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Post by andy on Jan 13, 2008 14:04:18 GMT -5
Hi to all you carb wizards! I recently fitted a mikuni vm34 roundslide carb which works o.k but i feel is not responsive enough when you snap the throttle open,it seems to go 'fluffy' until the revs catch up.Looking at the slide cutaway tells me 2.5,the mainjet is 240 and i think the pilot is 25,the needle is set on middle.If any of you have experienced similar i would be grateful of any advice,[i'm thinking of going to 3.5 on the slide].The plug colour looks fine.The only difference to stock is a large k&n air filter and a straight thro' open goldstar muffler,just had a fresh re-bore and new wiseco piston and kept to stock 9:1 compression.I realise there are infinite set-ups with jetting,needles etc so your input based on your pesrsonal experiences would be gratefully appreciated.
Thanks,
Andy
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Post by colinjay on Jan 14, 2008 16:32:31 GMT -5
Andy,
It depends on what you mean by "snap the throttle open". If you are snapping the throttle fully open from low revs, the problem may not be the actual jetting, but a loss of vaccum, and therefore the abillity to draw fuel via the main fuel system as the carbie comes off of the pilot system. This is a common problem with all engine, and especially 4 stroke singles fitted with carbies that do not have an accelerator pump, to give a small squirt of fuel into the venturi to overcome the inability to draw the fuel via the main fuel system jets.
If the engine responds well to normal throttle use, i.e. not snapped wide open, the the jetting is probably very close to correct. A jump from a 2.5 slide to a 3.5 is probably a bit excessive, normally you should only go one size increment at a time, note; a 3.5 slide cutaway is the leanest std cutway made for Mikuni carbies and is not that commonly used, most engines in my experience use from a 1.5 to a 2.5 slide.
Also you may need to look at the jet needle, as it has an effect on the responce through that 1/4 to 1/2 throttle area, possible lift the needle up one notch to clip position 4 (the clip grooves are numbered from top to bottom, i.e. 1 is the top and 5 is the bottom groove).
Out there on the net there are copies of the Sudco mikuni book that will give you a good rundown on the way the VM series carbie works.
CJ
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Post by andy on Jan 15, 2008 9:45:30 GMT -5
Thanks Colin,yeah i did mean snapping the throttle open off the bike with obviously no load on the engine.reading your comments and listening to local chat on the subject points to one or two similar comments, 1,running a bit lean and 2,maybe either go a bit smaller on the slide/ or m/c a bit off the bottom face which would in turn reduce the size of angled cutaway.then lift the needle one notch and give it a go.like yourself i dont think its a million miles away,just that it could be improved on.thanks.
Andy
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Post by caferacercarl on Jan 17, 2008 0:59:41 GMT -5
as colin said, needle jet is critical, is it a two stroke carb? if so the needle jet will be wrong [the thing the needle drops down into] two stroke tubes generally only have one hole in the side, four stroke ones usually have about 16 tiny holes, and the fit between the two is different from 2 to 4 stroke. regards carl.
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Post by andy on Jan 17, 2008 16:06:10 GMT -5
Thanks lads,i've just turned 1mm of the bottom of the slide on the lathe and lifted the needle 2 notches,not road tested yet due to heavy rain but even warming the bike up it feels a bit more like how it should on the throttle response and starts a bit easier,pretty sure the fueling was a bit weak.carl did you mean mainjet or the tube it screws in ?2 stroke? its a vm34mm and jetted for a 500single,the slide moves against a large spring not fixed like stock carb with solid link it does not have the pumper on it either.i found the stock carb a bit temperamental and a bit worn so i changed to something a bit more simple.what are you lads running,stock or different?.thanks for the help.regards.
andy
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Post by colinjay on Jan 17, 2008 21:48:28 GMT -5
Andy, The following internet adress is for a diagram of the VM Mikuni (Ian William Tuning in Adelaide SA). www.iwt.com.au/VMdrawing.htm Item No. 11 is the needle jet. As Carl said, ther are two different types of needle jet used in these carbies, the Primary type (2-stroke) and the Bleed type (4-stroke), the bleed type needle jets give better fuel atomisation that the primary type and therfore better "Off Idle" responce, which is a problem with 4-stoke singles. If you have a look at some of the other pagese of Ian Williams Tuning web site there should be pictures of both types of needle jet. I have run EI carbies for the past 20 odd years on my hotted up engines, but, I have just fitted a 38mm VM Mikuni to my racer (with the Primary type (2-stoke) needle jet). I have used this carbie because of cost, $10 from a mate years ago. As the engine is tuned for flat out running, and Off Idle responce isn't a major concern, having the wrong type of needle jet is not a major worry to me. CJ |
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Post by andy on Jan 19, 2008 13:56:54 GMT -5
Thanks for the help lads,having took the needle jet out it shows 1 hole and obviously for a 2 stroke! I'm a bit annoyed with the place i had it from as i specifically told them the bike and model so as to get the jetting about right.needless to say i will be having some stern words!Again it shows the benefits of discussing these problems with more knowledgeable and experienced members,I could have been trying all sorts of alterations and still not looked at the needle jet.I'll let you know the results later.
Regards,
Andy
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