CM125C Starting problem

[Beeza-Geeza]

Purchased a non runner CM125C over a year ago
Would just about start on full choke and full throttle

Fully stripped down once in lockdown, engine completely taken apart and put back together with new bearings, piston/rings, gaskets, seal etc
Both bores were in very good condition so did not need a rebore

Finally after stripping, welding, painting and polishing, it is back together again

Still will only start on full choke and throttle!!
Timing marks adhered to during rebuild, tappets adjusted correctly, new twin coil, leads and plugs fitted

Purchased a generic 125cc carburettor which is a clone of the fitted Keihin one and that also needs full choke and throttle.

Question
If the CDI or pickup were at fault do they just fail or can they be intermittent?

Spent more than I will ever recoup but it has been fun during lockdown, be nice to actually get it working

Any advise would be appreciated
Regards
Ade from Nottingham

[Nobby the Bastard]

No. They either work or fail completely.

ignition coils can be fine when cold but fail when hot but they get hot because much higher voltage.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[Beeza-Geeza]

Also, it is not in the book what spark plug cap to use 1, 5 or 10K
using a 5K at present with a CR7HS medium heat plug

[Nobby the Bastard]

[stinkwheel]

Is it getting enough petrol through the tap?

Do you have the airbox and filter and exhaust on?
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Easy-X]

Replaced carb, what jetting did you go with? What the new carb came with or the factory spec?
____________________
Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Beeza-Geeza]

[Beeza-Geeza]

[Beeza-Geeza]

[jeffyjeff]

It does start with full choke and full throttle (oxymoron there!), suggesting a fuel system issue. Still, I would verify ignition timing is right and that spark advance is not way out of whack. The different spark plug cap resistance values are for radio noise suppression.
____________________
History shows again and again how nature points out the folly of men - BOC

[stinkwheel]

Many bikes wont run properly without the airbox in place.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Easy-X]

How's the compression?
____________________
Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[stinkwheel]

You've been painting... Do you have a good earth connections? Paint doesn';t conduct very well. So between the frame and engine, coils and frame and the battery frame earths. You often need to scrape some of your nice paint off behind the relevant fittings. I often fit an earth strap between the engine and the frame.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[GettinBetter]

[Robby]

You've had the whole thing apart, so there is a big list of things to consider. You also haven't told us how it runs when it starts - is it full throttle just to tick over, or will only run at high revs and no hope at low revs?

There are a lot of things in the rebuild that would make me ask questions, like why both fitting new pistons? New rings I can understand, but why replace the Honda originals for what I assume are pattern replacements. Did you measure the old and new piston rings to be sure they needed replacing, and the new ones are any better?

There is also no mention of cleaning up and lapping in the valves, which is a standard part of a top end refresh.

That, combined with my naturally pessimistic attitude, makes me wonder if you committed the ultimate sin of the top end rebuild, and didn't get all of the gasket surfaces spotlessly clean.

Check compression, check valve timing.

[Beeza-Geeza]

A few updates

Bore was good, pistons replaced because I damaged them, bar underneath them to remove the clutch put dents in the bottom (I know, should have used a wider plates)

Compression - I do not have anything to compare the results to but LHS 8 bar 116psi RHS 7.5 bar 109psi so a little different

Valves removed and were lapped in, medium then fine

Earthing - Battery to crankcase to frame with paint removed

It's got a lever choke and will start on full throttle on full choke, can get it to about half throttle but erratic then dies, not a beat with no choke

BUT, have found that the existing carb it came with which did look as old as the bike itself is not a Keihin, googling HL IC T W shows it is actually a Fujian Hualong Carburetor (never heard of it) and jetting is Main 90, Pilot 35 not the listed 98/38 (so probably never ever worked on this bike) "it was working, honest guv"

Replacement carb was listed for a CM125C 1982 (mine is a 1983) so wrongly assume it would have been jetted for the bike it was listed for, jets on that were main 90 and pilot unknown (can't find a thing on it)

So
Ordered the correct jets, not sure if 90 to 98 & 35 to 38 would cause sure a drastic failure but I'll soon find out

Thank you for all your comments and help
Ade

[bypass2]

with it being the wrong carb might not be allowing the right amount of petrol whats needed try adjusting the float heights to allow more petrol. what about the battery power is it good

[Beeza-Geeza]

[stinkwheel]

[GettinBetter]

[Robby]

Whether or not the compression readings and carb settings are bang on, they are in the right general area so it should work just fine.

I'm assuming engine timing is right to get the compression tester to give a reasonable figure. Any chance the valve clearances are wide open, so the valves are barely opening?

[Beeza-Geeza]

Well, I would not have believed it if I had been told it and not done it!!

New jets arrived today
Got back from work, swapped the 90 main for the 98, the 35 pilot for the 38

Tried on full choke and throttle, nothing, even worse than before

Tried with no choke and slight throttle as she bust into life
quick turn down on the tick-over screw and she's ticking away nicely

Would not have thought such a little change in jets would have such a massive affect

Still bits to go back on and then for a road test (after a MOT of course)

Cheers for all your help
Ade

[Nobby the Bastard]

That's a huge change in jetting.....
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[GettinBetter]