1990 316 garden tractor Onan, loping

Our good ole 316 has started loping at most all throttle settings.
Onan 18 hp. Have never touched the carb yet. New plugs, fuel filter. Fuel pump and and coil 2 years ago. Fresh fuel.
Any ideas please.! !
Thx, Paul

A COMMON problem with those Onans.

Carb has some clogged passages and/or Welch plug (Key #26) has fallen partially out, or not sealing.

Also, the intake manifold is made in two pieces, and sealed together with silicone rubber. Sometimes, the silicon rubber deteriorates and allows air to leak in an lean out the mixture.

<img src = "http://oi66.tinypic.com/b3pks2.jpg">

Thank you Bob, I'll dig into it. Oh boy, Here we Go! !
Paul

Along with what Bob said check the manifold
bolts and if they are real loose you might
as well plan on gaskets. We have three
Ingersol tractors with Onans that we got
real cheap because they would not run right
and all they needed was the manifolds
tightened up and/or gaskets.

I agree with Butch. Manifold gaskets and the manifold seal should be checked. There was a problem with the intake manifold seal
failing. Factory told us to check all manifold seals for leaks. If found we were to reseal at no cost to customer. Also carb may just
need a good cleaning.

I have a P218 and P220 Onan, both on Wheel Horses, have had trouble with both surging at any speed and have had success with spraying carb. cleaner around inside the top (air horn) while running half throttle. Let it set an hour or two, blow it out with compressed air and your good to go. There's a small hole (air bleed I think) in there that becomes restricted. Also my 218 ran lean, started slow, needed to warm up before you did anything and the exhaust smelled like an old cat. converter. I put up with that for a few years and finally opened up the main jet a hair and it runs good now, over 1800 hours on it.
Be careful if you open up the carb., I priced a kit a few years ago from Wheel Horse and it was around $180. I think.

Well, about a month later, all that you gents have mentioned has been cleaned, repaired, adjusted and put back together. All is fine
now. You were all spot on with your comments. Here is what I found: The Carb was working OK but needed a good cleaning. This was done and reassembled with a new gasket kit. The the source of the air leaking, as you said, was the silicone seal of the aluminum manifold halves. I found that the little cast aluminum studs of the lower half (4 of 6) had sheared and not gripping the halves together. The tops of these went through proper sized holes in little flanges on the upper half casting and were peened (sp) over like a tubular rivet.
I cleaned up all the stud locations on the flanges, drilled and tapped the 6 spots for 6/32 X 3/8" S.S. panhead machine screws;
Reassembled all with Hi-temp Locktite at the screws and Hi-temp silicone along the manifold seams. I let it sit for a day or so waiting for the intake and exhaust tube gaskets to arrive and bolted the carb onto the manifold.
It was interesting seeing how much additional "stuff" had to come off to get at the intake manifold itself. The intake Manifold is overlaid with the exhaust system. To get at the bolts for the intake manifold, the exaust piping AND muffler HAS to be removed.
to remove the muffler, two bolts behind the front PTO clutch need to be removed. So, the PTO has to be removed. WOW!!!!
Once these are clear, all is wide open to do the repair.
Typical destroyed items that were replaced on reassembly were 2 intake manifold gaskets; 2 exh. tube gaskets; and two smallish
(1-1/2") U-bolts for the 2 muffler clamps.
She runs like a charm now. Thanks all for your help ! !
Paul