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CBF 250 Carb Issue Part Two

Blog Date - 14 April 2014

Further to my original post - CBF 250 Carb Issue Part One

I'm making very slow progress. Very slow.

As per the manual I found online and the recommendation of reader "Ron" I had set my pilot screw set to 2 and a quarter turns out. I am NOT at all experienced with setting up carburettors. In around 400,000 miles I've never had the need to, they've always just "worked". I'm sure if I did know how then I could have improved the running of my various machines over the years but this is the first time I've ever had a carb issue big enough to frustrate me. I have much to learn.

Due to my lack of knowledge and experience I was loath to play with the pilot mixture screw more than a few minor degrees. I did not want to get it all wrong and either burn a valve out from being too lean or cause massive flooding with too rich. And yet the problem persisted.

The correct method of setting a pilot screw follows thusly, according to given wisdom. Turn the idle screw in, if the revs fall turn it out. Keep on turning the screw in either direction until peak revs are reached. Then adjust the throttle stop screw to the correct tickover revs, and repeat the in-out process. The idea is simple, when the mixture is just right the revs will be at their highest, too much or too little juice and the engine slows a little. When the revs are highest the mixture is correct and you bring the idle down by adjusting the throttle stop. Simples.

Not on my bike. First of all it won't tick over properly. Even when the engine is mad hot the pilot screw is so fiddly to get to so by the time I wormed my watchmaker's screwdriver in and burned my hands on the scorching motor, it's stopped ticking over and stalled. If I can finally persuade the engine to keep running long enough with the odd blip I find that each time I screw the pilot out the revs rise so much I fear melting something. I'm sure the experienced tinkerers among you know exactly this kind of scenario. Manuals are written by engineers in laboratory conditions working on a bench motor. Real people work in back yards covered in oil and rain with worn out engines and worn out tools.

But. I do know this pilot screw is a FUEL screw. Some pilot screws let more or less air in, others let more or less fuel in. Mine lets more fuel in the more I open it, the more I turn it outwards. I also know that lean fuel, not enough, is dangerous, it can overheat an engine and burn out valves. Whereas a rich mixture, too much fuel, may cause the engine to run rough but it won't damage the engine. I decide to go for broke and seriously richen it up. Not the recommended 2 and a quarter turns out, but 4 turns out.

Woohoo! Well, sort of anyhow. The bike is still running wrong, especially when cold. Typically an engine may take 30 seconds to 3 minutes to warm enough to come off the choke and tick over. The gf's Chinese Keeway needs choke on a cold day but will tick over fine after a few moments with no choke. The CBF 250 will start with the choke but not even attempt tickover until 5, maybe even 10 MILES later. That said, if the bike is fully warm it will tick over.

Buuuuuuuut. After a motorway ride the tickover is steady at 1,500 RPM. After a while in the town, when the motor has cooled, this falls to 1250 RPM. This doesn't happen in my experience. It's possible that the revs may be 50 RPM higher when super warm but not 250 RPM. It's still not right.

I have 2 possible explanations I need to explore. First off the mixture may still be lean. Richening the mixture has improved the tickover no end so maybe I've not gone far enough. I can't make it any more richer though, I've turned the screw out a little more and if I go any further the screw will actually be at risk of falling out of the carb. Which begs the question, what is the problem anyway?

Carb makers do actually know what they're doing. Keihin, or whoever made the carb, would know that this size jet in that size hole with this type of motor and that sort of filter will tickover just fine. The adjustment in the pilot screw is to cover the small variations in manufacturing and wear over the years. I know the jets are standard so I should only need to alter the pilot maybe 1/2 a turn, a turn at the absolute most. It is more likely that there is a blockage in the pilot fuel circuit that is not allowing the correct amount of fuel to flow.

There is another, more sinister possibility. When my bike is cold it rattles like hell. Even when it's warm it's not exactly sewing machine quiet. I don't know for sure but I suspect I have a dose of piston slap. This means the piston is somewhat loose in the barrel. In itself the given wisdom is not to worry about it, it's not burning oil, it's not getting any worse, it's just one of those things, "They all do that sir". It might suggest though my compression may be low. 

Compression? As the piston comes up the barrel to squash air and fuel it needs to be sealed, kind of like a bicycle pump as you squash the air into the tyre. If that seal is weak and some of the mixture escapes, it's possible the engine will not fire. At low revs, tickover, the mixture has time to escape. When the engine is cold the gaps are large, hence the loss of tickover. When the engine is hot the piston grows and the seal is improved, hence good tickover. I guess I should do a compression test...

Anyhow...I have no answers yet. I shall do a compression test, cold. I shall strip the carb, again, and see if anything is blocked. I shall report back my findings...


Reader's Comments

Tom McQ said :-
This sounds like the hell I've been to.
1/1/2000 UTC
john de ville said :-
Swap the carb with latchys?.................if Latchys is running right then it should in principle make your engine run right......if it runs the same then its not the carb........just saying loik.
1/1/2000 UTC
Ren - The Ed said :-
Hi John

For those who don't know, Latchy is a friend who also owns a CBF 250 and his runs perfectly. John, I agree it's a sensible notion - BUT - Latchy's bike is running just fine and there's an old saying, if it ain't broke don't fix it. It's possible that by removing his carb he could create a whole hill of trouble for himself and I don't want him to risk that.

Now...if there's anybody out there with a spare CBF 250 carb lying around, do let me know!
1/1/2000 UTC
bern said :-
Hi, any fix on the carb yet? mines doing the same, it's my first bike and was just getting into biking and now i am going crazy over this.
1/1/2000 UTC
Ren - The Ed said :-
Hi Bern

Sorry bud...nope! I have fitted a larger pilot jet (.45 upped to .48) and it certainly has improved the overall carburation. The bike feels "right" now. HOWEVER...the damn thing refuses to tick over until fully warmed. 4 miles...phut. 8 miles...dum dum dum phut. It takes about 10 miles of steady riding before tickover comes in.

Most CBF 250's I know of never had the tappets done. So they go tight. Mine was so tight that there was no gap whatsoever. I've sorted that now but I suspect it is possible the valve seats are partially knackered. As such when it's even slightly cool it might not have quite enough compression to tick over. When it warms either expansion causes the valves to seat better or the motor can tolerate the poor seal.

Thing is, it's a fair old task to remove the head and have new ones fitted. I'm pondering whether or not to just live with it and call it "character" or strip the head and find someone to replace the seats.

Crazy? It's driving me INSANE!!
1/1/2000 UTC
bern said :-
thanks for the info, will drop it into mechanic and hope it's not to expensive.
will let you know how i get on. thanks.
1/1/2000 UTC
The other cbf 250 owner (Latchy ) said :-
Why don't we whip out the valves, inspect them and grind them in with grinding paste if needed , yes you will need to re shim I know but wouldn't it do the engine a world of good
1/1/2000 UTC
Brian Simpson said :-
My CBF 2504 also has a carb problem. The tickover increases to 3000 rpm soon after starting.
When traveling with say 5000rpm on the clock as I close the throttle and depress the clutch approaching a junction the revs stay at 5000 rpm and cause problems.
I have removed and cleaned the carb and carried out tests for air leakage, and used a vacuum cleaner to test the vacuum operated piston.
Also where is the thermostat for the carb heater?. There is no voltage on the heater and I cannot find the thermostat on the bike.

Email : brian.radioham@gmail.com
15/6/2016 4:07:29 PM UTC
Ren - The Ed said :-
Hi Brian. If you read around the website a little further you'll see what I had to do to fix it. I'm afraid it is likely to be worn valve seats. This required removing the cylinder head and grinding the valves... A lot!

Cheers, Ren.
16/6/2016 7:43:45 PM UTC

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