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Mikuni VM 34

Submitted by david_evans on Sat, 15/01/2011
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Commando

I have a single Mikuni 34mm fitted to my 1970 750 Fastback and I've jetted it according to the info in the American "Tech Digest" That recommends a 3 cutaway slide. This is giving me spitting back until the bike is well warmed up indicating a weak mixture. The American Norton forum has a thread with someone suggesting a 2.5 slide in the 750. Does anybody out there in Commando land have one of these carbs fitted to a 750 and can you tell me what slide you have fitted? TVM

Submitted by eddie_simpson on Sat, 15/01/2011

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hi david,

i have fitted mikunis to my t160 and they are set up perfectly. along the way i tried different slides and found with mikuni s that to be a bit of a red herring. if your carb set up is fine when warmed up i would look at a better choke arrangement where you can raise the tick over as well as enrichen until the bike ticks over fine. look how the choke on the suzuki gs slide carbs workas themikuni choke circuits work,unlike amals.use platinum plugs also if you can source the correct heat range for your engine. i also had the dreaded popping , spitting back and rougth running from tick over to 3,500 rpm and i eventually found my pilot jets were one size too large. i was convinced it was too weak but it was some smoke from the exhaust when i came to a halt that made me suspect pilots. it still smokes a little and i probably can go smaller but the bike runs great.i know its a different bike but the basic principleis thesame.

Submitted by david_evans on Sun, 16/01/2011

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Thanks for that Eddie, you say to look at a better choke arrangement but the choke on a VM is either on or off. I will fit the 2.5 slide and give it a go. I was surprised by large difference between the cutaway on the 2.5 when lined up with a 3, a much greater difference than the Mk2 Amal

I've used platinum plugs with good results. I got some from Wallmart when I was in the States a couple of years ago, "Bosch platinum 2" with two electrodes. I can't find any details of heat ranges but they do work well and were quite cheap.

Submitted by eddie_simpson on Mon, 17/01/2011

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hi david,you have the 2 stroke choke system with with the cam bracket so its either on or off. look at the gs750 which have the same choke plungers but different lift systemwith a rod and friction lever. ihaveread somewhere where some onehas modified your arrangement to a proper choke system on a triple so do some research.i hope your slide change cures your carb tune but i still think you need a better choke system. the plugs i have are ngk and end with eix and they are about £7 each, regards eddie

Submitted by david_evans on Mon, 07/03/2011

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I have just got the bike back on the road after servicing the forks and took the opportunity to fit a 2.5 slide. It starts really nicely and ticks over much more smoothly than it did before. Looks like the INOA Tech notes need an ammendment. Pity I had to fork out nearly 20 quid for a slide and then return to the original. Anybody want a No 3 slide, as new, going cheap.

 


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