Author Topic: Needle jet shroud height changes? Mikuni VM38.  (Read 2712 times)

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Offline John Orchard

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Needle jet shroud height changes? Mikuni VM38.
« on: October 12, 2018, 06:59:30 PM »
Can anyone give me a basic guide to changing the height of the shroud on the top of a 2-stroke Mikuni VM38 please.

For a given throttle setting, would say, shortening the height of the shroud reduce fuel flow at higher rpm? For example.
Johnny O - Tahition_Red factory rider.

Offline mboddy

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Re: Needle jet shroud height changes? Mikuni VM38.
« Reply #1 on: October 12, 2018, 09:02:26 PM »
The shroud is 8mm on a VM38 and 4mm on a TM38.
General recommendation is to add 4 to the number when going from VM38 to TM38. So P-2 becomes P-6.
I think the 8mm shroud is only that tall to increase suck through the nozzle at low throttle openings which is where the VM performs poorly.
If you fit a UFO slide stuffer in the VM38 you won't need the 8mm shroud because it will suck harder.
One bike we fitted UFO to we were able to solder up the pilot and it ran fine because it could suck enough fuel through the nozzle at idle. (VM38 with 8mm shroud).
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Offline John Orchard

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Re: Needle jet shroud height changes? Mikuni VM38.
« Reply #2 on: October 12, 2018, 09:41:59 PM »
The shroud is 8mm on a VM38 and 4mm on a TM38.
General recommendation is to add 4 to the number when going from VM38 to TM38. So P-2 becomes P-6.
I think the 8mm shroud is only that tall to increase suck through the nozzle at low throttle openings which is where the VM performs poorly.
If you fit a UFO slide stuffer in the VM38 you won't need the 8mm shroud because it will suck harder.
One bike we fitted UFO to we were able to solder up the pilot and it ran fine because it could suck enough fuel through the nozzle at idle. (VM38 with 8mm shroud).



Thanks :-) Yeh l ran a slide stuffer in a VM34 on an RM125B, l dropped the pilot from 45 to a 15, it ripped off the bottom end !

l had heard that the shroud varied the affect differently at either high or low rpm, l think l'll stick with the 8mm that is currently in it, the original needle jet in this VM38 (Kawa OEM) had a 4mm high shroud (and round head main jet), l was considering the need to return to it.
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Offline dont

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Re: Needle jet shroud height changes? Mikuni VM38.
« Reply #3 on: October 13, 2018, 08:11:53 AM »
I read a couple of years ago a technical report from perhaps Penton originating early 70's about cutting down or slotting the shroud on a bing carb in a sachs 125, changing idle jettng and improving throttle response at low revs. By the time I got spares I couldn't find the article to duplicate

Offline Jumbo J

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Re: Needle jet shroud height changes? Mikuni VM38.
« Reply #4 on: October 14, 2018, 10:39:27 AM »
I'm interested in understanding this a little better. Can some explain what the "shroud" is exactly and same for a slide stuffer
thx
« Last Edit: October 14, 2018, 12:10:51 PM by Jumbo J »

Offline LWC82PE

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Offline John Orchard

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Re: Needle jet shroud height changes? Mikuni VM38.
« Reply #6 on: October 14, 2018, 11:51:29 AM »
https://thunderproducts.com/product/u-f-o-ultimate-flow-


Thanks, l just ordered another one, this time for the 38 on my KX/KDX400 :-)
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Offline Jumbo J

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Re: Needle jet shroud height changes? Mikuni VM38.
« Reply #7 on: October 14, 2018, 12:18:47 PM »
https://thunderproducts.com/product/u-f-o-ultimate-flow-optimizer/

thx Leith - so the "shroud" and the "slide stuffer" are one in the same then?

from the diagram i can see it suggests there would be less mixture turbulence generated by directing more of the the air flow horizontally through the throat of the carb and reduce virtiical disturbance.  Never heard of this issue before

Offline tony27

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Re: Needle jet shroud height changes? Mikuni VM38.
« Reply #8 on: October 14, 2018, 01:01:54 PM »
https://thunderproducts.com/product/u-f-o-ultimate-flow-optimizer/
I have 1 of those sitting here, don't fit the new mikuni slides, possibly may fit in a snowmobile slides if they are thinner walled

Offline John Orchard

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Re: Needle jet shroud height changes? Mikuni VM38.
« Reply #9 on: October 14, 2018, 05:14:56 PM »
https://thunderproducts.com/product/u-f-o-ultimate-flow-optimizer/
I have 1 of those sitting here, don't fit the new mikuni slides, possibly may fit in a snowmobile slides if they are thinner walled


Can't you file it to make it fit?
Johnny O - Tahition_Red factory rider.

Offline John Orchard

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Re: Needle jet shroud height changes? Mikuni VM38.
« Reply #10 on: October 14, 2018, 05:16:47 PM »
https://thunderproducts.com/product/u-f-o-ultimate-flow-optimizer/

thx Leith - so the "shroud" and the "slide stuffer" are one in the same then?

from the diagram i can see it suggests there would be less mixture turbulence generated by directing more of the the air flow horizontally through the throat of the carb and reduce virtiical disturbance.  Never heard of this issue before


No, the stuffer is screwed to the bottom of the slide, the shroud is an extension of the needle-jet, into the venturi.
Johnny O - Tahition_Red factory rider.

Offline bazza

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Re: Needle jet shroud height changes? Mikuni VM38.
« Reply #11 on: October 14, 2018, 05:18:57 PM »
On my big bore Honda XL/Maico I cut away half the shrowd and got rid of the hesitation off the bottom
« Last Edit: October 15, 2018, 02:38:59 PM by bazza »
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Offline John Orchard

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Re: Needle jet shroud height changes? Mikuni VM38.
« Reply #12 on: October 14, 2018, 05:39:12 PM »
On my big bore Honda XL/Maico Icu away half the shrowd and got rid of the hesitation off the bottom


What made you think to do that Baz? So does the shroud reduce the drop in pressure over the needle jet, so by you cutting it, you increased the vacuum (reduced atmosphere pressure) to eliminate a lean stumble?
Johnny O - Tahition_Red factory rider.

Offline bazza

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Re: Needle jet shroud height changes? Mikuni VM38.
« Reply #13 on: October 15, 2018, 02:40:00 PM »
Some one told me to try and it worked, on my B44 with mikuni i had no hesitation down low so it is standard.
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Offline tony27

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Re: Needle jet shroud height changes? Mikuni VM38.
« Reply #14 on: October 15, 2018, 06:53:25 PM »
Shrouds on 4 strokes seem to be shorter in general