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Post by scootlife on May 31, 2015 8:30:53 GMT -5
90GTVert, do you know what piston to valve clearance should be for a "49cc" scooter?
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Post by 90GTVert on May 31, 2015 10:35:15 GMT -5
I have yet to see any sort of official spec for that. I can find minimums posted for big engines, but not for our little scoots. My best guess is to treat a minimum sort of like squish clearance and use a percentage of stroke. 2% is generally a street safe squish clearance. 44mm stroke would put that at 0.88mm or roughly 0.035". Maybe moving up to 3% (1.32mm or 0.052") would be a safer target since squish clearance really is looking at rod stretch and thermal expansion, but not accounting for valve issues. If you want the big engine specs, and by big I mean autos not big bore 50s, minimum is usually 0.060" some racing apps or more commonly 0.080-0.100". I've never seen any sort of formulas, just specs. Kinda reverse engineering the big engine specs and assuming they're for roughly a 3-4" stroke it ends up as 2.5% to 3.3% of stroke. If nothing else at least make sure they aren't currently hitting just turning the engine over. This won't save you from rod stretch, expansion, or valve float, but at least you won't kill it on startup.
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Post by Fox on May 31, 2015 12:28:49 GMT -5
Well I didn't want to start a disagreement. I was just thinking that maybe scootlife's piston made contact with the head and broke/bent something.
I think you took usmcdoc's comment the wrong way.
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Post by scootlife on May 31, 2015 19:23:14 GMT -5
I finally got a chance to try the drops of oil, and I don't have any compression. Checked gaskets...all don't show any signs of leaking. I think it must be a valve staying open or bent. Not sure what to do at this point.
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Post by katastroff on May 31, 2015 19:28:08 GMT -5
Open it up and see.
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Post by scooterpimp on May 31, 2015 19:34:52 GMT -5
If your good on the timing ? Time to pull the head for inspection. , like Katastroff mentioned.
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Post by scootlife on Jun 1, 2015 9:16:29 GMT -5
I have pulled the head. No visible damage to valves, cylinder, rings or piston. Measured ring gaps and they are at .008 as calculated using 90gverts advice for a 52mm piston.
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Post by scootlife on Jun 1, 2015 9:16:51 GMT -5
No signs of a gasket leak either.
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Post by katastroff on Jun 1, 2015 9:27:30 GMT -5
Time to check for leaky valves since you got it opened up.
Hold the head with the side that faces the piston down. Spray WD-40 in the intake then in the exhaust port and see if anything is dripping out from the valves.
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Post by scootlife on Jun 1, 2015 15:40:05 GMT -5
Noticed a pool of oil coming from the crankshaft seal behind the flywheel. Don't think it's related though, right?
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Post by katastroff on Jun 1, 2015 15:41:45 GMT -5
Probably not, but you'll need a new seal. I would do both sides.
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Post by scootlife on Jun 1, 2015 16:38:34 GMT -5
Found this stuck in my intake valve. Must have been when I drop a few drops of oil in the spark plug hole to check the seal. Oops! That's why I was getting no compression and not the 60psi I started with. Still stuck at 60 psi though. Did the WD-40 test to check valve sealing and I have no leak there. I'm stumped.
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Post by scootlife on Jun 1, 2015 16:39:58 GMT -5
Pic won't post but it was the tip of my oil dispenser.
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Post by katastroff on Jun 1, 2015 17:13:41 GMT -5
Throttle fully open when you do the compression test?
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Post by scootlife on Jun 1, 2015 17:28:02 GMT -5
After I put it back together, the compression test showed 60 psi. Out of pure chance....(I should play the lottery tonight)....I put the spark plug in and tried to fire it up.......and VRRRROOOOOM!!!!! JACKPOT!! Purrs like a kitten. Pouring rain here in CT tonight so I won't be able to test the upgrades. 52mm HOCA BBK and HOCA Big Valve head. Can't wait until the weather breaks.
Thanks everyone for all the help...looks like it ended up being a chunk of plastic keeping the exhaust valve wide open.
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