Onan P218 surges after it gets hot

[ALCO 3]

Good afternoon all, I'm new here and have a question hopefully someone can help me with. 

I have a 518-H with a ONAN P218G engine and after if gets hot with the PTO engaged it starts hunting...bad. It does NOT do it when its cold or with the PTO disengaged. It has new intake manifold gaskets, carb was cleaned twice and installed with new gaskets. I sprayed carb and choke cleaner all over the intake manifold when it was running and found no leaks. Any ideas or if someone had this issue any help would be greatly appreciated. Oh yeah it's a NIKKI carb.

I also have a 516-H with a new carb (aftermarket) on it and thought about swapping carbs and see if it still does it. ?

[lynnmor 7,363]

Just above the idle mixture there should be a welch plug, is it still there?  I can't quite see it in the photo.  Did you pop the welch plug out, clean in there and replace it with a new one?  You have the idle screw limiter cap on, did you remove the screw and clean in there?  Did you adjust the float per the Onan manual?  Is the throttle linkage in the original positions?  Did you try new spark plugs?  Did you adjust the valves and know you did it right?  Did you check the compression cold and then again while hot and having the problem?

[ALCO 3]

Thanks lynnmor for getting back to me. To answer your questions, the welsh plug is there and I did not remove it. I do not have a new one. I did remove the idle screw inspected it and cleaned it, it's fine. The engine idles perfectly. I did check the float and it's ok even though it's kind of a funky way to do it. The throttle linkage is back were I found it not sure who was in there before me. the engine had no spark when I got it. Had to replace the coil. Plugs are new from Cummins. Valves are set correctly. I did not check the compression but I will. Like I said it only does it after about 15 - 20 minutes. One thing I did notice today was that the rpms increase like it's leaning out. Could it be the manifold opening at the seam when it gets hot?? I did take the carb off the other machine and I'm going to try it tomorrow.

[lynnmor 7,363]

Now that I know that the RPM is increasing when hot it may be the ball spacer in the governor.  Next time you use the tractor, run full throttle (3600 RPM) and see if it overspeeds when it gets hot.  Be sure that the throttle stays put during this test.  If it overspeeds, get back to me for help with that fix.  By the way, I am across the river in York County.

[ALCO 3]

Well that's exactly what it was doing. I had a tach on it and when it went past 3600 and pushing 3800 I shut it right down. That's a pretty scary speed. I know the throttle is not moving and the high speed stop is set at just under 3600 (when it's cold) It's almost like a diesel running away. I've seen that and it's quite intimidating. I checked the compression and it's 145 front cyl and 135 on the rear. That seem kind of high because my other one is around 115 on both the front and rear. The last owner had the wrong spark plugs in it, he had short reach with a gasket. I managed to get the threads cleaned up and the RS17YX installed. But that was when I first got the machine. I'm also in the process of resealing the intake manifold. Why? I don't know seemed like and interesting project but I did notice that when you removed it from the engine and removed the carb I could see the two halves move apart ever so slightly.  I see your in Red Lion that's not to far, I'm just across the river just North of the Armstrong Plant on 441.

[lynnmor 7,363]

Here is your problem.  Call Boomer for the parts you need and he will help you as well.

 

The wrong plugs can damage the tapered seat in the head, so check that while you have the heads off removing the carbon.  The high compression readings probably are a resiult of carbon buildup.

 

I split the intakes and drill thru for #6 stainless steel screws with washers and locknuts and use this sealant.

 

I ride the bike trail from time to time, probably close to your house.

[ALCO 3]

Yep that is definitely my problem. I removed the gear cover this morning and the flyball spacer just about fell off the cam gear. I found a youtube video from Mmrbeef that is very informative and he mentioned Boomer as well. I'll call him later this afternoon. 

As for the manifold I found the through hole in the lower half is almost the exact size for a 8-32 screw and I had a bunch of them left over from another project so I used them. I thought it came out so well that I did the one from the 216 as well. I have a small mill and since I still had it set up to drill out the one I'd just do them both. 

Thanks for you time and help. 

Oh yeah, I'm right near River Front Park. If you turn right before the RR tracks and go up that road I'm the last house.