Loss of compression? Advice for a friend...

[sparklepants]

I am helping someone with their scooter. This might be a long one - my apologies.

They drove to a gas station (he and a friend on the back), now it won't start. It was running for 7 minutes and had trouble getting up the last hill before the gas station.

I arrived and checked a few things like gas lines, carburetor, etc., which all seemed connected and flowing.

The oil had been low (not even moistening the dipstick, low) so he added some.

Assuming it might be the battery, I tried to foot crank it.and there was almost no resistance whatsoever. The last time I tried, it would catch, then push the engine over after moderate pressure. Now, no notable pushback from the engine when footcranking.

On the surface, I would presume that the piston rings were toast, or some such. But I suppose it could be worse, like crank shaft or any other compression killer.

Piston rings are cheap, but it is fairly labor intensive to change them, and honestly there's no guarantee that is the problem.

The owner weighs over 250lbs, and he rides with another sizable (200lb+) person on the back.

I feel That even if I fixed it, the problem will occur again in another 1,500 kms. I suspect this machine just can't take the load - 50 ccs is probably just not enough.

Anyone else have thoughts on this?

[humanshield]

450lbs is an awful lot of work for a poor little 39mm piston

I suspect you're right

[acvw74]

Might check the valves...if it was overwarm from overweight poorly adjusted valves could cause a loss of compression.

[sparklepants]

Nov 27, 2016 11:18:49 GMT -5 acvw74 said:

Might check the valves...if it was overwarm from overweight poorly adjusted valves could cause a loss of compression.

Good call, I was thinking of checking that before I replaced the entire piston and housing. Thanks for the reminder.

[mrhooper]

Sounds like tuliped valves common with cheap metal they use for valves.

[humanshield]

To put a bit of perspective on this........

450lbs per 49cc's for the scooter would be the volumetric equivalent of my 3,300cc Van carrying 30,294 lbs !
Hint: You need a tractor / trailer to haul a load like that.

.....or your car trying to tow 13 Honda Civics behind it

[mrhooper]

Yes but you have to remember there is a gear redutions to counter that

[greggkinney]

my advice would b a 80cc bbk (47mm,72cc) with head and a naraku v2 touring cam. lap the valves and use the touring cam nit the a9. they claim more power down low and mid range.

[dexameth]

I found a friend's bike that was the same way, the compression that is. Older Yamaha big wheel or whatever, those enduro / dual sport bikes with the fat tires. Super cool bike, but when I kicked it nothing happened, and seemed like WAY too low of compression. Turned out to be the spark plug was loose. We think his dad did it to keep us from riding it. I ended up finger tightening it and rode it up and down the road for a few.

May sound weird, but have you checked the plug?

I'm also thinking maybe the piston burnt up ):

Whatever it is, do a BBK. I've been running a 50mm setup for at least 5000 miles now. It'll help them with acceleration most definitely. Also, when you do the BBK, get all new parts; find a kit that comes with all the parts. Head, piston, jug, you get the idea. Then, most parts will be new (hoping it's not the crank).