Camchain and tensioner seen up close in a cutaway bike engine

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CBF 250 Carb Issue

Blog Date - 03 March 2014

I do love the CBF 250. Light. Nimble. Easy to ride. Excellent handling. Economical. It's all the things I look for in a bike. A little more performance would be nice but power adds weight and weight looses economy, that makes the price too high. No, it's as good as it can be.

Except mine has a carburettor issue. In every gear in every situation it feels perfectly jetted, mixed, balanced and happy. Yet the goddam thing will not tickover properly.

Let's say the bike is warm, hot even, after a hop down the motorway. I pull up to the lights and the revs fall to 1,500. That's fine, the book says it ought to be 1,400 plus or minus 100, I prefer it on the high end. As I sit at the lights the revs gradually fall to 1,400. Another minute later it's 1,300. Now the lights change and off I go. At the next set of lights it's the same story. At traffic lights the fall is so slow that it rarely drops enough to stall before the lights change but if it's a long cycle then the revs will fall to 1,000 then stall.

If the bike is cold or just doing town work the stall comes sooner, often within the time the lights remain red.

honda cbf 250 carburettor in bits on a table
Nope...can't see owt wrong with that...

Turn the tickover up? If I do that then after a motorway run the engine revs far too high, 3,000. Also rolling off the throttle it feels as though the throttle is not closed. This is common if the tickover is too high, try it on your bike (if it's not fuel injected) and you'll see what I mean. So no, I am certain it's not simply an incorrect tickover setting. Again if this were the case then no matter what the tickover screw is set to the revs would remain there, they ought not to fall slowly.

Sticking slider? Possibly. This is a CV carb and it is perfectly possible the slider attached to the diaphragm falls normally until it almost reaches the bottom. Maybe something stops it falling all the way to closed then the pulses, vibration and shaking of the motor nudge the slider down gradually. I have this under consideration but there's an argument against this. It's too slow. The pulses and vibrations come thick and fast, even at tickover, it might take a few seconds to nudge the slider down but not a minute or more.

Diaphragm? Possibly. I have inspected the diaphragm and found it complete and in good order. Since doing this I have read it is possible to have tiny, minuscule holes that the eye can't see and I admit I wasn't looking for these. The incorrect seal, being so small, may possibly explain the gradual fall of the tickover too. There's 2 problems with this thought. Firstly to re-check I'll have to remove the carb, again. Secondly if these holes are so small I won't see them anyhow, and a new diaphragm is...ahem...steady...£102. Yikes! That's an expensive experiment. If I was sure it was the diaphragm then I'd happily pay, but as yet I'm not.

looking into the inlet port and tract on the cbf 250
Coooooool...this is the last thing the fuel/air sees before it falls into the piston and gets burned.

Incorrect choke fitment or setting? Yes...and no. I have checked the choke plunger and it works as it should. Secondly if the choke was incorrectly fitted this would affect ticker at all times, not a gradual drop off in revs.

Mixture screw? Again the same as the choke. If it were incorrect it would affect things at all times.

Air leak? I found a simple way to check from a website. Using an unlit blow torch I allowed the gas to flow around the carb. The idea is if there is an air leak it will suck in the gas and the revs will rise. If I put the gas into the air inlet this is what happens. If I move the gas around the carb, nothing changes. This eliminates the idea of a leak. Nice one.

We're getting really complex now. It it possible the idle mixture is too rich and this cools the motor at tickover. As the motor cools the tickover falls? I've set the mixture screw to the manual's 2 and a quarter turns, to no avail. I've checked the choke. I've done the tappets and they're all sort of right and so on and so on. I'm getting frustrated now. There are many things to try I guess but each one in turn requires stripping the bike down, removing the carb and putting it all back together again.

My friend, SL, has the same bike and his has more miles. I started his bike the other day and after a few minutes to warm the motor I left it ticking over. It sat there at 1,200 rpm, dum-dum-duming away for 5 or 10 minutes. It didn't miss a beat. It was perfectly smooth. It made me want to cry. I have thought of asking if I can borrow his carb to ensure 100% that it is at least a carb issue and not something completely random in the motor. It's a big ask though. No...I shall plug away with mine.

Any ideas? What have I missed? I've stripped the carb and used an air line to blast out every orifice and narrow passage. The float is plastic and non-adjustable. The jets are standard as per the manual. I've cleaned the air box and the air filter is in fine order. The tappets were out, they have been done and although not perfect I'm happy they're close enough. The motor runs just fine off tickover and rides as sweet as can be.

ARGH!

Reader's Comments

john de ville said :-
Electrical fault maybe ? cant give any more advice sorry.
1/1/2000 UTC
Ron said :-
I have been thinking about this tickover problem and can't come up with a definitive answer. However, a few things you might consider:

The bike runs well in all situations bar tick-over, and even then only under certain conditions, so not much would seem to be wrong! I doubt an electrical problem personally because all else seems fine. However, is the spark plug the right type and new? Cheap to eliminate this idea.

The throttle stop and throttle cables have been checked, but make sure that when the twist grip is "shut" the butterfly really does hit the throttle stop. Push / pull cables should both have slack so you are sure the throttle isn't being held open slightly. Similarly the "choke" cable and mechanism.

Idle mixture is only significant at tick-over, not at mid and high revs. This bike won't tick-over when cold. The standard 2 1/4 turns out of the idle mixture screw is a general setting - try a 1/4 turn (or more) either way with a warm engine and see what the revs do.

Its easy to suggest these things but of course I have no idea if they are relevant!
1/1/2000 UTC
John said :-
Did you get this sorted ?
I'm having similar issues with mine
1/1/2000 UTC
Ren - The Ed said :-
Hi John. Ermmm. Nope. Not at all. In fact it's getting a little worse. Grrrrr!

I have a cunning plan. My mate has a CBF 250 that works just as Honda intended. I am going to try and pluck up the courage to ask if I can, er, borrow his carb for a while, see if that helps. If not I'll try the HT coils and maybe a few other bits.

I am considering the possibility of worn valves seats too but then compression seems acceptable, I might be clutching at straws.

I hate just not knowing what the problem is and it's driving me insane! I wouldn't mind but the bike is fine in all other areas apart from tickover. GRRRRR!
1/1/2000 UTC
Ger said :-
Hi if you remove the recirculate valve and remove the resonator you will get what you want.
1/1/2000 UTC
Ger said :-
Also it sounds like you may have air leaking in front of the carb between the carb and inlet leaning it out raising the rpm intermittently.
1/1/2000 UTC
Ren - The Ed said :-
Hi Ger. By recirculate valve I am guessing you mean the emission controls, basically the airbox sucks air from the engine cases, therefore burning any oil mist rather than pumping it into the atmosphere. This entire system was removed from my bike as I struggled with the issue. I have of course blocked all the associated holes to the carb and fitted an crankcase breather filter.

As for air leaks, I am aware of this. I have checked for leaks and to make double sure I've put grease all around the join between the carb and the rubber spigot. I've also refitted the spigot and checked the gasket.

What has made a big difference so far is removing the choke cable...? Perhaps, possibly, the cable was not fully releasing the choke? So why would this make the engine run lean, choke is designed to run rich surely?

It is rather too complex to explain here but due to the design of the choke plunger on this model I believe it is possible on partial openings that the choke will allow additional air in but not additional fuel.

1/1/2000 UTC
Bob said :-
Re: choke, the engine is just as likely to die (in neutral with no load) from being over-rich as over lean.
Have you looked at the air cut off valve? If it has one it'll be on the side of the carb.
I would recommend doing a compression check on the engine hot and cold, just to be sure. I had a CLR125 that ran fine but was a bugger for not ticking over and taking a lot of cranking to start, turned out to be a worn bore.
1/1/2000 UTC
Ren - The Ed said :-
Hey Bob. There's no air cut off valve on the CBF 250 carb. Compression was 125 PSI when I checked it, acceptable but not exactly good. It's definitely a possibility. That said the bike ALWAYS starts really well even on cold and wet days, it just won't tick over.

That said since messing with the choke it is a LOT better. It takes a lot less time to warm up enough to tick over and when it has warmed the tickover is a lot more stable. I'm 90% sure it's a mixture issue but as I'm a bit of a numpty with tuning I'm rather blundering around in the dark.

It's working well enough now to not make it worth the effort of stripping the engine. IF a good carb comes up at the right price I may buy one.
1/1/2000 UTC
David said :-
I know you said you have checked your valve clearances but this was exactly the cure for my bike when it had the same problem. I bought it with this temperature-related-idle-speed issue and did a full service on it.. I didn't strip the carb, or do anything special... just a normal service plus valve clearances. There was practically no clearance on the exhaust valves at all. As soon as they were reshimmed and the bike put back together, it was peachy. Starts easily, pulls well. I hope you get your CBF sorted out, if you haven't already!
1/1/2000 UTC
Ren - The Ed said :-
Hey David. I had the same issue on my CBF 125 and I thought of all kinds of horrible problems but when I finally set the tappets correctly everything was back to perfect. When I bought the CBF250 I though it would be the tappets too but regrettably it made no difference.

However...in the interest of being thorough the 250's due a service soon so I shall be checking said tappets and adjusting as required.

And nope, the bike's still giving me grief but I've just learned to live with it now...
1/1/2000 UTC
Arturas said :-
How is you carb?
5/4/2016 6:45:38 AM UTC
Ren - The Ed said :-
Hello Arturas. After many many miles and many more struggles I finally took the cylinder head off and re-ground the valve seats. This fixed the problem.

It seems after all my battles it was never a carburetor problem but worn valve seats.

We live and learn, sometimes the hard way.
5/4/2016 7:16:16 AM UTC

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