Proper Jetting and Fuel/Air Screw Position Mikuni VM-34 on a 1972 Yamaha XS2 (650)

[Hanniusms]

LATEST UPDATE: 03/25/24

Main Jet: #180
Pilot Jet: #27.5
Air Screw: .5 - .75 out from closed
Needle Clip: #3
Needle: 6F09
Needle Jet: 159 P-6
Slide Size: 2.0
Air Jet: Removed

Called SUDCO today who is the Mikuni Carb West Coast Distributor. They build and sell custom carbs for any application.

I told them there is still a flat spot from 0 to .25 throttle. And if I let the bike engine break with no throttle it bucks slightly as it slows.

They suggested upping the Pilot Jet to 30 and leaving everything else the same (except the air screw of course) and see if that smooths it out.

That is the next step. I’ll post results when I swap them out and take a test ride.

 

[WideAWAKE]

30 on the pilot, probably need to back out that air screw too.

180 main might be a little small but you can plug chop and find out.

needle and jet look good.

 

[Hanniusms]

30 on the pilot, probably need to back out that air screw too.

180 main might be a little small but you can plug chop and find out.

needle and jet look good.

What do you think would be the best size main??

#185, #190?? I want to get the engine running its best without overheating or over working it.

I am at 2500 ft elevation in Tucson AZ

 

[WideAWAKE]

What do you think would be the best size main??

#185, #190?? I want to get the engine running its best without overheating or over working it.

I am at 2500 ft elevation in Tucson AZ

I tend to take a reverse method to dialing in carb jets (especially mains). I see what I can get to run, then go big until it bogs then back off until it’s right.

Especially when doing plug chops, I don’t want to be finding out it’s lean and trying to bump it up.

I would sort out the studder first, then the mains.

I ended up with 195s with a 750 kit and open pipes. I don’t think it’s all that much more than stock.

If I had the jets, I’d stick 190s in and work backwards.

 

[Hanniusms]

UPDATE 03/30/2024

So I decided to put in #190 mains and the #30 pilots.

Bike started great and ran smooth at idle.

Took for test run and it still has some slight hesitation from 0 to .25 throttle and some very slight bucking when letting it engine brake with no throttle.

Figured I’d go up to #32.5 and call it fixed.

Put in the #32.5s had hard time starting and ran terrible was hunting for idle (up and down past 1.2k by 100 rpm each way)

Took it for ride and it ran terrible from 0 -.25 throttle. Ran like a champ once the #190s kicked in.

Bucked like a bronco engine braking.

I don’t get it?? Anyone have any answers??

These are cheap chinese jet kits off amazon so I can kinda dial it in and then buy mikuni jets once I know what I need.

I’m wondering if the #32.5s were stamped wrong and they were smaller or they were drilled wrong but I can’t imagine them both being wrong.

I had to up the air screw to 1 turn with the #30s and then with the #32.5s it was back firing and I had to dial it back down again.

I’m thinking they are stamped wrong and maybe are #27.5s or #25s

I’ll try going up to #35s and see what happens.

Any suggestions??

 

[WideAWAKE]

Only buy genuine jets.

Who knows what comes from those kits. You can measure the openings if you have a small set of pin gauges, at least you can verify consistency. But anftermarket jets are a crap shoot.

If the 30s were almost there, put em back and raise your needle one clip

 

[Hanniusms]

Only buy genuine jets.

Who knows what comes from those kits. You can measure the openings if you have a small set of pin gauges, at least you can verify consistency. But anftermarket jets are a crap shoot.

If the 30s were almost there, put em back and raise your needle one clip

I called up SUDCO this morning and ordered

Mains #185 and #190
Pilots #30 #32.5 and #35

Should have them by end of week. I will put them in this weekend and make a few more test runs.

I only bought the cheap amazon ones to kinda get a ballpark range before buying the Mikunis.

Was like $10 for 10 jets where Mikuni is $5 for 1 jet.

 

[WideAWAKE]

If you have a local motorcycle shop they likely have em on the shelf and are cheaper.

Or sell em at the same cost with no shipping.

 

[Hanniusms]

UPDATE 04/09/2024

Received the Jets from Sudco

Installed #190 mains
Installed #35 pilots

From 0 - 1/4 throttle there was still a slight flat spot and when allowing engine braking a slight bucking very slight but still there.

From 1/4 to full throttle bike ran beautifully.

Yesterday I ordered #37.5 #40 and #42.5 pilots.

Should have them by this weekend and will bump up to #40 to start and adjust either way to fine tune after that.

 

[WideAWAKE]

Where is your air screw set?

A slight adjustment should rid your problem.

 

[Hanniusms]

Where is your air screw set?

A slight adjustment should rid your problem.

About .75 a 1 turn out

 

[Hanniusms]

UPDATE 04/15/2024

Received the Jets from Sudco

Kept. #190 mains
Installed #40 pilots

From 0 - 1/4 throttle there was still a slight flat spot and very slight hesitation, but when allowing engine braking the bike ran perfectly and did not buck or stutter at all.

From 1/4 to full throttle bike ran beautifully again like before because of the #190 Main.

Air screw is @ 1 full turn out.

I took the bike out for a trial run and did a 15-20 mile test loop. No back firing, no smell of unburnt gas was noticed. Ran very good besides the slight hesitation off the line from 0 - .25 throttle.

I am thinking of bumping up to #42.5 pilots (I already have them) and this should solve the last bit of hesitation in the throttle.