VM38 pilot jet?

Been running my 74 850 for 18 months now with a Mikuni VM38. Very happy with how it has worked out starts first kick good fuel consumption and good top end.
The only very slight problem is it runs slightly rich at less than 1/4 throttle. Have moved from a 40 pilot all the way down to a 32.5 still very slightly rich.
No engine performance mods and fitted with peashooter exhausts. Have searched this forum and the internet and have seen values from 30 to 45 qouted.
Would other VM38 users comment on the pilot jet sizes that they are using?

Raise the needle 1 notch(lower the clip 1 notch). Pilot circuit is primarily for idle.
If raising the needle cause cutting out or studdering at higher rpm, then a leaner needle or needle jet may be needed.

Slightly rich at low rpm my be a good thing and maybe even proper. What is the RPM at the level and is this giving a good rate of charge? Not that it has anything to do with the charging system, I am just questioning if you are simply running at to low of an rpm.
Again, this being a "Slight" issue with richness and the emensity of adjustments with these carbs may suggest both leaving well enough alone or delving into jetting sea.
Your Choice.

Anyone choosing to experiment is couragous and info is welcome. But beware, No two bikes suck mixture exactly alike and what Is right for some is rarely right for others when dealing with Mikuni's.

pvisseriii said:

Raise the needle 1 notch(lower the clip 1 notch). Pilot circuit is primarily for idle.
If raising the needle cause cutting out or studdering at higher rpm, then a leaner needle or needle jet may be needed.

Slightly rich at low rpm my be a good thing and maybe even proper. What is the RPM at the level and is this giving a good rate of charge? Not that it has anything to do with the charging system, I am just questioning if you are simply running at to low of an rpm.
Again, this being a "Slight" issue with richness and the emensity of adjustments with these carbs may suggest both leaving well enough alone or delving into jetting sea.
Your Choice.

Anyone choosing to experiment is couragous and info is welcome. But beware, No two bikes suck mixture exactly alike and what Is right for some is rarely right for others when dealing with Mikuni's.

Click to expand...

Thanks for the advice, I feel better now that I can stop obsessing with a clean conscious ;-).

As far as Idle goes i keep mine at about 1000 to 1100 since with loads of riding in town and lights on it keeps the battery happy.

Hi I have just fitted a vm38 and my bike is running very lean I wonder if you would be good enough to advise me as to to set up for your carb, I would be happy with a little rich on tickover.

Thanks bobby. 1976 850 mk3.

Huge carb.

What kind of clearance do you have under the backbone?

Can't be much room for the cable to make a turn towards the headstock?? (i'd love to see a picture).

Anyway, I would think that a #40 idle jet would be the number unless you changed to a cam with more overlap; the signal strength wouldn't have changed. A tad rich IS a good thing as it smooths the transition to the needle/needle jet.

What are you running for a needle and needle jet?

PM me if you need jets/needles, I have a good selection of the 159 series of needle jets and a larger selection of needles (tons of mains and idles).

RS

rocker1 said:

Hi I have just fitted a vm38 and my bike is running very lean I wonder if you would be good enough to advise me as to to set up for your carb, I would be happy with a little rich on tickover.

Thanks bobby. 1976 850 mk3.

Click to expand...

Hi will post the info on my setup plus photo tommorow morning (Lights in Garage out and all my notes etc in there :-()

For info on the Mikuni Carb and the principles of tuning go to

http://www.mikuni.com/pdf/vmmanual.pdf
This is great for info on what influences the Air Fuel ratio at different throttle openings an essential first read.

This is a list I found that has a collection of settings for VM Mimuni's on many bikes including 750 and 850 Commando's

http://www.braigasen.com/Mikuni_jetting_chart_four_stroke.htm




Interesting to note the variations in pilot sizes especially looking at the 850 Mark 111 pilots of 25 and 27.5 for a VM36 and 40 40 for the other 850's

Cheers Paul

A late addition to the list of info above is that this link which relates to DellOrto carbs actually has a much clearer description of the various stages of carburettor cation from idle through to WOT.(Better than the Mikuni document which has quite a few errors and overprinted corrections.

http://www.ducatimeccanica.com/dellorto_guide/dellorto_1.html

rocker1 said:

Hi I have just fitted a vm38 and my bike is running very lean I wonder if you would be good enough to advise me as to to set up for your carb, I would be happy with a little rich on tickover.

Thanks bobby. 1976 850 mk3.

Click to expand...

Hi there as promised my 74 VM38 settings

Main Jet 280
Pilot Jet 32.5
Needle Jet 159 P-2
Needle 6DH3 (middle notch)
Slide Cutaway 2.5

Let me know what you have...

Cheers Paul (Photo to follow)

Road scholar and plj850.

Thank you chaps, My Commando is at my mates workshop as he has rewired it for me. I will see him during the week and find out what jets I have to play with. Hope to have it all done for the 30th as I want to ride it to the Talmag trial at Hungry hill in Aldershot.

rocker1 said:

Road scholar and plj850.

Thank you chaps, My Commando is at my mates workshop as he has rewired it for me. I will see him during the week and find out what jets I have to play with. Hope to have it all done for the 30th as I want to ride it to the Talmag trial at Hungry hill in Aldershot.

Click to expand...

Feel free to PM me

Cheers Paul