Correct Jet Sizes for GPX Jialing 125 and Mikuni 26mm

[JHZ]

Hey guys,

Just installed this engine and this carburettor into my MadAss. Seems to work pretty nicely, but it's idling way too high. I've tried adjusting the idle screw, but it seems to make no difference so I'm guessing my jettings all wrong. Can anyone recommend a pilot and main jet size for the combination?

Cheers,
JHZ

 

[DvDRip]

check that the throttle cable it properly seated in the throttle slide (where it goes in the top of the carby) and that it's around the right way

 

[JHZ]

Will do mate. Are you asking b/c you believe the jetting should be right and it must be something else? Or is it just another common cause that I should check first?

 

[Cactus Jack]

He's telling you that the slide should be able to drop to the bottom of the venturi far enough to cut the airflow right off and stall the engine ... The slide is actually a variable air CHOKE ... IF you can't drop the idle speed right off there's something mechanically wrong with the carburetor ... jets only do what air rushing past them tells them to do ...

On one side of the slide bore wall there's a small pin sticking out ... It HAS to be aligned with a groove that goes all the way up the side of the slide ... If it isn't , the slide gets hung up on the pin and held open which accounts for the high idle speed that won't respond to turns of the idle screw ...

 

[JHZ]

That makes sense, I'll check tonight when I get home. I'd still like some confirmation of the jet sizes so if anyone knows what they should be please post them and I'll check to see what I've got at the same time.

Cheers,
JHZ

 

[thump*140]

The other thing to check for, is that you havent installed the slide into the carby ass-about... the slide should have an angled cutaway facing towards the intake side... ie: the angled cutout should be facing towards the air filter. This will also have the same effect you are describing, but it will be utterly screaming if you do get it started with the slide in the wrong way round...
high idle due to cable being dislodged, or adjusted too far, is a very common thing to do..
easiest way to check if this is the case, is with the engine turned off, turn the throttle, then let it go altogether.. you should hear a metallic clunk from the carby, signifying the slide is physically hitting the idle stop. if it doesnt make this noise, the cable is adjusted too tight for whatever reason, and isnt allowing the carby slide to close up to it's adjusted point.

 

[thump*140]

oh and starting jet sizes, should be around a 29 pilot jet, #92 main jet, and the needle should be on the middle clip...
That is just a starting point... and given you ride the madass on the road, with constant throttle settings, a slightly larger main jet (#95-97) may be the go... just to make sure you dont run it too lean...
try the 92 and go from there.

 

[headsmess]

the slide should have an angled cutaway facing towards the intake side... ie: the angled cutout should be facing TOWARDS the air filter.

just double check what you wrote

other than that, everything is correct releasing throttle quick should produce a nice clunk

also, you should have a few degrees of slack on the throttle before you feel the pressure of the slide spring... more than likely your stock cable is a tad short and will need replacing... or adjusting, if thats possible... some are, some arent...

 

[thump*140]

just double check what you wrote

other than that, everything is correct releasing throttle quick should produce a nice clunk

also, you should have a few degrees of slack on the throttle before you feel the pressure of the slide spring... more than likely your stock cable is a tad short and will need replacing... or adjusting, if thats possible... some are, some arent...

My bad, didnt describe that overly well... I meant the "INTAKE" side of the CARBY itself... which is the air filter side... the OUTPUT side of the carby is closest to the engine and intake manifold... ie: air goes in INTAKE side, fuel/air mix goes out OUTPUT side..

 

[JHZ]

Checked the cable and the slide, both good, even tried disconnecting the throttle cable entirely to make sure, but it's not the problem.

While I was at it, I also had a look at the jet sizes, #95 main jet and #15 pilot jet. I'm guessing the pilot jet is way too big (small?) and that's the problem.

 

[Brocko]

As Cactus stated before mate the pilot jet wouldnt be the problem.. I had the same problem as you after cleaning my carb and it was high idling because the cable wasnt properly seated in the top of the carby. Make sure the cable runs in nice and straight directly down into the carb. Also take your air filter off and have a look straight into the carb.. The gap should be very small - thus reducing revs. Try adjusting your idle screw all the way out and if this doesnt do anything it must be one of the problems the others said, short cable etc..

Check everything twice..

 

[JHZ]

Well I changed the pilot jet this morning to a #20 and the idle dropped to a more reasonable level which is great. But I did notice that adjusting the idle screw still doesn't do anything, which does seem to indicate that there's a problem with the idle adjust as you've all been saying.

I'm not sure what the problem is though, there is a definite clunking sound when I let go of the throttle so it seems like the slide is hitting the idle stop and I've noticed that the slide does move up and down when I adjust the screw, but even with the screw turned all the way out, the gap at the bottom is still present, so maybe there's a problem with the slide. I'll check it out further tonight and see.

 

[headsmess]

another thing...what type of carby? or, choke, moire importantly?

the "flap" chokes cut air and tend to kill the engine, whereas the "piston" chokes make the engine rev out at idle.... they bypass the slide completely , adding extra air(and fuel), causing a "high idle"

if its a piston type choke, you want it DOWN for running.... just an idle thought...

ok, its a genuine mikuni(fake) 26.... choke lever down for running, up for starting...

 

[JHZ]

It's a "genuine" Mikuni 26 with the lever choke. I didn't realise that it's a seperate circuit, maybe I've got it in the wrong position. Part of me hopes that's it because it'll fix the problem, but it'll also mean I've been a complete douchbag the whole time.

 

[thump*140]

It's a "genuine" Mikuni 26 with the lever choke. I didn't realise that it's a seperate circuit, maybe I've got it in the wrong position. Part of me hopes that's it because it'll fix the problem, but it'll also mean I've been a complete douchbag the whole time.

dont feel too bad mate, if it is a case of 'choke on'...
i started rejetting my blitz before i figured out which way the choke worked... forgiveable for a noob, but i started playing with bikes in 1982...

 

[JHZ]

Yep that was it. :#)

Thanks for all the help guys!

JHZ