guy that set it up and changed it over most likely had to unhook the governor completely or modified it in some way for it to ever idle at all. If its the original carb it was never designed to idle, It my not have a ideal jet hole in it.

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Guest
Thanks all for the suggestions. I tried adjusting the idle needle but it had no effect on the shifting RPMs. Didn't try adjusting the power needle yet. The slot on the end is worn and difficult to turn. I plan on purchasing another needle. Strange that the idle needle was included in the carburetor rebuild kit but not the power needle. Here's another observation. The oscillations go away with a little choke. Starved for fuel, right?

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D-17_Dave
If turning the adjustment has no effect on the engine, then look for a blockage in the idle flow. Could be before or after or at the idle needle. Pulling the choke slightly stops the surge= a slightly lean mixture. It's the condition I described, we just haven't hit on what is causeing it yet.

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Guest
Dave - How about this for a possible reason the mixture is lean. I installed new intake manifold gaskets and a new carb to manifold gasket at the same time the carburetor was rebuilt. Could I be getting a leak at one of these joints? Maybe I need to tighten the bolts again.

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D-17_Dave
Yes, definately COULD be part of the problem. However, try to properly diagnose the problem through proper methods. This may keep you from wringing off the bolts. With the engine running try useing an acytalene turch w/ only the gas turned on and move the tip arund all of the gasketed jionts slowly. If the rpm's rise then you have an intake leak and you can tell about exsactly where it is. You can also do this w/ carb cleaner or other fairly cumbustable liquid. It just shows up better for me when the product your useing is already in a gas form. Less mess and chance for too much product to build up and flash. This test would apply for all gasketed areas from the carb to the block. Any leak would cause a lean condition and by replaceing the leak with a fuel will stop the influx of air and ritchen the fuel/air mixture. Again, we've seemed to have identified the problem. Lean mixture. It's ust a matter of isolateing the cause. The problem could still be in the carb. But if it will run strong under load and stay reaved up it's not a fuel supply problem. If you've been into the carb. it might be worth the effort to recheck the float level.

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Guest
Thanks Dave. I'll give all the stuff you mentioned a try. I especially like the method of using gas to find a leak. One that I never heard of before. I assume propane can be used as well as acetylene. Correct?

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HubbardRA
Beeser, I have been using carb cleaner or starting fluid to check for vacuum leaks for years. It works great. If you use a plastic tube on the can, you can pinpoint exactly where the leak is. Be sure to also check to see if you have a worn throttle shaft. This can also cause mixture problems. The starting fluid will also show if air is being sucked in around the throttle shaft. So will the gas.

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Guest
Sorry about the delay in posting my progress in trying to find the Onan hunting problem. Another issue came up with the starter circuit. More about that in a separate thread. Looks like you guys nailed another one. I sprayed some starting fluid around all of the newly gasketed connections. Nothing. Then I sprayed all around the carburetor. Again, nothing. After having a cup of coffee and looking again at the parts breakdown of the carburetor it finally dawned on me that there was a small foam type washer included in the carb kit that I forgot to put on the throttle shaft. Sure enough, directing the starting fluid in this area had the effect you guys were talking about. Is it likely that the foam washer is all that is needed to seal the leak or is something more elaborate necessary?

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