Tuning Bike Carbs

[Guest billynom8s]

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.

 

 

Hi Peter

 

Thanks for that sound advise...just what i needed guidance wise and fully understandable.

 

My timing should be spot on from rolling road i had last year with the dgas carb fitted so i can follow your guidlines to the letter.

 

Cheers

 

Nick

[Guest salty_monk]

You should remember though that the needles & mains work together... i.e. the needle "restricts" the hole in the main jet on anything less than wide open throttle. Obviously as your needle diameter & taper won't be changing the larger hole will let more petrol flow for a given vacuum (or lift of the slides).

 

It is really better to get the main right first but being 5 or 10 off probably won't have too much effect.

 

You can also do "chops" i.e. hold constant throttle for a period of time & then clutch in cut the engine & coast to stop.... you can then look at the colour of the plug to see if you are rich or lean in that part of the throttle.

 

Judging by your experience I would say the mains are probably too big & you need to get them balanced properly. Hose trick will help but bike carbs are quite sensitive & benefit from the use of something like a Morgan Carbtune or similar.

 

There seems to be a real attitude of "drill out the main & throw them on" with bike carbs in the Kit car world. If any of these people had ever worked on them with bikes where you can much more easily feel the effects they'd know it's not that easy....!

[Guest woolly]

thanks for the information guys

i have jumped in and ordered some zr6 carbs and will set about making a manifold later in the week.

what is done about the choke, one person said to me that its not required it will start anyway, would suggest its a bit rich ? old sus and SB carbs would never start cold without choke if correctly set

 

 

woolly

[Peter Bell]

Mine will only start on choke, just pull it out DON'T touch the throttle and off it goes. I used a standard choke cable but a pull and twist one would be better, I have to use a clothes peg to hold it out initially, seems a pretty common accessory for bike carbs!

[Guest robin]

i have used a pull and twist choke mounted under the steering column. works a treat. got it from last years peterborough show for about £7. Tifosi i think?

robin

[speedtripledan]

I,ve just converted my 2.1 pinto to bike carbs from efi, using fireblede carbs as i already had them and bought a gbs manifold. The engine ticked over streight away but will not rev cleanly.

I,ve found a rolling road near me who has a car and a bike dyno.

I,m getting him to tune the whole thing seems to know his stuff, also advised me there is some power to be had from a vernier cam pulley, which i have now ordered.

 

The car is booked in for next sat will post results and bhp figures

 

Dan

[Guest woolly]

sorry silly question time.

 

what is screw b please on the picture mine are zx6r carbs

i fully understand the balancing or think i do

is it ok to block off the return ? fuel pipe i have a pressure req fitted.

on the top rear part of the set between 12 & 34 carbs is a t piece 4mm short pipe and that if for ?

 

i started to make a manifold but running out of talent.

 

i purchased the carb to engine connectors which have a bolt to engine flange, do you think its worth buying the RH manifold and welding a flat plate to the pipes to be able to use the connectors seeing that some people have had the odd carb fall off, as i now see how it could happen.

 

thanks for reading

 

woolly

[Guest robin]

hi woolly,

srew B is i beleive just a fuel float chamber drain for removing rubbish/water from the carb without having to remove them.

i think th small bore 4mm pipes are for carb heating and can be removed as they shouldn't need them.

 

i noticed that the carbs have been spaced using the exact same method as 'cryomans' as used on my carbs. any link?

 

robin

[Guest woolly]

helpful link for bike manuals

 

woolly

 

http://www.carlsalter.com/motorcycle-manuals.asp

[speedtripledan]

Hi,

My car has been to the dyno now with some interesting results when it 1st got there it was running rich as hell with the main jets taken out to 1.6mm by gbs after much fettling and the mains reducing to 1.3mm with a few other carb mods it run up with 102bhp at the wheels according to the dyno guy this was approx 20bhp up on standard once that was sorted a little adjustment of the cam pully gained another 6bhp overall 108bhp at the wheels approx 130 at the flywheel.

My Engine has a fuel injected head standard cam and standard efi dizzy.

The dyno guy was really impressed with the gbs manifold said it flowed well and for the price was unbeatable.

My car was there for 4hrs all in and he charged me £160 that included welding a lamba sensor ring into the exhaust to he could get a better read out..

 

What it also proves is there is a lot of crap info about claiming all sorts of magic BHP figures for pintos i was told by someone that 150bhp was possable from a standard pinto my engine is in perfect condition as it was checked today before the run and got nowhere near that

 

Not tried the car on the road yet due to the loverly weather we are having.

 

My advise to anyone messing with bike carbs is get it set up on the dyno as you will save a lot of messing around !!! there is a bit more to it than you think.

 

I was really impressed with TORQUETUNE in new ollerton (next to Gbs factory) give him a call on 07976961442

 

I,ll be going back when i can afford a modified dizzy fr32 cam and some head work, be interesting to see what that produces.

 

Dan ( waiting for the snow to melt)

[Guest woolly]

robin thanks for the info i found a manual for the zx6r and now understand a lot more.

 

Dan interesting about the main jets as the standard ones are 1.55 anyway. due to the weather i am still waiting for my gbs manifold to appear.

what i have decided to do though is to use the bike carb rubber coupling in stead of the jubilee clips, as the proper couplings have a ridge built in to help keep then in place.

i have made a plate to weld to the gbs manifold with the ports and mounting holes, this will also allow me to use the 4 vacuum take offs

 

as my 2.1 pinto is just about f2 spec the cam makes it a bit rough low down but measured 136 at the wheels so the idea of the carbs is to regain some low down smoothness, time will tell.

 

woolly

[Guest SEgan]

the mains reducing to 1.3mm with a few other carb mods it run up with 102bhp at the wheels according to the dyno guy this was approx 20bhp up on standard

 

Hi Dan, some very impressive figures from a standard head and cam, I am very interested in the 'other carb mods'. A 1.3 main jet sounds very small compared to the sizes others (including me!) are using, did the other mods include blocking the air corrector jet? Did you do anything to the slide lift holes?

 

Sean

[speedtripledan]

Hi Dan, some very impressive figures from a standard head and cam, I am very interested in the 'other carb mods'. A 1.3 main jet sounds very small compared to the sizes others (including me!) are using, did the other mods include blocking the air corrector jet? Did you do anything to the slide lift holes?

 

Sean

The air corrector jets were blanked off, the car was to rich at the high revs you could smell it let alone see it on the dyno pc. By blanking the air correctors and changing jets to 1.3 he gained 10bhp it went from 92 to 102 bhp then a turn of the cam pully strangely retarding it a few degrees out popped another 6bhp .

 

Just took the car up the road for a quick spin and my god what a differance the car was spinning through 1st 2nd and part of 3rd (damp road) only ever used to spin 1st.

The guy i took it to knows the pinto in side out he has done a lot of work for stock car racers and the like also he is a bike nut and has a bike dyno so he knows that side inside out too.

 

The money spent on the pulley manifold fuel pump and reg would have been a waste without the dyno run.

Dan

[Guest SEgan]

Thanks for sharing the info , have fun,

 

Sean

[Guest Techmage]

Found this link which may be helpful if your doing or thinking of a a Bike carb conversion

 

http://www.totalvauxhall.co.uk/resources/t...4.tech60697.pdf

 

There seems to be nothing about the air corrector jet though. Is it normal to block them or does

it depend on which bike the carbs come from ?