XS360 with Carb/Jetting Issues will XS400 fit?

[coal miner]

SO THE QUESTION THEN IS... Does anyone out there know for sure the 400 carbs would fit, and be tunable? Or have or know of a place that can make or has 360 diaphragms (again JBM is out)? Would there be any specific year models of 400 carbs that wouldn't work, and i need to stay away from?

ANY OTHER CONSTRUCTIVE INSIGHT YOU CAN PROVIDE?

I should probably start out with the back story of this issue. I apologize for the length.

I started out with the stock carbs back in 2012, but had diaphragms that were badly cracked. Bike would run but not run very well. I hit up JBM and got the closest diaphragms they had to what I needed, but were definitely not the right size (was too big outer diameter), and they don't offer the correct size for the XS360. This diaphragm worked to some degree, but it was always impossible to get both the vacuum leaks stopped, and to get the diaphragm perfectly centered where the slide wouldn't rub or drag slightly. This would cause the slide to return slower than the other cylinder and the RPMs would hang up. So it just ran lousy.

About 6 months ago, i got back to working on this bike (it is one of 6 in the garage), and decided since XS360 carb parts are so difficult to find, I would install a pair of VM32s, and then if nothing else I could tune them to work well enough. I have always been able to get it to run, it just really bad. The thought behind the VM32s was to go smaller diameter, to increase intake air velocity at lower RPMs and pick up a little extra torque, which should be better for around town riding. So here is where I am to at this point.

I have had the motor rebuilt, new rings, valve job, etc. Compression is at 135psi Left, 140psi Right (pre rebuild was 35 Left and 105 Right), Both exhaust pipes are within 12degrees (with a temp gun) after running. The timing is dead on, the valves are exactly in the middle of the adjustment range. I am 1000% sure I have no vacuum leaks.

Currently I have VM32s installed, I have tried running every pilot size from 25 down to 12.5(currently installed today) with a 140main. Even with the 12.5 it runs so rich, its smoking out of both cylinders terribly bad. Today i experimented, trying to lean it out to stop the smoke. I ended up with the air filters completely removed, the air mix screw out 3 full turns, lowered the needle to the lowest (leanest) setting, and removed the caps on the carb tuning ports, and it was still running rich enough to have visibly wet plugs, and developed soot within a matter of minutes, and will use fuel quicker than my CB750F out of my remote fuel supply. It will not maintain idle without holding the throttle open.

At this point I am looking at going back to a more stock setup, given the issues in the past locating diaphragms for 360s (which will be an issue again if trying the XS360), I am leaning towards trying XS400 carbs, and should then be able to locate 400 diaphragms moving forward.

I have read everything there is about the carbs similarities on this website, and other sites out there. I have read a couple threads on 360s on 400s, with people indicating 360s are the better carbs, but without diaphragms that option is now out. I haven't found anything about 400s on 360s, probably due to the lower production run of the XS360. Without having a set of 400 carbs in hand, I am limited to reading and cross referencing part numbers etc.

If this was any other bike it would be in pieces on eBay. But my grandfather bought this bike new in 1976, rode it for 9 years and parked it in the barn. I have easily spent 3 times what it is worth, but it was never about the money.

 

[Wolfe_11B]

The reason the VM32's aren't working is because they are way too big. The proper size VM for use in a XS360 would be the VM28. You could probably get the VM30's to work, but it's still more carb than the motor needs.

Sell the 32's and grab some 28s.

Or grab these from a respected forum member, at a good price nonetheless- http://www.xs400.com/forum/showthread.php?t=15020

 

[coal miner]

Thanks Wolfe, I shot a message over to xschris, hopefully i can snag these from him.

Where exactly did you find the information indicating the 28s would be ideal? Is that in the Mikuni graph that has aperture diameter on left, and PS on bottom,

http://www.mikuni.com/pdf/vmmanual.pdf

Section 4-1 Pg7? The 360 has roughly 26-27hp stock, so for numbers sake say 30hp, 15hp/cylinder~15PS/cylinder. Putting it around a VM26-28.

 

[Wolfe_11B]

Don't get too caught up with comparing diameters of carbs. It has more to do with flow rate, venturi design, etc. The round slide and flat slide carbs are all different beasts compared to CV carbs. To be honest, a direct equivalent to the stock 360 carb would be the VM26. That carb would get you easier tuning. But the 28 will be an upgrade performance wise.


And the graph you showed spells it out perfectly

I've had a bunch of experience with flatslides in my other powersports adventures, so I always do the face palm when I see guys throwing huge carbs on small motors. I give you credit though, as most say 'it will work fine' but then they disappear, never saying if they worked or not(because they didn't work). It's a common theme on about 20 different motorcycle forums I visit.

I think you would be pleased with the aftermarket carbs once you get them sized right. CV carbs are great for daily commuter bikes(when they are working), but nothing beats slide actuated carbs throttle response and raw power delivery! SO basically, go with the 400 carbs if you just want something that works, or go with some smaller VM's if you want something that feels better(just have to put some tunning in them)

 

[coal miner]

OK, so for the sake of contributing to the greater good, here is what I have accomplished so far.

First and foremost, the XS400 (80-82) carbs will fit, and use the same throttle cable. The OD of the engine side of the carbs is the same as the XS360. The OD of the air filter side of the carbs are larger than the 360. The spacing between is the same. The length of the carbs are longer by several mm's.

If you are still using the stock location for the battery you have to use the 360 air intake tubes. This is due to the intake tube "crossover/vent tube" is farther from the carbs on the 400 intake tubes, and also due to the extra carb length. This requires you to heat up the 360 tubes with a heat gun and stretch them GENTLY so they will go on the larger filter side of the 400s, but with some massaging they will fit. Beside that, the rest of the fitment is simple plug and play. I am using some Emgo/stock filters in stock airbuses.

I started with 25pilots, idle mix at 3 turns out, floats at 26mm exactly 135mains (thanks to xschris for that info). The bike fired up and idles nicely, it pulls well up to around 6krpm, then it falls on its face. I think it gets lean up top, but I need to get some fresh fuel, clean plugs, and play around with it some more. I only got about 20mins with it tonight, i should get some more time tomorrow. I have several sets of genuine mikuni pilots and mains so I'll start with increasing the main jet to 137.5 and see how it responds.

With the way it is running thus far, I'd say that the late model 400SOHC carbs can be made to run a 360. I will report back what I find tomorrow.

 

[xschris]

Pilot jets should be 42.5 for the xs400 carbs. Have you synced them with a manometer yet? I would run the stock air filters as emgos are the worst filters made Unless they are oem style.

 

[coal miner]

i synced with gauges. they are within 1cmHg relative to each other

 

[xschris]

6k is where you are running off the jet needles/needle jets. You may need the raise them. The style of xs400 carbs you have use non-adjustable ones. You would need to shim them to raise them (rich en them) up. I would do some plug chops to see if it is lean or rich before making changes.

 

[xschris]

Also make sure the rest of the bike is in good tune. Points, timing, valves adjusted. The points bikes use a mechanical advance ignition. Make sure it is free and lubed.

 

[ensoroni]

Any recommendations on a place to get VM28's? Will they fit right into my carb boots or do I need to find an entire kit.

I ride a 1976 XS360

 

[Wolfe_11B]

Although I dislike the company, give Speedmoto Co a call. Tell them you want VM28's to go on an XS360/400. They do sell a complete kit to bolt on for the 400's, but they are 26's, which might be best for you on the 360 anyway. But they won't have a problem giving you 28's either. They include the proper manifolds and cables. They will pre jet them, but just be prepared with extra jets, since nothing can truely be 'pre jetted' properly.

 

[markjs]

I've had good luck going over old cracked diaphragms with grey RTV sealant. Got a KZ750 with bad cracks running well for the cost of a tube of that thinly applied to both sides of the diaphragms.