Carb needle setting and idle problem...

evogav · Oct 30, 2009

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 · Oct 30, 2009

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 · Oct 30, 2009

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 · Oct 30, 2009

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 · Oct 30, 2009

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 · Oct 30, 2009

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 · Oct 31, 2009

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 · Nov 2, 2009

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 · Nov 2, 2009

evogav said:
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 · Nov 2, 2009

marts1 said:
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.

marts1 · Nov 2, 2009

Put a few sqirts of lub into the throttle cable before you connect the carb back up.

xlite · Nov 3, 2009

evogav said:
II 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.

I got a carb from Dax that behaved just like that and I isolated the problem to the cap on top by swapping with other carbs. Duane sent me a replacement immediately which worked fine. I never did figure out exactly what is wrong with that top but it's definitely bogus.

PS All the carbs I got from him say NTCC instead of NT and seem to have somewhat crooked and cross threaded tops but except for that one the others work fine.

GearNut · Nov 3, 2009

Venice Motor Bikes said:
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!!

Be cautious to not let the cleaner spray get sucked into the air cleaner. This will cause a false reaction.

bairdco · Nov 4, 2009

your cable might be too tight, also. try messing with the adjuster at the carb while you pull lightly up and down on the cable till you get hardly any freeplay. then take off your air filter and make sure when you turn the throttle it opens the slide up all the way.

evogav · Nov 4, 2009

Its really strange. I've had the top off again and have made sure theres no restrictions and i've even ptfe'd around the thread. I thought i'd cured it but all of a sudden the revs rose skyward again. I can feel that the slide is still working when i use the throttle.

I noticed that when i rock the bike from side to side (like you do to feel how much fuel you got left) - this causes the revs to rise dramatically. Literally went from tickover to 3/4 revs in a second or two.

I'm wonderin if i should just get a replacement carb whilst i strip this one down???

Gonna take my air filter off first and get it running for a short while while i look to see if i can see it sticking or anything.

george_n_texas · Nov 4, 2009

I was talkin about the fuel filter bein a paper element not the air filter.
I bought some Lucas 2cycle oil and it advertises no more clogged filters...must be a common problem for them to put that on the label.

GearNut · Nov 4, 2009

Just out of curiosity, is the little pin that guides the throttle slide actually long enough to engage the groove that runs the whole length of the slide?
I have had similar issues as you are having when the pin was either worn down too short or it fell out alltogether. These conditions can let the slide spin around in the slide bore and wreak havoc with any and all throttle positions, especially idle speed.

Egor · Nov 4, 2009

These little carbs are a pain in the rear. I have had a few that will not fetch up at all. As the bike breaks in it will usually settle down, then again it will sometimes go the other way. I am the one that started adding the air bleed to the float chamber in order to add some fuel to the idle. This carb is missing an idle circuit all together. You can try this and see what you think, I will not run any of my bikes unless the hole is in there. Have fun, Dave

http://motorbicycling.com/f13/stock-carb-mod-1494.html

evogav · Nov 9, 2009

FIXED!!! .wee.

It was an air leak. Got it running today and sprayed carb cleaner around various parts - as soon as i sprayed near the inlet manifold, revs rose and when i sprayed on the engine side of the inlet, it cut out. Turns out there was no gasket left top or bottom - only the remains of the gasket around the bolts.

The most shocking part of this is that the intake and exhaust sides of my grubee engine have nice wide ports but the exhaust and intake flanges are roughly half the size.

I got my dremmel out and widened the intake pipe - took about 3mm off each side but none off the top or bottom as there is only roughly 3mm alloy to make a seal with the engine.

Didn't use a gasket either - just put on some gasket paste instead.

Well its running better than ever now. No air leaks etc and has a nice throaghty sound to the intake - deffinately has more power low down now too. The carb got a complete strip down and clean too - i was shocked to see how much cr@p had gotten into the air filter housing and around the choke area.

I used sealant to seal up the intake to carb and also around the slide to slide top.

No more air leaks and running sweet.

Cheers for all the help guys. Lots of helpful hints here in this thread,

Gav.

bairdco · Nov 9, 2009

congrats.

you might have problems further down the road if you're not using a gasket on the intake. any auto parts store will have sheets of gasket material for cheap that you can make your own out of.

Carb needle setting and idle problem...

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