I have an older OS 48 4 stroke that I picked up from a club auction about a year ago. I've rebuilt the engine and replaced the bearings and the carb (newer OS 55 carb).

Today I was running it on the test stand and it seemed to run fine except for an inconsistancy in high RPMs. I got the high speed needle set at about 10K, backed off to about 9700 and then let it run for awhile checking it periodically with the tach because it just didn't sound right. It sounded like it would lean out and then slow back down ... It would idle fine. The RPM's varied between 10K to as low as 9400 RPM.

It did the same thing with the old carb, which is one reason I decided to replace it (that and the fact that I wasn't getting much more then about 8800 RPM and the needle valve was shot).

So, what can cause an inconsistant high RPM? Bad valve spring? Valves out of adjustment? Air leak? Any idea where I should start looking? Or is this normal ... I'm fairly new to the hobby (about two years) and even newer to 4 strokes.

Thanks in advance for any help.

Don't know, normally you see this on a new engine being run in?

I can't tell for sure without seeing and hearing the engine, but my first guess would be an air leak either on the engine, in the plumbing, or in the tank.

Ditto Nathan's post. One other thing to check is to make sure the Crank Case vent is clear. If it is plugged or restricted, you get some dificult tuning problems. One of our guys was trying to get his plane going with an older Mag 52. He had plumbed the vent into a bottle with some cotton in it to keep the oil out of the cowl. It wouldn't hold a steady RPM and had little power. He clipped the line to a couple inches and away it went. Might check this out.

Don

I had this same exact problem on my Saito 65. It ended up being a tiny bit of slack in the throttle pushrod. I had drilled out the servo horn to accept my steel pushrod linkage and the hole was just a little to big. This combined with the engine vibration caused the throttle to be erattic at certain rpms. You could just put your finger on the linkage to stop the vibration and take up the slack and it would run fine. Also it would idle nice and smooth and transition just fine but when you decided to leave the throttle alone at mid to full throttle it would jump around. Almost sound like it was surging.

Hope this helps you

Jay

It is probably getting hot from excessive ring friction, then the heat is causing it to detonate which only makes it hotter. I would just run it in at a very rich setting, then try again.

Do you have a picture? How did you get a 2 stroke carb on the 4 stroke? The carb needed to feed a 4st would be a bit smaller than a carb for a 2stroke. You "rebuilt the engine", did the ring and/or cylinder get replaced. If so, it nneds to run real rich during the breakin period.

I completely missed the 2 stroke carb. Unless he ment a 52 carb and had a finger check.

Don

Sorry, my error, it's a 52 carb (4-stroke) OS no longer makes the carb for the 48. The piston and ring were not replaced in the rebuild. I got the engine from a club auction for $10 and replaced the bearings, they were rusted. I saw no other damage and it looked like it had little run time on it.

After replacing the bearings it fired up and ran fine - except for the appearent surging at high RPM. That's when I decided to change the carb - mainly because I couldn't stop the fuel leakage around the needle valve.

It still surges so I've got another problem. Based on all of the feedback I'm guessing it's in the fuel lines ... probably a little pin hole because it only seems to surge at higher RPM's ... it's on my test stand right now so I'll replace the fuel lines and give it another try. I'm stuck at home right now and haven't been able to make it to the flying field because of medical issues ... You've all been a huge amount of help! THANKS!

This is a common problem with the OS 52. I have four of them. Every time I think I have the problem solved, it is not long before the problem recurs. It might be carb ice in the intake?