Can somebody help with my daughters CG125 again. My last post confirmed that problem was fixed and all was going well until it ran out of fuel. Now that could be coincidental.

So the bike will not restart , so I re-cleaned the Carb . Checked for a spark which ok . Now this is where I thought the plug should be wet with the amount of trying to start it that I was doing , but it was dry ( maybe a wiff of fuel) . The engine is turning over fast on the starter motor then , and this is difficult to describe, sort of stalls . Its almost as if a valve has touched the piston. Another go and the same thing happens . Normally about 4 cycles of the stater motor.

Hmmm I thought . I'll do a compression test as the fuel isn't being sucked in so it seems.

I got 60psi and I read I should have at least 100psi !

So off with its head and barrel to find no damage of any kind ! The barrel still has nice honing marks with no horizontal scratches . Piston seems in good shape and the rings which weren't stuck.

Valve seats seem good though I reground them in with valve grinding paste. They were both straight.

So , put the barrel and head etc back in place and with the Carb and exhaust off did another
compression test and got 80 Psi .

Carb and exhaust on for a restart but nothing apart from that odd stalling / stopping of the starter motor turning it over. Incidentally this didn't happen on any of the starter motor compression tests.

Baffled now . Its almost as if the timing is too far advanced firing when the piston is not at the correct spot with a little fuel , and so just burning it ? But it has a CDI and the pulse coils haven't been moved. Or is it just down to the low compression and so not sucking in enough fuel?

Any ideas would be gratefully received .

 

thought the plug should be wet with the amount of trying to start it that I was doing , but it was dry

I think this is where I'd start
Check the fuel line - is fuel getting into the carb ?
If it is, check the needle and seat in the float bowl to make sure you have fuel getting to the plug.
If it is, check the plug for fouling.
That's where I'd start.

 

Hey thanks for the reply. You're right . I can drain the float bowl and so I know I have fuel getting through to the carb. When I had it apart every jet was clean.

OK . Here is my latest theory. Every time i turn the engine over on the starter motor without the HT lead connected ( like when compression testing ) I dont get any ' stalling ' where the starter motor sort of stops. So I think that the fuel being sucked in, is being ignited to early on the up stroke and just burning off . That would explain why the stater motor stalls as the piston is being forced down the barrel at the wrong angular point. And also the reason why I had a back fire when I poured some fuel down the bore. The only reason that would happen on a cold engine is if the spark fired off with the exhaust valve open , and I now know the valve and seat are good.

The pulse coil is giving me 240 Ohms and that cant be moved. The rotor ( with the reluctor ) of the generator is fixed via a wood-rough key,so that cant move .

There's no way to check a CDI except by replacing it . So ebay here I come ?

Another test will be to turn it over on full choke with no HT lead and see if I get a wet plug ? And then again you can often convince yourself you know what it is so feel free to criticize

 

Do you know at what point the spark plug fires? Take the plug out, hold it near the cylinder barrel with insulated pliers, so you can see the plug fire and once it sparks check the timing marks to see if they are where they should be. There should be a TDC mark on the rotor. Set the engine on TDC on the compression stroke (place finger in plug hole and feel for air pressure whilst turning the engine over to ascertain this) then put a pencil or similar into the plug hole to make sure the piston is at the top end of its travel. It may be that the ignition is advanced as you suggest and you may need to adjust the position of the ignition trigger unit. If your bike has the old contact breakers then you can turn the backplate to achieve the correct firing position of the points. Keep us up to date on how you get on.

 

Well , a short update.

I think , I know I've found the problem . While waiting for the CDI I almost forgot my theory behind the reason for changing it , so when the wrong one turned up I thought I'd better think of something else.

With the original CDI in place , I was able to use a strobe gun to check the timing and to my surprise the timing marks lined up!

Then I it dawned on me that if the flywheel had slipped a few degrees with a woodruff key failure , that would advance the timing .

AND.. that is what has happened ! I took off the flywheel to find it had slipped over the key , which only protruded about 0.75mm . It was 90° advanced . SO waiting for a new flywheel with a undamaged slot , and key to fix the problem .

 

That indeed is what the problem was. Another flywheel was fitted and after the fuel got through , the bike started up and ran normally .
So it was :
Stopped
Funny stalling noise on turning over
No fuel on plug
Got spark
Carb cleaned recleaned
CDI pulse coil not moved and resistance ok

"Highwaypatrols" help here
Strobe light showed timing OK as the marks lined up,at starter motor speed and so CDI was good
Checking timing marks on flywheel to position on piston with a top dead center tool ,
Problem found
"T" on flywheel was 90° advanced ( and so "F" )

Sounds easy now, but took me weeks and a brain wave about the stalling and how it was causing the fuel to be ignited and so consumed and with the piston no where near the top, stopping engine rotation .

Hope this summary helps people in the future