Carb needle setting and idle problem...

evogav · #1 10-30-2009, 10:25 AM

Hi Guys & Gals...

I've done over 700 miles on my MB now and lovin every minute of it. My £4 daily petrol cost in my car now lasts me for 2 weeks on my MB. I've had a few problems and all but one have been simple roadside repairs.

I altered the circlip on my needle yesterday - my needle has 5 notches and it was set in the middle. I lowered the circlip (further away from throttle cable end of needle) so my needle now sits 1 notch higher.

Performance wise its amazing - an 18mph WOT Hill i encounter on the way to work can now be done at 26mph and it accellerates still!!!

BUT - i cant get it to idle smoothly. The idle screw either makes it rev too high or cuts the engine - theres no happy middle. Also, the engine is really smooth when i'm on the gas but as sool as i let off the throttle it seems lumpy and feels like its going to die.

Before i changed this, it would idle lovely but felt a little gutless and took a while to get up to speed.

What do you think?

Gav

GearNut · #2 10-30-2009, 10:41 AM

My only thought is to double check for intake leaks. With the taper of the jet needle (or lack there of) at the top of it, it really should not have affected your idle mixture that much.

Venice Motor Bikes · #3 10-30-2009, 11:35 AM

I concur on the 'air leak'.
My other thought is, that because the needle position is now so high, that it's allowing too much gas to be drawn up @ idle.

Try a larger jet size (to allow more gas @ WOT) & return the needle to the original setting. (to cure the idle & rough engine braking)

chrisme · #4 10-30-2009, 03:18 PM

This is the problem with carbs that only have one jet. They don't like running well at WOT, half throttle, and idle. Pick 2, and be happy with it. Mine runs great between 1/2 throttle and WOT, runs really rich at idle, but 95% of the time the engine is running is between 1/2 and wot, so it is worth sacrificing a bit with idle.

marts1 · #5 10-30-2009, 03:46 PM

Strange that moving the c clip would give you more top end speed. I also tryed different settings and found more power and more top end using 3rd from top.

george_n_texas · #6 10-30-2009, 07:57 PM

I threw in some berryman b-12 in my fuel today and ran much smooter...I ran 16:1 for about 2 gallons breakin I imagine may have gummed up the jet a bit? At any rate when I cracked open the top I had a bit of crusted residue in the combustion chamber which is why I put the berryman in. I don't know if berryman increases octane or not but it cleans carbs well and is less than $3 fro 15oz can. That's enough for about 15 or so gallons of fuel!

Just_Gassit · #7 10-31-2009, 12:30 AM

Search this site for carb mods, there is one about adding a small hole between the float bowl and slide. This adds a small "idle air" fuel vapor to provide a crude idle jet circuit that is missing in this China carb. I did it to mine and idle did improve as did off idle responce. Worth doing if you are handy and have small drill bits.

evogav · #8 11-02-2009, 11:41 AM

I put the clip back in its original position (the middle) and it still has eratic idling. I think its got something to do with the cap at the top of the carb where the throttle cable screws onto. Maybe cross threaded or something. Its bizzare. Sometimes i slow and clutch in and all is well but other times the revs rise up to about half throttle.

Gonna strip the carb one evening and have a good look.

Cheers for all the suggestions.

Venice Motor Bikes · #9 11-02-2009, 12:24 PM

Quote:

Originally Posted by evogav

I put the clip back in its original position (the middle) and it still has eratic idling. I think its got something to do with the cap at the top of the carb where the throttle cable screws onto. Maybe cross threaded or something. Its bizzare. Sometimes i slow and clutch in and all is well but other times the revs rise up to about half throttle.

Gonna strip the carb one evening and have a good look.

Cheers for all the suggestions.

That really sounds like a air leak!! With the engine running, spray starting fluid around the engine gaskets & intake manifold, where ever it starts running fast is where the air leak is!!

Bikeguy Joe · #10 11-02-2009, 03:52 PM

Quote:

Originally Posted by marts1

Strange that moving the c clip would give you more top end speed. I also tryed different settings and found more power and more top end using 3rd from top.

It all depends on how yoiur engine is running before you change the clip.

Different mixtures (Gas to oil) and different altitudes will also affect the overall scheme of things.