Honda 250 Carb issue has me bummed out [Archive]

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AndreJ

5th June 2018, 00:59

Hi Guys,

I ride a Honda CBX250 Twister (known as CBF250 in some markets) and I am from South Africa.

I am having an issue with the carb that I am unable to diagnose.

First off, the bike has an aftermarket exhaust on it. The bike is also at 46 000Km. The motor was rebuilt by Honda by the Previous Owner. The carburetor is so clean, I will eat out of it. There's nothing wrong in the carb.

The bike had a #142 Main Jet in the carb when I got it. (factory is #138 - It was probably changed somewhere in it's life due to the aftermarket slip-on)

On partial throttle and idle the bike is very happy, no issues at all. After 3/4 throttle the bike starts to starve it feels like you're closing and opening the throttle continuously.

Now, here's what I've done in order to diagnose the problem.

1st, I removed the air filter to test - No difference
At full throttle with choke on - No difference

So I went up from the #142 to a #145 main jet.

It feel ALMOST perfect but it still starves at >3/4 throttle. again, air filter out and choke on with full throttle makes NO difference. - Choke makes it feel a bit worse though.

So I purchased both #148 and a #150 main jets in HOPE that I manage to sort the bike out!....

Installed the #148, it feels exactly the same as the #145. same applies to choke and air filter removal.

I then installed the #150. Now. Now, it over fuels! The bike sputters and pops even at partial throttle (anything more than 1/2 throttle).

BUTTTT with the air filter removed, it revs freely to 10grand with no issues - which means that my #150 main jet is too BIG!

I am now currently lost for words.

I have stripped the whole thing down, checked the air box, all the hoses the carb, everything. I found a o-ring in the carb at the idle jet that was perished, It was replaced. Needle works perfectly and moves up freely when air is blowing over it. The diaphragm is perfect. The carb does not have an adjustable air jet and the needle and floats are non-adjustable.

It is a Keihin VE carb.

Guys please help me diagnose this issue!?
https://farm5.staticflickr.com/4344/35755986483_3cc5f11ff5.jpg (https://flic.kr/p/WtCM4z)

Thank you,
André

slofox

5th June 2018, 12:41

Have you tried the #138 jet?

FJRider

5th June 2018, 14:54

I have stripped the whole thing down, checked the air box, all the hoses the carb, everything. I found a o-ring in the carb at the idle jet that was perished, It was replaced. Needle works perfectly and moves up freely when air is blowing over it. The diaphragm is perfect. The carb does not have an adjustable air jet and the needle and floats are non-adjustable.

Thank you,
André

Did you do this before or after the #150 jet was tested ... ???

AndreJ

5th June 2018, 22:11

I have not tried the 138 jet. But, I think if it under fuels with the 142, testing a 138 would be pointless?

I have tried all jets, the stripped the bike down and inspected everything (replaced the broken o-ring) then tested all the jets AGAIN with similar results

husaberg

6th June 2018, 18:43

Hi Guys,

I ride a Honda CBX250 Twister (known as CBF250 in some markets) and I am from South Africa.

I am having an issue with the carb that I am unable to diagnose.

First off, the bike has an aftermarket exhaust on it. The bike is also at 46 000Km. The motor was rebuilt by Honda by the Previous Owner. The carburetor is so clean, I will eat out of it. There's nothing wrong in the carb.

The bike had a #142 Main Jet in the carb when I got it. (factory is #138 - It was probably changed somewhere in it's life due to the aftermarket slip-on)

On partial throttle and idle the bike is very happy, no issues at all. After 3/4 throttle the bike starts to starve it feels like you're closing and opening the throttle continuously.

Now, here's what I've done in order to diagnose the problem.

1st, I removed the air filter to test - No difference
At full throttle with choke on - No difference

So I went up from the #142 to a #146 main jet.

It feel ALMOST perfect but it still starves at >3/4 throttle. again, air filter out and choke on with full throttle makes NO difference. - Choke makes it feel a bit worse though.

So I purchased both #148 and a #150 main jets in HOPE that I manage to sort the bike out!....

Installed the #148, it feels exactly the same as the #146. same applies to choke and air filter removal.
Also check the Float level.

I then installed the #150. Now. Now, it over fuels! The bike sputters and pops even at partial throttle (anything more than 1/2 throttle).

BUTTTT with the air filter removed, it revs freely to 10grand with no issues - which means that my #150 main jet is too BIG!

I am now currently lost for words.

I have stripped the whole thing down, checked the air box, all the hoses the carb, everything. I found a o-ring in the carb at the idle jet that was perished, It was replaced. Needle works perfectly and moves up freely when air is blowing over it. The diaphragm is perfect. The carb does not have an adjustable air jet and the needle and floats are non-adjustable.

It is a Keihin VE carb.

Guys please help me diagnose this issue!?
https://farm5.staticflickr.com/4344/35755986483_3cc5f11ff5.jpg (https://flic.kr/p/WtCM4z)

Thank you,
André

If the issue is at 3/4 throttle try raising or lowering the needle.
Some of the Hondas are not adjustable but you can raise them with a washer under the clip.
Personally I would put it back to how you got it and put a washer (say 1mm thick) at a time under the needle clip. then try 2mm then 3mm.
When adjusting it always go back to the original setting afterwards just to confirm it is actually better.
Carbs have as well as having overlapping circuits also have air corrector circuits that work over narrow ranges.
But stick with the basics first.
As it has a CV carb you can also make the air slide admission hole slightly larger or make the Diaphragm springer stronger (but only do this last)

AndreJ

7th June 2018, 01:12

Thank you! I was looking at the needle when I had it all apart and wondered if this idea would work. Now you have confirmed that I can try it, I am going to give it a try TONIGHT!

I currently have the #145 back in the carb as it gives me the best feeling out of all of them. Will try the washer idea with this jet first and see how it goes.

F5 Dave

9th June 2018, 10:15

You can get some pretty thin washers don't go crazy at first but the needle is very likely the issue due to new pipe. Obviously clean carb also means you can blow through the pilot and air holes. Blow on a tube if you don't have compressed air.

On a cv carb blowing in the inlet at the top will raise the carb slide if it is not perished.

awayatc

9th June 2018, 18:52

Sounds like carb may not get enough fuel....
You say floats are not adjustable?
Can't bend the tab?
See at what level floats shut of fuel... ( clear hose from bowl drain up...)
Also is fuel filter in fuel tap clean...?

AndreJ

11th June 2018, 18:44

Hi guys.

First of all, I cannot bend the tab on the float, its plastic. I have already replaced the fuel hose from the tank to the carb, new in-line fuel filter - feels exactly the same with or without the filter. the fuel petcock trap is also clean. And yes, I have checked that the slider moves up with air and it does and I have cleaned and blown out all the jets and holes. The carb and jets are CLEAN.

Ok, I have tried some washers:

The washers I used is 0.5mm thick.

With the #145 it feels the same as without one.

#148 with 1 washer feel dramatically beter! But there is still a little bit of hesitance.

#150 with or without a washer feels horrible the bike pops and spits with it.

So I tried the #148 with 2 washers, also feels horrible.

So I'm back with the #148 with 1 washer and still the bike isn't running as it should, although it is a lot better.

husaberg

11th June 2018, 19:00

AndreJ

11th June 2018, 20:02

Are you at altitude?
Next step i would try is to preload the slide spring a little bit.
Failing that admit defeat and buy a jet kit.
https://www.6sigmajetkit.com/honda-cbf250-jet-kit.html

Yes, I am at altitude, 1724m.

How do I preload the slide spring?

husaberg

11th June 2018, 20:12

Yes, I am at altitude, 1724m.

How do I preload the slide spring?

Stick a big washer under it

AndreJ

11th June 2018, 20:31

Stick a big washer under it

I'm gonna put the bike on a exhaust gas analyzer first and see what that tells me. Then I'm gonna have to get serious with this carb. I'm getting rather tired and taking it apart and reinstalling time and time and time and time again.

husaberg

11th June 2018, 20:33

I'm gonna put the bike on a exhaust gas analyzer first and see what that tells me. Then I'm gonna have to get serious with this carb. I'm getting rather tired and taking it apart and reinstalling time and time and time and time again.

Cant you just loosen the clamp and twist the carb to one side to access the top or the bottom?

Odd thing is altitude requires leaner jetting than std.

AndreJ

11th June 2018, 23:50

Cant you just loosen the clamp and twist the carb to one side to access the top or the bottom?

Odd thing is altitude requires leaner jetting than std.

Yes, I can, but access still isnt easy and my clamps etc is starting to show wear on the screws which I hate.

By bike has a air injection system - https://www.motorcyclespareparts.eu/en/honda-parts/2006-cbf250-motorcycles/f-16-1-air-injection-control-valve

I wonder if this might not be the cause of my problems...

AndreJ

12th June 2018, 03:22

Just got off the bike....

I bypassed the smog system, checked the fuel petcock trap, its clean, removed my inline fuel filter. The bike feels exactly the same, no difference whatsoever!

Hugo Nougo

12th June 2018, 10:28

So, just maybe it's not a carb issue.

AndreJ

13th June 2018, 19:30

Also been thinking this, but what else would cause this?

I mean at partial throttle and at lower RPM the bike behaves PERFECTLY. And with the #148 and the single washer the bike feels so much better than with any of the others so that is an indication that I am on the right track isn't it?

pete-blen

22nd June 2018, 22:12

At 46,000 replace the needle & emoltion tube...
possably worn to hell putting the vacume all over the place
that picks the fuel up though the main jet...
And as already stated.... fuel level....

another point.... the pilot jet... it has a bearing on full throttle as it dose
not suddenly shut off after 1/4 throttle...

the same bike/model can come with diffrent jet combinations for diffrent climates
cold... big pilot / small main..
hot... small pilot / big main..
but both have about the same air/fuel ratio though out the rev range...
Its to do with starting and warm up...

..

Vinz0r

31st October 2018, 19:55

I just had a quick skim through the posts, but... are you sure you're not just running too rich with the bigger main jet? You say that you don't really see an improvement with a bigger main jet, so maybe go for a smaller main, and see how it performs.
Just because you have a slip on, doesn't mean that you necessarily have hugely increased air flow. Also, you're at altitude so might need to lean it out a bit.

Vinz0r

31st October 2018, 19:58

On partial throttle and idle the bike is very happy, no issues at all. After 3/4 throttle the bike starts to starve it feels like you're closing and opening the throttle continuously.

To me this sounds like bogging rather than leaning out.

AndreJ

15th February 2019, 18:05

Morning Guys!

I didn't want this thread to die without a resolution so I thought, let me update it!

First off, Thanks for your replies during my struggles!

Guys, it turns out, I have been dumping WAAAAAY to much fuel into the carb!

I went down from the 148 main jet I had in it to 132 and ultimately, Yesterday went down to a 128 main jet. I also removed the washe I installed in the needle and removed the wire mesh wrap around the air filter.

The bike is fixed! It revs straight to 10grand for the first time since I've had it! I am so relieved!

NOW I can actually feel how my bike ACTUALLY performs! I mean it is only a 250, so you will need every drop of power that it can produce, I only ever had about 40% of it, and I'm not a featherweight...

Here she is, so happy!!!:
https://farm8.staticflickr.com/7892/47079549941_94bd38929b_b.jpg (https://flic.kr/p/2eJfZYi)cbxxx (https://flic.kr/p/2eJfZYi)