Re: Mikuni Carb

Posted by Garenne on Aug 01, 2021; 5:17pm
URL: http://silk-motorcycles.221.s1.nabble.com/Mikuni-Carb-tp5001990p5002207.html

A much delayed update on my progress with this Mikuni carb.  I think I am now 95% there and I am now just going to ride it to see how we get on.

It has been a most frustrating procedure never having started from scratch with no initial info to go on.  I had assumed that the carburettor had been purchased suitably jetted for the Silk.  This was way off the mark and probably took me down a lot of blind alleys and expense on parts.  The main problem turned out to be the air jet which feeds air to the needle jet when the throttle is closed.  As supplied it was 2.0mm but I am now running 0.5mm which helps a lot on the partial throttle openings.  Obvious now but I don't think I am even half way up the learning curve on carb tuning yet.  I have done over 2000 miles now with a few jaunts of 150-200 miles in a day and it has been worth all the time and effort getting this bike back on the road.  I've not even tried sizing the main jet yet as I have been using a dummy which just has a large hole.  This was recommended in the useful Super Tuning Mikuni manual, only sizing the main jet having got all the intermediate stages right.  I have only got to where the bike is obviously over rich a couple of times and it is way over the speeds I am likely to be riding at.  It is doing at least 70mph before this occurs.

Anyway the settings that I am now running are

 Mikuni Carb VM34-275      34 mm bore

Slide 3.0
Needle 6DH2 Clip Position – 2  
Needle Jet 159 P – 8
Main Jet - probably around 350
Pilot Jet 32.5
Air Jet       0.5mm
Carburettor Float Ht    24mm

Hope this helps if anyone else goes down this route.  It does go well now with very progressive feel to the power delivery.  Through these tests the fuel consumption has been hovering just below 50mpg with the lowest at 45mpg on one particular setting.