Honda CG125 rough idle when warm

[CharlieShimm]

Hi guys,

Hoping you can help me here.

I’ve got a Honda CG125 98, completely stripped back with aftermarket exhaust and cone filter etc. The bike idles ok when cold after starting on choke and warming up and teasing off the choke. No issues opening up the throttle. But after a short 20 minute trip, the idle seems to get worse. It gets very inconsistent and sounds like the engine is struggling to turn over and will occasionally cut out.

The plug is fine, it has a new tank with no obvious fuel delivery issues. I’ve adjusted the fuel mixture a couple of times when warm and have ended up with what seems like a nice idle, but after a couple of days it seems to end up rough again intermittently. I’ve not inspected the carb yet.

I stopped at the petrol station today when the bike was warm and had a bit of kick back when starting the bike, along with a couple of popping noises (more of a large water drop type noise) before the bike started, but after that it was idling and running perfectly for the short distance to my house - Any idea what the could mean?

More than anything I’m trying to understand what would cause the idle to go rough, after riding for a short period.

Any help appreciated cheers.

[WD Forte]

Wild guess If its a CDI ignition type and not carb related.
I'd been looking at the CDI source coil and/or pulser to see if it's breaking down
as it gets hot
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[Easy-X]

Yeah, it sounds more timing related with the kickback. Valve clearances would just be engine rough after 5mins or so.
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[Nobby the Bastard]

Put the standard air box and filter back on it
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[Robby]

Lots of things that could be the problem. Getting a small-engined old bike to idle properly can be an utter pain in the arse, particularly when they will run just fine everywhere else.

If it is carb related, then it will probably sort itself out. The bike is running and in use, petrol is moving through it and petrol does a good job of cleaning carbs. I wouldn't disturb it in case you introduce another problem by messing with it. I would make sure the mixture screw is set close to where it is meant to be in the workshop manual - just in case an over-rich idle mixture is compensating for an air leak in the inlet rubber.

Then it's a case of doing a full service to get to a good baseline for diagnosis, and checking/cleaning anywhere that the wiring loom earths onto the frame and battery. Make sure the valve clearances are right while you're servicing it.

Using the standard airbox and filter is a good idea. Pod filters fuck up fuelling.

[CharlieShimm]

Thanks guys.

I checked out the valve clearance the other day and it was way tight, like seemingly no clearance at all. So I adjusted to the correct clearance (maybe a little on the loose side) and I the idle appears to be much better now when warm. Although there’s definitely tapping coming from the top of the engine now where as there wasn’t before, is that a big problem? I heard a bit of tapping isn’t a bad thing.

Could valve clearance have been the issue or is this just coincidence?

[Robby]

The tapping is good, it means there is some clearance between the pushrod and rocker.

[davebike]

CG is a clatery engine fine one that not and the Valves are too tight !
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