mikuni 34mm carb jet sizes

hi folks i have a 74 850 with mikuni 34mm and pair of dunstalls , the pilot jet was getting stuck on me

so i have to eventually butcher it out as it was locked in had to drill and scrape our the excess an awful task last night

right ok, the book calls for 230 main jet and 40 idle/pilot what is ur opinion ?

i have electronic ignition and just had complete topend rebuild 1k ago,

i did notice beforehand the plugs looked a little black too ?

so any help with air screw setting be great too ?

i will try upload pics later

regards,Brian

There is a whole bunch of threads on here discussing Mikuni jet sizes, just do a search. For what it is worth my opinion would be, #40 is too big, #25 or #30 most likely to work.

SteveA said:

There is a whole bunch of threads on here discussing Mikuni jet sizes, just do a search. For what it is worth my opinion would be, #40 is too big, #25 or #30 most likely to work.

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thanks Steve, there were 40s in her just thought id put same again, she ticked over great and starts first kick everytime

35s work so do 40s. A fresh top end will require almost constant tuning until rings and valves seal/seat properly.

Best

I settle on 37.5 pilot for a 750

If the motor runs well at low throttle openings, you have probably got the jetting right at that level. The main thing is to get the needle jet size and the needle taper and position right. The main jets should always be rich. It takes patience to get the jetting right, and jets and needles cost money. You need to think about it logically and understand which jets do the metering at various throttle openings. As long as the main jets are too big, they do not affect the jetting at low and mid throttle openings. The taper on the needles is there to compensate for loss of vacuum as you opem the throttle. Slow taper needles can be good on a road bike.

folks i tried 25s as pilots and wont tickover and will only run on choke ?

When you are jetting your carb, you need the correct size jets, So you buy three of each type which are around the size you need, and try each of them in turn, from largest to smallest. Jets and needles cost money and that is probably the reason many motorcycles are poorly tuned.

Head said:

folks i tried 25s as pilots and wont tickover and will only run on choke ?

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#25 pilot jets are way too small, I used to ship the single Mikuni kit with 40s, 35s are also a good choice for Commandos with a stock air filter.

seattle##gs said:

I settle on 37.5 pilot for a 750

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Great suggestion.

Goodness is when the air screw (pilot air trim) is off its seat between 1 and 2 turns out, this will give you the ability to adjust more broadly for ambient temperature.

Best

RoadScholar said:

#25 pilot jets are way too small, I used to ship the single Mikuni kit with 40s, 35s are also a good choice for Commandos with a stock air filter.



Great suggestion.

Goodness is when the air screw (pilot air trim) is off its seat between 1 and 2 turns out, this will give you the ability to adjust more broadly for ambient temperature.

Best

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thanks buddy, question for u, i have 260 mains and 35 pilot, she starts first kick when cold or hot,i have air screw at 1 and a half,i do notice if i leave choke on too long she smothers herself after 15 seconds or so,i do have 45 thou pilot will this help ?, also how do you set air screw im only guessing at moment,she idles at 1200 revs, she has had full top end rebuild 100% 900 miles ago now, pulls like a train too

Head said:

thanks buddy, question for u, i have 260 mains and 35 pilot, she starts first kick when cold or hot,i have air screw at 1 and a half,i do notice if i leave choke on too long she smothers herself after 15 seconds or so,i do have 45 thou pilot will this help ?, also how do you set air screw im only guessing at moment,she idles at 1200 revs, she has had full top end rebuild 100% 900 miles ago now, pulls like a train too

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I recommend a 40 idle jet, no larger. A 240 main will be a tad rich, a 37.5 might be ideal. Your throttle cable should have 2 to 3 millimeters of slack when you establish an idle in the 900 to 1000 RPM range; at 1200 RPM. You should have a 6DH3 needle with the clip in the middle groove (position 3) and have a 159 O-8 needle jet.

Best.

Looks like I was out on my pilot jet comments, sorry about that! I do have #27.5 pilots in my twin carb set up, but with velocity stacks and no air filters, and of course idle isn't my priority and I don't kick it into life! Though it fires up pretty rapidly on the rollers.

I had interpreted the OP also had twin 34s, maybe I got that wrong too, though I doubt that makes as much difference as an air filter.

#40 is still large in my experience, and #45 to me is 2 stroke territory, but I'm being told it can work.

With that size fitted, I would expect to be able to start it with no fuel enrichment! I fitted a VM32 to my AJS single road bike and had exactly that experience in initial tests with too large a pilot, idle was the priority on that set up and on the right pilot it starts great and tickover it is great.

I have used 6DH3 needles on my Norton, though I have a slightly different one in at the moment they are pretty close, but I have never used anything as small as a O-8 needle jet, though a pair of used VM34s I bought did come with a pair.

I use mainly in the P4/P5/P6 range and I use 200 to 220 main jets, though I have been as low as 180. This doesn't surprise me much because my guess is you all have 2.0 air jetsfitted , which is a stock Mikuni fit, and it should be adjusted to suit the application. Look at the manual marinatlas posted a link to, and it will tell you where you need to be, but expect nearer 1.0 than 2.0. Too large an air jet will cause high rpm lean out. This is typically compensated for with a larger main jet, which only partially works and generally robs you of high-end power.

Whether you will ever spend enough time at full throttle/high rpm to detect the problem is another matter.

Road Scholar has clearly spent enough time with this to be confident in his recommendations and choices, it is not my intent to argue, more to give an alternative perspective.

With a race bike set up, I do have a different objective, and a different compression, cam and exhaust choice.

update folks,

i am running 260 mains and 35 pilot at the moment ,
she starts first kick with choke for 5 to 10 seconds ,
then i do have to bring idle screw up slightly until she warms up,once warm she is perfect
even when hot starts first kick and ticks over at 900 to 1k
took her for small ride and pulling like a train no dead spots
i have the air screw at 1 1/2 turns out seems to be the sweet spot
just fitted new plugs
if i was able to upload a video i would thats another on the to do list
i want to thank all of you for ur huge help here
it is a great site
go raibh mile amaith agat