Mikuni VM36 help

Status
Not open for further replies.
I'd recommend a #40 idle jet, that will give you more adjustment with the air screw. a 159 O-8 needle jet and a 260 main, needle clip in position 3 (middle). Everything else you have jibs with my chart.
 
Currently out on a ~15 mile ride. I take that back out the fuel dripping, it's as dry as a bone!

Will update properly when home.
 
Last edited:
Right, that was a very successful hour long ride! Roughly 38 miles done with a bit of stopping to admire the view.



Girlfriend reluctantly took a short video of me leaving!

I'm sure there's a load of room for improvement but she seems nice & responsive through the ranges. Managed to briefly wind her up to 70mph @ 3k-3.5k RPM. Seems to sit well at 30mph at 2k RPM in 2nd or 3rd.

NO DRIPPING FUEL! So pleased about that.

Now to give her a well deserved clean up for the local town show tomorrow morning.

& then address the other jobs on the list! First & foremost.. isolastic/fuel tank mounts.
 
Last edited:
RoadScholar said:
I'd recommend a #40 idle jet, that will give you more adjustment with the air screw. a 159 O-8 needle jet and a 260 main, needle clip in position 3 (middle). Everything else you have jibs with my chart.
Click to expand...
Interesting, I listen to your posts about tuning Mikuni carbs :)
I edited by previous post about the jetting I have, looked in my note book and I had a 6DH3 needle but have since switched that out to a 6DH4.
I don't see much difference in the graph between the two, so may not matter too much?
Mikuni VM36 help


You have me thinking I may try a bit leaner O-8 needle jet which may allow me to bump up the pilot to a 35 or 40 and still have a nice burn on the plugs.
There's so many combinations of settings on these mikunis its almost endless.
 
Last edited:
Chuck_Wagon said:
Interesting, I listen to your posts about tuning Mikuni carbs :)
I edited by previous post about the jetting I have, looked in my note book and I had a 6DH3 needle but have since switched that out to a 6DH4.
I don't see much difference in the graph between the two, so may not matter too much?
Mikuni VM36 help


You have me thinking I may try a bit leaner O-8 needle jet which may allow me to bump up the pilot to a 35 or 40 and still have a nice burn on the plugs.
There's so many combinations of settings on these mikunis its almost endless.
Click to expand...

There are about 21 different needles available from Mikuni for the VM carb series (30mm to 38mm--I believe). The differences I have seen in the Mikuni catalog appear to deal with the angle/width of the taper and the length of the taper. I can't prove it, but I'm under the impression that a 6DH3 in a 159 O-8 needle jet can be equated to other combinations of needles and needle jets. The 6DH3 is commonly known as the 4 stroke needle, although I use a 5F21 in the 26mm kits I offer for R50 and R60 BMWs. For the Norton, Triumph and BSA conversions I used to sell, to the BMW R75 to R100 kits that I do sell, the 6DH3 is the only needle I use.
 
Chuck_Wagon said:
Interesting, I listen to your posts about tuning Mikuni carbs :)
I edited by previous post about the jetting I have, looked in my note book and I had a 6DH3 needle but have since switched that out to a 6DH4.
I don't see much difference in the graph between the two, so may not matter too much?
Mikuni VM36 help


You have me thinking I may try a bit leaner O-8 needle jet which may allow me to bump up the pilot to a 35 or 40 and still have a nice burn on the plugs.
There's so many combinations of settings on these mikunis its almost endless.
Click to expand...

I am under the impression that the first two identifying digits on the needle needle are about dimension and taper 'D' is slower taper than 'F' - so 'F' is richer, the second two are about the finish. '6' is about the size carb the needle fits (it's length) . - ''D' is about the taper, 'H3' or 'H4' is about whether the needle is hard anodised or plated. If you lower the needles by fitting the clip in the top notch, the motor should cough when you rev it on the stand. If it doesn't, your needle jet is too big. Perhaps you are running with a blocked the air-cleaner ?
 
Luke, great that the carb overflow issue looks like its dealt with, that had to be solved before you could go further.
current settings:
260 Main Jet
30 Pilot Jet
6DH3 Jet Needle, e-clip on number 3 of 5
159 P2 Needle Jet
2.5 Throttle Valve Slide
2.0 Air Jet
Air screw: 1 & 1/2 turns out
New NGK BP7ES plugs adjusted to 0.025" thou gap.

Based on what I know, and RoadScholar's wisdom, all I would do is buy a 159P-0 & a 159O-8 needle jet.
Put a 35 pilot jet back in and the new 159P-0, turn air screw out at 1-1/2 and run it, try adjusting plug burn with only the air screw.

I want my air screw to be out 1-1/2 to 2. If you think still a bit rich, put in the 159O-8.

I played with pilot jets, 45 to 40 to 35 to 32.5 to a 30, before I got wonderful looking plugs with a 30.
each bike is different.

I'm going to get around to trying a 159O-8 and up my pilot to a 35 just because I can and its fun to fiddle, a larger pilot jet is a good thing as long as the particular bike likes it.

Whatever you do, stay a bit on the rich side rather than too lean.
 
Cheers all! Will address the fuel tank mounts ASAP & look at the isolastics.

I will also continue to fettle with the carb & give those bits a go, Chuck.
 
A loose top head steady or broken rubber could be the cause of the carb having intermittent overflow issues.
Too much shaking going on rattling the carb floats around?
 
Just in case you missed it, I did a little post here to thank you lot for your help.

What's the best way to run the bike to get an accurate plug colour?
 
Luke Turner said:
Just in case you missed it, I did a little post here to thank you lot for your help.

What's the best way to run the bike to get an accurate plug colour?
Click to expand...
If you are talking about main jet settings then you need to find a straight peice of road
Best with a slight uphill gradient if possible
Nail the bike in 3rd gear
Hold it above 3/4 throttle
Then pull the clutch/cut the motor at the same time, coast to a halt without letting the motor turn over
Then take both plugs out and check them there and then
But these days it's harder to get a good reading from the plug colour
Others on here may know a better method than this
Cheers
 
Luke Turner said:
Just in case you missed it, I did a little post here to thank you lot for your help.

What's the best way to run the bike to get an accurate plug colour?
Click to expand...

Very difficult to get a good colour read without lead in the fuel!

So the best way....is to add something like TetraBoost....Tetra Ethyl Lead additive, which is what was used in old leaded fuel mixes.

Or buy some Avgas for your 'microlight'!

Or go to a dyno and test it with the fuel you are going to actually use!
 
Last edited:
Or race gas or even lead substitute available here in Canada which is handy when out on the road for more than a tankful .....
 
baz said:
But these days it's harder to get a good reading from the plug colour
Others on here may know a better method than this
Cheers
Click to expand...

Recent experience says it is bloody nigh on impossible! Even with a loupe and a strong lamp you can't see down inside the plug to see what might or might not be there!
 
Craig said:
Or race gas or even lead substitute available here in Canada which is handy when out on the road for more than a tankful .....
Click to expand...


For most racers 'race gas' is 50/50 unleaded and avgas.

Specific racing fuels are available of course, but not only is it expensive, both to buy and ship, not all of it is leaded! Here in France Elf 102 race fuel is unleaded, more than 5€ a litre and you need to buy 50 litre tubs with 100€ shipping!

In the UK try these guys, who have a deal with the CRMC to deliver fuel to race meetings...... https://classicfuelsolutions.co.uk/
 
Last edited:
The last 5 gallon USA can of race gas 116 octane with lead was almost $125 CAD out the door .... I mix it with High test pump gas 91oct. so one can lasts me almost 2 seasons I use the K100 treatment so it lasts over winter , yup expensive but the bike seems to love this mix and works so well , I don’t mind since Norton is a Sunday morning rider mostly .....
 
Luke Turner said:
Luckily quite sheltered in the garage but everything in the gardens been bolted down!

I'll be there even if I'm not on my dad's Commando. Will no doubt be wearing my black Norton hat so if you see me then say hello.
Click to expand...
Off subject slightly, but were can I get one of those hats Luke?

Cheers,

cliffa.
 
Status
Not open for further replies.
Back
Top