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Norton Commando single Mikuni carb on 850

Submitted by graham_green-buckley on Sun, 28/05/2023
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Commando

I know that there has been plenty of discussion on this before and I know some will say don’t bother with a Mikuni and stick to twin Amals but I have it and it is installed. The problem I am having is that it is running rich. It is an Mikuni VM36 with an MAP 2 into 1 manifold and pancake air filter. Wanted to get the kit from Allen’s performance but after waiting for 15 months and constantly being told it was arriving next week I gave up and bought it from the States. According to Norman Whites restoration manual the pilot jet should be 35 and the main jet 290. At that it was blowing out unburnt fuel and the new plugs were covered in soot. I dropped the pilot to 25 and the main to 260. It starts ok with a bit of choke and will tick over albeit a bit lumpy. Adjusting the idle mixture can get it to stall in both directions but is ok ish in the middle but there is an occasional misfire. If I rev it up it picks up ok but does blow a bit of back unburnt fuel out above 5000 revs. I took the spark plugs out and they are very sooty almost as bad as when it had the bigger jets in. Question is can I go lower in the jets and has anyone found a good set up I can try?  As further info the engine is newly rebuilt by Norman White. It has Boyer Branden electronic ignition and an Alton starter. Everything is tight and I have set the timing dynamically. Not sure where to go next. Any assistance appreciated

Submitted by tim_harrison on Sun, 28/05/2023

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Whilst this doesn't help you, most people I know with a Mikuni run on a VM34. I have an 850 Mk3.To start mine I have to be on full choke and then keep it on for several miles. Your VM36 will be pushing out more fuel but this shouldn't be a problem if correctly jetted. The 25 pilot jet and a 260 main jet seems to be the sizes that most agree on, although one person I know went down to a 20 pilot jet. It is probably the enricher that is clogging your plugs but you would expect this to burn off after a few miles. Probably best to give the bike a long run and then adjust the mixture and idle and see how it is then. Have you checked that the needle is secure and in the correct position which is usually mid point. The needle can come adrift and would make for rich running.

Thanks for the info. I intend to take it out tomorrow as long as it is running ok to ride. I did notice that when I put in a smaller pilot jest it certainly my needed more choke to get it running from start up. Needle is ok in mid position. I will give it a reasonable ride then whip the plugs out and see what it is like. I don’t mind having to warm it up on the choke as long as it runs ok after a couple of minutes warning up. 

Looks like I'm late to the party with my reply. I run a VM36 on my 750 which was running rich. I've lowered the needle as low as it will go so the clip is in the top notch and the air screw is only half a turn out. Plugs look pretty good when checked nowdays. I can't remember what size my mainjet is as I've not changed it for 30 years. My Mikuni has been on for longer than 30 years now. The bike does take a while to warm up from cold and spits back a but until it warms up. For some reason unknown to me the bike won't run on NGK plugs as they keep failing so I just use Champions which last for years in my bike. I had a brilliant trouble free run to the NOC rally in Aberfoyle this June and did 700 miles over the weekend with speeds up to 90 when I got a bit childish after being overtaken by a modern Triumph who could not loose me in the bends. Anyway, just have a play with needle positions.

@ Donald, Glad it’s not just me that can’t resist tailing modern bikes, Ducati scrambler last week (but then I ‘lost’ the oil in the reservoir - another story now seemingly resolved) :-)

Submitted by george_farenden on Wed, 09/08/2023

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Hi,

Something to consider?

I have VM carburettors on my old Kawasaki (VM22's) and I was having great difficulty getting them to balance and getting the mixture right.

Cue reading replies on a forum, and it turns out it is very common for the choke plunger seals to stop sealing properly when they get old. The VM36 has a very similar choke system to the VM22.

For my bike/carbs, new plungers are no longer available. The fix (for me) was to get the plungers fitted with 'Viton' seals by a guy called Rob Meggitt, and since then the bike has behaved perfectly. Type Rob Meggitt zed1015 into Google for more info.

Seeing the original posters issue, it could well be that the choke plunger is the problem, so could well be worth installing a new one (if available) or see if Rob can renovate yours.

Regards, 

George 

 

 

Submitted by stevan_barrett on Mon, 28/08/2023

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Hi Graham,

It has been a while since you first posted about your issues with your 36mm mikuni. I hope you are getting it sorted!! 

I also have an 850 Commando mk3, also fitted with Boyer Branden ignition and with the 36mm carb fitted. I got the bike with this carb on and it had been fitted by Norman White. I found like you that the bike ran rich and it seemed to be throughout the rev range. 

I changed the air filter to a K and N and upped the pilot jet from 25 to 32.5 at the same time, this gave no improvement. So I changed back to the 25 pilot jet, lowered the needle one position from the centre groove to the next one up ( now the second groove down from the top ) and also changed the throttle slide to one with a no 3 cutaway ( it was a 2.5 cutaway ). The main jet is still at 290. I rarely run anywhere near full throttle but am a little nervous of going too weak.

When warm the bike now runs very well, no flat spots and picks up cleanly. After my last trip out I came back on the motorway and ran at high speed for a short while and everything was good!

The bike starts very easily, but I do find the transition from running on the starter system to normal running quite difficult. If turned off too soon the bike will splutter and often cut out. If left on too long it feels awful and the starter system needs to be turned off quickly, which is difficult because the lever is hard to find with a gloved hand.

Cheers, Steve

 

Submitted by neil_wyatt on Mon, 28/08/2023

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I just can't see the logic in this when a Mk1 32mm Amal will do the job without the grief and for  less cash.

All you need is a stay up float. You can even fit the pre 850 air filter.

Mine went up from 44 mpg to 68 and no meddling. (From twin Amal's.)

Submitted by jan_nelder on Tue, 29/08/2023

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I also have an 850 with 32mm Amal Premier, which runs pretty well with no adjustment needed so far, and handlebar choke, which isn’t needed for long , mostly.

 



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