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Lost power

ksj2171979

New member
I have a 25R75. It has lost half of its power. I have replaced the spark plugs. The power came back momentarily but was lost again.
What could be the issue?
 
Sparkplugs provide a gap for a high voltage pulse to jump.-----The sparkplug does not make that pulse.-------So start with this simple inspection / test.-----Does the spark jump a gap of 5/16" or more , yes or no ?----A testing device bought or homemade would be used to determine that.------Sounds like the points under the flywheel need cleaning.
 
Please describe 'lost half it's power'. Does the engine bog and die? Will it take off normal then bog and run at low RPM? Is it misfiring or just low on RPM? Does this happen at all engine temperatures or only when it's hot or cold?

You need to work on some troubleshooting and remember this equation -- Compression + Spark + Fuel = Power
Plain and simple on a 2-stroke, if you have proper spark and compression with correct fuel delivery it should run well.

Spark
** Using an air gap tester, will the spark jump a 7/16" gap while cranking the engine? To do this, remove both spark plugs and make sure the plug lead you are NOT testing is properly grounded.

Spark Plugs
** What color are they? Brown, black and sooty, or clean. Brown is good, black and sooty means improper combustion, clean means you might have a head gasket leak.

Compression
** What is your compression number using a gauge?

If you have poor spark you need to inspect the coils. Clean all the grounding points and re-try air gap test. If it still fails you need use a multi-meter and measure the coils (sorry, I don't know the specific values for your engine). If the coil checks out fine you need to dig into the magneto, points, and condenser. There may be some cleaning, adjusting, or replacement needed on these ignition components.

If your compression and ignition system all test out fine, your next place to look is fuel delivery. When was the carb last cleaned? Is your high speed jet clear? Do you have any air leaks in the fuel system?
 
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