OzVMX Forum
Clubroom => Tech Talk => Topic started by: John Orchard on May 07, 2019, 04:37:52 pm
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Anyone filed Mikuni VM throttles slides into the richer direction? I'm thinking of filing the bottom of my VM38 slide, changing it from 2.5 to 1.5.
I found the following chart giving slide number cutaway height...
1.0 = 1/16"
1.5 = 3/32
2.0 = 2/16
2.5 = 5/32
3.0 = 3/16
3.5 = 7/32
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Can be done but there is the problem that you effectively lower the needle at the same time and can also bottom out the slide on the top of the needle jet.
Better to buy a new slide.
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Same as Sleepy said . I have played with those carbs on my triple . I bought 1.0 slides , filled the bottom cavity with a detachable insert and kept shaving away (just on the angle of course) till it got better .
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Yep done that & been caught out by "everything"l Possibility for the slide then to sit on top of the needle jet where it sticks up' Also a possibility that it could get past the guide pin in the side on full song then all hell can happen. Just take care that's all, & as said--best to buy a new slide. Maybe go richer on the pilot which might help the leaner cutaway in transition to the needle [or not lol]. Would have to lean off the air screw a little [maybe], but the richer jet could help via the compensating hole [one goes straight up [think that's it] & one comes off that zt an angle [which i think is for idle only] Confused -- hell i am!!.
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for some reason it remnds me of the stories i heard when i was racing speedway.....apparently old timers used to remove the needle from their carb completely as speedway is wide open at all times (if you can ride)
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Thanks for feedback Chaps, l will eventually buy a new slide, will test on an old slide first as l have a stack of 2.5's kicking around the shed.
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Can be done but there is the problem that you effectively lower the needle at the same time and can also bottom out the slide on the top of the needle jet.
Better to buy a new slide.
l guess l can just raise the needle by the amount l file off, l will check the needle-jet fouling issue, l took 1.4mm off the bottom of the slide, l could machine a slight 'pocket' in the bottom of the slide for clearance, maybe a small trim of the NJ shroud might not make too drastic change?
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Maybe go richer on the pilot which might help the leaner cutaway in transition to the needle [or not lol]. Would have to lean off the air screw a little [maybe]
l initially went larger on the PJ, then realized that my lean issue was at partial throttle, went big with NJ and small on the needle straight section, it helped, have sinced realized that slide cutaway works between PJ & NJ which l now have worked-out is my area of concern :-)
DT211 initially advised me that he runs a 1.5 slide, l should have heeded his words sooner ;-)
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Same as Sleepy said . I have played with those carbs on my triple . I bought 1.0 slides , filled the bottom cavity with a detachable insert and kept shaving away (just on the angle of course) till it got better.
Walter a triple fitted with slide stuffers (UFO?) would really rip off the bottom-end!!
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I have seen the UFO , but what worried me was, the screw going up from the bottom and you could not unhook the cable end without unscrewing the insert. I made a cavity to slide the nipple sideways and can now detach the cable if need to be, as per standard . Also being screwed from the top down will eliminate ever ,a bolt going through the engine . I started with a 1.0 , made a jig to fit the milling machine and retain the right angle and height on all 3 slides. I ended up somewhere between 1.5 and 2.0 . It cleaned up that fluff around 2000 RPM just before the power curve goes up rapidly :o.
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I have seen the UFO , but what worried me was, the screw going up from the bottom and you could not unhook the cable end without unscrewing the insert. I made a cavity to slide the nipple sideways and can now detach the cable if need to be, as per standard . Also being screwed from the top down will eliminate ever ,a bolt going through the engine . I started with a 1.0 , made a jig to fit the milling machine and retain the right angle and height on all 3 slides. I ended up somewhere between 1.5 and 2.0 . It cleaned up that fluff around 2000 RPM just before the power curve goes up rapidly :o.
Ah cool, l might think about making my own slide filler, l had a UFO item in the VM34 on an RM125B (that won an Oz title after l sold the bike to Ted). l was so surprised how the UFO kit made a 125, with a 34 mil carb, rip off from low rpm!
I seem to remember the retaining screw coming down from inside the slide, or l'd be worried about it going into the engine. With the increase in pulse signal in the venturi, l had to halve the pilot to a #15.
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Posted by: Hoony
« on: Yesterday at 07:42:59 PM » Insert Quote
for some reason it remnds me of the stories i heard when i was racing speedway.....apparently old timers used to remove the needle from their carb completely as speedway is wide open at all times (if you can ride)
HOONY
only 1 reason to take the needle out
NITRO
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Posted by: Hoony
« on: Yesterday at 07:42:59 PM » Insert Quote
for some reason it remnds me of the stories i heard when i was racing speedway.....apparently old timers used to remove the needle from their carb completely as speedway is wide open at all times (if you can ride)
HOONY
only 1 reason to take the needle out
NITRO
yeah i heard that story too, the sponge soaked in nitro in the slide and when it went to the stopper on top of carb
the nitro was released..... don't know if that one is true as where would it go as it was captive at top of carb ?????
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There was the story of Bert Flood when he was getting Ginger Molloy's Bultaco for the speed record on the caller highway thought that the way to jet around a lean jetting issue caused by a leaking crank shaft seal was to totally remove the main jet.
There may have been a sniff of Methanol in the fuel
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Yes Hoony. you could tell who was using that trick at Golden City. They would idle around to the start line painfully slow then holeshot easily. I recall one top liner telling me ( at the party in eaglehawk mum & dad had after the boxing night race every year, he had consumed more than a couple !!) that it was worth 1 tooth on the engine.
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Well l filed the slide, easy, no problems, l actually filed-off more than the engine ended-up needing (made it too rich at 1/8th throttle), so l just filed the cutaway a bit to bring it back :-)
The slide (that now travels downward further) didn't touch the needle-jet (NJ) shroud, but l flatened the angle on the bottom of a 7mm drill-bit and gave the underside of the slide a recess for more clearance from the NJ, just incase l find, at some time in the future, that l have to run the slide lower at idle.
One thing to keep in mind, as the slide centre (the bit that holds the needle) sits lower (equal to the amount you file-off), the needle will sit lower into the NJ, l had to change the clip position from groove 3 to groove 5 (bottom groove) to keep things equal. Should l ever need to raise the needle further in the future, l'll just put a shim-washer under the clip :-)
My filing may (yes it was) have been a little uneven at first and my idling rpm was all over the place, a few strokes on some course emery-paper to get it square, got things right and she runs so sweet now.
So there you go, don't be afraid to experiment :-D