mikuni sudco VM34 jetting (again)

Status
Not open for further replies.
alan hodge

alan hodge

Joined
Oct 10, 2017
Messages
1,121
Country flag
well i like the sudco VM34/K&N filter/non-ethanol gas rig i chose for my 72/73 850 as it starts first kick and runs smoothly throughout the range and idles great.... but as some of you'uns have said and as i see when looking at Champion N4G or NGK BP7ES tip it is too rich in the real world what are you who have this combo doing for jetting to lean out just a hair without buying a pocketful of jets and taking this one out and putting this one in etc etc etc over and over up and down the road a hundred times.. if i have to change anything i would appreciate the advice of someone who has been down this path so i don't have to stub my toe on the same rocks...……..PS i am also even corn-sidering putting twin amals or even waffle evolushuns on …..all to get a nice golden tan plug tip instead of chocolate brown...……..little ol' OCD me.
gennelmens..let the re-marks begin please
 
Run good?
Fuel economy as expected?
Any other signs of too rich?
Strong smell from exhaust while underway?
 
Not so easy these days to tell how a bike is running just from plug colour
Unless it's really white and burnt or black, black/oily
After you have started the bike do you turn the choke/enrichener jet off immediately?
What sort of riding are you doing when you cut the engine to do a plug check IE what throttle position?
Cheers
 
runs perfectly makes me smile 500 miles since build 'done' plugs just somewhat sooty chocolate brown not oily at all maybe I chasing the impossible dream of khaki colored plugs tips maybe I need to find something else on this Spit Mk V/early 60s E Type Jag with 2 wheels to fret over maybe stop thinking of ways to spend $$$ on it...so far never ridden it over 4k rpm and 55 mph I am low and slow rider
 
Alan , I been running with Mikuni VM34 for almost 20 years , when I run pump high test my plugs look a little bit rich by old reading standard , when I run my special mix of 30% 116oct. leaded race gas and 70% high test pump gas , I get that nice tan colour we all like to see and I smile .... one can lasts me 2 years this way, it is expensive to buy here , last can was more than $100 for 5 USA gallons , worth it in my opinion , but I`m not riding to work every day .... my offer is still good if you want me to get numbers off my carb jets for you to compare , it sounds that if you not right on you very close ... maybe try moving jet needle clip one notch to lean it a little and check plugs after a run
 
I figure if you are just a bit rich I think I would leave all the other jets and settings as they are except for the needle jet. Mikuni VM34 uses 159 primary type needle jets. Example: If you have a P-2 jet in it, try a bit leaner one, P-0 or O-8.
mikuni sudco VM34 jetting (again)
 
If you don't plan on going over 55mph or 4000rpm you really don't want twin Amals on there mate
Just stick with what you have I'm guessing you get 60mpg ?
 
How about trying a slightly hotter plug ? (BP6ES) and see how it affects the colour?
 
you fellers are the best got a set of BP6ES might give those a try also have access to race fuel locals run in their stock cars and 110LL for aeroplanes…….but that somewhat in cornveeenient……..maybe you'uns just saved me $400
 
Last edited:
alan hodge said:
you fellers are the best got a set of BP6ES might give those a try also have access to race fuel locals run in their stock cars and 110LL for aeroplanes…….but that somewhat in cornveeenient……..maybe you'uns just saved me $400
Click to expand...
Aw shucks..
 
If you have a 3 cutaway slide? manually file the leading edge to make it a 3-1/4 instead of going to 3-1/2. If improved but not FULLY successful finish the filing to 3-1/2. If it fails to clean up your plug on light cruise, just buy a new slide...

or borrow a 3-1/2 before you start to play
 
Last edited:
I found this note in my mikuni spares it is credited to “Road Scholar” for VM34
No doubt this what I used for start point on my ‘72-750 .... you could still try raising clip one notch on needle ..

230 main jet
159-P-0-8 needle jet
6 DH3 needle
35 idle jet
2.5 cutaway slide
 
sudco folks tell me….250 main...35 pilot...159-PO needle....6DH3 needle jet...2.5 slide....2.0 air jet
 
try the clip trick , cheap and easy .... pretty sure my air jet is 1.0 , however would have to look to be certain about any of it, other than slide size , which is 2.5 and I think main is 220 .... it been on there a long time to remember with what left upstairs .... you just looking for a tweek , right
 
I made an edit on the size of the main jet my 850 customer uses.

The 250 is too big, but then you said, in essence, that you don't use the main. The P-0 is also too rich, some of my 850 customers have asked for O-6 needle jets.

If you let the engine idle for even 20 seconds the plugs will soot, not a big deal as the soot disappears quickly unless your valve guides or rings are bringing oil to the party.

The jetting that Sudco supplies is rich for liability (real or imagined) reasons, Rocky Point's jetting was also a bit rich, but adequate and a good starting point for individuals that know how and when to lean the mixture. One of my 850 customers (with stock cam and exhaust--whatever that is) was very impressed with:

Main: 230
Idle: 35
Needle jet 159 O-6
Needle: 6DH3, with clip in position 4
Cutaway: 2.5

He lived at sea level and rode when the ambient temp was mid 70s (F) or above, probably not above 90.

If I had the brass to sell such jetting in an 850 Mikuni kit, it would have been received by an individual that regarded a screw driver as a machine and would have created, or missed, at least 4 air leaks; I don't have enough ketchup to eat an engine rebuild.

Start rich in all stages, train your reproductive parts to be sensitive to the slightest changes in engine behavior. Or do as Full Auto did and purchase a wide band O2 sensor kit and save your reproductive parts for their primary mission, which takes lots of practice, also.....
 
Last edited:
i think before i throw a whole bunch mo money at this machine i will try the B6 plugs and/or lower the needle one notch all the while looking for/feeling for leanness
 
I think if you trying to lean it a tiny bit , you want to raise clip one notch ... would try that first then maybe change plugs , not both at same time ... the clip would be my first action ...
 
I am a great fan of single Mikuni 34. I am at sea level in Seattle. I used to use the 2.5 slide always but have changed to the #3 and the plugs stay clean a lot longer. The driving is a lot of in town and congested freeways. Otherwise stock Sudco jetting. P-o needle jet
 
Status
Not open for further replies.
Back
Top