85 rm 125 tiny window of power ..ideas??

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sr5bidder

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Oct 27, 2008

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• Jul 1, 2009

• #1

Hey guys I got one of my project bikes back for repair from the guy I sold it to about 2 yrs ago.

Problem is it has no power in the low revs..... just a tiny powerband ...then nothing after that in higher yet revs.
it never had power down low anyhow but when it hit the powerband it pulled strong till fully reved out..

I got 90 psi compression, I think is low (my kdx200 is in the 155 range)...

I'm not convinced it is the piston/rings as it holds 90 at the gage forever. (no leakdown)

do you think this an indication of bad reeds.

and the clymer manual I have does'nt spec the compression this bike should produce?? anyone have specs

Also it hardly smokes at all it may be to lean.... has anyone had power issues with a to lean condition?/?

 

C

Chili

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Apr 9, 2002

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• Jul 1, 2009

• #2

At 90 it is most defintely time for a top end. Most of our 125's had between 140-160 psi when fresh.

 

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sr5bidder

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Oct 27, 2008

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• Jul 1, 2009

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• #3

ok I'll break her down tonight and call the machinist to get the cylinder and piston from the back of the wifes truck at work ( pretty cool pickup /delivery system and free!!) and get it measured.

 

J

jsantapau

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Nov 10, 2008

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• Jul 1, 2009

• #4

sr5bidder said:

I got 90 psi compression, I think is low (my kdx200 is in the 155 range)...

I'm not convinced it is the piston/rings as it holds 90 at the gage forever. (no leakdown)

?/?

Click to expand...

If you are using a standard compression gauge they have a one way valve in them to be able to keep the recorded high pressure and 90 lbs is low

 

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spark250

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Feb 7, 2008

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• Jul 1, 2009

• #5

jsantapau said:

If you are using a standard compression gauge they have a one way valve in them to be able to keep the recorded high pressure and 90 lbs is low

Click to expand...

Lol! I was thinking the same thing. A compression test and a leakdown are two different things.

BTW why are you doing all this business with a machinist etc...

Just give it a quick hone and throw a new piston and rings in it!

 

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sr5bidder

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Oct 27, 2008

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• Jul 1, 2009

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spark250 I have not tore down yet and I use my machinist because he measures for me and everything he does come back beadblasted and super clean and cost $50 including the bore out

to hone would be to guess, its on its 1st oversize now .50 and there is 5 more sizes to go all the way to 2mm over.

If the cylinder and piston are in spec then I'll do rings if not I'm going to .75 weisco


Does any one have knowledge of adjusting the powerband to lower rpm's by increasing compression ratio..shaving 20thousanths off the cylinder head????

I want to make this more woods ridable for the owner

 

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spark250

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Feb 7, 2008

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• Jul 1, 2009

• #7

aside from porting you could get a pipe and gear it differently. 125's are gonna be pipey though. You could change where the power hits but it will never be a torquey feel...

 

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sr5bidder

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• Jul 2, 2009

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well the piston had some nice scratching on the intake side and a bit on the exhuast side the owner ran it with a broken exhuast spring and I think it sucked some nasties in ..the filter had not been cleaned at all...

while inspecting everything I found some slop when pulling up and down on the flywheel so I need to find a source for the crank bearings and seals too..

the connecting rod big and small end is in good shape I think I'll bolt the head back on and seal the intake and exhuast and power wash before I take it out of the frame...this is the second bike this guy has brought me that was filthy nastey

 

J

jsantapau

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Nov 10, 2008

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• Jul 2, 2009

• #9

sr5bidder said:

Does any one have knowledge of adjusting the powerband to lower rpm's by increasing compression ratio..shaving 20thousanths off the cylinder head????

I want to make this more woods ridable for the owner

Click to expand...

get the bike in good working condition first,engine tight,exhaust good shape and repacked and jetted well for your conditions, then adjust from there.
You can use a thinner base gasket(or omit it altogether if the gasket surfaces are good enough to seal with some Threebond) to feel what the bike will be like without machinining at first

 

T

TRexRacing

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Jul 23, 2002

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• Jul 2, 2009

• #10

Jetting well will give you as much as you can get from it.

 

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sr5bidder

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• Jul 3, 2009

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• #11

well the owner basicly said I could have the bike seeings how the shock and forks are in need od service , clutch cable, piston, main bearings.... maybee an old air cooled kdx200 engine conversion?? I'm going to hunt for a while