Carb options for Redline XS650

[patentgeek]

I'm trying to put a long dormant Redline framed XS650 back on the road with some modern upgrades. The engine is a 1970 and it appears to be in stock tune.

I considered running a single Mikuni VM carb on one side to accomodate a high mount exhaust on the other side, but have discovered the single carb interferes with the frame. Rrrrrrrr.

The distance between the head intake to the main tube (red arrows in the pic) is approximately 135mm.

I really like the Mikuni TM34-B120 carburetor kit from Topham in Germany, but I'm not sure if it will work with my frame.

I'm pretty certain the Mikes XS Mikuni VM34 kit would fit.

Thoughts? I'm open to other options as well. The bike will be used for casual street riding, so I don't need anything crazy. Thanks in advance for the help.

 

[jpdevol]

I suspect you may have to use "angled" intake manifolds. I have not seen a set of those in quite some time. Below is a pic of KR mounted on a '70-'71 XS that appears to have the intakes angled outward.

 

[grizld1]

Angled boots may not be possible, but angled spacers to give conventional boots enough outward tweak so the intakes clear the backbone? Maybe possible. Love the look of those 34 mm. flat slide pumpers from Topham's too, though I first became aware of them after I'd banked a pair of TM36/68s.

 

[patentgeek]

It wouldn't be incredibly difficult to modify the MIkes XS adapters to angle outward.

 

[arcticXS]

A somewhat unusual alternative would be individual DellOrto pumpers. Sort of like the VM, but with accelleration pumps. As used on the BMW R90S and many Moto Guzzis.

PS: I like your camshaft end caps

 

[patentgeek]

A somewhat unusual alternative would be individual DellOrto pumpers.

I'm familiar with those through BMW R90S and Guzzi twin ownership, but I'm trying to avoid "unusual alternatives" that will require a lot of effort to set up. For my purposes I'd rather be a lemming and do what many others have done before me. . . .

 

[arcticXS]

https://www.heidentuning.com/xs650-...burator-kit-dellorto-phf36-pumper-detail.html

 

[jpdevol]

https://www.heidentuning.com/xs650-...burator-kit-dellorto-phf36-pumper-detail.html

Those may be the "ticket" if the manifolds are angled (appears so in pic), but "the new set is with stainless steel clamps and new type manifolds with O rings. note: picture is not correct!". IMHO, I'd change the filters to 4" long, round K&N

 

[grizld1]

If you want carbs with a deep well of experience on the XS650 to draw from, the Mikuni VMs are your best choice, but most owners have to do some tuning. The VM 34 is the most common choice, but the VM32 will pass all the fuel that a stock motor needs. What counts is unobstructed venturi, and the open throat VM32 gives you more of that than a BS34 with throttle plate and shaft reducing the effective size. I now run 33 mm. flat slide dedicated 4-stroke pumpers on a cammed and ported 700 cc. XS650 motor. With 17/32 sprockets, a well calibrated instrument reads 118 mph at 7K rpm with lots of throttle left. The TM36/68 carbs had good manners down low, but the 33s are smooth as butter at 1500 rpm in 1st. You'd find the 32s more easily tunable, too. IIRC Niche sells a VM32 kit for the XS650.

I'd recommend the 33 mm.pumpers, but the only offering from Mikuni now is the TM33/8012 for singles. One carb would have to be line bored to let the throttle shaft pass through. Push-pull cables are needed with these, and running two pairs would be problematic. I was lucky enough to get my hands on an old set of banked carbs that were sold by the unknowing as salvage, modified the throttle shaft and brackets for 120 mm. center-to-center spacing. Getting mid-50s/gal. on 10% ethanol, high 50's on straight gasoline when I can find it. The smaller venturi lowered fuel consumption from the 48 mpg that the 36s delivered.

Should have put all that in the past tense, since my helmet is currently wall decoration, but hoping for a few more rides.

That's a beautiful frame, hoping to see the finished bike!

 

[patentgeek]

If you want carbs with a deep well of experience on the XS650 to draw from, the Mikuni VMs are your best choice, but most owners have to do some tuning. The VM 34 is the most common choice, but the VM32 will pass all the fuel that a stock motor needs . . . . . . The TM36/68 carbs had good manners down low, but the 33s are smooth as butter at 1500 rpm in 1st. You'd find the 32s more easily tunable, too. IIRC Niche sells a VM32 kit for the XS650.

Yes, the VM34 seems to be quite popular so less likely to be "lost in the desert" relative to baseline tuning specs. Niche Cycle only seems to offer a VM34 kit.
I was curious as to the VM32's as smaller diameter can result in higher velocity flow, better mixing and more low to mid-range torque. Strange that nobody seems to offer a VM32 kit, but I expect "bigger is better" drives carb sales.

Thoughts on the Mikuni TM34-B120 carburetor kit from Topham in Germany?

I'm trying to determine dimensions of the Topham kit. I suspect the TM flatslides don't extend as far back as the VM roundslides so I might be able to fit them on my frame. Maybe with offset air filters or mounting a custom air box on the center tube.

 

[patentgeek]

Can't remember if I've posted a pic of the Redline to this forum. James Banke installed the R1 front fork with custom triple clamps. And James also built the wheels using his hubs and Excel flanged alloy rims. The project was sidelined during COVID but it's back on the lift.

 

[grizld1]

If it helps, the length of the TM36/68, spigot end to end of intake bell, is 100 mm. Topham doesn't post the numbers, but the 34s won't be longer than that. Too bad the bean counters at Mikuni America don't import the 34s and don't support some of what they do import with parts.

 

[patentgeek]

If it helps, the length of the TM36/68, spigot end to intake bell, is 100 mm.

That's promising. Do you happen to know the approximately distance between the intake bells?

 

[drgonzo]

I really like the VM34s I got from Gary Hoos. They were plug and play for me. Plus, he may be familiar with your frame.

 

[650Skull]

looking at the pic in post #11 there is a lot of space between the front of the head and the frame. The XS engine can be tilted 15% forward without any problems, the Ascott was built like that

I cant see engine mounts on the bottom to the frame, so that may make a difference to the thought i'm thinking. That is tilt the engine 15% forward and this will gove you a bit more room to mount the carbs. ill still need to angle them out though but the overall look and fitment may be better.

yea i know it means changing your front, top and rear mounts but that would't be a big job. Will move the center of gravity slightly as well

 

[patentgeek]

The XS engine can be tilted 15% forward without any problems, the Ascott was built like that

Creative suggestion, but that would require fabricating new engine mounts, which is a non-starter.

 

[650Skull]

Fair enough.............since i wrote that post i done a rough mock up of the tilt and it would give an extra 40-50mm between the engine manifold and Frame.

 

[patentgeek]

Helpful dimensions from the Niche Cycle site.

EDIT: After I ordered and received my TM34 carbs, I measured the intake diameter (D) as 54.8mm which is about 3mm smaller than the 58mm shown in the chart below. This impacts air filter selection.

Strange that Dimension "B" is 100mm for the TM36 and 83mm for the TM34 but increases to 93mm for the TM33, and then back down to 83mm for the TM32. I wonder if that's a misprint?

The VM34 appears to have approximately the same length (93mm vs. 83mm), so maybe no significant space savings with the flat slide configuration.

 

[grizld1]

No misprints there. Mikuni varies lengths of the intake bell on their carbs for reasons I don't pretend to understand .Yep, the variation in a lot of cases appears unrelated to venturi diameter.

 

[arcticXS]

IIRC, 62 mm. I believe the OD on the air filter side is 58 mm. Then 120-58=62

That's promising. Do you happen to know the approximately distance between the intake bell