I have a 1973 Commando 850 Interstate which has had a few issues since I bought it. Having now stripped, cleaned and rebuilt the carb, I have got it starting second kick but once it warms up it’s not running too well and plug indicates rich.
I am new to old bikes and don’t really know where to start with carb setup and wondered if anyone could offer any advice. It’s fitted with a single Mikuni VM36 with:
Slide - 2.5
Main Jet - 290
Pilot Jet - 35
Needle Jet - 6DH3
Needle - 159 P-2
Needle Seat - 33
Any advice gratefully received. If anyone can recommend anyone in the Aberdeenshire are who may be able to help with setup then please say also.
I also run a single vm36 Mikuni on my 850 commando
My kit came from allen performance
Heres a useful guide hopefully
My trick for starting /running this set-up
1/ kick through a few times to free clutch
2/petrol on carb enrichment lever down then a few more kicks
3/ ignition on start her up, I then screw in the throttle stop screw(big brass thumb screw l/h side) half a turn and pull the enrichment lever up (the off position) after she has warmed up a little you can back out the thumb screw to get the idle you want
I generally use the enrichment lever even on a warm engine.. as soon as it starts lever off (up)
hope this helps
Thanks for this Steve
I have found these people helpful for Mikuni parts
I run a Mikuni VM36 too. Will nip out to the garage & check what settings\specs everything is. Would also recommend Steve @ Motocarb in Liverpool.
1974 Norton 850cc Commando MK2 Roadster with a Mikuni VM36:
Throttle Valve - 2.5 (2.5)
Main Jet - 290 (260)
Pilot Jet - 35 (30)
Jet Needle- 6DH3 (6DH3)
Needle Jet - 159 P-2 (159 P2)
Needle Seat - 33 (Unsure what this is? Float valve? 3.3?) (Main air jet - 2.0)
(Air adjuster screw: 3 half turns out)
(Idle adjuster screw: 9 half turns out)
I've just put my specs in brackets by the side of yours to make reading it a little easier! I'm also based in England so similar altitude/environment etc.
I haven't long had it set up like this & haven't ridden it a great deal either to really test it out - my plugs look a little rich still so I'm going to try screwing the air adjuster screw in 1/4-1/2 turn to see if it leans up the mixture a little.
Small differences only, and really appreciate you taking the time to respond. Will give my settings a check at the weekend.
Thanks also to you and Steve for the contact at Motocarb. Sure this will come in handy soon
https://youtu.be/T2w-vSyA4eQ I dialled the air adjuster screw back by a 1\4 turn & fitted my new Andover downpipes & silencers. Had a good hour ride around Warwickshire this morning & that's the best she's ever ran, without a shadow of a doubt. Really chuffed.
Great video Luke and she looks and sounds amazing. I’ve been out on my Triumph TR5 Trophy today which is running great so onto the Commando next and your help and advice is appreciated.
I too am a member of the 'VM36 Club' here is what I have
Main Jet: 280
Pilot Jet: 35
Needle Jet: P2
Clip Position: 2
Air Jet: 2.00
Tri Spark EI, K&N RC-0850 Air Filter , straight through pea shooters. Warm starting and normal running are good, but like others the on/off enricher makes the transition from cold starting to normal idle a bit hit or miss. If I get it wrong the plugs need cleaning. I am going to try a 32.5 pilot Jet next. Reading the various Mikuni tuning guides online, they mention treating each circuit (Main,Pilot etc) as a separate setup.
Thank you all for your input. Had another play today and managed to get the bike starting and idling well with just some adjustments of the idle and air mixture screws.
Interestingly, I had to clean the plugs after the initial first cold start with the choke as they were very black so similar issues to Gerald.
Now starting and running okay up to around 5000 RPM then gets a bit lumpy and runs out of pull so still some work to do. Main difference between my setup and those above is the main jet which I have a higher number. Anyone have any ideas the impact this may have?