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mikuni 44 phh taper bored


smugley

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I'm after any information on taper bored mikuni's  ?

I have recently picked up a set of triple mikuni 44 phh S4 out of USA . They have had a taper machined into to bore's that run from 44 @ the choke to 52 @ the outer opening ( standard is 48 )  . From what I can find this modification was done in the day as a " cheat " in certain classes of racing as larger carbs weren't allowed . It is rumored to flow up to 20% better or almost the same as a 50 phh . I am trying to find out anything I can regarding this modification as even some mikuni experts haven't seen this before .

Cheers Doug

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The classes that are in Australia, some, state that Carburetor bore size is free.

And other classes that run Fuel injection, allow the boring of throttle bodies, and corresponding butterfly plates, ( these can be machined mch thinner too)

 

The airflow into, and out of, where rules allow, can also have included angle tapers.

 

Re the 44 Miks,? You'd be wanting to make sure there is enough body meat to support them, ie vibration etc.

 

One common ploy, swap the top covers over, or apply incorrect stickers!!!

 

My cars are all standard,,,,, my standard!

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This mod was and is still done in Japan I am lead to believe. I know the mod cost a lot of yen and I believe it is just one shop/expert that carries out this modification.

 

I have seen similar mods done on dellorto 40mm to make them 48mm.

 

Either way it was a practice that was done in the 80's as far as I can recall but it might have been done in the 70's too but I know it was done in the 80's.

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This mod was and is still done in Japan I am lead to believe. I know the mod cost a lot of yen and I believe it is just one shop/expert that carries out this modification.

 

I have seen similar mods done on dellorto 40mm to make them 48mm.

 

Either way it was a practice that was done in the 80's as far as I can recall but it might have been done in the 70's too but I know it was done in the 80's.

 

Not sure when my were done but it would have been a long time ago from the colour of the alum .

From my research there is someone in the the USA that dose this mod too but at about $600 per carb it's not cheap .

I would like to know how well or if this mod actually works ?

Dose anyone have first hand experience ?

Cheers Doug

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Why so expensive? Sounds relatively simple, must be more to it! ???

I wouldn't mind having a look at them if I could Doug

You live between work and home for me  ;)

Hi Simon

You show me yours and l will show you mine  ;)

Pm me your number .

I found some more out this morning , they were done about 30 years ago !

This mod gives about 7-8 % increase in cfm .

As far as cost goes it takes a long time to set up as it is done in a lathe using a 4 jaw chuck .

 

Also looking for a set of 50mm mikuni velocity stack's if anyone has a set lying around ? The standard 44's are no good .

Cheers Doug

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Can get you trumpets for 50 Miks, $120 each, original new, they are 50mm long

I recently got some in for a Dato 2000 owner.

 

The reason for boring out the 44's, was they were lower cost and plentifull compared to 50's ($2500 approx per)

 

We've seen a couple of bored ones crack just behind the mount face, they cannot be welded, Devcon putty works, but is ugly. As long as crack doesn't go up and through to any progression tubes/passages, they will be fine.

 

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Can get you trumpets for 50 Miks, $120 each, original new, they are 50mm long

I recently got some in for a Dato 2000 owner.

 

The reason for boring out the 44's, was they were lower cost and plentifull compared to 50's ($2500 approx per)

 

We've seen a couple of bored ones crack just behind the mount face, they cannot be welded, Devcon putty works, but is ugly. As long as crack doesn't go up and through to any progression tubes/passages, they will be fine.

Jason

They haven't been bored threw the venturi just up to it . Its still 44 mm at the butterfly . So no problems with strength in the body .

I can get 50 mm mikuni  trumpets  for $70 each over seas ,  just wanted to see if anyone has some in oz ?

Cheers Doug

 

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Jason

They haven't been bored threw the venturi just up to it . Its still 44 mm at the butterfly . So no problems with strength in the body .

I can get 50 mm mikuni  trumpets  for $70 each over seas ,  just wanted to see if anyone has some in oz ?

Cheers Doug

 

Efi hardware make some great full radius velocity stacks and they can be cut to the length you require.

 

http://www.efihardware.com/products/1072/ram-tube-velocity-stack-50-x-150mm

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Yes yes, go for the full radius ones, much betterer airflow into, compared to OE units!

 

I have a mandrel to go in a lathe to machine the correct lip angle face relative to the flat disc sides in thin brass shim plate, to produce butterfly plates, bit tricky, but possible.

 

They can be through bored , but not huge amounts, for a 2mm diam increase it is a 1mm cut, biggest issue is going into the progression channels if cut too much.

The taper boring Smugley has is all about included intake angle design, there is a magic degree amount, , over whatever length.

Hence the gentle flare out on an EFI trumpet, before the full radius turn lip.

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