125cc Zongshen revs dropping when hot

maqqx · 05-03-2020, 07:11 AM

Hello everyone!

Last year I've bought a second-hand Chinese 125 motorbike, which is basically a clone of 1970s Honda CG125 (156 FMI pushrod engine). It was running fine, until recently.
Couple of days ago I noticed the engine revs dropping after I ride it for a while and then put it at idle. Sometimes the revs drop so much that the engine dies down. In such a case I can restart it with a single kick after just 10 seconds.

I've checked the valve clearence and it seems OK - 0.08 in and 0,10 out (metric)
I've cleaned the carb (Chikuni VM 22)
I've checked the fuel filter and all the fuel lines
I've repleaced the spark plug, just in case
I've even checked the engine's compression - its 142 psi when hot, a bit in the lower range but still OK I guess

I thought it might have been starving on fuel so I check the fuel level in the bowl and it was holding at 3 mm below the edge of the carb body.

I've also checked the fuel-air mixture at idle and looks OK to me - the spark plug electrodes are brown ("coffee with milk" color).

There's no electronics except for the ignition module, which is located under the seat and does not get hot.

It really buggers me that I can't find a solution. Any help would be greatly appreciated.
And sorry for my clumsy English - if anything I wrote is hard to understand I'll try to re-word it.

NzBrakelathes · 05-03-2020, 07:22 AM

Air leaks? Use brake clean spray around carb and intake
The rubber intakes do tend to leak air and also the round slide in carb wears n plays funky idle etc

maqqx · 05-03-2020, 07:55 AM

I'll check the intake for leaks, thank you.
As for the slide, it's almost new, so I think it's a bit too soon for it to be worn out, but I'll give it a look too.

Stoo2k · 05-03-2020, 12:12 PM

Does it not have a CDI unit?

maqqx · 05-03-2020, 12:24 PM

I think so.
It has a module called "electronic ignition module" which is the only piece of eletronics on this bike.

Stoo2k · 05-03-2020, 04:58 PM

Yeah, that the fella. I would think if what you're saying is correct that this is breaking down under load when hot (heat = higher electrical resistance)
You can get good unrestrictive units for only a few pounds.

maqqx · 05-03-2020, 06:05 PM

Yeah but now to tell those unrestricted units from the rest?
There are several model available, but most of them lacking any description.

Stoo2k · 05-03-2020, 06:46 PM

Quote:

Originally Posted by maqqx

Yeah but now to tell those unrestricted units from the rest?
There are several model available, but most of them lacking any description.

True but a bit of internet searching will have it sorted soon enough.

maqqx · 05-04-2020, 06:52 AM

I did some research and from what I see there are no resctrictions in these modules. The only difference may be ignition type (AC or DC) and build quality.
Some of them are marketed as "tuning/racing cdi modules", but all they do is changing ignition timing a bit.

maqqx · 05-05-2020, 09:18 AM

Are we online again?

There's one thing I've missed or maybe I should say "failed to measure correctly": the fuel level in my Chikuni is 14 milimeters below the rub of the carb body.

maqqx · 05-28-2020, 02:14 PM

Three weeks later and I'm still struggling.
I've corrected the fuel level (now it's 3 mm from the carb body) and set the air/fuel mixture again. I've also drained and rinsed the inside of my fuel tank cause there was some rust.
Judging by the color of my spark plug the mixture is correct at every rpm range.
Yet, the engine is still stalling after a few km ride.
I'm out of ideas.

maqqx · 05-29-2020, 01:25 PM

Is that possible my mixture is waaay too rich and I'm just flooding the engine?

Megadan · 05-29-2020, 02:28 PM

Poor idle and stalling when the motor is hot, but otherwise runs fine if you are riding?

That condition usually indicates a too rich idle mixture. A very common mistake many newbies to carb tuning make is that they set the idle mixture on a cold engine. As an engine heats up the fuel atmoizes in the cylinder better, thus requiring less fuel, that is the purpose of the choke for a cold engine, to richen the idle until the motor warms up.

If this is the scenario, try turning in the idle mixture about a quarter to half a turn and see if it changes. Ideally you should ride the bike for several minutes until it is fully up to temp, and then adjust the idle mixture. It's tricky to do with air cooled bikes because they are prone to overheat while just sitting there running, so it is usually a good idea to have a good strong fan blowing on the motor.

If the idle mixture is set properly, then when you start up a cold engine and the air temps are above 10 Celcius (50F) it should start with minimal to no choke, but might idle low. Once the bike gets up to temp the idle will normilize. If you start it cold and it idles perfectly, especially with no choke, that is a good indicator.

Goob · 05-29-2020, 08:38 PM

Quote:

Originally Posted by Megadan

Poor idle and stalling when the motor is hot, but otherwise runs fine if you are riding?

That condition usually indicates a too rich idle mixture. A very common mistake many newbies to carb tuning make is that they set the idle mixture on a cold engine. As an engine heats up the fuel atmoizes in the cylinder better, thus requiring less fuel, that is the purpose of the choke for a cold engine, to richen the idle until the motor warms up.

If this is the scenario, try turning in the idle mixture about a quarter to half a turn and see if it changes. Ideally you should ride the bike for several minutes until it is fully up to temp, and then adjust the idle mixture. It's tricky to do with air cooled bikes because they are prone to overheat while just sitting there running, so it is usually a good idea to have a good strong fan blowing on the motor.

If the idle mixture is set properly, then when you start up a cold engine and the air temps are above 10 Celcius (50F) it should start with minimal to no choke, but might idle low. Once the bike gets up to temp the idle will normilize. If you start it cold and it idles perfectly, especially with no choke, that is a good indicator.

Megadan...That is what I was going to say! You stole my thunder (not a surprise). I had this issue when tuning a carb. Ran great until it got fully warmed up, then would idle lower and lower, and just die sometimes. Increased the size of the pilot jet (mine is on the air side) to lean it up. Just wasn't getting enough air. I went from a 22.5 to a 30 on a VM26-606. If you have a VM26 or similar Flange mount that has the Pilot jet on the fuel side...make the jet smaller I suppose so less fuel, more air.

maqqx · 05-30-2020, 10:17 AM

I have a Chikuni VM22 with the pilot jet on the fuel side.
Every time I tried to set the idle mixture the engine was warm (I think right in the operating range) but I'm going to give it another try.

And now for something completely different

What size of the main jet would you guys recommend for a 125 cc? I have a #105 installed and at W.O.T the bike is slightly jerking sometimes. Too rich?

My elevation is 1000 ft.

05-28-2020, 02:14 PM	#11
maqqx Join Date: May 2020 Location: EU Posts: 21	Zongshen 125 - poor idling and stalling when hot Three weeks later and I'm still struggling. I've corrected the fuel level (now it's 3 mm from the carb body) and set the air/fuel mixture again. I've also drained and rinsed the inside of my fuel tank cause there was some rust. Judging by the color of my spark plug the mixture is correct at every rpm range. Yet, the engine is still stalling after a few km ride. I'm out of ideas. Last edited by maqqx; 05-28-2020 at 03:06 PM.
	0

05-29-2020, 02:28 PM	#13
Megadan Join Date: Apr 2017 Location: Omaha, NE Posts: 8,030	Poor idle and stalling when the motor is hot, but otherwise runs fine if you are riding? That condition usually indicates a too rich idle mixture. A very common mistake many newbies to carb tuning make is that they set the idle mixture on a cold engine. As an engine heats up the fuel atmoizes in the cylinder better, thus requiring less fuel, that is the purpose of the choke for a cold engine, to richen the idle until the motor warms up. If this is the scenario, try turning in the idle mixture about a quarter to half a turn and see if it changes. Ideally you should ride the bike for several minutes until it is fully up to temp, and then adjust the idle mixture. It's tricky to do with air cooled bikes because they are prone to overheat while just sitting there running, so it is usually a good idea to have a good strong fan blowing on the motor. If the idle mixture is set properly, then when you start up a cold engine and the air temps are above 10 Celcius (50F) it should start with minimal to no choke, but might idle low. Once the bike gets up to temp the idle will normilize. If you start it cold and it idles perfectly, especially with no choke, that is a good indicator. __________________ Hawk Information and Resource guide: http://www.chinariders.net/showthread.php?t=20331 2018 Hawk 250 - Full Mod list here. http://www.chinariders.net/showpost....62&postcount=1 2024 Royal Enfield Shotgun 650 https://chinariders.net/showthread.php?t=34124
	2

05-03-2020, 07:11 AM	#1
maqqx Join Date: May 2020 Location: EU Posts: 21	125cc Zongshen - poor idling and stalling when hot Hello everyone! Last year I've bought a second-hand Chinese 125 motorbike, which is basically a clone of 1970s Honda CG125 (156 FMI pushrod engine). It was running fine, until recently. Couple of days ago I noticed the engine revs dropping after I ride it for a while and then put it at idle. Sometimes the revs drop so much that the engine dies down. In such a case I can restart it with a single kick after just 10 seconds. I've checked the valve clearence and it seems OK - 0.08 in and 0,10 out (metric) I've cleaned the carb (Chikuni VM 22) I've checked the fuel filter and all the fuel lines I've repleaced the spark plug, just in case I've even checked the engine's compression - its 142 psi when hot, a bit in the lower range but still OK I guess I thought it might have been starving on fuel so I check the fuel level in the bowl and it was holding at 3 mm below the edge of the carb body. I've also checked the fuel-air mixture at idle and looks OK to me - the spark plug electrodes are brown ("coffee with milk" color). There's no electronics except for the ignition module, which is located under the seat and does not get hot. It really buggers me that I can't find a solution. Any help would be greatly appreciated. And sorry for my clumsy English - if anything I wrote is hard to understand I'll try to re-word it. Last edited by maqqx; 05-28-2020 at 02:06 PM.
	0

05-03-2020, 07:22 AM	#2
NzBrakelathes Join Date: Jun 2016 Location: Hell Posts: 2,408	Air leaks? Use brake clean spray around carb and intake The rubber intakes do tend to leak air and also the round slide in carb wears n plays funky idle etc
	1

05-03-2020, 07:55 AM	#3
maqqx Join Date: May 2020 Location: EU Posts: 21	I'll check the intake for leaks, thank you. As for the slide, it's almost new, so I think it's a bit too soon for it to be worn out, but I'll give it a look too.
	0

05-03-2020, 12:12 PM	#4
Stoo2k Join Date: May 2020 Location: Plymouth Posts: 8	Does it not have a CDI unit?
	0

05-03-2020, 12:24 PM	#5
maqqx Join Date: May 2020 Location: EU Posts: 21	I think so. It has a module called "electronic ignition module" which is the only piece of eletronics on this bike.
	0

05-03-2020, 04:58 PM	#6
Stoo2k Join Date: May 2020 Location: Plymouth Posts: 8	Yeah, that the fella. I would think if what you're saying is correct that this is breaking down under load when hot (heat = higher electrical resistance) You can get good unrestrictive units for only a few pounds.
	1

05-03-2020, 06:05 PM	#7
maqqx Join Date: May 2020 Location: EU Posts: 21	Yeah but now to tell those unrestricted units from the rest? There are several model available, but most of them lacking any description.
	0

05-04-2020, 06:52 AM	#9
maqqx Join Date: May 2020 Location: EU Posts: 21	I did some research and from what I see there are no resctrictions in these modules. The only difference may be ignition type (AC or DC) and build quality. Some of them are marketed as "tuning/racing cdi modules", but all they do is changing ignition timing a bit.
	0

05-05-2020, 09:18 AM	#10
maqqx Join Date: May 2020 Location: EU Posts: 21	Are we online again? There's one thing I've missed or maybe I should say "failed to measure correctly": the fuel level in my Chikuni is 14 milimeters below the rub of the carb body.
	0

05-29-2020, 01:25 PM	#12
maqqx Join Date: May 2020 Location: EU Posts: 21	Is that possible my mixture is waaay too rich and I'm just flooding the engine?
	0

05-30-2020, 10:17 AM	#15
maqqx Join Date: May 2020 Location: EU Posts: 21	I have a Chikuni VM22 with the pilot jet on the fuel side. Every time I tried to set the idle mixture the engine was warm (I think right in the operating range) but I'm going to give it another try. And now for something completely different What size of the main jet would you guys recommend for a 125 cc? I have a #105 installed and at W.O.T the bike is slightly jerking sometimes. Too rich? My elevation is 1000 ft.
	0