Mikuni VM36 single carb conversion

I've got several Mikuni's fitted to my Commando's, my 850 I use for most long trips has a VM36, it is essentially a plug and play bit of kit. What does happen however is that because of modern petrol (being crap thanks to the tree huggers and backhanders to governments) the plugs can foul up more easily, my starting procedure is to start the bike with choke (almost always a 1st or 2nd time start) as soon as its running off with the choke and warm up just on the throttle.

Old petrol (more than 6 weeks) can cause issues in my experience, after a meeting last year I took the advice of a fellow member and changed over the plugs (NGK) from BP7's to BP6's (hotter plug) if you use champions I think the heat range goes the other way (N8's) but I stand to be corrcted on that one. This seems to be helping starting and fouling issues.

We had a meeting at the weekend an more than one of the guys told me they had to fit new plugs before leaving (fouling due to petrol being a few months old)

I would suggest its more than likely not the carb at fault.

Here is the fitment chart for Mikuni carb conversions

The chart calls for a #40 pilot jet but everything I have read says that you have to go down to a #30 or smaller

http://www.ohiocaferacers.com/MikuniJetting.html

I have a Mikuni VM36 on my Mk IIA, but am thinking of going back to a MkI Concentric, the new Premier version. The Mikuni never seems to be quite right and there's a mind boggling array of jets and needles available.

It runs quite well when warmed up but cold starting is often a struggle. It wants choke up to a certain ambient temperature but above that it will only start with the choke off, so it's a bit hit and miss if the weather's sort of in between. Once started with the choke on it runs very lumpy and rich, but with the choke off it spits back or dies when you open the throttle until it's warmed up a bit. I find I'm missing having a tickler and variable choke but then I don't know any better, never having owned a modern bike.

Motocarbsays a VM34 is better for road use, but for a single VM36 recommends 280 main jet, 2.5 slide, 159.P-2 needle jet, 6DH3 needle, 40 pilot jet, (2.0 air jet, I think). These are the settings I'm using, with a K&N air filter and Champion N7YCC plugs. I've tried between 32.5 and 42.5 pilot jets but with the smaller sizes it's unrideable until the engine's properly hot.

I hadn't seen the Ohio cafe racers chart before. Their settings are richer for main and needle jets (and air jet?). Am I wrong in being surprised that they apply the same settings to single and twin carb set-ups?

A good site for Mikuni info. including jet charts and so on is http://www.motorcyclecarbs.com. If you follow the links you'll find a Mikuni tuning manual there (PDF format).

Or there's http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

If you're feeling well off you could go to http://victorylibrary.com/MIK-BT.htm.

I have main jet:320,pilot jet:35,needle jet:p8,needle:6DP1,air jet:0,7 and slide:2,5.

Workin but not very well,plugs are almost snow white and fourt gear i can reach about 4500 rmp.How does that air jet works?-only with main jet or also with idle jet?.

The plugs are champion 7 NYC and i think these should be ok.I have ported cylinder head and PW3 camshaft-maybe one carbuertor does not give enough fuel.The cylinder head air(gas/ mixture flow is 38% better now before porting).

Now i have changed 6DH3 needle and working much better.

If I understand http://www.motorcyclecarbs.com/Mikuni_Jet_Needle_Dimension_Ch_W121.cfm right (and I don't guarantee it, Mikuni needle markings seem to be a mystery even to the suppliers, as you'll see if you read the link) a 6DH needle has a more gradual second taper (farthest away from the clip) than a 6DP so would give a weaker mixture at first, whereas it sounds as if you needed to go richer. So how a minor change in apparently the wrong direction can have helped, I don't know. Once the needle's raised far enough you'll have effectively the same 6D needle in either case. I hope I've got that the right way round, feel free to correct me if anyone knows better.

Mind you, the P8 needle jet is quite large and 320 main is too so that should help. A P8 is not a bleed type so the air jet won't make a difference, according to Motocarb who I asked the very same question last week. He thought going from a P2 to P8 was too big a jump so we compromised on P6.

I'm surprised you have white plugs with such big jets, but then my motor is bog standard compared to yours. Presumably you don't have an air filter fitted?

My 850 seems a bit sluggish to pick up in the needle jet range so I've bought a 159-P6/6DP17 combination on the strength of Steve Elston's experience but haven't had chance to try it yet. Incidentally, Steve, are you using an air filter?

Thank you Rob,what are your settings?(idle jet,air jet etc)?.

Rainer

Previously wrote:

Thank you Rob,what are your settings?(idle jet,air jet etc)?.

Rainer

Mikuni VM36 with K&N pancake type air filter. Main jet 4/042 #280, throttle slide VM36/36 2.5, needle jet 159.P-2, needle position middle groove, needle 6DH3, air screw 2 turns out, air jet BS30/97#2.0 (I think - haven't checked - but I believe it's not important), pilot jet VM22/210#42.5. Fuel consumption between 35 and 62mpg (Imp gal.) depending on pillion/luggage/type of riding.

Seems rich on idle so I might try a smaller pilot jet, then larger needle jet as mentioned above.

I tried yesterday a #40 idle jet but i think it?s too large, between 32,5-35,0 could be better. I had earlier 2,0 air jet but I think it?s also too big. When I get a smaller air jet i must try with them (0,9-1,3).

I've now tried the new needle jet and needle and they were a disaster. As soon as the engine reached 3000rpm it started spluttering and died under load, so much so that I couldn't get up my steep drive. I tried the new 6DP17 needle with the old 159.P-2 needle jet and it was no better so I went back to the old 6DH3 needle with the 159.P-2 and all was well.

Funny how similar engines can differ so much.

I also tried a 45 pilot jet and although it looks OK on the Colortune it feels a bit rich to me so I'll go back to the 42.5 or maybe try smaller ones. Then I'll try the needle in different notches, maybe even a 159.P-0 needle jet which I have.

Ok,you are lucky man,my problems continue,i got my record today:80mails/h-there must be something wrong in my Mikuni...

With Q-5 needle jet and 6DH3 needle i finally got black plugs,even on second gear speed was 70m/h,looks good.

Hello,has anyone idea why Mikuni spitting petrol out from air jet on idle speed?.

I had a single Mikuni fitted to my 750 Commando and it worked really well after it had been fitted.I did not ride it for a while, and then it just would not start. I put some petrol down one of the plug holes and it fired, but it would not continue to run. I put back my newly Martin Bratby re-sleeved Amal Concentric's and it has been really good to ride. The Mikuni conversion or the so called new one is in a box in the shed, never to be used again. Why it just stopped working I do not know, and I never knew if it had petrol in it, and it did used dribble petrol out of the over flow tubing.

Sounds like a blocked pilot jet, all carbs suffer more from this with the new E fuels. If it was an Amal 25 you would drill it out with a 16 thou drill, no idea of the correct sizes for Mikuni pilot jets. The same rubbish fuel leaves crud on the float needle and stops the float valve from fully shutting and on a Mikuni the excess fuel exits via the over flow. Never lay up a bike with E unleaded fuel in the tank, drain and use in a fuel injected car or use super unleaded which is mostly E free.