almost no compression.... need help!!!!

[CR-FLAVOR]

fellas,

I just finished putting my 00 cr125 back togethor, thinking that a fault head gasket was to blame, but she still wont start, wont even bump start.

i know a compression gauge would help, but no much really since i can easily cycle the kickstarter with one hand. Any ideas for finding where the problem lies?????

a few months ago, i put in a new cylinder, wiseco topend, and phathead. took it out to the desert for a ride, ran great for an hour or so, then just died and never would restart. i know the phathead o-rings can let water thru sometimes if not heavily greased up, so that was my original thought after it died on that first ride since the new cylinder and top end ect....

any ideas???
when the pipe is off, and you put your fingers up in the exhaust port, should you be able to flip the powervalve up and down with no effort? mine is at what i would call the 3/4 open position.

really need some help, the bike is super clean and i need to get it sold.

[Vintage Dave393]

If the piston and ring clearances are in tolerance, I would suspect that you have a crank seal that has blown out. The bike would start and run with a stuck exhaust valve and your compression wouldn't be affected as much as you indicate.

A lack of compression can only come from a few areas. Start at the top. Spark plug, head gasket, ring clearance, piston clearance, hole in piston dome, base gasket, center case gasket, crank seals, hole in the case. That's it. If you check all of that, you will find your cause of low compression.

[magnethead]

sounds like a mis-fitting piston ring or very loose spark plug. pull the plug out and seal the plug hole with one hand and seal the exhaust with the other hand while somebody cycles the kickstarter. If it seals, it's the plug or exhaust. If not, look deeper.

[CR-FLAVOR]

good stuff, keep the replies coming, i plan to test all tips then post the results tommorow pm......thanks!

[Vintage Dave393]

It's not the exhaust valve. You could completely disable it and the bike would run. This is done all the time by shifter kart racers. You may indeed have a problem with the valve but it is separate from the bike not starting.

Start at the top of the list I gave you and work your way down.

[magnethead]

Is this a 2 or 4 stroke, i didnt understand?

either way, do what i said initially. If it's a 4 stroke, one of the valves might be hung half open. If it's a 2 stroke, there's not many things that could do it, as there is only 1 air exit possibily once the piston is above the cylinder wall ports- the spark plug.

If a piston ring isn't sat properly, I'm not sure how exactly to tell other than moving the piston at TDC and checking the slack. Anything over .005 (if even that much) is too much.

[Vintage Dave393]

It's a CR125. All CR's are 2 strokes.

[CR-FLAVOR]

a hunch tells me its the left side crank seal pulling air in preventing a clean burn

how on HIS green earth, do you get the flywheel nut off????? i have the bike in gear, with my 300lb buddy standing on the rear brake and the flywheel is still turning...... yes i am a jacka$$.... educate me.

[2fun]

Originally Posted by CR-FLAVOR

a hunch tells me its the left side crank seal pulling air in preventing a clean burn

how on HIS green earth, do you get the flywheel nut off????? i have the bike in gear, with my 300lb buddy standing on the rear brake and the flywheel is still turning...... yes i am a jacka$$.... educate me.

If I am thinking of the same nut you are talking about, you can often times use an impact wrench.

[CR-FLAVOR]

yeah, i was thinking the same thing, but wasnt sure if it would work since the flywheel is already turning.... time to go find a compressor.

[Faded]

Originally Posted by magnethead

If a piston ring isn't sat properly, I'm not sure how exactly to tell other than moving the piston at TDC and checking the slack. Anything over .005 (if even that much) is too much.

Big Dub, VD provided an excellent list to go from, like he said, start at the top. Use an impact gun to get the flywheel nut off. You can also do a search for "Leak Tester" to find directions on how to build a tool to help you find gasket/seal air leaks.

[CR-FLAVOR]

fademaster,

Yo big A, i was actually wrenching according to your email from way back about replacing the left side seal. I thought that with the bike in gear and the rear brake locked up, the flywheel would sit still.......learn something new every day

honda makes a tool called the universal holder, found it in the service manual.... but prob more expensive than its worth

I dont have a compressor with inpact gun, but im sure i can find someplace local that will do it for me with the bike in the truck.

i was looking at the directions to build the leak tester, looks fairly simple, time to steal the baby powder from the and get to it.

[Faded]

Originally Posted by CR-FLAVOR

fademaster,

Yo big A, i was actually wrenching according to your email from way back about replacing the left side seal. I thought that with the bike in gear and the rear brake locked up, the flywheel would sit still.......learn something new every day

Sometimes it will work, but your best bet is to hit it with the impact.

Soap/water or glass cleaner works a little better with the leak tester to help find air leaks. Good luck, and keep us posted on what factors you eliminate and/or what you find.

[ossagp]

20 dollars will get you a reasonably useable 1/2 inch air impact,,,,,,90 more will get you a compressor, try Kragen's or Schucks. If not get the wrench now and use the hose at the gas station like i did for years.

Sounds like you did not reconnect the linkage to the power valve.

Even with all that and the chance of a seal blown on the mag side, I cannot figure out why you are so far down on compression that you cant get it pull started. I have seen CR125's run without a mag side crank seal in place at all, and run pretty long too. I had a man bring me his son's bike that had been rebuilt by it's former owner,,they were at a loss as to why it reved up high when the clutch was pulled in. (on cr's this accentuates a leakey seal by shifting the cable and letting air get into the magneto chamber in greater volume). Off with the cover and the rpms went through the roof,,shut it off,,remove the rotor,,and NO SEAL. NONE! It had run for weeks.

I have seen them pull started with no rings and actually run (ok, wasnt a honda 125) . Anyway, I think you are going to pull the engine down again.

If you have a windowed piston, and it is facing backwards you could be losing enough primary compression to keep it from running.

[CR-FLAVOR]

i remember connecting the linkage on the PV for sure.

just for giggles, i drained the carb bowl, put the left side cover on, and kicked it a few times, it fires 1-3 times then dies, wont rev at all and sounds like a bike running underwater...... ever feel like sailing your bike off the nearest cliff?

[ossagp]

If the valve moves by the pressure of your finger in the exhaust port it doesnt sound like it is connected,,i am sure you COULD NOT have done it wrong though.

[CR-FLAVOR]

thanks guys,

I am using a brand new plug, and the valve doesnt move with finger pressure, i wasnt sure if it should or not.

all i can think of is making the leak tester and giving that a try, or perhaps replacing the left side crank seal first, since i have the crank seals and flywheel puller.

[magnethead]

the power valve is my next guess. I can offer only so much help due to my lack of 2 stroke knowledge.

From what I do know, the piston should move up over the intake & exhaust ports and the skirt keeps the oil from going out them. If you can see inside the cylinder with the piston at BDC and the exhaust off (or through the plug hole), scoot the piston up to just above the exhaust port, pull the plug if it's not out already, and use your thumb to see if there's any pressure while moving the piston from that point up to TDC.



Looking at that, I just don't see many places where total compression can be lost.

[hardlydangerous]

Make yourself a flywheel holder and a leak down tester

To make the flywheel holder you will need
A piece of 3/4" or 1" wide flat bar steel at least 30"- 36" long. (The longer the better)
Four 1/4" bolts at least 1" long
Six 1/4" nuts
1/4" drill bit and a drill

#1 cut two pieces of the flat bar 15" long (you can make it shorter if you want but it makes it a lot harder to hold)
#2 cut another piece 6" long
#3 drill a hole 1/2" from each end of each piece of flat bar
#4 drill a hole 6" from then end in each of the two 15" long pieces.

Now bolt it altogether like in the pic. Use two of the nuts as spacers between the 15" pieces. This will allow the flywheel holder to be adjustable for any size of flywheel.
When it's done it should look something like this (keep in mind I welded mine together instead of using nuts)







To use the flywheel holder simply insert the two bolts on the end into the two holes on the front of the flywheel to hold it from turning while you loosen and tighten the flywheel nut.

Here is my $40 (CND) leak down tester set up

1 adjustable freeze plug $5 depending on exhaust port size
1 low pressure VAC/PSI gauge $15
1 valve stem $2
1 plastic adapter $4 find a size that fits your intake
1 cap $2
1 plastic 3/16" fuel/vacuum hose fitting $2
small air pump $ 8

To make the tester I first bough a VAC/PSI tester from my local automotive store, then went to the hardware store and picked up the 1-1/4 adapter and end cap.
For my tester I just use a cheapy-do bicycle tire hand pump I picked up for $8.

To assemble the tester I used a bit of silicone on the threads of the end cap and tightened it onto the adapter or you can use ABS cement.
The drilled the hole in the end of the cap for the valave stem. I used a bolt on valve stem but you can use a regular valve stem from a car or ATV rim.
Next I drilled a 1/4" hole in the adapter for the PSI gauge fitting. I used some silicone to help seal it in or again you can use ABS cement.
That's it.

It easy to do the test but takes some time.
Just remove the pipe, clean up the flange (if you have an aftermarket pipe) or exhaust port (if you have the stock pipe)
install the freeze plug and tighten the wing nut. If you don't clean the port of all carbon it's going to leak.
Slip the carb out of the intake manifold and slip the tester in and tighten the clamp.
Pump up 6-7psi.
It should hold 6-7 psi for at least 10 minutes or longer . If it drops you have a problem and you need to spray the seals and gaskets with windex or a soapy solution to find the leak. When you find the leak repair it and test it again.









OR

http://www.imagestation.com/picture/...5/f9593e88.jpg

[hardlydangerous]

BTW I have had bikes come in with busted power valves. They were both Kawasaki's though. But still the main valve broke off at the pin area and that allowed the valve to slide forward enough tohit the piston. Once that happened the rings were damaged and the bike lost all compression.