ok... i'm baffled... advice requested...

leo

Member
Jul 20, 2015
250
0
16
southern wv
hi everyone. hope you're all doing well...

i have a pothole on a nearby hill that i use to gauge my low end power with. when i put on any new part, i run up the hill at full speed and see how far it gets past the hole. i put little chalk marks to compare with my gps info. if they correlate, we have a winner.
anyway, i recently checked my mountain bike and found that it seems to be loosing power. i cleaned the carb and gave it a general tune-up. double checked for leaks, and so on. i even got a SBP expansion pipe, but it still wont make it to it's highest point ever recorded, even with the SBP pipe.
as of now, it has around 2500 miles and still runs like a dream, other than this issue... my top speed on level ground doesn't seem to be affected at all.

what could cause it to gradually loose low end power like that? what am i missing?
 

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Slogger

Member
Sep 8, 2014
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4
18
nohio
You could try a new spark plug. I've been amazed at what old and dirty plugs can do to performance on little engines. Then take a compression tester to it, whatever it used to be I bet it's less.
2500 miles is quite a few for one of these things, depending.
 
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crassius

Well-Known Member
Sep 30, 2012
4,032
156
63
USA
You've got some miles on it - things that come to mind; seals not as supple as they once were, rings not sealing as well as they once did, carbon build up in cylinder, carbon build up in muffler - any of these can give small changes in power without making enough of a difference to call a mechanical failure.
 

leo

Member
Jul 20, 2015
250
0
16
southern wv
is there a fuel additive, or maybe sea foam, that will reduce/eliminate the carbon buildup? or is it a wait until rebuilding time, kind of thing?
 

MEASURE TWICE

Well-Known Member
Jul 13, 2010
2,449
541
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CA
The ambient air temperature if colder gives engine more power. That is since difference in density of colder air to what volume difference after combustion, expansion is greater. I'm always amazed of the extra power in the AM compared with hot afternoon times.

Also if engine is too hot it can loose power.

Some recent things have been said about two stroke engines and heat and not to let idle too long as there is no fan as on most four strokes. 95 degree heat for a while I was trail riding the engine really lost so much power. Also moving not idling on a four stroke cools it down and so at times, I just had to stop altogether an let it cool off and myself as well as it would not go.