Onan P216G Spitting Back Through Carburetor

[Skywatcher]

Greetings All

Have a Lincoln Ranger 250 portable welder in the shop with an Onan P216G that seems to have issues. The machine came in because the engine had no power and bogged down while burning a ⅛" rod. Turns out the exhaust valves had burnt out and the engine had virtually zero valve clearances.

Engine now has 2 new exhaust valves, intake valves refaced, all seats cut and valves set to spec (intake .005" exhaust .014"). The engine now has adequate power, but when burning a 3/16" rod with 225 amps, a lot of fuel spray spits back through the carburetor. Does anyone have any ideas as to what would cause this and how it could be remedied, or is this just the nature of the beast? Thanks and all the best,

Sky

Engine Information
Make: Onan
Model: P216G-1
Spec: 11421H
S/N: A007048592

[bgsengine]

Its fairly normal depending on amount of spitback. 2 notes: Most all small 4-stroke and 2-stroke engines have a certain amount of spitback - you do realize that as piston is coming up on TDC of exhaust stroke, the intake and exhaust valves are both momentarily open, no? Part of the reason on that overlap is the flow of exhaust out technically should aid in pulling fresh air/fuel charge into combustion chamber. However as intake is starting to open as piston is still coming up, a wee tiny bit of that pressure is causing some spitback.. maybe loosen the valve clearances slightly to closer to max limit may help alleviate that a bit as well.
Secondly, any exhaust back pressure is also going to cause reduction of that scavenging effect (exhaust flowing out helps pull fresh fuel /air charge in) so may be worth checking if there's any loose baffles inside the muffler, or the diffusion plates starting to plug up with carbon deposits.. even a damaged exhaust outlet pipe can cause a restriction ... same exhaust blockages can cause (usually not even noticeable) reduction of performance.
Finally, your onan should have a yellow or ivory colored plastic composite spitback plate that bolts down using the air cleaner base mounting bolts (to keep spitback from contaminating air cleaner element) - that is assuming it has the "square-ish" pancake air cleaner housing (think they have different design on the round or cyclonic filter housings) So, overall, if engine performs adequately, with no obvious issues, you should have nothing to worry about, since there's always a small bit of spit-back on any engine. especially when under load (throttle open wider, dontcha know? )

[Skywatcher]

Hi BG

Thanks for your quick technical reply. We learned about valve overlap and the scavenging phase of diesel engines in second year heavy duty mechanics. I also noticed this engine appeared to have more overlap than the average brand B L-head engine, so this is what I kind of suspected was happening.

As for the spitback plate, I was a little surprised to find this engine did not have one. I think that any Onan engine in JD equipment that I've worked on (like the JD 318 garden tractor) had them, but this engine just has the 3 short screws that hold the air filter base onto the venturi. And yes, it does have the flat square air cleaner housing.

My thought for the next test was to remove the single piece welded exhaust system and try the engine again, but what would be the possibility of heat shocking and warping the exhaust valves by running the engine with open exhaust ports? I've seen motorcycle exhaust valves get bent that way. All the best,

Sky

[bgsengine]

As for the spitback plate, I was a little surprised to find this engine did not have one. I think that any Onan engine in JD equipment that I've worked on (like the JD 318 garden tractor) had them, but this engine just has the 3 short screws that hold the air filter base onto the venturi. And yes, it does have the flat square air cleaner housing.

perhaps someone left it out or didnt re-install it, or perhaps it was cracked or broken and never replaced?

My thought for the next test was to remove the single piece welded exhaust system and try the engine again, but what would be the possibility of heat shocking and warping the exhaust valves by running the engine with open exhaust ports? I've seen motorcycle exhaust valves get bent that way. All the best,

Sky

doubt it would give you any heat shock problems just running it long enough to test, but I would highly recommend at the very least to install some straight pipes or extensions - first, chance of flame from exhaust lighting spitback on fire.. (not good, eh?) and second, exhaust fumes being drawn into intake would change operating characteristics and performance. do you know if your engine has pipe threaded exhaust ports as well as the bolt-on flanges? might be able to just thread in some pieces of iron pipe for the test run

of course, if you value your hearing probably want to wear hearing protection.. (obviously)