Carb Tuning Help, please!!

[Mexicano]

O.K. this is the story: I use a KOSO 30 m.m. with 105 main jet and 37 pilot jet, at full throttle the scooter feels great but at 1/4 throttle feels and sound like shit..it is like when you are using a BIG main jet, I have been playing around with different needles and clip position with not better results.

If I go for an smaller main jet it doesn't seems to get much better, maybe just a little BUT, if I do so, full throttle gets bad.

What can be the problem?..it sounds to me like a bad carb tuning...this has ALWAYS been a problem to me: I hate carbs!!!!!

Please help.

Thanks,

 

[xzburnoutzx]

I suggest you try to set the carb without any airfilter, because all filters are way different. I suggest to put the std needle in the 1 spot higher from the middle, then a 115 jet with a 40 pilot. Although I havent messed with a 30mm:S...28mm max at the moment and it worked like that very well. (without air filter). And the mixture screw 1.5 half turns from the bottom end. Oh end let you motor heat up before adjusting anything.

Greets, Lempzu...

 

[DRXRACING]

O.K. this is the story: I use a KOSO 30 m.m. with 105 main jet and 37 pilot jet, at full throttle the scooter feels great but at 1/4 throttle feels and sound like shit..it is like when you are using a BIG main jet, I have been playing around with different needles and clip position with not better results.

If I go for an smaller main jet it doesn seem to get much better, maybe just a little BUT, if I do so, full throttle gets bad.

What can be the problem?..it sounds to me like a bad carb tuning...this has ALWAYS been a problem to me: I hate carbs!!!!!

Please help.

Thanks,

As with all KOSO carbs we had ran in the past, stock needle leanest position and pilot smallest you can get. I suspect the carb needle could be changed to fix this problem. The OKO or Kehin Knock offs are much easier to tune, especially the power jets.

 

[Mexicano]

I suggest you try to set the carb without any airfilter, because all filters are way different. I suggest to put the std needle in the 1 spot higher from the middle, then a 115 jet with a 40 pilot. Although I havent messed with a 30mm:S...28mm max at the moment and it worked like that very well. (without air filter). And the mixture screw 1.5 half turns from the bottom end. Oh end let you motor heat up before adjusting anything.

Greets, Lempzu...

Thanks!

My needle has 5 slots, do you mean to set the clip it in the second slot from top to bottom?

I have tryed before with 115 main jet and feels like to much gas!, even with out any air filter!

What is the effect with BIGGER pilot jet?

Regards,

 

[Mexicano]

As with all KOSO carbs we had ran in the past, stock needle leanest position and pilot smallest you can get. I suspect the carb needle could be changed to fix this problem. The OKO or Kehin Knock offs are much easier to tune, especially the power jets.

This sound good because the engine feels like to rich now, at least at iddle, top end is good!!

So, the smallest pilot jet, the less fuel for the iddle, right???

And about the neddle, I got a set of 4 different neddles from scooter center, and switched to the fatest trying to lean the mixture.

Do you think I'm going in the right direction trying to make it lean just for iddle?

Thanks!

 

[xzburnoutzx]

Wel I bought the set of jets for a Rotax 125ccm with that Carb, worked exellently that way. Yes the 2nd from the top.

 

[Big B NRG]

Thanks!

My needle has 5 slots, do you mean to set the clip it in the second slot from top to bottom?

I have tryed before with 115 main jet and feels like to much gas!, even with out any air filter!

What is the effect with BIGGER pilot jet?

Regards,

Pilot jets determine the gasrespons/gaspick up form idle. The idle jet can be alittle to small, just that you won't have the BWOAAP sound (to much air). Just a crisp and clean gasreaction/gaspick up.

I think you need a thicker needle. That's fatter on the end. You proably have a really edgy needle?

 

[StepVino]

.. at full throttle the scooter feels great but at 1/4 throttle feels and sound like shit.. ...I have been playing around with different needles and clip position with not better results.

If I go for an smaller main jet it doesn seem to get much better, maybe just a little BUT, if I do so, full throttle gets bad.

Mexicano, what about a slightly larger slide cutout? I was having close
to same problem, different carb with round slide though. Raised cutout
by 0.040" ~1mm, and it seems to have solved the problem.

Found this:
http://scootrs.com/tech.cfm?tip=tunecarb

"The slide not only monitors how much airflow goes into your engine (its main job). But it has various angles cut on the bottom of the slide to monitor airflow at low RPM’s. This is referred to as slide cut away. The slide cut away is measured in 4.0, 5.0, 6.0 etc. (see attached chart). The higher the number, the larger the cut away the leaner the slide setting is.

The slide cut away generally effects the jetting in the ¼ throttle range at almost the same throttle position as the needle diameter effects. The slide cut away is usually predetermined by the engine manufacture or engine tuner. As a general rule do not change the slide cut away unless instructed to do so by a skilled engine tuner.

For ¼ throttle jetting adjustments it is easier to adjust the needle diameter."

I kind of disagree with this a little.. If the scooter runs bad at 1/4
throttle, but well otherwise, and the cutout effects off idle to 1/4
throttle.. then it seems like the cutout is the one to change, not
the needle.

 

[Mexicano]

Pilot jets determine the gasrespons/gaspick up form idle. The idle jet can be alittle to small, just that you won't have the BWOAAP sound (to much air). Just a crisp and clean gasreaction/gaspick up.

I think you need a thicker needle. That's fatter on the end. You proably have a really edgy needle?

I'm using the FATEST I got...can I get better using an smaller pilot jet?

 

[Mexicano]

Mexicano, what about a slightly larger slide cutout? I was having close
to same problem, different carb with round slide though. Raised cutout
by 0.040" ~1mm, and it seems to have solved the problem.

Found this:
http://scootrs.com/tech.cfm?tip=tunecarb

"The slide not only monitors how much airflow goes into your engine (its main job). But it has various angles cut on the bottom of the slide to monitor airflow at low RPM’s. This is referred to as slide cut away. The slide cut away is measured in 4.0, 5.0, 6.0 etc. (see attached chart). The higher the number, the larger the cut away the leaner the slide setting is.

The slide cut away generally effects the jetting in the ¼ throttle range at almost the same throttle position as the needle diameter effects. The slide cut away is usually predetermined by the engine manufacture or engine tuner. As a general rule do not change the slide cut away unless instructed to do so by a skilled engine tuner.

For ¼ throttle jetting adjustments it is easier to adjust the needle diameter."

I kind of disagree with this a little.. If the scooter runs bad at 1/4
throttle, but well otherwise, and the cutout effects off idle to 1/4
throttle.. then it seems like the cutout is the one to change, not
the needle.

Thanks StepVino, I will check this also.

 

[Big B NRG]

I'm using the FATEST I got...can I get better using an smaller pilot jet?

Mm, got a pic of that needle? I don't think it's your PJ.

 

[StepVino]

Sorry to but in.. just had another thought for maybe a temporary
fix.

There should be 2 holes in the throat for the idle fuel. One
one just under the slide, one just after, closer to the motor.
You could try plugging the one under the slide. Should be easy
since it's very small.

The hole under the slide supplies extra fuel, just as the slide opens.
Before that, it supplies air to the fuel coming out of the second
hole, past the slide. You might have to lean out your idle a bit
if you try this...

 

[Mexicano]

Mm, got a pic of that needle? I don't think it's your PJ.

This are three neddles that came in the set, the FATEST is inside my carb now, I will take pictures of it, as soon as I pull out the carb, next week.

I will try to tune my carb next week but would like to start all over again, do you think a good combo coul be 115 main, 35 pilot jet and the fatest neddle in the leaner position???

I know 115 main sounds very small for a 110 c.c. engine with 30 m.m. carb, but I live at 2,300 meters over sea level!!!

Thanks for your inputs.

Thanks!

 

[Mexicano]

Sorry to but in.. just had another thought for maybe a temporary
fix.

There should be 2 holes in the throat for the idle fuel. One
one just under the slide, one just after, closer to the motor.
You could try plugging the one under the slide. Should be easy
since it's very small.

The hole under the slide supplies extra fuel, just as the slide opens.
Before that, it supplies air to the fuel coming out of the second
hole, past the slide. You might have to lean out your idle a bit
if you try this...

Thanks again for your help, I will try all the info you provide first, very useful info at the link!!! but there is one thing I don't understand, the instructions said to perform neddle and pilot jet tuning WITHOUT main jet, is this right?????


Regards,

 

[Big B NRG]

Take a look at this needle

It comes from a mikuni carb that was mounted on a 125cc engine. Comes alot closer to your engine, then those 70cc engines..

Typenumber is: 5H38

These needles fit Keihin/Koso carbs, no problem. Try to looks for a keihin needle that looks something like that.

 

[str8dum]

what are the needle #'s you are running on your carb now. fattest needle really doenst tell us anything. Its the taper that matters.

 

[StepVino]

Thanks again for your help, I will try all the info you provide first, very useful info at the link!!! but there is one thing I don't understand, the instructions said to perform neddle and pilot jet tuning WITHOUT main jet, is this right?????


Regards,

I think that is only to test your needle jet size/needle thickness.

The needle should have enough flow to flood the engine at around
1/2-3/4 throttle, when the main jet is removed. A.G. Bell says this
test should be done with the needle clip in position 1, full lean.
If the engine keeps running past 3/4 throttle, then your needle
jet is too lean. I'm assuming this could also mean that the needle
is too thick.

According to Bell, this should be the first test, after checking
the float level.

 

[Mexicano]

what are the needle #'s you are running on your carb now. fattest needle really doenst tell us anything. Its the taper that matters.

My needles doesn't carry a PWK number( N427-46-XXX)....they have only a two digit code.

 

[Mexicano]

Take a look at this needle

It comes from a mikuni carb that was mounted on a 125cc engine. Comes alot closer to your engine, then those 70cc engines..

Typenumber is: 5H38

These needles fit Keihin/Koso carbs, no problem. Try to looks for a keihin needle that looks something like that.

Thanks BIG B, I will try to get some 4 different needles from the leaner available.

Regards,

 

[tiba_karotsu]

i think i know what your problem is, i have tuned a KOSO 30mm on my MHR Team engine & i didn't make it work b'coz here are the reasons;

1. the jet block where the pilot jet, emulsion tube & main jet is housed in is probably porous b'coz of air pockets (poor casting) & fuel shortcircuits from the float bowl to the carb inlet which messes up the jetting.

2. the rubber o-rings that seal the channels where fuel pass through might also be leaking, it happened to me with this brand new KOSO carb, i couldn't believe it myself but the o-rings are shot.

3. the rubber o-rings on the jet block that mates up to the carburator body doesn't seal up hence fuel is sucked up directly from the float bowl.

this only happened with the KOSO carb i bought, i got so frustrated with it that i wanted to pound it with a sledge hammer, the OKO & keihin knockoffs are better than this & much easier to tune.