No compression...CR125

[sdog30]

Alright...I'm pretty frustrated with this rebuild...it's been a nightmare.

02 CR125 engine rebuild....went with Wiseco lower-end kit and upper stock piston kit. Got the engine back together and in the frame, but I can't get ti to turn over.

Things I've tried...new spark plug, tightening the the spark plug boot, I get a spark when grounding the plug to the head. I can smell gas in the cylinder, I cleaned the carb and peckock valve, pulled the fuel line and gas comes out when the valve is on. Used a meter on the wiring, seems good, even disconnected the shut-off switch to eliminate it. Seems to have pressure when I kick it over, but not getting a reading on the pressure tester at all. I never checked the gap on the new piston ring before I installed it, so was wondering if that might be it. Or the exhaust valve?

[landricscustoms]

timing off ? rings gaps aligned wrong?? stuck valve?? its a start

[union]

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timing off ? rings gaps aligned wrong?? stuck valve?? its a start

Im going with the rings arent right since in a two stroke there arent any valves.

[butcher bergs]

I know a guy who managed to put his pistons in backwards and had all sorts of problems.

Just sayin'...

[Marc]

If you don't have any compression reading using a gauge I'm afraid you are going to have to pull the head and cylinder to see what's wrong.

[Paul_E_D]

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it'll run for a bit with the piston in backwards.

I have had a togh time starting after a full rebuild due to some kind of reversion in the carb. I had to bump start in on a long hill to get the fuel flowing in the right direction.

if you can feel compression whe you kic it, perhaps your tester or method is flawed? If that's accurate, i'm guessing head gasket/oring, base gasket, or gase gasket is not seated correctly

[Stromper]

Try starting fluid see if she will pop

probably lots of oil in there from assembly

piston ring may have been damaged and stuck in the groove during assembly

[Paul_E_D]

I don't like starter fluid, but good point about the ring.

[sdog30]

The piston has an arrow that points toward the exhaust and I made sure the ring was seated with the gap around the little stop in the notch of the piston, even made sure that the "n" printed on the ring was facing up. There's only one ring.

I did a full cleaning of the carb (it was already pretty clean) except the air/fuel mix and the choke. Took apart and cleaned the peck-cock.

I used all new seals, the one between the cylinder and case said in the manual to have the word "up" facing up, but there was no printing on the aftermarket replacement gasket or the original that came off. There were indents, so maybe I have the gasket upside-down? I never measure the gap of the ring, so could it be too much?

The compression tester is from auto zone and when I blow air into it from my compressor, it reads preassure. Could a exhaust valve cause the piston not to have enough preassure?

[Stromper]

If the piston seals and goes up then you will have a pressure reading.

Also if the head gasket is completely f'd then could leak there and you should hear it his unless it dumps into the head bolt area

could the read be in backwards no air in no compression

[Kurlon]

You're holding the throttle FULL OPEN when testing for compression, right?

[lrrs313]

Yeah, make sure you've got the choke on, WOT, four or five full kicks in quick succession. Either that or plug the intake and exhaust ports completely air tight.

[sdog30]

Holding it wide open, yup...I'm pretty sure the reeds are in right, there's only one way the housing fits in.

I pulled the cylinder, no scoring, still honed, the ring was still in the correct position on the piston...the exhaust valve was closed and I found out it doesn't really prevent the bike from having compression.

[sdog30]

Just put the ring in the cylinder to check the gap and it looks correct. Anybody know the proper orientation of the cylinder head gasket? The book said to have the printed "up" facing up, but the wiseco gasket doesn't have any imprint. Anybody know which way the gasket goes?

[Stromper]

You may/ probably have a warped head.....

Get a double thick pc of glass
Place wet/dry 220 on it facing up
little water
gently work the head in a figure 8 pattern then rotate 90 degrees do again

examine the surface to see that entire surface is being touched and ergo
flat

if not rinse and repeat

[Paul_E_D]

Warped head or cracked cylinder are possible, but was it running before you tore into it? If so, that kinda points elsewhere. Does the head have locating pins? If on of those is poorly seated it could be your problem.

[sdog30]

You guys won't believe this...so after taking the cylinder apart again, re-cleaning and inspecting the carb and checking the reeds, I don't think it was any of that.

I put it all back together today and tried to just pop start it instead of kick start, and viola it started. After the initial start, I was able to kick start it, but it wouldn't stay running at idle, so I played with the idle valve and it seems good now. I guess I should have tried pop starting it before totally dismantling it, but at this point, I'm just psyched it runs. I was running out of things to check.

[Paul_E_D]

Bingo! I believe it obviously. This happens often with a full rebuild. Like I said, some kind of air bubble or reversion in the fueling tract requires some extra pull to get fuel flowing in the right direction.

[Stromper]

no compression, somehow you did something different and got some compression

Its not ZEN its physics

[lrrs313]

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Sometimes just disassembling/reassembling works. Something goes back together just a little bit different and its fixed. That said, Paul is right. Often after a complete rebuild they take a lot to get refired.

[DuncanMoto]

I know a guy who managed to put his pistons in backwards and had all sorts of problems.

Just sayin'...

That's just an urban myth.
Noone is really that dumb.

[david135]

i have a similar problem with my 2001 cr125 i just rebuilt the engine a few days ago and i had it started it was running so i turned it off let it cool down and started it up again and went for a ride ( low rmps for break in run) and it shut off after about 5 mins and i lost compression in the bike and now it wont start again