Onan 5500 wont start ... runs ok once going

Question

My Onan 5500 wont start anymore. Been messing with it and figured out it is rich/not getting enough air. Have it narrowed down that if I slide the intake butterfly position lever so that it is open a bit more it starts. If the butterfly is open all the way it runs perfect. If it is open about 20% it is still good but seems like it is wanting a little more air (the sound is a bit different but it still runs perfectly smooth). When it does not start the butterfly is only being slightly cracked opened maybe 10%. It seems like it drifts down to this 10% when running and still runs but it wont allow it to start. I can't really figure out how the lever(?) grabs onto the pulley(?) to open the butterfly. I see when I go to start it that it moves the lever but never grabs onto the pulley to actually pull the butterfly open yet nothing seems obviously broke or missing. If I force it open by rotating the pulley it stays open as again the pulley just slides without grabbing the lever. Hopefully that makes some sense. Here are some pics.

The black somewhat L-shaped piece is what I am calling the lever and what I am calling the pulley is the arc shaped piece it connects to. I don't understand how it ever grabs the pulley when it trys to move it as there is just two loose washer like rings on both sides:

Butterfly 100% open runs great/sounds very strong:

Butterfly 25% open ... still runs great and sounds strong but close it a bit more and sounds muffled yet still runs but wont start again after shutting generator off:

Its not the filter as putting the filter back on does absolutely nothing. its also not the altitude setting which also seems to have no effect. When it wont start the light flashes three times so I don't know if this is telling me anything specific.

What might be wrong? Is something missing or broke on the lever?

clouddriver · Answer

The butterfly valve you are referring to is the choke, which is usually operated by a bi-metallic spring. When the engine is cold, the bi-metallic spring holds to choke closed so that the engine gets some extra fuel for starting. The lever you refer to is connected to a vacuum operated diaphragm which is supposed to pull the the choke open somewhat as soon as the engine starts to allow additional air. The bi-metallic spring then slowly opens the choke the rest of the way as the engine warms up.

With the engine cold, the choke should be closed and if opened by hand should spring back to the closed position when released. If this doesn't happen, the bi-metallic spring may have failed or a linkage has become disconnected.

You should also see the diaphragm pull the choke open when the engine starts. If this doesn't happen, there may be a hole in the diaphragm.

kpackpackkelley · Answer

Give us the full model and spec letter and we'll try to find you a service manual.

wa8yxm · Answer

Also your altitude adjustment may affect that, you might try increasing it.Mine runs kind of ****ty for the first minute or so till the choke warms then it runs fine.

nuggetthunter · Answer

Had the same exact problem with my 5500.  It was the vacuum pull off shown in your picture.  Purchased a replacement and installed.  Runs great now.  Part number for the vacuum breaker is A043B658.

twvette · Answer

Cloud - thanks for the detailed info. My choke never moves as the generator is trying to start. Maskes further sense as the genrator actually does "start" but it sputters for 5 to 10 seconds and slowly dies as not getting the air it needs (choke never opens). The choke also does not return if I force it open, it just stays open but does seem to very slowly drift to near closed as it runs with time.

Kpack - It is model 5.5HGJAB-1038C Spec C. A Service manual would be awesome.

nugget - Thanks for the part number. Looked on web and for others it is the dome looking part with blue grommet(?)on the end. Looks like easy repalcement, true? I can see this diaphram part move the black lever it conencts to shown in the first pic but it never results in it opening the choke (just moves within the semi circle slotted piece on the other end). Maybe the diaphram is cracked so it still moves just cant generate any force to move the spring? Did yours move at all? Just wanting to make sure its not the bimetallic spring/linkage ... dont know if anyone can tell from the pics ... wish I would have taken a video now before putting in storage hundred miles away.

Thanks everyone for the continued input!

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Onan 5500 wont start ... runs ok once going

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