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  #1  
Unread 09-07-2011, 04:25 AM
tonyed tonyed is offline
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Default 38mm Mikunis - the long and short of it

Has anyone reprofiled the long inlet bell mouths of the 38mm powerjet mikunis to be like the short version. If so, does it work?

Frepin has a frame tube that makes the long version difficult to mount and remove.



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  #2  
Unread 09-07-2011, 06:02 AM
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G JONES G JONES is offline
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Quote:
Originally Posted by tonyed View Post
Has anyone reprofiled the long inlet bell mouths of the 38mm powerjet mikunis to be like the short version. If so, does it work?
Frepin has a frame tube that makes the long version difficult to mount and remove.
Hi Tony - I've not used "ordinary" 38s for many years - but the R/S 38s I've been using for years are shorter on the inlet side - and work very well - I machined a radius on the inlets a long time ago - whether it made any difference is difficult to know - but I think it makes them look faster...
You could always chop the frame tubes - must get your priorities sorted here..
I'll pop a hacksaw blade in the post tomorrow....
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  #3  
Unread 09-07-2011, 04:47 PM
tonyed tonyed is offline
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Trouble is, George, the frame holds the engine in on which are mounted the carbs

The short bell mouthed 38s in the photo are not reverse slide ones, so I am tempted to modify my long bell mouths to suit. It does appear that everything else is the same.

Cheers
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  #4  
Unread 09-07-2011, 07:40 PM
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G JONES G JONES is offline
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Quote:
Originally Posted by tonyed View Post
Trouble is, George, the frame holds the engine in on which are mounted the carbs
Minor detail
Quote:
The short bell mouthed 38s in the photo are not reverse slide ones, so I am tempted to modify my long bell mouths to suit. It does appear that everything else is the same.
Cheers
Hi Tony - I guess it depends on whether you are happy to chop the carbs - if it was me - I would do it - but then - I only ever look at "function" - if it does what I want it to do - then it has to happen....
The only downside is a possibility of loss of performance (guessing that's unlikely)
I have seen some folks fit "clamp on" air filters on those carbs in the past - would be a bit more awkward with the chopped carbs - on our sidecar - we use an enclosed air box to keep the carbs away from any road dirt & grit which you always get with open carbs....
Cheers - Gordon.
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  #5  
Unread 09-07-2011, 08:51 PM
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TerryM TerryM is offline
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Tony,
My Nico came with carbs that were shortened at least as much as your second photo. I used them and ones with the standard bellmouth and I could not tell any difference at all.
Terry
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  #6  
Unread 09-08-2011, 03:33 AM
tonyed tonyed is offline
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Thanks for the opinions and advice. Think of the weight saving

They are going on the lathe.
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  #7  
Unread 09-08-2011, 02:17 PM
170plus 170plus is offline
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Have seen quite a few 38's shortened - I think Lea G does it and everyone copies! Or buy 26J bodies and puts the older internal in them..They'll be lighter but I'd be surprised if there was a difference that was detectable by seat-of-pants dyno!
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  #8  
Unread 09-08-2011, 06:18 PM
Rob Hinton Rob Hinton is offline
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Yamaha are not dills!
They wouldn't make TZ350F-H carbys with long bellmouths for no reason at all.
It would be interesting to find out Yamahas perspective on this as well.
Does anyone know the project manager?
I'm in the "can't tell the difference" brigade, my bike runs 26Js as well as 40mm Lectrons and not even the dyno knows the difference.
Hinto
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  #9  
Unread 09-09-2011, 07:01 AM
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Twinshock750 Twinshock750 is offline
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I'm in the can't tell the difference either, camp.

On the dyno back to back 26J and long bell mouth bodies, no difference.

The curve was just about identical (so close it didn't matter). The 26J's gave me the part throttle feel I like, but that wasn't the body length, more the slide/needle set up.

Al
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  #10  
Unread 02-15-2017, 08:59 PM
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Hinto's probably right- in that there must be a reason that the long 38 bodies were used by Yamaha, though it's not obvious to us, even with the impartial dyno data.

Shortening the bodies is a popular mod in the UK, and Lea G does believe there's a couple of HP in it, though that is combined with other intake mods he does, though it is very noticeably at the expense of low-end pull. For an average ride, on a near-standard bike, long or short, it makes no difference.

Cutting a long set down to make them fit in the frame is a personal choice, potentially devaluing a set of 3g# carbs...why not buy some old eBay snowmobile 38's (long intakes), cut them down (or buy short bodied 38's, used) and transfer the power jet/bowl/internals over. Flogging the original bodies would probably pay for the exercise!
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