Mikuni Carb

[Garenne]

Has anyone tried a Mikuni carb on a Silk?   A new 34mm Mikuni carb came with the boxes of bits that I have eventually managed to turn into a reasonable representation of an early working Silk.   I have not seen any mention of Mikuni carbs in this forum and assumed that I was on my own trying to sort it out.  I was too miserly to lash out on a new Amal carb having already spent more than intended but it would have made life a lot easier as it turned out.   If anyone is interested I have so far come up with the following settings although work still needs doing to get it more to my satisfaction.  

I had hoped that the previous owner when buying the carb had it set up to emulate the 34mm Amal but it proved to be set very lean in the middle range.  It still runs very hot according to the temp gauge and I am still richening it up although I now have a nice brown colour on the plugs at last.  In light of recent posts on the forum I have yet to try diluting the radiator content from the present 50/50 antifreeze.    It needs a bigger needle jet to get the needle down to the middle range of adjustment.  It is just that it isn’t as easy as with the Amal going to the next number, Mikuni offer a much greater choice.  The main jet isn’t yet finalised as not many miles have been put on the new pistons and bores.  However it was worth the wait and exasperation as it is performing better than expected if a little more loudly than I would like.

Mikuni Carb VM34-275      34 mm bore

Slide 2.5
Needle 6FJ6 Clip Position – 5  (Came with original carb)
Needle Jet 159 Q – 5
Main Jet 350
Pilot Jet 30
Carburettor Float Height 23mm

[hendred]

Running an Amal on my Silk so unable to comment or help on that front.  Strongly recommend you dilute the water/ antifreeze mixture.  50% antifreeze is similar to the Rock Oil Kool XL which caused overheating..  i did a rapid 90 mile run today and the engine stsyed cool using just plain water - I drain the engine down over winter.  I found the exhaust very loud too.  There is supposeed to be wadding around the gauze tube in the tail piece.  Mine was nearly sll missing.  Replaced and much quieter and the engine seemed to run better too.

[Smokey_Paul]

Just replaced mine too amazing difference in sound and I think less clouds lol

[Garenne]

I have been unable to find the standard settings for the 34mm Amal carbs anywhere.  Anyone able to post those up so that I can try to do a direct comparison to help in setting up this Mikuni.  There is lots of information on Mikuni components dimensions but I cannot find the same for Amal.  I have ordered a MK2 two stroke needle since it was only a tenner which I will be able to measure but I now need to know the standard needle jet for the 34mm carb.   I am a bit unconvinced about having a double tapered needle as I am having the most trouble in sorting out the small throttle opening settings.  It is going well at 1/4 to 1/2 throttle giving an easy 55mph at 1/4 throttle in top and consumption working out at over 50mpg so far.  However riding it in town is definitely a pain at present. Changing pilot jets and cutaways hasn't helped much so far.    

[ukpco]

If you go to the silk-motorcycles.org website, under the technical specs tab there are the carb set up details you want.

[Garenne]

Red face time.  I had looked at the tech page but somehow took the 34mm as 32mm.  Thanks for the info.  Now just waiting for the needle to see how the setup compares.  Will post results if successful.

[Garenne]

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.  

[Smokey_Paul]

Thanks I will post those on the carb page to keep safe.

Let us know if you change anything.

Cheers Paul