vm28's on an xs360

[aus_xs360]

Project Update! ignition installed, VM's tuned and bike run on the dyno.....VICTORY!

she running and running well! currently putting out about 23HP (see Dyno results attached).

jetting details:
-Mikuni VM28's
-Pilot jet: 22.5
-Main Jets:165

Mod details
-reverse megaphone shorties with an (off the shelf) baffle
-K&N pod air filters (as big as I could get them)

my final thoughts on the VM28: while the 28s give a notice power boost, they are still slightly too big for the low range. it will bog when I snap on the throttle up to 1/4 throttle but fine from 1/4 to full. just need to smoothly roll the throttle on at the low end.

VM26's may be a better option for the xs360 but will result in a slight HP drop, not sure how they would go on a xs400 but probably slightly better. overall I'm pretty happy and look forward to a summer of braap'ing

 

[drewpy]

sweet glad you got it sorted

 

[NewHavenMike]

Itd be nice to see a comparison with the bs34s. Those numbers seem kinda low but i really have no idea. Figure the 360 has 35(?) hp and youre talking a 30% loss from the drivetrain?

Dont get me wrong here. I love my 360 and i think its a quick little bike and ill never ever sell it. It just seems like it would have alittle more output... then again, the 360 was probably dynoed at the factory directly from the output shaft and absolutley perfectly tuned.

 

[xschris]

xs360 bone stock has 27hp. I saw a dyno sheet from an old mag when they where new.

 

[xschris]

Found the sheet. Looks like 26hp at rear wheel.

 

[NewHavenMike]

Oh ok thats more i like it. Seems like ever the me i see a spec sheet theres any number from 29 to 39hp listed. Same goes for C/Rs.

So 23hp is a pretty decent number. Makes me wonder what mine puts out.

 

[xschris]

I would think the stock setup is a better one. With a modified bike as much as that one some may think it will make much more than stock power. Yamaha engineers designed these bikes to make as much power as they could and keep fuel and emissions as little as possible.

 

[aus_xs360]

26hp out of the factory....40 years ago....23hp now.... I'm pretty happy with that. It reminds me of an old top gear episode where they took some old super cars and did a now and then comparison of hp and they had waaaaaay worse decreases in power.

Ideally I would have used the stock carbs but they were in really bad shape. the vm's were an experiment....and not a cheap one.

 

[aus_xs360]

Thought I might give an update on how the lil xs360 has been running over the Spring/Summer/Autumn period.

she's now done about 1000kms since the top end rebuild, Pamco ignition and VM28 experiment. the engine is running strong with ridiculous compression, my riding buddies feel a massive force of pressure coming from the exhaust when I gun it along a straight!

However, with those VM28s it still runs rich and I'm forever having to clean my spark plugs just to get it started on cold mornings. don't get me wrong, I've enjoyed the punchy acceleration I get from them. But I cant help but wonder....would I get better reliability and go through less spark plugs if I had a set of original BS34s? and not having a decent set to test before, did I really get any more performance from the VMs?

These questions have kept me up at night, and the slow warm ups and constant spark plug cleaning have, if I'm honest, made me think that the bike isn't worth the trouble and I should ditch it for another project.

BUT!!!!! last week I actually found someone wrecking a xs360 in Australia (they are very very rare here). I've bought the carbs and they will hopefully be delivered later this week. I'll run them through an ultrasonic cleaner and get some fresh jets, will probably require new jetting with the pods and exhaust. if I'm feeling particularly flush with cash I might even take it on a dynotune and see what the HP difference is!

I'll report back on my experience and we can once and for all put to rest the ongoing debate of Mikuni BSs Vs VMs.....well...maybe.

 

[aus_xs360]

So I thought I might report back on my experience of Mikuni BS34 stock vs. Mikuni VM carbs. the VMs are very popular on the xs650, and from all reports they work great. When I got my project xs360 I knew the carbs were completely junked and would need to be replaced so I tried out a set of VM28s.

After much effort, a few runs on the dyno, I got them pretty good. But, they ran very rich and were boggy at the low end.
Earlier this year I managed to get hold of some original BS34s for the xs360 that were in ok shape, I've now cleaned them up and installed them....so how do they compare?

Bs34s are far more reliable and easier to tune BUT the VMs provide a significant boost of power in the mid to top end. As my bike is for daily rides and weekend jaunts in the country, I'll be sticking with the BS34s. If I was using it as a track bike then the VMs would make a lot more sense....horses for courses.