Mikuni VM36 help

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I've set it at 19mm. The SUDCO manual says 17-19mm.

Chuck_Wagon said:
Mikuni VM36 help
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acotrel said:
If your carb is flooding, you will never fix the problem by changing jets. However, if it is not flooding - when you fit an air-cleaner, you might need to go down a size in needle jet. If you lower the needle, you should be able to force the motor to cough when you ride the bike, then raise the needle at least one notch and get good performance.
The first things to do when tuning a bike, is to get the float levels right and replace the needle jets. With old bikes, the needle jets wear. With petrol as fuel that is bad.
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Yeah I never thought I'd cure the overflow by changing jets, I said I've fitted a whole new float valve assembly & that I've got various other main & pilot jets to help with running.

http://www.allensperformance.co.uk/vm36-4-carb-data-page

Do you mean the jet needle or needle jet?
 
Chuck_Wagon said:
I have a VM36 on my 74/850, this is the jetting I have, my plugs look perfect.

2,150 feet above sea level.
260 Main Jet
30 Pilot Jet
6DH3 Needle, e-clip on number 3
159 P0 Needle Jet
2.5 Throttle valve slide
2.0 Air Jet (BS30/97)
Air screw: 1-1/2 turns out

Personally, I think a VM36 is too big of a carb, a VM34 is better suited for a Norton in my opinion.

We all know there is more to a nice running ride and good-looking plugs than just carb settings.
Your carb should not be flushing fuel out of the overflow tube, somethings not right there.
Measurement is from top of the rib to top of the float arm. Lift it another mm?

-correct valve adjustment
-finding true TDC with a degree wheel (only have to do this once) and setting correct timing.
Nortons will start and run with the timing way out of whack, make sure its set correctly and retard it just a tiny bit if its giving you kickback.
-good coils and correct plug leads and plugs.

Electrics count big time.
I use a RM23 single-phase Lucas stator with a Podtronics single-phase regulator/rectifier with my 2MC capacitor still in circuit.
Boyer MK4 analog electronic ignition with 5 ohm Dyna coil, solid copper core plug wires, non-resistor plug caps and non-resistor NGK BP7ES plugs at .025-.026” gap.

All that movement I see with the engine is just weirdness with the video on youtube I hope :)
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Blimey, I'm only 197ft above sea level here, bit of a difference!

What do you think of how she sounds in the video Chuck?
 
Luke, I don't think this will affect your low end running much, but note that Chuck mentions the Air Jet. You don't. For some reason a lot of people don't seem to register that the Mikuni has one.

They come from the factory with a 2.0, which Chuck says he has fitted, and I would bet most people have and don't know. This size is generally too large for any 4 stroke, but it mainly only affects higher rpm, where too large an air jet can cause lean out. Some users will never really notice an issue, making adjustments to the main jet because there is interaction. This way you can end up with too large a main jet. And 280 is pretty large.

Overall your jets look large, and that will have some effect on your low end running.

Maybe try Chuck's P0 before changing the slide, one it is cheaper and two a 2.5 slide should be fine. The mixture screw setting should be set at a high rpm for best running, I expect you will end up nearer Chuck's 1 1/2 turns than your 1.

I have a pair of 36mms and have gone down to a 0.7 air jet to improve high end running. On my 34mms I didn't have to reduce it as far as that to get what I needed.

Fullauto on here is probably the only guy I can recall reducing the air jet size and he reported good improvements, I think he went to 1.2 or something like. I recall he was advised on this topic by Comnoz. The Victory Mikuni manual gives an indication of suitable sizes, but I think still errs on the larger end.


I did find with my 34mms that getting the float height right did help stop fuel coming out of the vents. I am not 100% sure why, but fuel venting didn't stop completely until I reduced the air jet on the 34mm to around 1.2, however it only showed after a track session with periods of erratic high end running!

No point using my float height figure (which I don't recall off the top of my head anyway) because my engine mounting and inlet manifolds puts the carbs at a steeper angle. The standard figure should work fine for you, but I agree with you and Chuck by going right to the lower fuel height end of the range to start.


(Chuck should also note from my comment that I won't agree with him that a single 36mm is too large! Really I think a single 34mm is too small. But horses for courses, you won't notice much until high rpms in high gears.)
 
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SteveA said:
Luke, I don't think this will affect your low end running much, but note that Chuck mentions the Air Jet. You don't. For some reason a lot of people don't seem to register that the Mikuni has one.

They come from the factory with a 2.0, which Chuck says he has fitted, and I would bet most people have and don't know. This size is generally too large for any 4 stroke, but it mainly only affects higher rpm, where too large an air jet can cause lean out. Some users will never really notice an issue, making adjustments to the main jet because there is interaction. This way you can end up with too large a main jet. And 280 is pretty large.

Overall your jets look large, and that will have some effect on your low end running.

Maybe try Chuck's P0 before changing the slide, one it is cheaper and two a 2.5 slide should be fine. The mixture screw setting should be set at a high rpm for best running, I expect you will end up nearer Chuck's 1 1/2 turns than your 1.

I have a pair of 36mms and have gone down to a 0.7 air jet to improve high end running. On my 34mms I didn't have to reduce it as far as that to get what I needed.

Fullauto on here is probably the only guy I can recall reducing the air jet size and he reported good improvements, I think he went to 1.2 or something like. I recall he was advised on this topic by Comnoz. The Victory Mikuni manual gives an indication of suitable sizes, but I think still errs on the larger end.

I did find with my 34mms that getting the float height right did help stop fuel coming out of the vents. I am not 100% sure why, but fuel venting didn't stop completely until I reduced the air jet on the 34mm to around 1.2, however it only showed after a track session with periods of erratic high end running!

No point using my float height figure (which I don't recall off the top of my head anyway) because my engine mounting and inlet manifolds puts the carbs at a steeper angle. The standard figure should work fine for you, but I agree with you and Chuck by going right to the lower fuel height end of the range to start.

(Chuck should also note from my comment that I won't agree with him that a single 36mm is too large! Really I think a single 34mm is too small. But horses for courses, you won't notice much until high rpms in high gears.)
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Hi Steve,

I do have it jotted down that it's got a 2.0 air jet.

It was running a 290 main jet before & it must've ran as my dad rode it for 4-5 years. I do have a 260 main jet to hand so I could drop that in to see how she goes out of curiosity?

I've found it hard to get an accurate measurement of the float height but it's roughly around 19mm, I may bend the tab a little more to be on the safe side.

Cheers.
 
Luke Turner said:
& I was under the impression the main jet handled the top end range?
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It does. And it is above 3/4 throttle opening that you expect to choose the right main jet size, so it is tricky to do on the road and best done on the dyno with a sensor up the exhaust to monitor unburnt fuel. With it too rich it may run a little cooler, perhaps not so important in Blighty most of the time, but will not rev as freely as it should and it won't deliver the power it should, and you will be wasting fuel. Too lean can melt stuff! This is why the air jet should also be considered in the mix, see below. Make sure your ignition is 100% correct before testing too much up there.

The days of 'plug chops' to check mixture are over with unleaded fuel. If it is genuinly too week at the top you will notice a slight acceleration if you close the throttle a little from wide open. You may find it difficult to find enough room on public roads to do this! But since all fuel (apart from cold start enrichment fuel) goes through the main jet, a wildly large or small main jet will have an effect at other throttle openings. The main jet will probably end up smaller than you imagine.

On the Mikuni the air jet bleeds air into the needle jet area. Too little air and it will be rich on needle and needle jet, too much air weakens the mixture, and when you have the throttle wide open at higher rpm under load this can weaken it too much, some call it 'high RPM lean out' and is a slope rather than fixed relationship. It seems like the obvious fix to increase the main jet size, but you are masking the problem and you actually need to get the air jet right first.

Use Chuck's settings as an indicator, he has a smaller main jet than you and is at greater altitude, where you might expect to use a larger one. You might want to try a 250 or 240, maybe smaller, certainly worth putting the 260 in right now. But with a 2.0 air jet if you go too small with the main you might be too weak at high rpm under load! Not critizing your dad at all, but he may not have ridden at higher throttle openings and available fuels have changed too. So what was right a few years ago may not be now.

Amals don't have these jets. It is part of the legendary Mikuni 'tunability', which really means there are very many more opportunities to screw up!

I think you are just trying to get it 'running nicely' and you may never want to run it at 7000rpm all day, but if the jetting is right across the range it will just be so much more pleasant to ride all day.
 
On an Amal I increase the main jet until it 8 strokes and then go down 2 sizes and then I know I am in the right ball park for fine tuning. Its a good check as to what 8 stroking feels like to just leave the main jet completely, the bike will run normally until 3/4 throttle and then the 8 stroking will start. No idea if this carries over to a Mikuni VM36 but leaving the main jet out part at least should.
 
Cheers Steve!

I'm back in the garage now so will strip the carb off, clean it out, fit a 260 main, a 30 pilot, the e-clip on position 3 of 5, fit the finer fuel filters I've got & see how she goes.

I'll get a more accurate measurement of the float lever & if it's at 19mm then I'll leave it where it is. Hopefully these finer fuel filters will stop the particles if they're clogging the float valve up.
 
Also buying a whiteboard for the garage to jot down all this info.. the basics of which needles & jets effect which circuits, which way is leaner & richer on the needle etc..
 
Blimey Luke, won't need a carb for tomorrow if it stays like this, just put a sail up and will be there in a flash. Given up in the shed and locked down for today. Missus has gone negative about ride over, so maybe on my own. Whoopee !
 
Pushed it n rode it said:
Blimey Luke, won't need a carb for tomorrow if it stays like this, just put a sail up and will be there in a flash. Given up in the shed and locked down for today. Missus has gone negative about ride over, so maybe on my own. Whoopee !
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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.
 
I.M.S. = Idle mixture screw, which your Mikuni doesn't have, it has an "air screw", I refer to it as an "idle air trim screw".
 
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.

New inline filters, paper rather than mesh.

Here she is after first kick..



Seems a lot calmer & revs nicely up to 3-4k then back down smoothly.

Will suit up & take her out.
 
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& still dripping fuel.. I disassembled the float valve assembly, carb cleaner, compressed air, reassembled & set the lever to 19-20mm.

Will ring Steve at Motocarb on Monday as I don't know what's causing it.

Is it possible that if my isolastics are worn or my fuel tank isn't mounted with the correct rubbers that vibration could be the cause..?
 
Luke Turner said:
Blimey, I'm only 197ft above sea level here, bit of a difference!
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LOL, yes a bit of a difference. When we go ripping off into the Alberta/B.C. mountains it can get to 5,500 ft above sea level.
Mikuni VM36 help


Ride sounds improved with last video, time for another test ride.
check your top engine mount for tightness/broken rubber. Loosen the bolts and push the mount forward a bit to preload the rubbers & then tighten it down.
The tank should tighten down enough that it doesn't move, if you can rock it around you need more rubber spacers on the tank studs.
 
Mikuni VM36 help

If tank is moving around, does it have the foam bits 12,15 on it?
Thicker foam bits, need more rubber spacers, 20?
I had to glue about a 1/4" of hard rubber on the inside curve of the back hold down clamp, 23, before it would clamp solid on the underside of the frame backbone.
 
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