vm16 carb issues

[dj.chicken1]

Ok so i have got a vm16 carb on the bike it came with a 85.5 main jet i think it was bogging over every range in the throttle

i went to a 61 size main jet and it runs alot better from idle to about 3/4 throttle

i saw this on another thread soem said this
My carb came with a 70 jet in it and it wouldn't go past half throttle without bogging . I went to a 95 jet 100 % better with just a bit of a off idle stumble. Today I went down to a 92.5 jet adjusted the air screw to the highest ldle about 1 3/4 turns out from seated . Bike has never run better , smooth idle no four stroking anyware with a nice crisp exhaust note

i am a bit confused if i am donig the right thing.

if it is bogging at 3/4 throttle to full throttle and it doesn't get going untill i back the throttle off should i go a bigger or smaller jet ?

any help with this would be awesome

thanks Zach

 

[Bikeguy Joe]

"Bogging" is kind of a weird term people use....it can be interpreted either way, the best way to get a handle on it is to look at the plug and see if it's too rich, or too lean.

 

[dj.chicken1]

its more like it will fire from 3/4 below properly and once you put full throttle it just dies unless it gets abit of momentum going first

 

[Bikeguy Joe]

Sounds a bit lean....if it's too rich, usually it will break into a four stroke, and not "bog".

 

[headtrama]

Try turning on the choke when it starts to bog and see if it will let you go to full throttle . I am running a 80 main jet now in mine and it runs well . Temperature will affect jetting as well as humility. Get in contact with Ivanh . He is in Australia to and I am sure he can help you out some . My jetting and his jetting are different same carb but different ends of the map .

 

[ivan H]

Hi mate, 61 will b 2 small (lean). I'm at 77.5 main. This may b couple posts cause using phone & run outta screen. Learn 2 read ur plug & exhaust note. Plug should b strong white coffee color, including flat part of earth electrode, blackish on the curve. If ur getting a tinny crackle exhaust sound over 3/4 throttle ur jet is 2 small, it'll also take a while 2 die down when u back off & pull clutch in when lean. If ur plug is grey ur dangerously lean.

 

[ivan H]

They come stock with 70 main, 25 pilot. If its idling ok dont worry bout pilot 4 now. Ur better off 2 start a bit rich & work down as u wont do harm, & needle height will affect WOT (until u get the jet right) so adjust the clip all the way up 1 position at a time b4 going 2 next jet down. U said it came with 85?, go down 1 size (2.5 increments) at a time & start each time with needle clip in center. Rich will 4 stroke & have a dull, flat exhaust sound, & fume.

 

[ivan H]

A motor thats a bit lean will run good downhill & produce hi revs downhil but lack power & run flat everywhere else & exhaust note is tinny & crackly. A bit rich will 4 stroke downhill & on flat but run better uphill under load, exhaust note is heavy dull sound, but ur plug is the best indication as 2 how ur jetted. When u get the main jet close, select the right 1 by which gives highest top speed WOT on a straight. That should get u close. Cheers

 

[ivan H]

Go 2 Mikuni corporation's web page, they should have a PDF file on tuning VM carbs, if they dont go 2 mikunioz.com & u'll get it there. If u have trouble getting both WOT & mid throttle positions in sync, try blocking off the air hole at the very bottom of air intake with the filter off. Only the bottom 1, the other is 4 choke circuit. It will mean jetting down as it stops air getting 2 the bleeder holes in the needle jet. Good luck with it. Cheers

 

[superharry]

To find out quik, cover a portion of your air filter, if the engine wants to die, its rich, if it picks up, a lean condition. My carb (vm22) came with a 95 main, and I have it all the way to a 126. its still a bit lean, but I ran out of jets!

 

[ivan H]

Both Headtrauma & Superharry gave good advice, quickest way 2 get an idea which way 2 go. Do a plug chop with a new plug, bring motor 2 temp & do a long enough run 2 color the plug, keeping throttle only in problem area, then with throttle still in problem area, pull clutch & hit kill at same time. Let cool & check plug, it'll tell u. Cheers