engine wont rev over 2600 rpm

[crh023]

I have just acquired an early 80's mercury 50 hp 4cyl 2 stroke. serial # 0A103077. When running at WOT it wont rev over 2600 rpm and sounds like it is dropping cylinders at that point. have thoroughly gone through the fuel delivery system and completely rebuilt the carbs. It starts right up and idles reliably with a slight sputter. squeasing the primer bulb at WOT only chokes the engine. I performed a compression test. 134lbs. on #1,2&3. 125lbs on #4. The resistance values of the trigger check within spec of the manual. The low speed circuit of the stator checks within spec but the high speed circuit tests high at 150 ohms. I have no way of checking the switch box. From what I've read these engines are prone to stator and switch box problems. Any advice would be greatly appreciated. Thanks in advance.

 

[Laddies]

Re: engine wont rev over 2600 rpm

Have you checked for spark on all cyls. with a spark gap?

 

[crh023]

Re: engine wont rev over 2600 rpm

No I haven't. Where can i find one of those? I've heard about them but never used one before. Should I check it in the water at 2600 , at idle, or just cranking it? How big of a gap should it jump?

 

[crh023]

Re: engine wont rev over 2600 rpm

I did ground all 4 plugs to the block and cranked it over. All 4 plugs sparked but how strong they were I don't know.

 

[crh023]

Re: engine wont rev over 2600 rpm

Ok. I got a hold of a spark gap. All 4 cylinders spark across a 1/2 inch gap when cranking.

 

[crh023]

Re: engine wont rev over 2600 rpm

Ok. I decarbonized this engine and now it will rev to 4000 rpm's. I removed the powerhead and found the exhaust tube almost completely plugged with carbon. The 2 inch tube was restricted to about a half an inch. What kind of symptoms would this cause? Could this be my problem? I chipped out all of the carbon and cleaned it really well. I just need to locate gaskets to reassemble and then i can try it. Im keeping my fingers crossed.

 

[j_martin]

Re: engine wont rev over 2600 rpm

Unless you had a white knuckle grip on it, nothing should change when you squeeze it while it's running. If it flooded out, you have a fuel issue, either fuel pump failing (diaphragm leaking gas into the crankcase) or needle or float problems in the carbs.

That would coke it up like you found in a short time.

hope it helps
John