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Post by stevec5000 on May 10, 2008 12:42:32 GMT -5
>These off-the-shelf Mikuni 2-stroke carbs are inexpensive (brand >new), readily obtained, and are successfully used in many 4-stroke >applications.
I never heard of anyone using a 2-stroke carb before. I wouldn't have thought it would work or that you could find the proper jets but it sounds rather easy. Not as easy as my HS40 that I just bolted on and didn't have to do anything but change the main jet but still pretty easy.
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Post by spindlehead on May 10, 2008 21:49:55 GMT -5
All of the aftermarket Mikuni VM series carbs and most of the TM series (flat slide) are "primary choke" types and not "bleed" types -- refering to the type of needle jet. Primary choke type carburetors are normally intended for 2-strokes and bleed types are normally used on 4-strokes.
I guess calling the VM34-168 a "2-stroke" carb wasn't entirely accurate, but it is a primary choke type. In some folks minds, using a VM34 or VM36 or TM38 or similar is "using a 2-stroke carb on a 4-stroke." I'm encouraged that there are a bunch of folks like andy, nsman, curly and carl who have had success using them on SR's.
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Post by caferacercarl on May 11, 2008 2:38:08 GMT -5
I 'spose I should remember that being in the industry that the last thing I want to do with my spare time is experiment blindly,I envy those of you who have understanding partners that don't mind you fiddling with the bike, mine's over it, even as a job as there's no money in it.probably why she's left with the kids...
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Post by andy on May 12, 2008 7:25:21 GMT -5
Hi spindlehead,just catching up on recent posts,your cold starting issues are a carbon copy of mine but i am not to concerned cos after my initial stone cold start of the day my bike is fine.If the bike doesnt start within 2 kicks on full choke i turn it off,sometimes she does sometimes she dont.Carls comments are spot on,there are better carbs out there and depending on your viewpoint the vm34-168 haves some limitations but overall i'm not looking to change mine [again!]and if your sticking to a mild state of engine tune i dont think there is any big advantage.
Regards,
Andy
P.S. Curly,if your watching this post any results with your trials with this carb type would be gratefully received.
1]
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nsman
New Member
Posts: 25
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Post by nsman on May 12, 2008 9:25:33 GMT -5
I switched main jets from a 260 to a 240 and it now runs better in the midrange,and spins easlily clean to red line. I am now happy with my set up and this is how it will stay.It does take a few kicks now to start from cold but it starts first kick(even wearing slippers ;D )all day when warm.
I have a White bros head pipe and white Bros super trapp muffler for exhaust.
Also I am using a intake manifold off a Yamaha wolverine(4KB avail aftermarket for $50) atv,install it upside down and it bolts up perfect and the carb sits straight and the ports line up almost perfect.It also spaces the carb back far enough for the cable to fit without any stange bends.
My next test will be for fuel economy,early indications are that it is within the 20-25 km/litre range...
I will let you know.
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Post by spindlehead on May 12, 2008 11:17:00 GMT -5
andy- Thanks again for the feedback. I'm suprised you can use the choke at all -- I get nothing if I try it. I'm going to try to geek up an insert for the start circuit inlet to reduce the effective "start jet" size. I'll let you know if it works.
nsman- great news! I'm glad the 240 jet worked out so well.
I took her out for about a 50 mile ride yesterday in mixed city and highway conditions. I haven't put any fuel back in yet but I will today and report the fuel economy. I noticed some slight bogging between 1/2 and 3/4 throttle in 4th gear when accelerating to freeway speed. I'm not thrilled with the throttle response right now, either. There is a noticable lag then abrupt power application when I get back on the throttle after a downshift or engine braking. maybe some of that can be tuned out with idle mixture adjustment but I have a feeling that it's partly a limitation of a round-slide carb without an accelerator pump. I could probably work around it with clutch technique too.
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Post by andy on May 13, 2008 23:49:24 GMT -5
O.K Spindlehead heres where i'm at.took the bike to work today and gave it a good blast on the motorway.midrange seems fine but definitely holding back at full throttle in top and the plug is showing a lightish grey which tells me weak mixture,reckon first off i'll try the next size up main [i've got a 250]and shift the needle to the last notch [richest] i'm currently at 4th notch,will then work from there possibly going richer on the needle jet.i'm determined to get the mixture right cos i dont want to undo all the effort of the topend rebuild,i'm talking burnt valve.i've also noticed my oil temp gauge is showing a bit hotter as well which stacks up with my weak mixture theory.i reckon topend should achieve 90mph + comfortable.i personally think delving into the needle area requires a bit of tecnical advice from the guys from mikuni's r&d dept.fuel consumption is a side issue with me i'm sure it will look after itself once the mixture issues are sorted.if all else fails i'm gonna go down the dyno route but right now i'm still punching! Andy
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Post by curly001 on May 14, 2008 0:31:03 GMT -5
Andy, yes I have been watching this thread.
Why would you use a VM34 when you could use an early XT/TT carbie which is 34mm bore and set up for a 4 stroke. (Feel better now, got that off my chest.)
You can,t tune these carbs by the seat of your pants. You can get them to start easy run cleanly up top, but as for the mid range and under load you need to use a exhaust gas analizer.
You need to bite the bullet and get it dyno tuned.
Here's my VM36 specs
Pilot 35
Main 230
Slide 2.5
Jet (needle) P2(188) custom drilling 1or2 thou larger
Needle 6FJ4/7 Top clip with a shim
Curly
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Post by andy on May 14, 2008 8:19:42 GMT -5
Cheers Curly,I'm sure i'm gonna end up on the dyno.....sometime! but for now I still enjoy playing, even if i am slowly dissapearing up my own backside!
Andy
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Post by spindlehead on May 14, 2008 10:57:27 GMT -5
Thanks for chiming in, Curly. And many thanks for posting your VM-36 specs! Was there a special reason for chosing the 188 type needle jet? My chart lists that needle jet type as being for a 32mm flange mount Kawasaki application. (just curious)
My main reason for chosing the VM34 was that I wanted to start with a brand new carb. I wanted to make sure all of my issues had to do with tuning and not the carburetor itself. I also wanted to be able to use standard jets. The stock VM34SS I have on the bench uses a funky oem pilot jet. I also like the screw top for needle adjustments and the easy access to the idle and air screws while sitting on the bike. I guess those reasons (or excuses) have more to do with convenience than performance.
Oh, I'm sure I'll end up on the dyno eventually, too, but I'm still getting some value out of seat-of-the-pants tuning for now.
BTW, My fuel economy from my 50-mi mixed city/freeway/highway ride was 38.4 mpg (16.3 km/liter). My oil temperature never got above 180 and the sparkplug was dark brown.
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Post by stevec5000 on May 14, 2008 11:49:22 GMT -5
Wow, 38.4 mpg is really low! I'm getting 60 MPG.
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jeffz
Full Member
Posts: 215
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Post by jeffz on May 14, 2008 12:27:34 GMT -5
What size air filter is everyone running with the after market Mikuni's?
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Post by stevec5000 on May 14, 2008 12:39:22 GMT -5
I don't remember the size but I got a pod filter at the local VW hop up store called BAP Import Parts and it's a perfect fit on the carb intake. They only had one size so there wasn't much choice. Before that I had a foam filter like a sock with a coil spring in it and it filtered well but didn't hold up to the sun and the foam kept shredding and falling apart and made a mess all over.
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Post by spindlehead on May 14, 2008 13:26:55 GMT -5
Yeah, 38 mph is pretty low. The 60 your getting is close to stock. I'm obviously pushing a lot of unburnt fuel out of the tail pipe. Hopefully, the leaner needle jet will help. Part of it might be my riding style. I was pretty agressive on the throttle.
I have a cylindrical K&N about 3.5" DIA x 6" long.
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Post by andy on May 14, 2008 23:55:13 GMT -5
Blimey mate 38mpg is thin!It makes the dyno more appealing cos within a few tanks full of juice you will have recovered your dyno expense.Seriously tho' its got to be a worthwile considereation with the ever increasing price of petrol,its up to about 1.15p/litre in the u.k which equates to over a fiver/gallon....depressing!
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