Rm85 Motocross Bike Stalling When On Load.? Edit

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Apr 5, 2016
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#1
i recently bought a 2 stroke motocross bike for my partner (she can ride fine), i test drove it through the gears tested it pulled away fine everything seemed well apart from it did not idle. got home opened up the idle screw a touch everything seemed well, change the gear box oil, changed the spark plug, cleaned and oiled the air filter (noticed air filter cage was missing)

took the bike to the track and my parter couldn't take off, i gave it a try and same thing it is running fine no flat spots just as soon as you go to take off it dies. i managed ot get it going by feathering the clutch and revs and done a lap, took it back to the van. cleaned the carb out (was clean) re assembled and still same problem, opened air screw and kept testing got to and extra 2 turns out and the bike pulled away (still with a struggle but ridable) got down the first stright as the bike was riding fine hitting the power valve riding good then lost all power, then a few seconds later got power again and ran fine again only to do the same again down the road...

yesterday i stripped the carb, soaked in cleaner over night blew out it out with an air compressor and re assembled. set air screw to stock setting and started bike seems running fine but still as soon as i go to take it off it dies...

the idle screw seems to be extremely far in to make the bike idle
when choke is pulled up bike stalls as normal
dont know when last piston
sound stupid but could it be the air filter cage missing tried removing air filter but still same problem.
or the gear oil used, normally use 10w-40 but used some motocycle gear oil.
(last two seem ridiculous but im lost, dont want to change top end and find out its crank seals or somthing.

any idea? im stumped
 

truespode

Wheelie King
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#2
Check the fuel lines. Sounds like it may be starving for fuel or air at some point. Fuel lines are cheap to replace just to rule it out.

Check the reeds as well.

Ivan
 
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#3
Check the fuel lines. Sounds like it may be starving for fuel or air at some point. Fuel lines are cheap to replace just to rule it out.

Check the reeds as well.

Ivan
blew through the fuel supply tube, and pushed the float up and watched the carb fill with fuel... im thinking the air filter i didnt put much oil on it but going to wash it and put it back on without oil, dont know what im looking for in the reeds?
 

truespode

Wheelie King
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#4
I seriously doubt it is the air filter. If you ran it without the filter once (don't do that again) and didn't notice anything then there isn't any blockage. Also, make sure you keep the filter oiled properly. I prefer No-Toil and use the pre-oiled filters and wash them. I used to be able to get a year out of 5 and that was changing after every ride (back when I rode twice a week).

The reeds should be flat and not be curled up and not have any cracks.

Also, check the rings. Probably have to pull the topend to do that. If you aren't familiar with it have someone who is a good mechanic help you. I mean good and not someone who uses the tools from the discount bin at Ace Hardware.

I am not a great mechanic and can only give some basic advice to check.

Good luck.
 
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#5
im happy to change a piston, someone suggested it could be the crank seals, iv actually done a bottom end aswell just wanted to avoid doing the piston if im going to have to do the bottom end, was going to clean exhaust and have a look at the rings through the exhaust port, never done reeds before guessing its worth a look before anything else.
Thanks a lot.
 
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#7
quick up date, iv stripped the bike, it has excessive play in the bottom end (flywheel side) alot of play on the conrod left and right but no up and down. however the cylinder looks like a badly abused dog toy. its had a liner fitted at some point (badly) the power valve is actually hitting off the new liner causing it to stay open, the head was gasket sealed in place so much i had to beat it off with a hammer and evidence it was still blowing past. the head gaskets where in there but the liner is very very rough causing damage to the rings almost instantly. basically i bought a dud.

also has the lower mechanism from the k4 model and the upper parts of the k5/k6 yet to figure out how im going to make this work i'll update when i figure out!
 
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