Hello all,

I've got a '98 F3 that I've recently put back to stock jetting (138/140)... the previous owner was running 142/144.
I took the opportunity to replace pretty much everything in the carbs while I was there using a refurb kit, including needles back to stock.
It still has the K&N from the previous owner.
I suspect it might have some aftermarket cams, because when I did the valve clearances a date and 'BO=210' was etched into the cover.
The carbs are spotless, they were ultrasonic cleaned twice and are regularly seafoamed.
The bike idles great, barely needs the choke to start.
It revs happily with everything up to about 3/4 throttle.
But if you snap the throttle open quickly from idle, it stumbles and bogs.
Same behaviour with 1.5 turns, 2 turns and 2.5 turns on the screws.
Any ideas? I'd hate to be chasing my tail if this was just a feature of the cams (if it has been recammed).

Cheers!

 

Are the carbs sync'd?

 

Hi, thanks for the quick reply.
Sorry, I should have added in my first post - the carbs are synced to within 10mm

 

Your symptoms are classic out of sync symptoms.
I'd get them as s close as you possibly can while keeping your idle at 1,200.
then your pilot circuit needs to be dialed in to aprox 2.5 turns out

Did you check your valve clearanclearances?

 

Thanks again for the reply.
The carb sync is bang on (well, as close as I can get them, with a 10mm spread when the spec is 30mm at 1200 rpm - used a multimeter on a coil to double check the rpm too).
Valve clearances are all good too, reshimmed where necessary.
I've had them dialled from 1.5 all the way to 2.5 and it doesn't seem to change this behaviour...?

 

Have you put in fresh plugs?

 

Plugs are iridium, clean and the gaps checked. Not new, but less than 3k on them.
New gas in the tank.
Checked the resistance on the leads and the coils, all within spec.

 

Sounds like you are being thorough.
is it possible you have a vacuum leak? One easy way to tell is with the bike idli8, take an unlit propane torch and open the valve and stick it down around your carb boots. If there's a leak it will suck in the propane the your RPMs will increase.

 

I agree with hamlin but will also add, an exhaust head pipe can also cause a stumble.
Is the gas fresh?

 

Many thanks for the replies.
Carb boots are new this week - the bottoms were rock solid and spinning, even at full clamp so I swapped them all out. I'll check for a vacuum leak there, but with new boots sealing good I don't think there is a problem there.
Gas is fresh this week.
Tomorrow, I'm going to put back on the original needles and jets as a sanity check. Will take it from there I guess! I am starting to wonder if the previous owner drilled the carb slides.