Misfiring and not revving 140

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Spicerack

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Alrighty. Here goes. This is for a Zongshen 140 kickstart only motor.

This is a bike I've got no history on, so don't know if it ever ran with this setup. Doesn't look like it's been changed about but you never know.

It runs terribly, popping and misfiring at idle with very occasional moment when it seems to run a bit better then carries on misfiring. It won't rev much at all above idle and tends to die when you try and rev.

Choke makes very little difference. Possibly a tiny bit better but still not good.
Cupping hand over air filter makes very little difference.
Changing pickup timing makes very little difference (I made it adjustable).
The timing light shows it is sparking at the "F" mark just advanced from TDC so about right?
Changed needle position and no discernible difference.
The only thing I haven't done is alter the mixture needle- 1.5 turns from bottom (factory setting).

Brand new NGK plug.
Carby cleaned and no problems.
Excellent compression- if kick start feel is to go by!
Valve clearances done.
No intake leaks.
Fresh fuel.
Cam timing is spot on the marks at TDC.
Earthing between engine, frame, cdi, coil etc is good.

The carby is one of those 22-26mm oval throat Mikuni's.
Main jet is 95, pilot jet is 15. I thought maybe a bit small?

So any ideas?

Symptoms of running stupidly lean?
Something wrong with the valve timing/seal?
Coil/CDI stuffed?
Stator problem?

Must admit I'm a bit stumped and am about to start buying new coil/cdi etc an go through the swapping process.
 
my guess is a blocked or partially blocked Pilot Jet, or Pilot jet port where the fuel flows into the carby infront of the slide
remove the jet and check all the little holes are open, including the ones across the jet.
then look in where the jet screws into, there should be a tinmy hole that goes through to the carbys' main bore
the #15 i have found to be a bit small too, i usually find a #20 will fix any hesitation problems

with it running ok occasionally, i am guessing it has a vacuum leak somewhere as well.
maybe a warped carby mounting face ?, a warped fuel bowl? a leaking insulator to manifold o ring ? or a leaking gasket between the manifold and head.
start the bike and spray wd40/degreaser etc on all the carby to manifold joins, manifold to head etc
if the idle changes ion any way then there is the leak.


once the pilot jet is clean and working nice, eg no hesitation. then you'll need to adjust the mixture screw to find it's sweet spot.

then there are the valve clearances that would need checking/adjusting too
 
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Thanks for the reply. All jets and passages are clear in the carby. Been through it all and I've done it a few times- I'm not a teenage noob! :)

There doesn't appear to be an intake leak- done the spray test.

As mentoned above, valve clearances were done and have been double checked.
 
ok with the carby cleaning,
did you take the needle jet out and clean the tiny holes across it?
were the ports all blown out with compressed air, and you checked that air was coming out through the holes in the base of the carby ?
did you take the mixture screw out while you were blowing it out?
have you checked the float level?
is the throttle cable good ? eg no cracks in the outer
is there a good boot sealing the cable to the top of the carby?
what notch is the needle clip in?


i'd start by fitting a 20 Pilot Jet, and adjust the mixture screw.


is the exhaust gasket new? the one where the pipe meets the head ?
if not fit a copper crush ring there and make sure it's tight.
that should stop the popping

have you checked the packing in the muffler ?
 
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my67xr said:
ok with the carby cleaning,
did you take the needle jet out and clean the tiny holes across it? yup
were the ports all blown out with compressed air, and you checked that air was coming out through the holes in the base of the carby ? yup
did you take the mixture screw out while you were blowing it out? yup
have you checked the float level? yup
is the throttle cable good ? eg no cracks in the outer all good
is there a good boot sealing the cable to the top of the carby? yup
what notch is the needle clip in? tried every notch with no difference


i'd start by fitting a 20 Pilot Jet, and adjust the mixture screw.
yup, I'll try to find on tomorrow if I can escape from work!

is the exhaust gasket new? the one where the pipe meets the head ? yup
if not fit a copper crush ring there and make sure it's tight.
that should stop the popping

have you checked the packing in the muffler ? looks fine for a pretty much straight through muffler
Click to expand...

See above! :)
 
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If you've checked everything that you've stated the one thing you haven't stated like Craig said is I'd be going with an exhaust leak it can cause the bike to carry on like a **** and run lean. Take the exhaust off, check for a gasket/leak, even sometimes you can just do up the bolts tighter and it will crush the gasket a tad more to seal properly. I've seen china bikes not have a gasket before.
 
Thanks, I'll double check the exhaust gasket. It doesn't appear to be leaking but worth double checking anyway.

Not sure what's going on Craig- I answered all your questions within the quotation- it comes up on my screen. But basically yes I checked them all, all jets, passages etc were clear (carby cleaner, wired and compressed air), tried every position for the needle etc. I'll try to grab another pilot jet and see how I go!

Thanks for the responses guys.
 
ok, it's now all showing up since you've edited it


once you fit the new Pilot jet, adjust the mixture screw
if you need instructions let me know and i'll post them here

then you can start troubleshooting it a bit more

have a look at this thread, it explains how to check your stator

if you want to know the measurements on your ignition coil,
post up a pic to show which one you have and i can post the resistance measurements so you can check it before you spend money buying more parts that you may not need


have you pulled apart all the electrical plugs/socket and sprayed them with wd40 or contact cleaner to check they are all good ?

in your original post you asked for symptions of it running lean
if it is, it will have rev hang, eg start and run the bike, rev it a bit and the revs will stay high for a long time then maybe drop back
 
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My67xr- Not sure what happened with the edit- I went back in and bolded all my answers to make them stand out more!

Thanks again- I'll be having a look tonight (hopefully- if I can get another jet).

You're right with the lean stuff- this one is misfiring constantly at idle and not revving much at all. Might not be lean- unless it's crazy lean. Certainly not rich as there's no smoke puffing out at all.

I sohuld be fine with adjusting the mixture screw- I'm used to playing with 4cyl bikes so this should be easier, right? ;)


I'll check the stator- I'll have a look at the volts it's generating as well. It's a YX150 6 pole style stator.

All the contacts are nice and clean and earths all good- between motor, stator earth, cdi earth(s), coil and frame.

Thanks again for the advice. I'll update this thread with the fix when I get there (see, think positive!).
 
the Mikuni copy 22/26mm carbys are a bit painful to adjust the mixture on
a carby spinner will help you a bit

and checking the stator with a multimeter, you will need to check the Resistance of the coils (Ohms)
 
my67xr said:
the Mikuni copy 22/26mm carbys are a bit painful to adjust the mixture on
a carby spinner will help you a bit

and checking the stator with a multimeter, you will need to check the Resistance of the coils (Ohms)
Click to expand...

Must admit I've never seen one of those spinners before! Easy enough to adjust the mixture with my special home made screwdriver anyway.

Yeah I had a look at your sticky on how to test the stators- useful stuff there!

I ended up buying a cdi, coil etc kit so at least I'll have spares if they're not the problem. I also like problem solving by swapping parts- much easier. Just have to wait a week for it to turn up now...
 
She lives! All fixed and goes really well.

So what was the solution? Well I wish I knew! It's one of these though:

Head off, valves not leaking, clearances done again for goods measure.

Took all electrics off, re-cleaned all contacts and checked all components- all measured up fine. Even took plug cap off.

Took carby apart again. All good. the pilot jet says 15 but a .35mm jet drill passes through our so maybe someone plaited with it. A 20 jet has been ordered. Had anyone tried to buy one in Perth recently? ?? "Oh yeah mate they're forged from the finest Elvish high tensile brass that they use in their unicorn bridles so that will be $20 and will take two weeks to get here" ffs!

Where was I? Oh yeah when I saw the pilot jet was larger I turned the mixture jet out another half turn so it's now 2 turns out.

Put it all back together and she started first kick. Runs really well as is.

So the fix was one or more of the above!

Thanks for the help guys. Always good to bounce ideas around!
 
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yeah the .35mm hole is very close to a #20 jet.

and yeah leaning it off would have been enough to help it.
 
my67xr said:
yeah the .35mm hole is very close to a #20 jet.

and yeah leaning it off would have been enough to help it.
Click to expand...

You did mention that before, didn't you? Perhaps I should have just tried that first- but never mind, at least everything has been double checked now!:lol_hitting:
 
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