Since this comes up periodically, rather than everyone starting from scratch to provide answers each time, I searched for jetting information posted over the past 5 years. I simply cut/pasted relevant information from a number of posts (without credit and/or blame for the authors) of what appeared to be successful settings, making only minimal changes in some places to improve readability. Consider this post to be a first draft of a Wiki-carburetipedia for people to modify wherever needed.
To be most useful I think such a list should consist of a list of "best" suggested settings, possibly followed by comments on the effect of changes to those settings (e.g. use of a weak needle in a GP). However, tuning information common to carburetors used on all motorcycles (e.g. the importance of setting the the float level; how to read a plug; etc.) is dealt with in quite some detail in other threads so repeating it here would only make it harder to find GS-specific information.Amal 1-1/2" GP
Needle 3GP (set at the middle notch)
Needle jet 109
Main jet 360
Throttle slide cutaway 4
The settings for your carb are correct for the Clubman specification with 2442 and 2446 cams.
a #3 or #4 is commonly used on a muffled road GS.
Most GPs work well on road GSs with a #350 main jet at sea level.
Standard size air jet is 0.125", 1/8"
a racing GS with an open mega works the best with a #6 or #7 slide used along with a leaner GP6 needle.
The weaker needle should be marked 3GP6 at the top (you may need a magnifying glass to read the markings)
this needle measures 0.075" at the thinnest point.
The part nr. for this one is 316/408.
The std needle (this one is prefered for road use with silencer) is marked 3GP at the top.
This one measures 0.095" at the thinnest point at the bottom.
The part nr for this needle is 316/465
GP1 mixture screw adjusts air and the GP2 mixture screw adjusts fuel. Usually you adjust this in neutral with a warm engine, try to hold at 2500rpm and adjust in and out untill highest rpm is found,
Stock 1 1/2" GP needle jet is a 109, Lift the needle up a notch if you try to use a 108, for a start.
stock book main jet is 350.
I did try the 3GP6 needle last summer, though it was clearly not the right move. Legend has it that if you can get them to run on the (leaner) 3GP6 needle, they are better, but I didn't fiddle with the needle jet or slide to make it work. Assuming it does. (standard silencer - leaner needle usually used with open megaphone
My engine was running rich at 1/2 throttle, so I changed from a 109 to a 108 needle jet.
That resulted in a little stumble at low throttle openings, so I changed from a 4 to a 3 slide and that cured it. Unfortunately I can not remember if I tried to change the needle setting before changing the slide.Amal 10TT9
1-3/32" for the BB32GS, CB32GS and DB32GS
1-5/32" for the BB34GS
1-3/16" for the CB34GS, DB34GS, and DBD34GS
The BSA Gold Star Maintenance manual
has the same settings for all of the above for both touring and scrambles:
Main jet 360
Needle Jet .l09
Needle Position 3
Slide 7Amal Monobloc
3-1/2 slide is a good cutaway to begin with.
A 240 main, 25 pilot and .106 needle jets should be good. Start with the needle clip in the middle notch.
Gold Star literature calls for a 1 /16 Monobloc
, with a .1065 needle jet, 260 main jet, 4 slide and needle position 3. Modern fuels seem to like things a little leaner.Amal Concentric Mk 1
The rule of thumb when moving from Monobloc
was to drop the main jet size by 10% so 300 became a 270.
to use a 2-stroke carburetor change spray tube to straight and order up date kit #622/235 called conversion kit 4 stroke
Here's what's worked for me on my 1038 at sea level.
main jet 380, needle jet 622/122/108, needle 622/124, jet holder 622/128, pilot jet 30, slide 3.
only difference is my throttle valve is 3.5 so you may want to try that.
common baseline for the 38 is:
standard needle (start in the middle)
108 needle jet
main jet anywhere from 300 to 390
pilot jet 25 , 30 or 35
DB34 with a DBDhead and AMAL
1038 carb. 3,000 ft. there are 2 different jet holders for the 1000 series . The shorter holder and 4 stroke needle do not work well, the needle can foul the main jet. I am now down to 106 needle jet, needle in the middle. 107 jet was loading up on the over run. Main jet is my issue now, 330 was not too bad, 350 misfires as soon as the needle clears the jet, currently trying 320 main, no verdict yet. I have a 3.5 air slide, but feel a 4.0 might be better. Current. settings give me great acceleration up to 6,0000+ in the lower gears, a little rough on the overrun just off throttle, not sure of the main jet jet.Amal Concentric Mk 2
The standard jetting is pretty close, 3.0 slide, (only one needle) needle in the middle position. 30 ro 35 pilot jet, 49 starteer jet, 320 main jet...
Mark 2 Concentric
Main Jet: 320
Pilot Jet: 30
Needle Jet: 107
Throttle Slide: 3 Heavy chromed brass type
Throttle needle: middle positionMikuni VM
i HAVE A 38 mikuni...is the jetting still 25 pilot 220 main R0 needle jet 6dh3 needle #3 slide?
Jetting is the same for both 36 and 38. Some years ago, Allens told me the jetting they set the carbs up with is:
Main Jet: 240
Pilot Jet: 35
Needle Jet 159 series 'P8' (though I suspect the 38mm would possibly need the 247 series needle jet, in the same P8 size, depending on the carb body)
Needle: 6DH3 (pretty standard needle for a 4 stroke)
Slide - forgot to ask