2008 Honda CBF250 (Aka CBX250 Twister) Idle issue

[robertpnz]

Hey everyone, i have a 2008 Honda CBF250 with 22000km that just wont idle properly. It can be quite hard to start when cold and even when i use choke i have to give it a little bit of throttle. Once it is warm and at a good operating temperature the idle will sometimes hang at around 3000-4000rpm and sometimes come down slowly or not come back down at all. (The standard idle is 1500 +/- 100rpm)

https://www.youtube.com/shorts/VCIhN6fAB30

I couldn't find any vacuum leaks between the carburettor and the engine.

Spark plug is good

Petcock seems fine although i will check again. Non vacuum type.

I have had the carburettor out and given it a good clean with carby cleaner and some air. Its a Keihin VEA2A CV carb

The main jet is standard 142

Pilot jet is standard 45

The float needle valve seemed ok as did the non adjustable float. The only thing i did notice was that when measuring the float height as per the workshop manual i measured a float height of about 22mm at the highest point of the float (it should be 18.5mm) But i am not sure how this could be as it is a fixed non adjustable float? I am unsure if i am taking the measurement from the wrong place as the manual is Spanish to english translation. I am using a vernier gauge as i do not have the correct tool.

I checked the diaphragm and slide and that all seemed to operate correctly.

Things i am going to check next.

Choke circuit.

Still need to check the small fine mesh fuel filter at the inlet of the carb as i didnt realise it was there.

Valve clearances

It does have a very small fuel weep on the float bowl as the gasket is knackered. Ive got a carb kit on its way from ebay.

Any help would be much appreciated. Thanks all.

Rob. New Zealand

[Nobby the Bastard]

How are you checking for leaks on the rubbers?

It sounds very much like they are knackered.
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[robertpnz]

I was spraying a little carb cleaner around the intake boot with the engine running and listening for any change in rpm

[jaffa90]

""It does have a very small fuel weep on the float bowl as the gasket is knackered. I""
Hi, are you talking a fuel weep at the top of the float bowl?

[robertpnz]

Yes, just at the gasket seal where the bowl joins to the carb body. Do you think this could be the cause/part of?

[jeffyjeff]

Are you sure the float is nonadjustable? Most of the floats I have experienced have a bendable tab that bears against the needle when the needle is fully seated.
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[robertpnz]

Yes its the non adjustable type. All plastic and no metal tab for the valve to sit on

[Nutty]

Air screw should be 2 1/4 turns from bottom , and have you checked the Choke Valve

[jaffa90]

[robertpnz]

Ok cool. I will check all of these things. I am just away from home at the moment but i will keep you all updated.

[Robby]

I'm assuming that when you cleaned the carb, you removed the mixture screw along with its washer and o-ring to clean that area out properly (it's a tiny hole so it clogs). Did you set the mixture screw correctly afterwards? I tend to also give it an extra half turn out, particularly on older engines.

In my experience, an idle which is slow to drop back down is usually an air leak, sometimes a mixture valve which is screwed in too far.

Carb cleaner isn't fantastic for checking for leaks like that. One of stinkwheel's recent suggestions is an unlit blowlamp, which makes sense. Flammable gas is better than vaguely flammable carb cleaner. Be sure to waft it around the throttle shaft seals too.

[robertpnz]

Hey all, so i checked a few things on the bike today.

I checked fuel mixture screw and it seemed to be in ok condition. The hole was also ok and not blocked. I put it back in an screwed 2.5 turns from the bottom ( a quarter more than standard as i would rather to rich than lean)

I also checked the choke circuit. That also seemed ok.

I was able to removed the small fine mesh filter at the fuel inlet to the carb. But it looked ok and couldn't see any gunk.

I also checked the valve clearances. The 2 intake clearances were near perfect both at 0.11mm. But the 2 exhaust valves were 0.07mm and the other was less than 0.04mm (didn't have any smaller feeler gauge) The clearance is meant to be 0.15mm so i am going to get some thinner shims and see how that goes.

Thats all i have tried thus far.

[robertpnz]

Hey all, just an update on the bike. I changed out the valve shims on the exhaust valves and put a new carb float in and its idling much better now. There is a minor surge at 6000-7000rpm that is still there and i am thinking about changing the main 142 jet for perhaps a 145 or 148.

[The Shaggy D.A.]

https://bikesandtravels.com/biker.aspx?ride=822
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[RiderJake]

[jaffa90]

Hi, did you bench check the carb?

"""I suggest you bench test the carb with a pipe and small header tank because i suspect the needle valve is not shutting off the fuel supply on tick over / idle engine speed."""

A higher than normal fuel level will raise the revs.

Also as mentioned changing jets will make no difference.

[robertpnz]

I seemed to have fixed the tickover. I didn't bench test the carburetor. In theory the bigger jet should give it more fuel in the higher rpm range as i suspect its leaning out. And the only reason is suspect this is if i put the choke on it stops surging. But thats the theory anyway. Reality might be different. Open to more suggestions

[The Shaggy D.A.]

[Robby]

A compression test would be useful to find out if the valve seats could be a problem, from Shaggy's link. I would do that before fiddling with the fuelling.

My thinking is: The carb settings are standard, and surging does not seem to be a common problem on this bike. The carb is clean. So the fuelling side is probably working properly.

At the same time, you know it's been running for some time with insufficient valve clearance, and someone else with this problem had to lap the valves to make it behave.

Minor surging is a pain to diagnose, because it can be caused or affected by lots of minor things interacting, including the complex mathematics of pressure pulses shuffling around in the intake and exhaust. At least it's a single cylinder bike so you don't have differences between cylinders to account for.

[robertpnz]

Ok i will do a compression test and see how its looking. Could be a case of having to whip the head off and re lap the valve seats. I took the tank off and removed the long filter on the petcock. It had bits of dirt stuck in it. I removed it all put the tank back on and still no joy with the surge issue.

Here is it running after replacing the float and re doing the valve clearances. It idles fine once its up to a good operating temp but until then it wont idle. But idling better than before.

https://youtube.com/shorts/5ZHEHhNmTM8

[robertpnz]

Hey, i did a cold compression test and it came back at 132psi. I also managed to temporarily diagnose the surging problem by pulling off the overflow pipe on the left side of the carb. This overflow pipe runs down underneath the bike and it seems the airflow under the bike was upsetting the pressure in the carb.

[jeffyjeff]