Midrange carb tune

[TheMonkey]

VM34mm carbs from Hoos on a stock motor. Default jetting from Hoos. See pic for exhaust set up.

Bike runs really well for spirited riding, always starts immediately. I set up a wideband AFR meter. Generally runs between 12.5 and 14.0 which all feels good. Heavy throttle stuff ~13

Except midrange cruise at 20-25% throttle 2500-3500 rpm it is fat at 12.0. This feels like the place for efficiency to burn like 14.5.

And the other place is WOT gets super fat and stumbles <12 closer to ~11 or even fatter.

I’ve seen a chart in here that shows what part of different ranges get tuned with main jet vs needle and maybe has another axis with rpm? I can’t find it.

Any help with knowing what knobs to fiddle with would be appreciated. Thx

 

[jpdevol]

From the real Expert http://www.amckayltd.com/VM34-36.pdf

And the Mikuni manual

 

[jpdevol]

I think you would like having "the book"

https://hvccycle.net/sudco-mikuni-tuning-manual/

 

[TheMonkey]

Thank you !!

 

[grizld1]

VMs and early BS series carbies came very early in Mikuni history. Mikuni paid money to the French firm Solex for the initial BS carbies, and the VM design was bought from from the British firm Amal.

One characteristic of early carburetors is that component changes are poorly isolated. Example: a change of a single notch in needle position will as often as not force a change of pilot jets, and the smart tuner often takes advantage of strong secondary effects to ambush a defect instead of trying to confront it head-on. So: take those component-and-range charts with a pinch of salt,

Next: For some weird reason, whenever a problem crops up, we tend to blame the carbies. We can spend days trying to hunt down an electrical or valve train problem through the carbs while a pack of gremlins laughs at us. I've done it, knowing better, and I don't know one wrench of any level, from beginner to borderline genius, who won't admit to having done the same. Go here for a look-see on VMs for the XS650: www.amckayltd.com/vm34-36.pdf.

Monkey, JP means well, but I'm no expert, just a fairly advanced shadetree mechanic who's a pretty fair writer. Don't listen to that stuff. Keep on asking good questions and good answers may make it through the guff we amuse ourselves with.

 

[arcticXS]

VM34mm carbs from Hoos on a stock motor. Default jetting from Hoos. See pic for exhaust set up.

Bike runs really well for spirited riding, always starts immediately. I set up a wideband AFR meter. Generally runs between 12.5 and 14.0 which all feels good. Heavy throttle stuff ~13

Except midrange cruise at 20-25% throttle 2500-3500 rpm it is fat at 12.0. This feels like the place for efficiency to burn like 14.5.

And the other place is WOT gets super fat and stumbles <12 closer to ~11 or even fatter.

I’ve seen a chart in here that shows what part of different ranges get tuned with main jet vs needle and maybe has another axis with rpm? I can’t find it.

Any help with knowing what knobs to fiddle with would be appreciated. Thx

The rich condition at WOT indicates a smaller MJ is required
The slightly rich condition at around 1/4 throttle sounds like a different needle profile is required, that is thicker (leaner) in that region.
I guess you could first try leaning out the mixture screw as an initial attempt (that is mixture screw OUT) Normal position should be around 1.5 to 2 turns out from fully in (not tight, just where it stops) Do not go past 3 full turns out. If this improves things somewhat, without hurting the idle or running at small throttle openings, you could try a smaller idle jet as well, before starting to experiment with a different needle.

 

[grizld1]

Oh yes indeed, but here's the final in-the-saddle check. Roll the throttle wide open, then close it a little to the neighborhood of 75% to 80%. If the engine responds by pulling a little harder, go up a step on the mains. If the engine misfires briefly and settles down, come down a step--that's a rich signal.

Yeah, yeah, I know, a lot of other stuff can cause that roll-off misfire. That's why it's on the list of things that comprise the things that are in the rule that we should all subscribe to: Before carb work, everything else.

 

[grizld1]

Good advice from Arctic, as always, so this bit is just supplementary: Regard that needle jet as the heart of the carburetor.

On a street/road bike the NJ is the primary control in the cruising range of the throttle, where the engine spends most of its time. If the bike burns fuel like a
bigass SUV and reducing mains doesn't fix it, go down on the NJs. Oh yeah, they're spendy rascals. If you keep upping the mains to get rid of lean signals until the MJs are crazy big and the consequences are making a mess upstream, it's time to open the pocket book for a range of fatter NJs and leaner MJs.