Recipe - Installation of MikuniOZ single carb manifold and VM38-9

 I have pointed it out more than once, I am not a fan of those single carb setups on Yamaha XV engines, but I am a huge fan of empirical research and when I found out a while ago that there's actually a dyno at a garage near my workshop, this sparked the necessary bit of extra interest to finally get going. 

As the Turbo TR1 is for this instance more or less a standard TR1 with stock exhaust the results should be almost universally applicable to XV750SE, TR1, XV1000 and XV1100 models. 

Jetting / Setup:

• Pilot: 30 
• Main: 185 
• Stock needle, 3rd (middle position) 
• Air-screw: 1.0 turns out from a soft seat 
• K&N Pancake style air filter (63mm inlet diameter), type RC-0850 

 Additional bits:

• 3x VM38-200 inlet rubbers 
• 90 degree throttle cable guide
• 4x M5x16 allen head bolts for the float bowl 

Installation:

First was the (somewhat sad) task of de-turbo-ing the Turbo TR1 and swapping the clutch cover for an unmodified one as the old one incorporated the turbo oil-drain port. 

Installation of the manifold is fairly straight forward, just bear in mind, you will need 3 VM38-200 inlet rubbers as the stock ones won't work.

 

In my case I also had to install a stock exhaust again, as for the time being I had no other exhaust available.

Initially I tried to fit the carb without a 90 degree guide and as should be rather apparent from the picture below this did not yield satisfactory results.

In the end the matter was solved with a custom throttle cable, made at least in part from a left over SR500 cable. 

 

As by now I had the setup on the dyno (even though not on the old Turbo TR1), but on the Everyday TR1 and ran it against a pair of VM38s and a pair of TM38s, I can tell you that with a lot of tweaking on can achieve about 2/3 of the stock max. hp and torque compared to a well dialed in (and not worn out) Hitachi. Compared to a pair of dual carbs, it's a fairly hopeless affair. The pair of TM38s yielded almost double the horsepower even though it was limited by sucking through the frame and (unknown to me at the time) a damaged throttle cable, which effectively ruined the sync between the carbs at certain openings and a needle which was too lean. 

Further reading:

• Recipe for TM38 carbs 
• VM38-9 installation and jetting guide 
• AFRs at idle (a deep dive into what the air-screw does) 
• An even deeper dive into carb tech 

The XV SingleCarb manifold

To this day a year ago, I promised a friend to make a single-carb manifold for him. When he said "no pressure", I am not entirely sure, he was planning to be this patient about it.

The show stopper last time were the wrong-radius bends I was supplied... and then life and a workshop move got in the way. As can be seen in the picture, the radius is substantially smaller on my new bends.

So lets start with a fresh set of bends:

The more or less finished product. It's equal length and -radius and should allow both cylinders to get close to the same amount of air/fuel into each one. 

I'll finish up the welding next week as I simply ran out of time today. Also I have a few more ideas on how to design a few variants of this manifold for turbo- or supercharger use and take it from there.