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#279138 - 10/15/09 4:26 pm Move from MkII concentric to MKII smoothbore  
Joined: Aug 2001
Posts: 1,081
Phatt Bob Offline
BritBike Forum member
Phatt Bob  Offline
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Joined: Aug 2001
Posts: 1,081
Cheshire UK
Hi Folks,
A couple of Smoothbores became available at the right price, so I've swapped my old 36mm MkIIs for the 36mm smoothbores. I spent most of the afternoon at Santa Pod trying to dial them in with no success, only to have to put the standard ones back on in order to qualify.

As it's sort of like for like, I was wondering if anyone had any sort of fiddle factor to guesstimate Smoothbore jetting based upon standard jetting? Standard is 300 main, 106 needle jet, 2A1 needle on middle notch, which works perfectly.
I was recommended that for the smoothbore to start with 280 main, 107 needle jet, 2A1 needle notch 2. No joy.
Anyone done the swap?

Motor is 81*82 850cc 8 valver, 11:1 compression if that helps.

Thanks and regards

Bob


Phatt Bob
'95 Daytona 1200
'98 Daytona 1200 dragbike
ex-850 T140 Caff Racer, 850 Triton, Morgo T120, Starfire and Pretend Daytona 500 owner
#279187 - 10/15/09 8:04 pm Re: Move from MkII concentric to MKII smoothbore [Re: Phatt Bob]  
Joined: Mar 2005
Posts: 430
T. Sharp Offline
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T. Sharp  Offline
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Joined: Mar 2005
Posts: 430
Nashville, Tn.
Bob, first off, ditch the 107s and go to 106s (good tight new ones!)
Did you remove the air restrictors under the bellmouth for the needle jet circuit?
If the carbs are supposed to move more air, I think I would be safe and start off with a richer main than a leaner one!

#279430 - 10/17/09 4:27 am Re: Move from MkII concentric to MKII smoothbore [Re: T. Sharp]  
Joined: Aug 2001
Posts: 1,875
Ron - in California R.I.P. Offline
In Remembrance
Ron - in California R.I.P.  Offline
In Remembrance

Joined: Aug 2001
Posts: 1,875
California
Hi Bob..!!

There is very little difference between the standard and smoothbore Mk2's. So yes jet them the same, and yes it is safe to start one size main jet larger... but I suspect you will go back down again.. Keep in mind more air should** give a stronger signal thus it all is very close, jetting wise.

But I also suggest you do a fuel flow test. Remove the bottom plug and measure the volume of fuel per minute.. then cross reference that to how much fuel you need per minute based on the horsepower (per cylinder) you make..

Have fun... good luck..!

Ron

#279431 - 10/17/09 4:32 am Re: Move from MkII concentric to MKII smoothbore [Re: T. Sharp]  
Joined: Aug 2001
Posts: 1,875
Ron - in California R.I.P. Offline
In Remembrance
Ron - in California R.I.P.  Offline
In Remembrance

Joined: Aug 2001
Posts: 1,875
California
Hi Bob..!!

There is very little difference between the standard and smoothbore Mk2's. So yes jet them the same, and yes it is safe to start one size main jet larger... but I suspect you will go back down again.. Keep in mind more air should** give a stronger signal thus it all is very close, jetting wise.

But I also suggest you do a fuel flow test. Remove the bottom plug and measure the volume of fuel per minute.. then cross reference that to how much fuel you need per minute based on the horsepower (per cylinder) you make..

Have fun... good luck..!

Ron

#279898 - 10/20/09 7:23 am Re: Move from MkII concentric to MKII smoothbore [Re: Ron - in California R.I.P.]  
Joined: Aug 2001
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Phatt Bob Offline
BritBike Forum member
Phatt Bob  Offline
BritBike Forum member

Joined: Aug 2001
Posts: 1,081
Cheshire UK
Thanks for the advice folks,

That now concerns me a little, as I did try the 300/106/2A1 setup from the standard carbs with the needle in slots 2 and 3, and it was still rubbish.

May have to spend some time micro inspecting the carbs for differences, like jet positions and float levels. Still, got all winter now....

I'll try the fuel test too, Ron.

Thanks again

Cheers

Bob
Bob


Phatt Bob
'95 Daytona 1200
'98 Daytona 1200 dragbike
ex-850 T140 Caff Racer, 850 Triton, Morgo T120, Starfire and Pretend Daytona 500 owner
#280611 - 10/24/09 1:20 am Re: Move from MkII concentric to MKII smoothbore [Re: Phatt Bob]  
Joined: Aug 2001
Posts: 1,875
Ron - in California R.I.P. Offline
In Remembrance
Ron - in California R.I.P.  Offline
In Remembrance

Joined: Aug 2001
Posts: 1,875
California
HI Bob..!!

Oh my.. my mistake... I suspect that you may not be able to swap out the needles between the smoothie and non-smoothie.. I think you will find they are different lengths, and diffferent distances from the needle jet at the same throttle opening... but check that to be sure... all the other jets are the same types..

Cheers..!!

Ron

#281355 - 10/28/09 6:45 pm Re: Move from MkII concentric to MKII smoothbore [Re: Ron - in California R.I.P.]  
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Posts: 1,081
Phatt Bob Offline
BritBike Forum member
Phatt Bob  Offline
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Joined: Aug 2001
Posts: 1,081
Cheshire UK
Hi Ron,
Ahhh, thanks for that. I'll check. The 2A1 needle protrudes into the float chamber exactly the same distance on both. I did check that just in case. The only different part codes listed for the smoothbore are the slide, needle clip, Slide spring, needle spring and cable retainer (a sort of slotted bolt) and they're all OBVIOUSLY different, but I will check with the carb dealer on the recommended needle.

Maybe I don't want a 2A1.....

Cheers

Bob


Phatt Bob
'95 Daytona 1200
'98 Daytona 1200 dragbike
ex-850 T140 Caff Racer, 850 Triton, Morgo T120, Starfire and Pretend Daytona 500 owner
#281574 - 10/30/09 12:07 am Re: Move from MkII concentric to MKII smoothbore [Re: Phatt Bob]  
Joined: May 2006
Posts: 49
Seeley Offline
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Seeley  Offline
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Posts: 49
England
Bob,

Have you tried Dave Nourish?? He's a wealth of knowledge on smoothbores, and can supply both types of needle to suit!

Last edited by Seeley; 10/30/09 12:07 am.
#281628 - 10/30/09 9:49 am Re: Move from MkII concentric to MKII smoothbore [Re: Seeley]  
Joined: Aug 2001
Posts: 1,081
Phatt Bob Offline
BritBike Forum member
Phatt Bob  Offline
BritBike Forum member

Joined: Aug 2001
Posts: 1,081
Cheshire UK
Thanks Seeley...it was Dave who gave me the initial recommendation (after all, it's his motor!).

That's why I'm a little concerned, his recommendation for the standard MkII was spot on first time. I may give him another call anyway.

Cheers!

Bob


Phatt Bob
'95 Daytona 1200
'98 Daytona 1200 dragbike
ex-850 T140 Caff Racer, 850 Triton, Morgo T120, Starfire and Pretend Daytona 500 owner
#294675 - 01/19/10 12:43 pm Re: Move from MkII concentric to MKII smoothbore [Re: Phatt Bob]  
Joined: Aug 2001
Posts: 1,081
Phatt Bob Offline
BritBike Forum member
Phatt Bob  Offline
BritBike Forum member

Joined: Aug 2001
Posts: 1,081
Cheshire UK
Folks,
Bit of a development in the saga which I would like to get an opinion on.

A couple of days ago I sat down with a micrometer and vernier to check the difference between the standard and smoothbore carbs. needle diameters, protrusions into the jets and jet positions were all measured and found identical.

However, when it came to the slides there was one noticeable difference. Although on the surface edge of the No3 slides both were cutaway 3/16", but because of the manufacturing differences the standard slide was more like a No2 inside.

Let me explain. The lower wall thickness of the smoothbore slide is about 1mm, that of the standard about 4mm. As the cutaway is cut at a chamfer, by the time the air reaches the inside of the cutaway the cutaway is more like 2/16ths rather than 3/16ths.

Could be that the smoothbore needs a No2. Trouble is it'll cost me 100 for two new slides. This also correlates with the throttle opening position where the misfire and spitting occur, which should have given me a clue.

What do y'all think? Worth a risk?

Cheers

Bob


Phatt Bob
'95 Daytona 1200
'98 Daytona 1200 dragbike
ex-850 T140 Caff Racer, 850 Triton, Morgo T120, Starfire and Pretend Daytona 500 owner
#294689 - 01/19/10 2:01 pm Re: Move from MkII concentric to MKII smoothbore [Re: Phatt Bob]  
Joined: Nov 2005
Posts: 1,346
phantom309 Offline
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phantom309  Offline
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Joined: Nov 2005
Posts: 1,346
spfld vt
I tried the track carb last year and found it to work well but on the dyno I gained several hp by going up in carb size with a std mk11.up to 40mm and still likes it.


Tim Joyce
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