Mikuni VM34 Fuel Level

franko · Jun 14, 2020

I have a Mikuni VM34 that I am Trying to do a bit of fine tuning on.
I've worked my way down to a 25 pilot jet. It will adjust to a turn and a half out, then not take any adjustment after that.
I'm wondering if the fuel level is too high.
I got a fuel bowl drain plug from a snowmobile that has the nipple formed with it.
I attached a clear tube to it.
The fuel level comes up to the low part of the heads of the bowl screws.
Does anyone know what the actual fuel level is suppose to be on these carbs?
To me, I think the pilot circuit is rich, but I'm trying to figure out why.

seattle##gs · Jun 15, 2020

I am interested in the float bowl drain plug...is this a stock item or custom made? If I can buy them I want a few. any info?

franko · Jun 15, 2020

I got the plug from a snowmobile salvage yard. It is a stock Mikuni part for the VM34.
You can see one here. https://www.ebay.com/i/332519432250...CzSU59r1vAVcrHCgRT7YhKhjnwyEl4CgaAtN5EALw_wcB

acotrel · Jun 15, 2020

I once decided to drill the drain screws on my Mikuni carbs, so I could simply slacken then to drain the carbs. I found that had been done already.

NPeteN · Jun 15, 2020

franko said:
I have a Mikuni VM34 that I am Trying to do a bit of fine tuning on.
I've worked my way down to a 25 pilot jet. It will adjust to a turn and a half out, then not take any adjustment after that.
I'm wondering if the fuel level is too high.
I got a fuel bowl drain plug from a snowmobile that has the nipple formed with it.
I attached a clear tube to it.
The fuel level comes up to the low part of the heads of the bowl screws.
Does anyone know what the actual fuel level is suppose to be on these carbs?
To me, I think the pilot circuit is rich, but I'm trying to figure out why.

22-24 mm carb body to arm (sans gasket) subtract 1mm for gasket in place.
www.mikuni.com › pdf › vm...PDF Mikuni VM Manual - Mikuni American Corporation

concours · Jun 15, 2020

franko said:
I got the plug from a snowmobile salvage yard. It is a stock Mikuni part for the VM34.
You can see one here. https://www.ebay.com/i/332519432250...CzSU59r1vAVcrHCgRT7YhKhjnwyEl4CgaAtN5EALw_wcB

those are a Polaris snowmobile item, used all through the 90’s.

edit: I meant the drain hose barbed bowl plug were a Poolaris item. We bought ‘em for our Ski-Doo Rotax Mikuni’s.

franko · Jun 15, 2020

NpeteN: I couldn't get your link to work.
If it is for turning the carb upside down and measuring the tangs, I'm at about 7/8"
I don't know if it is just slop or other issues, but I think(but don't know) the 20 pilot jet should be too lean.
When I took the carb apart to refurbish it, the pilot jet hd the mikuni emblem and the number 168 stamed on it.
I haven't been able to find a reference to that number.
It ran rich, that is why I was working on straightening it out.

RoadScholar · Jun 15, 2020

A 20 pilot is absolutely too small, unless the choke circuit is leaking by or just wide open.
If your motorcycle appears to be running rich with a 20 pilot jet it is most likely because the needle jet/needle are still left over from the carb's snowmobile duty.

So, what are the numbers on the needle, needle jet and the main jet? What is number on the slide? Is there an air corrcetion jet, snowmobile carbs for 2-strokes, generally, don't use them.

Does the hose attached to the float bowl drain have a plug or a valve? Looks like the intention of the hose was to provide a way to drain fuel for when the machine was put away for storage, can't be sure.

I will say that it looks rough, but if the passages are clear you may be able to press it into service. I am happy to assist you with the correct jetting, but it may be a case of pay-me-now-or-pay-me-later especially if the slide bore and the choke seat are worn/corroded ; a jetting package with slide and seal kit will run between $60 and $70 + postage. A new carb with all the correct parts about $150, but there are very few vendors who know what those parts are and fewer yet who will work with you if changes are needed.

Best.

Peavey Jeltz · Jun 15, 2020

Try this http://www.mikuni.com/pdf/vmmanual.pdf Mikuni doesn't appear to specify a fuel level from the bottom of the float bowl, but rather float arm level measurements as measured from the base of the carb body. Depends on whether you have twin or single floats. See page 12 in the above linked pdf fig. 18.

As RS says, #20 sounds small. Jeltz runs a single VM34 on a 750 Commando and a #35 pilot is about right.

franko · Jun 15, 2020

Road Scholar: I lost my notes of what was in there when I bought the bike back.
It is a WM34 168 B. The drain, I bought to try to find the fuel level. I don't believe it is a carb from a snowmobile..
The Main Jet was a 250, I put in a 230.
The needle jet was P0, I put in a P8.
I started with a #40 pilot jet and have worked my way down to a #25 pilot jet in #5 increments.
The #25 responded to 1 and half turns out, but would not falter with more turns out.
I have the #20 ready to use, but I think I have another issue and don't want to try it just yet.
The slide is a 2.5.
The needle is on the middle clip.
The carb was cleaned in a ultrasonic cleaner and all passages blown through with compressed air.
The flange adapter is new is new.
The bike ran rich before.
The original pilot jet being a 168 has me baffled.
Thank You for any help.

RoadScholar · Jun 16, 2020

There is no such 168 pilot jet, the largest one I seen listed is a 140 in the SUDCO catalog; are you referring the the jet that lives in the small well just forward of the needle jet well?

I would have jetted the VM 34mm carb for a 750 at seal level with:

6DH3 needle with clip in position 3
35 pilot jet, bleed type
230 main
159 O-8 needle jet
2.0 air correction jet
2.5 slide

Do check the choke seat to make sure the lip is smooth and will make a good seal with the choke plunger's rubber bottom. The seats do corrode and the rubber does get brittle.

Best.

seattle##gs · Jun 16, 2020

MAP uses a #3 slide and a P-0 needle jet.

fiatfan · Jun 16, 2020

RoadScholar said:
There is no such 168 pilot jet, the largest one I seen listed is a 140 in the SUDCO catalog; are you referring the the jet that lives in the small well just forward of the needle jet well?

I would have jetted the VM 34mm carb for a 750 at seal level with:

6DH3 needle with clip in position 3
35 pilot jet, bleed type
230 main
159 O-8 needle jet
2.0 air correction jet
2.5 slide

Do check the choke seat to make sure the lip is smooth and will make a good seal with the choke plunger's rubber bottom. The seats do corrode and the rubber does get brittle.

Best.

Hi, what would you recommend for an 850, also at sea level? Of course for a 34mm.

Nater_Potater · Jun 16, 2020

fiatfan said:
Hi, what would you recommend for an 850, also at sea level? Of course for a 34mm.

I'm running this same setup, but with a #220 at 2400' MSL. As others have mentioned, it sounds like your enrichment plunger might have a failing/failed elastomer tip. They're easy to check, and relatively cheap to replace.

RoadScholar · Jun 16, 2020

seattle##gs said:
MAP uses a #3 slide and a P-0 needle jet.

2.5 + O-8 and 3.0 + P-0 aren't much different. the 2.5 will make the transition from idle circuit to needle metering a bit richer, but the difference at mid needle and the transition to the needle jet would be hard to discern without an eddy current dyno or riding with a wide bandwidth O2 sensor (search FullAuto's posts). In either case lowering or raising the needle will change the circumstances.

The beauty of the Mikuni is the large number of tuning options, the Amal Premiers' have a similar number, but, for example, the Premier idle jet retails for $15 to $20, the Mikuni idle jet retails for $5, or less; the rest of Premiers calibrated parts have similar high prices. My point is that with the single VM 34 you can more easily afford to experiment.

fiatfan said:
Hi, what would you recommend for an 850, also at sea level? Of course for a 34mm.

Start with: 35 or 40, 240, 6DH3 C-3, 159 O-8, 2.5. I suspect you will end up changing the main to a 230, possibly a 220, but when testing pay attention the the lean surge test.

One of my customers purchased a jet package for his 850 Mk3 as follows, I can NOT recommend this as a starting point!!:

Slide 2.5
Needle 6DH3, clip in position 3
Main 230
Idle 35
Needle jet 159 O-6

His engine was all stock and he had peashooters, not black caps, if you really want to try this work DOWN to it, do NOT start here.

The jetting I install for all my Mikuni kits favors a tad on the rich side which can cover a less restrictive, stock type, exhaust system and worn intake guides.

The jetting you select for a single Mikuni VM 34 should also take the ambient temp you ride in into consideration. The air and fuel will mix much more completely at summer temps then at spring or fall temps. With a 40 idle jet you will be able to adjust over a greater temp range then with a 35; couple that with needle position and you should be able to enjoy good performance over the entire riding season. If you ride in winter you may well need to get a bigger main and needle jet.

Best.

Geodimeter · Jun 19, 2020

RoadScholar said:
2.5 + O-8 and 3.0 + P-0 aren't much different. the 2.5 will make the transition from idle circuit to needle metering a bit richer, but the difference at mid needle and the transition to the needle jet would be hard to discern without an eddy current dyno or riding with a wide bandwidth O2 sensor (search FullAuto's posts). In either case lowering or raising the needle will change the circumstances.

The beauty of the Mikuni is the large number of tuning options, the Amal Premiers' have a similar number, but, for example, the Premier idle jet retails for $15 to $20, the Mikuni idle jet retails for $5, or less; the rest of Premiers calibrated parts have similar high prices. My point is that with the single VM 34 you can more easily afford to experiment.

Start with: 35 or 40, 240, 6DH3 C-3, 159 O-8, 2.5. I suspect you will end up changing the main to a 230, possibly a 220, but when testing pay attention the the lean surge test.

One of my customers purchased a jet package for his 850 Mk3 as follows, I can NOT recommend this as a starting point!!:

Slide 2.5
Needle 6DH3, clip in position 3
Main 230
Idle 35
Needle jet 159 O-6

His engine was all stock and he had peashooters, not black caps, if you really want to try this work DOWN to it, do NOT start here.

The jetting I install for all my Mikuni kits favors a tad on the rich side which can cover a less restrictive, stock type, exhaust system and worn intake guides.

The jetting you select for a single Mikuni VM 34 should also take the ambient temp you ride in into consideration. The air and fuel will mix much more completely at summer temps then at spring or fall temps. With a 40 idle jet you will be able to adjust over a greater temp range then with a 35; couple that with needle position and you should be able to enjoy good performance over the entire riding season. If you ride in winter you may well need to get a bigger main and needle jet.

Best.

Really glad I found this thread because I have a VM 34 on a 750 Commando which needs to be set up for the 4000-8000 feet elevation range (Colorado). When I bought this carb it had a 260 main jet (runs way rich) and I just bought a 250 and a 240 to try but I probably need to start with a 230 and maybe get a 220 also. Pilot jet is 25 (may need 30), needle 6DH3 position 4, slide 2.5, needle jet 159-Q-0. Would changing the main jet to a 230 and pilot to 30 be a good starting point? With the elevation changes hear I don't want to lean it out going down the hill. Thanks.

RoadScholar · Jun 19, 2020

Geodimeter said:
Really glad I found this thread because I have a VM 34 on a 750 Commando which needs to be set up for the 4000-8000 feet elevation range (Colorado). When I bought this carb it had a 260 main jet (runs way rich) and I just bought a 250 and a 240 to try but I probably need to start with a 230 and maybe get a 220 also. Pilot jet is 25 (may need 30), needle 6DH3 position 4, slide 2.5, needle jet 159-Q-0. Would changing the main jet to a 230 and pilot to 30 be a good starting point? With the elevation changes hear I don't want to lean it out going down the hill. Thanks.

I have to assume that your engine is in good shape. I which case:

The pilot should be good, but having a 30 is a good strategy; given the way you fine tune a Mikuni the idle circuit has a lot of bandwidth. The 220 main is a good starting point, I'd also get a 210.. 6DH3 I suggest position 3, think about trying position 2 if the transition from the idle circuit feels fat. The needle jet is way to large, try a 159 O-6. Slide, I think you would be better off with a 3.0, but if you can live with the 2.5 you will avoid the $30 price for a 3.0.

When your plugs stop fouling and the engine accelerates smoothly, constant speed is steady I suggest that you add 2 degrees to your full advance.

Best.

seattle##gs · Jun 20, 2020

I used to live in Lake Tahoe at 6200'. Plugs would foul all the time. I used the next hotter plug (NGK BP6ES) while riding at that altitude or higher and changed back to normal when I went to a lower elevation.

Mikuni VM34 Fuel Level

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