Mikuni manifold distance

I have read many times that the weak point of the Mikuni single carb conversion is the manifold and that an improvement can be had by lengthening the manifold. However no one mentions the optimum length...perhaps this is still theoretical?
** How much distance is required? What is too long?
** Should the space be between the head and manifold or between the manifold and carb?

I realize there is a real lack of space so this is almost impossible to do but I'm curious. Has anyone experimented with this?

Last week I finally got out of Seattle and had long stretches of road where I could run the Atlas up around 6000+ briefly. I noticed the acceleration began to back off at the higher rpms. Could be any number of things. The jetting is typical Sudco...37.5 pilot #3 slide P-0 n-jet 6dh3 needle 240 main. The clip was in the # position - slightly richer. I put the clip in full rich position and felt a slight improvement. Yes, I read about the unreliable butt dyno. The Atlas is currently running the best it ever has so I am reluctant to increase the needle jet size. It is performing beautifully at low throttle.

** Also, the problem of testing the main jet. Is it possible to test the main jet by approaching a long hill approx 5000rpm in third gear, suddenly open the throttle all the way and hold it there for the duration of the hill...repeat with different main jets and compare?

I use the DENSO Iridium plugs IW22 5307...best bang for the buck. About $11apiece and worth every penny.

Are you running points or electronics? I ran a single Mikuni on my street tracker for a while. The best performance gain was to remove the two inner bolts and fill the holes with epoxy, then blend the runners smooth. Sorry sold it and didn't note what jets. Good bottom end slacked off a little on the top, went back to two 32mm Amals. Somewhere I remember 9 3/8 was optimum length for Triumph, that was from the mouth of velocity stack to head or maybe intake valve.

Comnoz tested that a 36mm adapter is a big help on the 34mm carb installation.
That's what I'm using and it seems as strong as the Amals were. (I am using a TM flat slide).
Jaydee

I have a Boyer.
I can try filling in the two center holes, I have an extra manifold.
The manifolds come drilled for a 34 or 36 rubber adaptor. I believe they are the same manifold. Correct? To use the 36mm adaptor I would need to make an alu ring to glue on to the 34mm carb and fit to the 36 adaptor. Easy enough to do. Is this what you have in mind?

jaydee75 said:

Comnoz tested that a 36mm adapter is a big help on the 34mm carb installation.
That's what I'm using and it seems as strong as the Amals were. (I am using a TM flat slide).
Jaydee

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I made my own manifold after being very disappointed with a 36mm VM and manifold
I made my manifold from two mild steel welding elbows (38mm Id)
I left a big steeple inside the manifold where the two inner screws would be
I run a TM 40
There is no comparison between the VM 36 and a TM 40 it's night and day in terms of performance

baz said:

I made my own manifold after being very disappointed with a 36mm VM and manifold
I made my manifold from two mild steel welding elbows (38mm Id)
I left a big steeple inside the manifold where the two inner screws would be
I run a TM 40
There is no comparison between the VM 36 and a TM 40 it's night and day in terms of performance

Click to expand...

This.
Is the correct answer. If you're going to convert to a single carb, use one BIG ENOUGH.

concours said:

This.
Is the correct answer. If you're going to convert to a single carb, use one BIG ENOUGH.

Click to expand...

I've lost count of how many people have told me the carb is too big !!!

Works for me

baz said:

I've lost count of how many people have told me the carb is too big !!!

Click to expand...

Which is much better than being told you are too small

I've seen the 36mm VM Mikuni installed 0n 3 850s and all work well. Will it also work well on a 750?

I have not used a TM mainly because I'm well set up for the VM carbs including a pair of 28mm Mikunis on my BMW R60/2. Talking with Thunder Products, he developed the UFO to push the humble VM into flatslide territory. I can say the UFO works great and certainly cheap enough. It sucks so much fuel at idle and low throttle openings that I had to reduce the pilot jet to 17.5. I would have gone smaller but TP says that a pilot that small is prone to plugging. For in-city use I would not use it. If I lived in rural areas or was racing I would be happy to use it permanently. It provides noticeably more power for $75.

I have currently installed the TP Torque-Wing. I cannot really say if it provides a gain because I don't have a clear before-and after situation. The theory sounds good and I know the acceleration when quickly opening the throttle is very good. The pathway is not optimum as using dual carbs and I wish he provided the torque wing for Amals. I plan to install a pair in the R60 after it's up and running. That, a pair of iridium plugs, and a Boyer should really wake up the sleepy R60.