CB250 K4 carb queries

[mike73]

Hi all- I have been trying to get an old CB250 K4 (1973) back on the road. I cleaned out the carb and it runs quite well although it has all the classic signs of an air leak (revs increase spontaneously and stay high when throttle blipped) although I can't find it. The revs move around so much that the WD40 trick doesn't really help. Anyway Cable is new and throttles are closing by the way. However that aside I really have two questions-
1. I took the carbs apart and eventually cleaned and changed everything I could- including jets, O rings, needle and float valves. I noticed that the pilot jet sits beneath a rubber plug in this Kiehin 725b carb. My question is then how does fuel reach the pilot jet? I can't see a passage and the rubber plug seems impervious.
2. The air screw has little effect- probably because there is an air leak somewhere else, but I note that the textbooks say that fully screwed in is fully lean. Surely this would close the air passage completely and should therefore be fully rich?

Any comments on all this greatly appreciated. I did wonder if a leak around the diaphragm might account for this also- have checked carefully and there are no tears or holes in the diaphragm itself.
All the best
Mike
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[jjdugen]

If you are absolutely sure that the inlet rubbers are not leaky, then I would look carefully at the diaphram. These were always a bit iffy on the early Hondas.
Airscrew right up home cuts off the air flow over the pilot jet, no fuel, lean mixture.
Check your valve clearances too, even slightly tight can give some odd running symtoms. (Love these, BTW, probably the best small Honda that they have made).
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The CBR900RR has been sold. Aprilia Falco worms its way into my heart.
Try Soi 23 on Amazon for a good read.... Self promotion? Moi?

[Islander]

I learned to ride and passed my test on one of these many moons ago

There's workshop manual here:

https://downloads.hondatech.info/Moto/CB/CB_CL_SL_250-350/Honda%20CB%2CCL%2CSL%20250-350.pdf

One thing to watch with these is that the screws that hold the throttle butterflies onto the spindle don't come loose. I had one do this and it led to a top end strip down. Surprisingly no real damage done - the screw embedded itself in the crown of the piston. Easily fixed.

[reckless_b]

hondas this early don't have carb diaphragms and I think the pilot screw isn't a pilot air screw, screwing out will allow more fuel into the mix. My favourite method is to use a colour tune to set the pilot mixture and then to balance the carbs, quite an easy job on a twin although on these models you will be setting up the bell cranks inside the carbs, renove the top cover on the carb and you will see them there.
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40 years on the road on and sometimes off bikes with a very recent OFF

[Fizzer Thou]

The CB250K4 did indeed have CV carbs back in those early days of the '70s

https://i822.photobucket.com/albums/zz141/adelaw/My%20Bikes/HondaCB250K4001.jpg

A friend actually raced one at the Isle of Man and said that they were a bit difficult to tune for the altitude changes encountered there.Virtually the same engine was also used on the CB250G5 which I hankered after when I was a poor Navy College attendee.

For other Honda SOHC info look here www.sohc.co.uk
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[mike73]

Thanks for the input guys- butterfly screws are fine- incidentally I had one missing when I was rebuilding the carb but computer HDD mounting screws are a perfect fit. I guess I will change diaphragms and rubbers but they are so darned expensive!
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If you have eliminated the impossible- hit everything that's left with a hammer

[bikenut]

:karma: hi dude, are you based in north wales??

do you know about cmnsl, find your bike and see exploded views.

to clean the carbs, start at the tank!!

suspect blocked, gummed up carbs and jets.

centrifugal advance not sticking??

[mike73]

Hi -No, not in Wales although going to Lampeter next week. I have cleaned the tank- its was rusty and cleaned it throughly with ball bearings and tank cleaner followed by derusting with Deox C- great stuff by the way took it all back to shiny on overnight treatment. Also cleaned out crud from tap and filter and blown all through with carb cleaner. Replaced rubber gasket plate in fuel cock. Fitted new fuel pipes and incorporated filter elements in both lines to keep any crud I missed out of my shiny clean carbs.

I have now got my head around how the mixture screw gets lean when fully in and no air flow- but I still can't see how the fuel gets to the pilot jet.
Also Both my air screws/mix screws need to be almost all the way in. Does this tell me anything?
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If you have eliminated the impossible- hit everything that's left with a hammer

[bikenut]

:karma: hi dude some pics would be good.

smaller vw type fuel filters??

mixture screws 1/2 to 2 turns out usually.

i tend to run a gollon of fuel ( computer fans for cooling ) with a stron dose of redex carb cleaner..........

i leave jets etc in neat redex over night, goes in like cherry aid, comes out like coke!

air filter all ok and no air leaks ( tween filter and carbs and carbs to head )?

a bike this old would benefit from an engine flush, note that the clutch basket can act like a centrifugal crap trap, so clean this also.

centrifugal filter on this eng from memeory as well as ball and roller crank.

[mike73]

Hi Guys thanks for all the pointers- pleased to say its now sorted! I found 2 potential problems... I had set the float height to 19mm as per Haynes manual. A bit of exploration reveals that this refers to K2 bikes, the 725A and 7s5B carbs as fitted to the K4 need a float height of 25mm! (Anyone know the difference between A and B?)

Also found that the bike ran a lot better when the points cover was off. Seems one of the mounting screws had been replaced with a 25mm instead of the real 20mm screw and maybe this was interfering somewhere behind?? Not sure. Anyway cleaned, lubed centrifugal adv/retard system and cut screw down at the same time the float was adjusted so not sure which had the greatest effect... although my money is on the float height!
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If you have eliminated the impossible- hit everything that's left with a hammer