When I fire up the Onan Marquis 5500 in the mornings, it'll sound like crap for about five minutes or so. Sometimes it even stalls. Rough choppy idle, black smoke... then it settles in and runs like a champ the rest of the day. I'm thinking it's due for a valve adjustment... about 850 hrs on it, 2005 model. Regular oil changes, it's due for one now.

Thoughts?

Check the altitude adjustment. Mine did that and I discovered it was set waaay rich.

Thanks, I'll look at that. I was camping at around 3k ft. this weekend.

Bad gas? Try the Onan carb spray. It is supposed to work well. DO NOT USE regular carb cleaner. It will ruin your carb!

Mine was doing the same. then I replaced the plug, air filter and oil. Now it runs like a champ. The plug was the problem.

Mine did the same thing, used fuel injector cleaner in fuel tank.... Runs great

Clean the carb, clean the airfilter, change the plug. Regular maintainence.

I had/have the same problem and still had it this last weekend. All the responses that I got where about the same except for the fuel injector cleaner in the fuel tank. I have not adjusted the valves yet though. But ours is the doing the same as yours. If you find the problem please let me know. thanks

Choke adjustment?

I'll do the maintenance routine on it this weekend... oil/air filters, plugs. That should give me an idea of whether or not I'm on the right track. I'll post up with what happens.

Edit: FWIW, the Onan manual (pdf) does show that the plugs are supposed to be replaced @ 450 hrs. Ditto valve adjustment.

So, I replaced the plugs and fired it up. Still smoking a lot at first, running rich and rough. Settled into a nice idle after about two minutes or so, and the black smoke stopped. I'm thinking I still need to do the valve lash... it looks easy enough once I figure out how to get the thing to TDC. I'll order a service manual next week... I need one anyway in case it breaks.

Oil change, new air filter and fuel filter... seems good to go. I'm making a mental note to replace plugs at the same time I replace the air filter, every 400 hrs or so.

BTW altitude adjustment was at sea level. I cleaned the carb, lots of dust caked on it, didn't really affect anything but it made it look purty. Purty is good.

Checked the operation of the choke linkage when you first start it up?

I have the exactsame problem as you do. I did everything but the valves. Let us know if that helps.

Well I just went out and fired it up again. Started out rough and sooty, so I popped the cover and tapped the choke linkage with my finger... and voila, nice steady idle. I don't know if that means the linkage is sticking or the electric choke module could be failing... I'll hope for the latter as the carb and linkages were pretty caked with dust. I'll nuke the linkage with some carb cleaner and try it again tomorrow (it only happens on a totally cold start).

It is due for a valve adjustment anyway, the specs are .004" cold on both intake and exhaust... I didn't check mine with a gauge but I know they're looser than that.

FWIW here's what the plugs looked like (the one on the right is the one from the right-side cylinder, closest to the exhaust):

Click to view attachment

It's a dry soot, so it doesn't look like there's any oil getting past the guides or rings (whew!). Just rich. In this pic you can see both plugs have a slot worn in the conductor post:

Click to view attachment

I guess I should change the plugs more frequently, maybe?

Mine acted the same way. I sprayed the choke linkage with wd40 and then it has been fine for over a year.