Engine Trouble Engine only works under heavy load

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Apr 19, 2013
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ma
So I recently switched this engine to a new bike cause the old one was crap. For some reason it's lost 3-10 miles per hour on average. But that's not the biggest issue. I've tried various fuel /air mixtures and such, but for some reason the bike cuts out when it's not under heavy load a full throttle. So going up hills, fine. Accelerating, fine. but once it gets going about as fast as it's going to go (which is only about 19mph at the moment instead of the old bikes 30 despite being much heavier) it cuts out until it slows down a little, then runs/accelerates a bit, cuts out, slows, runs/accelerates, rinse and repeat.

At first I thought was because of air getting into the fuel line because it used to start bubbling every few minutes unless i loosened the gas cap for a few seconds, but I'm pretty sure i've sorted that out. Now it's just this cutting out and low speed thing.

It also sounds like there's something clanking slighty at the bottom of each revolution. but i can't figure it out. I looked inside and it's nothing above the piston and nothing obvious below it but i haven't stripped down the whole thing. Any and all ideas welcome.
 
As in the motor to the bike? That's super tight. Any other ideas? I'm gonna check for leaks tomorrow but I've used silicon gasket maker stuff around everything so that shouldn't be an issue.
 
shaking the carb with too much vibration will cause surging - not usually reduced speed tho

look carefully at all you did to see what changed between bikes
 
2 strokes like a rich mixture under load and a lean mixture at no load

so logically we can deduce that your jetting is too rich or possibly the air filter has poor flow through it. take it off and press it against/around your mouth so that it seals against your face and then blow thru it. You should feel next to no restriction. If there is restriction then get a good filter.
 
and maybe you are living at high altitude which requires less rich jetting to match the oxygen poor air
 
Thank you guys for the responses. There are no physical changes between the bikes.

My cities altitude is only 200 ft. I have also tried each setting on the jet needle (using the stock nt carb) and it always happens (though I do notice running leaner gives more power but makes the engine much hotter. At it's leanest setting I went to do a full throttle plug chop and when I killed it there was smoke coming through the carb.

Also I tried running it with no filter at all and with just the inner spongy stuff and no outter plastic to the air filter. Feels like it runs slightly leaner but still has the same issue under light loads.

I don't mind if it only does 20 cause that's not gonna get me killed. Jerking around and starting/stopping will.
 
It's possible I'm not doing the plug chops well enough but they never come out white or obviously tan. They're always black. Which made me think it was running rich. But at it's leanest setting the engine gets really hot compared to other settings and the plug still looks the same (and like I said I still get the no load issues)

I really appreciate your guys help. I've been avoiding asking for help while researching other people's issues but nothing has worked. I'm not as smart as jaguar when it comes to this stuff and this is my first engine build but I am technically minded.
 
changing the needle height doesnt change the mixture, except at partial throttle and idle.

everything says its too rich. get a smaller jet.

hot? gets too hot? sounds like your idea of "hot" is lukewarm. tepid, even.


clanking noise...either the main bearings are about to go, its plain old gear noise (possibly a cracked pinion gear), or the magnetos rubbing... could be others but theyre the first to spring to mind...
 
if the cylinders are basically the same then there could be a cranking pressure difference between the two.
use sandpaper and glass to make sure the head and cylinder sealing surfaces are truly flat.
other than that, it can only be that the main jet is too big for it.
maybe you had an air leak previously that compensated for the rich jetting, and you "accidentally" took care of the leak with the replacement.
 
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