38mm Mikuni Carb

Hi all,
I have found a VM 38mm Mikuni carb in good condition in my back shed, It has been sitting around from years gone by of my experience with motorcycling.
I currently have duel Amals which have the same old problem..... with worn slides.

I have heard that these 38mm mikuni single carb conversions work quite well on an 850, Does anybody out there have any Jetting specs for setting one of these carbs up a stock 850 commando?

I also have a conversion 2 into 1 manifold with with the letters "MAP" casted on it? It has the internal intake single side that measures 38mm throat, the spacing mounts for the rubber spigot carb to manifold measures approx 60mm! would this be a manilold for a 38mm carb or a smaller carb conversion instead????

Also, I would appreciate any feedback about the advantages & disadvantages of this conversion as compared to running the bike on standard dual Amal carbs.

By the way here is a Pic of my bike.

Thanks for all response.




Brewer.

Hi guy's, here is the Manifold, I am not certain that It is for fitting a 38mm Mic carb to though!!! the actual throat diameter of this manifold where the spigot manifold mates to is 38mm. the thread spacing is approx 60mm. I do hope someone has some answers?



Brewer

38 mm is a big carb, do a search and see what you come up with, I seem to recall some one commenting 38 was a bit large,my book says a 34mm flange is 60mm between centres,a36 is 70mm & a 38mm is .74.6mm.

Thanks for that spatt. I am hoping that someone on the forum, does have first hand experience with this conversion.

Brewer.

Try contacting Map they could help.

Brewer,

38mm is getting oversize for an 850. Most like mine, run a VM34. If your head is an RH10, it has 32mm manifold inlets tapering to 30mm at the head. The RH10 is a good head for strong low to mid range torque and a very stable idle. A 38mm carb is going to have jetting issues, and believe me there are dozens of combinations for the Mikuni. If you had a ported head, high compression and a cam to suit, maybe the 38 would suffice for high speed power but if its stock I suggest your not going to get much joy out of it.

the 60mm stud spacing is common to bigger Mikuni VM's. The adapator manifold is a rubber molding with a tube that sockets into the venturi of the carby.The carby is secured by a large hose clamp. Another thing to check is that the bigger Mikunis have a physically larger body and this may compromise the fit at the frame junction and make selection of a suitable air filter as well as the throttle cable having a decent fit as well.

Call Murray at CycleCraft Engineering and have a yak to him. He stocks a bunch of Mikunis and parts and may be able to advise what options you may have. If you go to a 36 or 34, stick to a 240 Main Jet, a 35 Pilot, a P2 or P4 Needle Jet, a 6DH3 or 6DH4 Needle, and a 2.5 slide. That will get you running on a stock 850 motor.

Mick

Thanks Horton & thanks Mick.
I did find the MAP website, It seems like the Manifold that I have is suitable for 32mm & 34mm single carb conversion.
Hey Mick, do you have a single Mikuni on your bike? how is the performance comparison to the original dual Amals?

Brewer

If you do end up buying a new carb think about buying a 34mm smooth bore,from what I have picked up on this forum is the mid range is even better than the standard VM mikuni,I cant give you a comparison but my bike 850,vm34,boyer starts up while slowly kicking over,once hot idles all day long,good mid range but it does lack in the flat out department.Some people dont like the fact that the choke is full on or off,if I was commuting on it every day it would probably become annoying but for weekend rides and touring a couple minutes warm up doesn't worry me. minor air filter issue but it is easily overcome.

Thanks splatt.
Good advice, I did hear that with the 34mm fitted on an 850 you do tend to loose a little from the top end performance, I thought that with the 38mm Mikuni you may get that back, but I have not heard anyone yet on the forum put there 2 bobs worth in.
What I must include that last night I saw the , 1970 Aussie Cult biker movie "Stone". The Commando scene in the movie is very powerful it pulls some very nice wheelies. I think the bike is a Combat, it has tremendous grunt!
It was great to see that movie again..... I have not seen it for about 25 years.
Brewer.

Hi Brewer,

yes I got a single VM34 on a stock 850 Mk2 Roadster. Good compression and good nick all round. Clocked at 172Kph, 6,000RPM, 22 tooth drive sprocket.

My only precaution with any single carb is getting the biggest airfilter you can fit in there. I could only fit a KN narrow pancake and had to peforate the back plate and add filter material over that to get adequate flow over 5,000RPM. I got a custom made manifold so it sits the carby a bit further back which closed up the space for a bigger filter.

Mick

Thanks Mick, I appreciate all of your advice.


Brewer

Hi Brewer,

Here's my 2 cents worth: I used to have a '74 850 that I bought with a 36mm Mikuni. It was in good nick, and ran ok. I no longer have that bike, but several years ago, I bought a MKIII that came with a 38mm Mikuni. It's on the same manifold as my '74 had - that manifold will take either the 36 or 38. I figured I would at least try it. Once I got it running, and put a few miles on it, I sold both the manifold and carb, and put on a new manifold that would take the 34mm VM Mikuni. I was contemplating using the larger manifold, and just going with a 36mm like I had on my previous bike. I can't speak for others, but the 36 worked better than the 38, hands down. Both were jetted correctly, and the engines were fresh. The 34mm has a better mid-range, and better throttle response than either of the larger carbs. I knew that would be the case, and I'm glad I just didn't stick on a 36 and be lazy about it. I think the difference in cost was small, and just a few more minutes to change everthing. You have to get both the smaller manifold, and the rubber carb mount to mount the 34, but the 36 carb costs more - so it was about a draw. Both my MKIII, and my '72 Combat run a single 34mm VM Mikuni. Not the most glamorous choice, but they both start and run fine, with no lack of upper RPM power.

good luck, whatever you do

Don W.

Brewer said:

... I have heard that these 38mm mikuni single carb conversions work quite well on an 850, Does anybody out there have any Jetting specs for setting one of these carbs up a stock 850 commando? ...

Click to expand...

The following is from the Norton Tech Digest for a 38mm Mikuni on an 850 Commando.
Main = 240
Needle jet = 159 Q-0
Needle = 6DH3
Idle jet = 50
Slide = 2.5
Idle air jet = 2.0

Hope this helps. I don't have personal experience with this set up, but I just bought a 36mm and was looking up the specs for it. I saw the 38mm specs in the next column and remembered this thread.

Thanks Brithit, very interesting conclusion. The manifold that I have is for a 34mm Mikuni, I just need to now! get a 34mm mikuni carb and the rubber manifold to try it out on my bike.

Also, thank you Corona, for providing the jetting specs for Setting up a 38mm single mikuni to run on a 850.

Ride on guys!

Brewer