SB50P Fires but Won’t Run

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tony.kelbert
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SB50P Fires but Won’t Run

Post by tony.kelbert » Thu Aug 22, 2019 7:38 am

My SB50P will not run. I can get it to fire as long as the starter is engaged but as soon as I let go of the starter button it stops. Here’s what I’ve done so far:
— Cleaned carb (tested every opening and cleared with carb cleaner and small wire)
- Replaced cylinder head and piston
- Oiled air filter and installed stock air box
- Cooked the exhaust on my grill to clear carbon
- replaced all ignition components (spark is super hot now)
- checked crank seals (no noticeable leaks)
- primed cylinder with gas, oil, etc.
- Replaced reed valve seals
- Replaced bystarter
Any insight you have would be greatly appreciated!

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Meatball
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Re: SB50P Fires but Won’t Run

Post by Meatball » Thu Aug 22, 2019 10:32 am

What is compression at?
I need to find some new haters...the ones I have are starting to like me.

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Re: SB50P Fires but Won’t Run

Post by tony.kelbert » Fri Aug 23, 2019 1:48 pm

Compression is at 50 psi. If I put oil down the plug hole it gets up to 100 psi. I just don't understand why it's so low.
- Cylinder/piston/rings are new
- No visible leakage on the crank seals
- I did not replace the cylinder head, but I did replace the metal head gasket
- I fabricated head-to-block gaskets and intake gaskets and neither appear to be leaking
- The reed valve has no visible openings and doesn't allow air through when I try to draw some through the intake tube
- I've cleaned the carb and cleaned the carb and cleaned the carb
- Air filter is new and oiled in stock sealed airbox
- Exhaust is free and clear
- Exhaust hardware and copper gasket are new
- Spark is hot
- If I hold a lighted match above the plug hole it shoots a flame when I turn it over so it has fuel.

Is timing adjustable on these scooters? It feels like the timing is retarded and firing when the exhaust port is open so it doesn't get any power.
Is it possible that compression will come up if/when I get the thing to fire consistently just from relative expansion between the cylinder and the head?

Should I just get a BBK for the thing and start over with a new cylinder/piston/head and gaskets?

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Re: SB50P Fires but Won’t Run

Post by tony.kelbert » Fri Aug 23, 2019 1:55 pm

The cylinder/ring/gasket kit I got:
https://www.amazon.com/NICHE-Cylinder-P ... way&sr=8-2

I've heard some of these are junk, any thoughts on this particular one?

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Wheelman-111
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Re: SB50P Fires but Won’t Run

Post by Wheelman-111 » Fri Aug 23, 2019 2:05 pm

Greetings:

If your gaskets and reeds aren't leaking, that pretty much leaves the ring seal as the reason for low compression. Rings have little notches in the ends that cradle a tiny metal pin in the ring lands - and it's pretty easy for these to become misaligned. Kits from the Far East are known to be imprecise, and the port windows often need chamfering to avoid snagging a ring or scraping the oil completely off the piston. The big-block kits are not reported to be much or any better. If you have a bore gauge, I'd pull it apart and measure for clearance and a good piston-cylinder match.

Timing isn't adjustable, as the trigger on the flywheel is keyed to the crank. If you didn't remove the right-hand flywheel, it should be fine. Anyway with compression at 50 timing is an unlikely contributor to the non-running condition.
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Re: SB50P Fires but Won’t Run

Post by tony.kelbert » Fri Aug 23, 2019 3:48 pm

Thanks for all your help so far. I'd be willing to bet the rings/pins are out of whack. I'll measure tonight, but if they are new parts are pretty much the only answer correct? Can I just get rings or would it be smarter to buy the whole shebang again?

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Re: SB50P Fires but Won’t Run

Post by tony.kelbert » Fri Aug 30, 2019 10:01 am

Solved the problem! I just bought a BBK on eBay and installed it. Compression is good, fires right up and I don't have any more low power issues. It will go 25 mph flat-out and 20 mph uphill with my 220 lb on board. Breaking in the cylinder/piston set up right now, but I'm excited to take this on a little longer ride soon.

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