Needle jet shroud height changes? Mikuni VM38.

[John Orchard]

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.

[mboddy]

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).

[John Orchard]

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.

[dont]

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

[Jumbo J]

I'm interested in understanding this a little better. Can some explain what the "shroud" is exactly and same for a slide stuffer
thx

[LWC82PE]

https://thunderproducts.com/product/u-f-o-ultimate-flow-optimizer/

[John Orchard]

https://thunderproducts.com/product/u-f-o-ultimate-flow-

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

[Jumbo J]

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

[tony27]

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

[John Orchard]

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?

[John Orchard]

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.

[bazza]

On my big bore Honda XL/Maico I cut away half the shrowd and got rid of the hesitation off the bottom

[John Orchard]

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?

[bazza]

Some one told me to try and it worked, on my B44 with mikuni i had no hesitation down low so it is standard.

[tony27]

Shrouds on 4 strokes seem to be shorter in general