Tuning Bike Carbs

[Guest billynom8s]

Calling the bike carb gurus !!!!

 

Im about to fit these zx6r carbs from G.B.S and after balancing them im going to try using a COLORTUNE kit to get the whole set up running near as i can to perfect.

 

Can anyone identify which of the 2 screws i need to be adjusting to get mixture correct ?

 

Thanks

 

Nick

 

[Peter Bell]

A. how many revs can a colourtune go to these days?

[Guest billynom8s]

A. how many revs can a colourtune go to these days?

 

 

Hi Peter

 

Thanks for identifying the screw.

 

Looking at the the instructions that come with colortune the highest revs quoted for "Rapid throttle operation" testing is 4000revs !!!!

 

Cheers

Nick

[steve in stockport]

That screw will only alter the idle mixture, the mixture throughout the rest of the throttle range is changed by the size of the main jet and the needle position and size, very similar to how SU carbs used to work.

Screw A's all the way in til they just seat then back out again 3 and a half turns, that should set your idle mixture.

Hopefully GBS will have drilled the main jets to about 1.65mm or thereabouts and set the needles, if not you will need to play around a bit.

 

Are you still in Bollington? If you are and need drills for the jets Cromwell engineering supplies nr B&Q in Stockport do drills in 0.05mm increments at about 70p each.

 

Steve

[steve in stockport]

It might also be worth putting some glue on the hose connecting the carb to the manifold as one of the big problems is that at idle the massive vacuum makes the hose pop off cos jubilee clips are too big for the job.

either that or make new clips from 6mm x 2mm bar - like the balloon clips Tricky used to make

[Guest billynom8s]

That screw will only alter the idle mixture, the mixture throughout the rest of the throttle range is changed by the size of the main jet and the needle position and size, very similar to how SU carbs used to work.

Screw A's all the way in til they just seat then back out again 3 and a half turns, that should set your idle mixture.

Hopefully GBS will have drilled the main jets to about 1.65mm or thereabouts and set the needles, if not you will need to play around a bit.

 

Are you still in Bollington? If you are and need drills for the jets Cromwell engineering supplies nr B&Q in Stockport do drills in 0.05mm increments at about 70p each.

 

Steve

 

Hi Steve

 

Thanks for the info...i checked with G.B.S and they told me main jets are drilled 1.8mm which from what i've read on here and locost website is big, but Richard at GBS says is fine !!

 

I also checked the needles and they don't seem to have the option of adjusting position via the base of needle so looks like i'm in for major session of messing about.

 

Yeap still in Bollington so thanks for the contact info for Cromwells

 

Cheers

 

Nick

[steve in stockport]

When you say you checked the needles did you remove the cap from the top of the carb and take out the needles and diaphragms, then did you pull the cap out of the centre of the diaphragm - very slowly and carefully, under a towel, maybe, to catch the little spring - that releases the needle.

You should find some notches with a small circlip - this is where you adjust them, circlip to the bottom, pointy end, to enrich and to the top to weaken

[Guest craig030774]

i had to get the proper bibe rubbers to manifols as silicon hoses kept popping off these have a lip on the inside and when used with the correct bike clips are great a good investment in my book. ps ebay is your friend as blo**y dear new

[Guest billynom8s]

When you say you checked the needles did you remove the cap from the top of the carb and take out the needles and diaphragms, then did you pull the cap out of the centre of the diaphragm - very slowly and carefully, under a towel, maybe, to catch the little spring - that releases the needle.

You should find some notches with a small circlip - this is where you adjust them, circlip to the bottom, pointy end, to enrich and to the top to weaken

 

 

Hi Steve

 

Yeap took the needle out as you described and its totally smooth at the base bottom end with no ridges or circlip's to offer adjustment.... not what i expected to see !!

 

Nick

[Guest robin]

ridges and clip should be at the top end inside the diapghram...

robin

[Guest billynom8s]

ridges and clip should be at the top end inside the diapghram...

robin

 

I got spring from diaphram then then a plastic seat that fits in end of spring to the take base of needle and no ridges or clips !!!!

[Peter Bell]

Mine just have one circlip groove but had a nylon spacer about 2-3mm long under the circlip which raised the needle, I took them out and replaced with washers trying different numbers til it seemed to give the best results. It was quite rich originally. Difficult to see on the photo but this is with no spacers, these are Mikunis btw.

[Guest salty_monk]

The no notches needles were to keep the emissions boys happy... supposedly so it was set & you couldn't tamper with it.

 

As Peter says you can use little washers to set them. One thin washer is about half a notch equivalent. All circuits affect the others so the bigger main jets will richen the needles up anyway so you may not need a lot. Be surprised it it was more than a notch equivalent.

 

Basically Screw A affects the pilot circuit so idle & very low throttle opening, that's all you can do with the colourtune. Needles affect from 1/4 to almost full throttle & the main jets affect the full throttle only. BUT they are all inter-related.

 

You also have to balance the carbs - that's very important too.

 

Set base 3.5 turns as Peter says, balance them then go for a run. You need to find the main jet that pulls hardest at full throttle. Once you have that jet you can then move to the needles to get maximum pull from them with no flat spots or stutter. Lastly you can tweak the pilots again using "highest RPM method" (tweak one to get highest RPM, reset RPM then move to set next one etc).

 

IN theory you should balance at each stage but you will probably be ok on those carbs just balancing the once.

 

Dan

[Guest billynom8s]

The no notches needles were to keep the emissions boys happy... supposedly so it was set & you couldn't tamper with it.

 

As Peter says you can use little washers to set them. One thin washer is about half a notch equivalent. All circuits affect the others so the bigger main jets will richen the needles up anyway so you may not need a lot. Be surprised it it was more than a notch equivalent.

 

Basically Screw A affects the pilot circuit so idle & very low throttle opening, that's all you can do with the colourtune. Needles affect from 1/4 to almost full throttle & the main jets affect the full throttle only. BUT they are all inter-related.

 

You also have to balance the carbs - that's very important too.

 

Set base 3.5 turns as Peter says, balance them then go for a run. You need to find the main jet that pulls hardest at full throttle. Once you have that jet you can then move to the needles to get maximum pull from them with no flat spots or stutter. Lastly you can tweak the pilots again using "highest RPM method" (tweak one to get highest RPM, reset RPM then move to set next one etc).

 

IN theory you should balance at each stage but you will probably be ok on those carbs just balancing the once.

 

Dan

 

Hi Dan

 

Thanks for the advice,much appreciated.

 

So from the sound of it im going to need to solder and re drill jets to 1.65 and try needles as they are with out any washers to add length as a starting point ?

 

Cheers

 

 

Nick

[Peter Bell]

Don’t worry too much about the mains at this stage, they only really have much effect at big throttle openings 3/4 + The mixture is controlled by the tapered needle most of the time, as it rises the gap between the needle and the hole increases as the diameter of the needle decreases, this gap controls the amount of fuel being sucked into the engine. At big openings if the main jet is too small then it will limit the flow, if too big it will run rich.

Assuming the timing is already correctly set start off by setting the idle screws all the same 2 ½ -3 turns out, then balance the carbs when it is running either with a flow meter or listening to the sound of the suck with a bit of rubber hose. One of the carbs is the master, probably the one with the cable connected so get the next carb in the line sucking the same amount by adjusting the throttle linkage screws, then the next and so on. You will have to adjust the idle speed as you do this as the revs will be going up and down. This will give you a good starting point, if there is a lot of spitting back through the carbs at idle and slight throttle opening or overrun then the idle mixture needs richening. If the exhaust is sooty after normal driving then perhaps the needles could be lowered to lean the mixture if it is sooty after a real thrashing then the mains are probably too big. Conversely if the exhaust is very pale and it struggles under load then the needles need raising and possibly bigger mains, but without a rolling road session it will be just trial and error, do runs then check plug colours to get an idea of what is going on.