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Post by humanshield on Jan 31, 2015 19:19:17 GMT -5
Hello,
I just installed a BBK from 02starrider purchased off ebay. Seemed like a very nice, quality kit. Everything went well. No problems.
Except...it won't start. This same scooter was running perfectly on the engine I just removed (just too weak).
Here's a rundown of what the situation is and a little background info.....
The starter is really struggling to turn the motor over, but that's not too odd considering the new cylinder and rings.
1). Carefully aligned the two small holes on cam gear to be perfectly in line with the cylinder head edge and the large hole towards the front of the scooter. Made triple sure that the "T" on the flywheel was aligned at the sharp pointer mark on the crankcase. Made sure the IN (intake recess mark) on the piston was at the top.
2). Carefully adjusted the valves (intake .003 inches and exhaust .004 inches)
3). Made sure I'm getting a nice, healthy looking blue spark at the spark plug. (I have a brand new C7HSA plug and stator and pickup)
4). Checked compression - 100 psi
5). Introduced a small amount of fresh gasoline into the cylinder
Still, not so much as a back fire or any sign of life from the motor.
I'm fairly experienced with engine mechanics and I'm 100% positive I'll eventually resolve this but I thought maybe some of you guys who are really good with the QMB139 motors and BBK's might have a suggestion or see something I missed.
One thing I DID NOT DO is use the nylon timing gear that came with the kit. I used the OEM steel gear.
Any suggestions?
Thanks!
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Post by katastroff on Jan 31, 2015 19:38:12 GMT -5
Looks like you know what you're doing, so i would check for those "DOH!!!" things. Ignition switch in the right position? Pushing the break lever? You got a kickstand kill switch? Ususally, when one knows what he's doing, the problem lies in stupid little details you wont be bragging about. I call it: the DOH!! factor.  |
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Post by humanshield on Jan 31, 2015 20:00:53 GMT -5
Thanks for the reply.
It will "crank over"...just won't start. Plus, there is a nice spark, so doesn't that rule out any kill mechanism at play here?
But you're right about the DOH factor. Happens all the time, and yes, it's hard to man up to those sometimes lol
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Post by niz76 on Jan 31, 2015 21:35:18 GMT -5
Hard to say- it seems like you checked all the important stuff. Does that compression seem a little low for a new top end?
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Post by katastroff on Jan 31, 2015 21:42:01 GMT -5
100 PSI compression, a bit low. That might just be your problem.
150 psi and up optimal.
120 to 150 pretty good.
90 to 120 not to great.
Below 90 get good shoes.
At 100 PSI you really dont have good compression. Could be a leak valve seat or something not torqued down enough. I hope its a torque problem. If not, try replacing the big head with your stock one and see from there.
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Post by humanshield on Jan 31, 2015 22:14:06 GMT -5
Hard to say- it seems like you checked all the important stuff. Does that compression seem a little low for a new top end? 100 psi cold. (Remember, it wont start). Normally, compression readings are taken when the motor is at operating temp at which point they will be higher than cold readings. Right? Since the engine has never run, the rings are not seated. So 100 psi cold, at this point is pretty darn good imo. I expect if and when I get it running and broken in, the compression reading when it's at operating temp will be 150+ On the other hand, if you're saying 100psi on a COLD engine that has never run (so rings can't possibly be seated) is low, that's a different ball game? YMMV |
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Post by humanshield on Jan 31, 2015 22:52:35 GMT -5
Tomorrow I'll pull the VC back off and re-check the timing and valve settings. I've actually built quite a few car engines and other motorcycle engines and I'm fairly certain I have everything right.
But if the valves were off, it probably wouldn't make 100 psi cold.
100 psi is good enough for a motor like this to start so it should be doing "something".
Funny, I built a Ford Mustang engine recently from the cam bearings out and after installing it it fired right up first key turn and has been purring like a kitten since...yet I can't get this monster complex QMB139 to start lol
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Post by niz76 on Jan 31, 2015 23:08:57 GMT -5
Lots of compression checks before an engine runs and rings get seated. 100 psi is about the minimum required to start these monster beasts lol. Props to you for even doing a compression check- most don't. Me thinks this psi reading may be pointing to your problem...  |
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nightriderrv
Scoot Enthusiast
Never Give Up- Ride all the time !!!!!
Posts: 177
Location: Western mountains Nebo NC
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Post by nightriderrv on Jan 31, 2015 23:14:19 GMT -5
Just to get a baseline when I put my 50mm BBK from 02 in the comp reading was right at 150psi I would agree with the others it's low at 100psi. I know all of it might be new but look at the head pull it off and do a liquid test on it to make sure the valves are sealing.
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Post by niz76 on Jan 31, 2015 23:21:10 GMT -5
Also check and double check your rings and that they're all in the correct locations- there's a bunch of 'em on those dang 4 pokes! Brent shows the order they're supposed to be in his kick-butt 4t BBK guide: 49ccscoot.proboards.com/thread/350/big-bore-kit-installationJust throwing some ideas out there for ya! |
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Post by humanshield on Feb 1, 2015 10:44:34 GMT -5
Katastroff.... And now for that "DOH" moment....Got up this morning determined to find the cuprit. After coffee and a donut.... Timing chain tensioner.I removed it, reset the ratcheted bar pushing it all the way in, re-installed it and it fired right up. Now it starts every time. Did absolutely nothing else. Thanks for all the replies and help guys  |
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Post by humanshield on Feb 1, 2015 10:49:09 GMT -5
Just to get a baseline when I put my 50mm BBK from 02 in the comp reading was right at 150psi I would agree with the others it's low at 100psi. I know all of it might be new but look at the head pull it off and do a liquid test on it to make sure the valves are sealing. Will do. I'll recheck the compression after re-checking the valve adjust. MAN this thing vibrates iike crazy. The 50cc engine it replaced was smooooooth as silk. |
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Post by Corrosion on Feb 1, 2015 10:49:32 GMT -5
Great Job!!!
Do another compression test, just for a verification. Of what a good number is.
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Post by katastroff on Feb 1, 2015 11:17:28 GMT -5
Katastroff.... And now for that "DOH" moment....Got up this morning determined to find the cuprit. After coffee and a donut.... Timing chain tensioner.I removed it, reset the ratcheted bar pushing it all the way in, re-installed it and it fired right up. Now it starts every time. Did absolutely nothing else. Thanks for all the replies and help guys Donuts, they fix everything!!!  |
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Post by niz76 on Feb 1, 2015 11:33:09 GMT -5
Yes the donuts fixed it! Glad to hear you got her running!
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