Won't run when hot?

**grotto** · 21st November 2009, 16:00

I've bought a 50cc two stroke which won't run when it gets hot.
Loses power as it warms up, then when hot won't run.
Does this signify anything specific to anyone?
cheers

**Sidewinder** · 21st November 2009, 16:08

Originally Posted by grotto

I've bought a 50cc two stroke which won't run when it gets hot.
Loses power as it warms up, then when hot won't run.
Does this signify anything specific to anyone?
cheers

is it brand new? is it chinsee?
model brand, specs and then maybe answers

**grotto** · 21st November 2009, 16:38

Yamaha TY50, 1978ish

**Teflon** · 21st November 2009, 22:52

coil might be fucked

**grotto** · 22nd November 2009, 09:22

The coil has one wire in which goes to both the kill switch and into the engine (to the stator?), and the HT wire to the spark plug.
Bike is not running a battery as no lights are connected.

**grotto** · 22nd November 2009, 11:53

did a search on coil testing...tested the resistance of primary secondary, both seemed within limits.
I thought about putting a coil of another bike on to see if that fixed problem, but my other Yamaha is a newer ('98) 100 cc, and the coil has a different part number on it. Can I swap them, or will it damage the coil?

**F5 Dave** · 23rd November 2009, 14:20

if they are both CDI type will be close enough. . . . Actually it probably is a points type, so visaversa.

More likely to be stator coil, but typically failures tend to cause the bike to stop sparking. This should be easy to test when hot. Sounds like the bike gets worse rather than stops dead.

OK so when hot it isn't seizing right. Lever still spins easily. Best to ask, was thinking an oiling fault.

Suspect airfilter clogged & more likely exhaust clogged from years of two stroke oil build up. Plenty of info on web of cleaning out pipes.

Check choke isn't sticking.

**grotto** · 23rd November 2009, 16:37

I've found another bike with the same coil, and tried that, but am not getting any spark.

I also did a compression test (cold) and it's only getting 85 psi (my other small 2 strokes hit over 100 psi).

I'm guessing it needs new rings, and maybe a piston, but is 85 psi enough to run?

I think I'll do the "no spark" problem through the electrical forum.

**soundbeltfarm** · 23rd November 2009, 18:17

not sure about your bike but have a look on thumper talk .
or find out what resistence your stator should be.
my boy has a ktm 50 and when hot did same thing and it was the stator .
resistence was bang on when cold but crap when hot.
when you check your stator make sure you unplug it from coil and take the spark plug plug off.
not sure on the compression but we run 125 - 135 with a new piston and rings .

**grotto** · 24th November 2009, 17:17

I've checked a couple of other 2 strokes for compression, and cold they are over 100, so I'm probably up for a top end rebuild.

Major problem is I don't get any spark, so maybe the stator coils are the real issue.

I've just got to track down a suitable Flywheel Puller to get to the stator.
Tried the bike shop but they're $58 and not in stock.

May be able to get one from a scooter shop for $20, but am not sure if it will fit

**F5 Dave** · 25th November 2009, 13:10

puller will be a std japanese one which will fit 90% of jap twostrokes. They usually aren't that expensive. Motion pro sell a good one, hmm here you go. any bike shop.

Scooter may be different if for euro brand scooters, but if for Jap should the the right one.

27mm one;
http://www.sudco.com/motionpro.html#flywheel

**oyster** · 26th November 2009, 10:25

Two strokes are an engine and a pump as far as their mechanical layout goes. And for the 'pump" to work it need good primary compression. this is effected by the fit in the bore. When it's cold the clearance is small and the oil visosity high so it seals well. When hot, the sealing of the "pump" is poor.
We used to have few really mouldy worn ot two strokes to ride on Oreti beach and the answer for them was to put in hot plug, carry a spare and run on high viscosity (50wt) oil at 16 or even 12 to 1.
My square four race bike with old chain saws in it runs great at 12 to 1. At 30 to 1 it won't even start at all. Slightly different reason, it has a rotary (not disc) valve induction which by nature has high clearance.

**warewolf** · 26th November 2009, 11:23

May not be relevant (2t vs 4t) but the '90s Triumphs had a similar symptom, would run fine when cold then as they heated up got worse and stopped. After waiting on the side of the road they'd cool down and run again. The fault was the ignition timing pick-up coil - resistance was fine when cold but not when hot.

**glegge** · 26th November 2009, 12:08

If you have an old 'in line' timing light, you can start and run the bike, then watch the light. if it stops when the engine conks out, you have a spark issue.
an inductance pickup type light may be no good, as there may be enough energy to fire the light off - even though there is not enough for a spark.
these in line lights used to be really cheep. not sure if they are still around though.

**grotto** · 27th November 2009, 18:32

Thanks for the link to sudco.
Knowing what size puller I need has meant I can track one down, should get it Monday.
I've got an old in line timing light, and will use that (once I get a spark).

cheers