Keihin FCR problem

Keihin FCR problem

I have been running my FCR carbs for a long time with no problems at all .
Recently they have developed a problem , there is a cough / hesitation at slight throttle opening .
This seems to be a common problem when tuning them and widely discussed on motocross forums .
I took them off & cleaned out the pilot jets but it has not solved the problem .
when I put it back together i adjusted the mixture screws as per spec , but to get the idle to rise to best running They are screwed all the way in .
One suggested mod is to tie the accelerator pump to the operating lever so it acts directly off the throttle instead of the spring , like this it seems to work well .
The bike starts , idles & runs well apart from the hesitation on acceleration .
If I give it a fist full from low revs it almost stalls .
As I said it has run well for a long time so I assume something has worn or gotten blocked .
The pumper mod & the mixture adjustment seem to be masking the real problem .
Any suggestions would be greatly appreciated

Ive put these on a couple bikes and if not used for a while the accelerator pump gums up, doesn't refill for next squirt. By the time ive taken carb off fuel has slowly filled the chamber and it appears to be working but several squirts in a row it cant refill quick enough. Cant open it up to clean so soak and spray carb clean and use marvel mystery oil in fuel is best I have found. Or it may be something else

The accelerator pump(s) may be leaking past the check valve, enough so that it has effected your idle mixture. You said that you cleaned the pilot jets, but they are very hard to clean, enough so that I'd advise you to just replace them, or try an ultra-sonic cleaning.

Quickly opening the throttle on non-CV carbs at idle with no load usually leads to a big cough, you must have had some BMW CV carb experience(s) as the CV carbs are the only design where you can do this un-needed trick and get a proper response.

Also, you, really, don't need the pumps. You can adjust them out of the game and see if that makes a difference. I'd look hard at the pump check valves and replace the idle jets.

Further to other replies, I believe from my own carbs that the needles wear in these. And the needle is responsible for a wide range of control. So, could be worn needles.

Thanks for the replies.
Looks like a rebuild may be in order

When you mention "a long time" - could you put some sort of measurement to that?
Time/distance?
Thanks

Are you sure it isn’t electrical?

I had this exact same problem under acceleration with my FCRs and it turned out to be a barely connected wire connecting to one of my coils. Only on acceleration.

Was bouncing around in there like a bee in a bonnet.

A less likely issue than the possible solutions already posted, I had these on a Honda 450 a while back (cough, cough) and I had your symptom. I found a very small leak in the diaphragm on one carb. A new diaphragm fitted with tlc was the solution.

Don't you need a letter of permission from The Queen to allow you to fit those on a Norton?

Diaphragm ?

Oops. Thought they were CV carbs......

Fast Eddie said:

Diaphragm ?

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The pumps on the FCRs have diaphragms, however, if they experienced a tear or a breach you'd expect the resulting leak to express itself on the carbs exterior.

Carbs have been on for aprox 8 years , bike gets ridden 3 or 4 times a week , min ride is 1hr , weekend rides around 3hrs .
Doubt that it is electrical as the accelerator pump mod virtually eliminates the problem .

Atlas Commando said:

Don't you need a letter of permission from The Queen to allow you to fit those on a Norton?

Click to expand...

I don’t think the queen will give a flying monkeys from what country you fit what parts on your British bike like how she has arts from masters from all over the world hanging in her home(s) not to mention knickers from Hong Kong !

Bernhard said:

knickers from Hong Kong !

Click to expand...

How dare you sir? You parliamentarian you...!

Interesting point ref the FCRs tho, out of all the mods and none standard stuff littering up my bike, it’s the carbs that generate the most comments from Nortonesque onlookers.

The problem seems to be common to both carbs , as both idle adjusters need to be all the way in to increase the idle speed .
The only thing I can see that is common is the accelerator pump .
I will o/h them anyway , but it would be nice to find out what happened .

Fast Eddie said:

Further to other replies, I believe from my own carbs that the needles wear in these. And the needle is responsible for a wide range of control. So, could be worn needles.

Click to expand...

It is unlikely to be worn needles or needle jets. Rich mixture usually just causes sluggishness - slower acceleration. A cough or gasp is usually a sign of too lean or an electrical problem. If your carbs are jetted for best power at mid-throttle , it is usually not possible to whack the throttles open in normal carbs - you should need to feed the throttle on. But the pumps in the Keihins should enable you to have extremely rapid throttle response, but with slightly more sluggish acceleration from the motor. If you have cleaned the pilot jets, I suggest the problem is either the pump or it is electrical. If you wind the throttle on and the cough occurs below about one-third throttle, then the motor comes on strong again when you get near half throttle, you might have a problem in the low running jets, but you seem to have eliminated that as a factor by cleaning the jets.

I wonder about using pumper carbs on a road bike. With a motocross bike, rapid throttle response is probably more important than slightly slower acceleration from the bike, due to the richer mixture. With a road race bike, a quick action twist grip is usually too fast for the carburation, but it allows you to get the throttle open without taking two bites and moving your hand on the twist grip.

The carbies have been excellent for years , throttle response has always been quick & smooth powerful acceleration through the range .
The problem has only surfaced recently .

Hi Dero,
I am using FCR on my Codo engine for more as 10 years, here are my comments:
When you receive the FCR kit, the jetting is to ritch, at least on a standard engine, but it works well, of course you can do it better, but it's difficult: if you change anything, you have to change mostly all the other stuffs, If you know that only for idle you have one pilot jet, one fuel screw and one air screw, for the middel rev's you have the main jet, the main air jet, the needels and so on, forget the idee that the main jet influence only at WOT, this is not true!! The influence of the main jet start at 1/8 opening on the FCR, In other words, if you put a larger main jet to adjust the mixture at WOT you will be to ritch on the needels...
Don't toutch the accelerator pump, the pump is mandatory on big carbs, there is not enough vacum created by the engine to suck fuel in the engine when you go to WOT that's why you have the accelerator pump to compasate the low vacum.
In case you use a standard engine, two things can happens, it happens to me at least: the needels and needels tube can worn out, change both!
The second thing: on the flat side you will see a round piece of metal with a hole in the middel, this is to create a vacum on the slide to keep the slide close to the intake side, BUT around the rond matal piece there is a rubber seal, and it's happens that on my carbs the seal disapear!!! I can not say if the factory forgot to put the seals but when I put the new seals on, no problems anymore...
If you use a high tuned engine like I have, then tuning the carbs is a different story, I spend day's on the carbs chearching different settings, and when you think "Now it's good" don 't beleieve it, search and you will find better, always!
Hope this help you
Yves