Turbo TR1 - 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 

Recipe - Yamaha XV 1000 TR1 with TM38-86

 The following recipe is not meant as a straight up, blindly follow it and it'll work recipe, but as something that will get that engine moving and save you from going through what is at the point of writing an eight year journey through TM38-land. 

If you just want the jetting that works for me on a TM38-86 (stock settings in brackets):

• Pilot: 17.5 (22.5)
• Main: 190 (230)
• Needle: 6FJ40 (6FJ41)
• Needle Jet: Q2
• Slide cut out: 4.0mm
• No Air jet 

On top of the carbs, I would recommend buying:

• 40 degree throttle cable guide
• 2in1 throttle cable 51-6506
• a set of allen head bolts as the stock JIS are very soft
• 4x M3x10
• 4x M4x16
• 8x M520 
• 2x 90 degree silicone reducer bends going down from 64 to 57mm 
• 2x VM38-200 inlet rubbers 

Assembly:

I started with unscrewing the splitter and then GENTLY clamping the cables in the vice using the splitter as a funnel to oil both sets of cables, as they are almost completely dry from the factory and become a lot smoother, if oiled properly. 

Then hooked up the cable and replaced all the hardware for stainless allen bolts with plenty of copperslip on the threads. Also a bit of grease goes a VERY long way on the paper gasket of the carb top and prevents it from sticking to the carb body and subsequently tearing apart.

The stock jetting of #22.5, #230, 1.5 turns on the airscrew is probably a good starting point, if you run huge velocity stacks or something like that, but after all those years of testing, it is too rich on the pilot and mains, yet the stock needle will cause some serious lean condition in the mid range. 

The difference between a 6FJ41 and a 6FJ40 is basically impossible to spot, which is good, because the needles are very similar in dimensions.

Swapping needles is a bit tedious and quite frankly it is easier, if you unhook the throttle cable from the little doohickey, which locks the needle in place and serves as a mount for the throttle cable.

... and in with those TM38s. Please not the zip-tie on the (short) throttle cables, this turned out to be one of those really important little modifications as it prevents the cables from snagging AND helps a lot with keeping them in sync. The (used to be) red velcro strap holds the cable splitter to the ignition box and again makes sure nothing moves around which helps tremendously with keeping everything in sync.

Conclusion:

When you swap jets, needles and do so many other things and there actually IS a 2-stroke version of the same carb, one can't help but start looking at that one, going with the specification of TM38-102:

• Pilot: 25
• Main:  340
• Needle: 6FJ40
• Needle Jet:  Q2
• Slide Cut Out: 4.0mm
• Air-Jet: 2.5 

In short, one could most likely save quite some money, by buying the 2-stroke version and just removing the air-jet that is threaded into the back of the carb and then only having to jet it to suit.

 

Quite frankly, I have spent a metric TON of hours on setting these and other carbs up and they work really, really spectacularly now. Even the mileage, most likely due to the improved mid-range has improved dramatically. I can ride the bike in a very laid back fashion and I managed to achieve a personal best of 5.77L/100km / 49mpg(UK), but a low 6-ish L/100km / 47mpg(UK) is absolutely realistic with this setup. Oil-temps stay under control on the Autobahn (!), with 160 kph (100mph) power-cruising being absolutely possible now. 

Further reading:

• 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