150 black max lossing RPM's

[firewolfe51]

Ok guys thanks for your help in advance!!!! 84 model 150 black max with 125 compression each. It runs great straight off trailer then after warming up after 10 or 15 minutes it loses 200 to 300 RPMs. It still runs smooth just not as much *** to it!!! and needless to say drops about 5 mph. Also seems to LOAD UP after few minutes of idle. CARBS are clean and plugs look good. HELP HELP HELP PLEASE driving me nuts!!!!

 

[SparkieBoat]

Re: 150 black max lossing RPM's

timing..fuel restriction..or possibly the timing...if that model has an idle stabilizer, remove it and adjust timing to 25 degrees at WOT..also check all linkage carfully..and make sure it is in sync

 

[James R]

Re: 150 black max lossing RPM's

You say the carbs are clean. This is something I always question but lets assume that that is correct. A common failure on these motors is switchboxes. If they are old you may be losing spark to a couple off cylinders. Have to do a spark test when the fault appears.

 

[CharlieB]

Re: 150 black max lossing RPM's

It is possible that a high speed winding in the stator is warming and loosing voltage, or the rectifier is warming and changing the stator output.

You can test both.

You need to make some long leads for your DVA meter so you can safely check high speed stator voltage while running at the speed that the condition occurs, watch for any droop in voltage as the motor warms up. Repeat test on both high speed windings to each switchbox.

Disconnect rectifier and repeat tests to see if any difference caused by the rectifier.

 

[j_martin]

Re: 150 black max lossing RPM's

Another possibility is a bad top bearing. As the engine warms up and the stator expands a bit, it might be contacting the flywheel lowering output. It doesn't have to hit hard enough to smear metal around, just a light touch will cut output.

Rare, but it happens.

 

[firewolfe51]

Re: 150 black max lossing RPM's

well for the carbs i cleaned myself so I know they are good, and I was afraid somebody would say something about the stator! I really dont think im losing cylinders due to engine still runs smooth but losing voltage is possible after warming up. It will be couple days before i get back on it but if anyone thinks of anything else let me know THANKS GUYS!!!!!

 

[j_martin]

Re: 150 black max lossing RPM's

While you're staring at it, detail (clean) all the related connections, beginning with the ground connections. You'd be amazed at how critical they are.