Have you tried a different plug? The new plug could be bad.

I removed the vacuum line from the intake manifold so I could spray starter fluid down the hole while I turned the engine over. Unfortunately my battery (relatively new) is shot.


I boosted the battery from a known good battery. I am getting 12.5 volts on the scooter battery when boosting. When I turn it over when while boosted and spray starter fluid in the intake manifold it wouldn't fire.


What should I check next? Is this a good enough test since the battery was shot while I tested it?

Fuel-Spark-Compression . . which aspect is deficient?

http://www.youtube.com/watch?v=-bh_f0ACQ_k

Anomalies include a sheared crankshaft-flywheel woodruff key, which throws off timing . . and possibly a 2-bolt intake valve cover orientation issue.

Hate to say it, but unless china did something stupid with the fuel pumps, you can run a 750cc lawnmower engine wide open on a vacuum pump.....john deere does it every day..... crank case pressure should be driving the pump off the crankcase vent, and that will rise with more rpm....

If it wont fire on starting fluid, i would be checking compression, if thats good, i would get a new CDI box/ignitor... you may have spark, but is it 1. Strong enough to ignite under pressure 2. Sparking at the correct timing. China is known for cheap unreliable cdi ignitions....

Thanks for everyone's help with this.


The battery was low. I was charging it until it said it was full but wasn't checking the voltage. I replaced the battery and fully charged the new one.


Fuel, Spark and Compression. I checked spark by pulling the new spark plug and grounding it while turning the engine over. I had good spark. Next I measured compression. Compression is 50 psi dry. There's your problem.


What next?

well, triple check your valve clearances. other than that, start with pulling the head. if one of the valves is bent, theres your compression leak. if its not one of the valves, its your piston rings. i would recommend doing a new piston at the same time as the rings, since they are most likely stuck in the piston. thats what happened to my 200cc roketa... stuck the rings from (my guess, not 100% sure) poor maintenance and overheating. there was carbon caked in the ring grooves of the piston, wedging the rings so they couldnt spring out to the cylinder walls and do their job...

I just put a new piston and rings in. What other things can I check before I pull the head? Make sure the timing is correct, check the valve clearance....

if you only have 50-60psi compression its either your valves or your rings, or your head gasket, which would become apparent as soon as you remove the head. also, did you have the cylinder honed to cut the new rings into the bore?

if you can get a Leak down tester (loaner tool from an autoparts store) that has the correct thread for your spark plug, thats the best way to diagnose low compression. hook it up to an air compressor, air everything up with the piston at top dead center of the compression stroke so all valves are closed, and make sure the crank cant move (breaker bar and socket on the crank nut or similar) and listen for leaks. if you only have 50-60psi, you will hear the leak..... if its coming from the crankcase vent, rings.... intake, or exhaust, its one of those respective valves.

Given the fact that I just had it all apart aren't the odds higher that some step hasn't been done correctly higher? For example valve adjustment, timing off, timing chain not put on correctly, what else...?


The valves held gasoline when I had it apart. The piston and the rings are new. The gaskets are all new.

Possibilities are endless...LOL its possible that your intake gasket didnt seal quite right, ran it lean, and burned a valve.... also, did you properly torque the head gasket? Also did you check the head surface for flatness with a straight edge?

Just going over the things i would in my head (i am a small engine/compact diesel mechanic....)

Still cant rule out stuck rings either..... easiest way to check that is pour a tablespoon or so of ATF fluid into the cylinder, spin the engine over (by hand if at all possible) to work the atf around the rings, if they are stuck and not too badly, they should free up and give your compression back, at least more than 50 psi....LOL

Oooh, one more thing...LOL Can i ask why the piston was replaced recently? What was the original failure? That may help the diagnosis... also, if you can post a video of what it sounds like when its turning over (with spark plug in) it may help as well....

More questions as i think about it.... is this an overhead cam engine? If so, do your timing marks on the crank and cam still line up? If one of the valves is open at the wrong time it will fubar your compression (obviously...LOL)

Just got back from O'Reilly's. They don't have a leak down tester in their tool loan program. I will look around for someone else who does.


The compression test result I gave was with a Harbor Freight gauge. Is that an issue in any way? Is it worth me getting a compression gauge from a tool loan program and retesting before starting other steps?


The head gasket was properly torqued following the shop manual. The head was placed against a steel straight edge and I saw no warping at all.


Single overhead cam with 2 valves which is water cooled.


I thought they lined up when I put it back together but that is why I asked the question. I can't see both marks without tearing everything apart again. There are two set of marks to line up. One on the sprocket of the timing chain which is covered up unless I pull everything apart again. The second on the CVT which I can get to with the engine assembled. So I might be able to adjust the valves without pulling it apart by finding TDC by the highest piston position with the two marks on the CVT aligned. I was careful and followed the shop manual.

Here is a good summary of how to do the valve adjustment so you can see how it looks:


http://www.chinesescooterreference.c...Adjustment.pdf


The engine is essentially a Yamaha YP250

bogieboy? Anyone?

i would double check the timing first off, then try the ATF down the spark plug and try the compression test again... it wouldnt hurt to double check the compression with a different gauge, because i have a comression tester from HF that doesnt always read right.... i have to clean out the shraeder valve in the end of the hose for it to work properly. however it was reading 0 psi...