Moving up to 38mm carbs, which manifolds should I use

[Plasticman]

I've decided to upgrade the breathing on my RD400.  This is my vintage road race motorcycle, so it has a race-only port job and the heads have been reworked.  I'm adding Swarbrick expansion chambers on the exit side and was thinking that 38mm Mikunis (VM38) will do the trick on the intake side.  I have to run round slide, VM type carbs to be legal in my racing class.  The problem is, I'm not sure which intake would be best for this upgrade.  Here are the two intakes, the stock looking intake, a product from UPP (see picture 1) and the Banshee type intake (see picture 2).  Which would work better for my application?  Or does it matter?

Picture 1


Picture 2


Rob

[m in sc]

either will work fine, but the second ones have a longer runner so depends on the length of the intake tract you want.  i would recommend supporting them at the back  of the carb  somehow though, i've torn manifolds over  a few years with them just 'hanging out' there, VMs are heavy. 

[1976RD400C]

I think the UPP ones are made of silicone, which is slippery ??   Well, I had them with my with 34mm carbs and no support of the carb. Took it to a track day. The track was kind of bumpy and the carb popped out of the manifold at full throttle. The sound it made was like blowing a motor and it scared the crap out of me.

[SUPERTUNE]

I think the UPP ones are made of silicone, which is slippery ??   Well, I had them with my with 34mm carbs and no support of the carb. Took it to a track day. The track was kind of bumpy and the carb popped out of the manifold at full throttle. The sound it made was like blowing a motor and it scared the crap out of me.

Made of urethane, so a lot like silicone. They last about 3 years here in Florida.
I just use some 60 grit sandpaper on my spigot on the carb to roughen it up a bit.
Chuck

[Organicjedi]

For what it's worth, some AHRMA guys I know like Tony Doukas swear by the UFO inserts. It's supposed make the round slide VMs have a flow more like the TMs.

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

[SUPERTUNE]

Rob, go look for a set of these...

'97 -01 Vmax 500 600 700 8CR-13555-00-00
They now make re-pop's from SPI, I don't know how good they are yet.
I always just bought used OEM Yamaha ones from ebay.

I also have my TSR/Barlow designed boost bottles for this setup too. If you do buy a TSR boost bottle from me I have some jetting specs to get you close on the 36-38's.
Chuck

[SUPERTUNE]

You will have to re-drill them...

[SUPERTUNE]

Jim's F-500 RD400 engine with the 36mm carbs using these manifolds.

[Plasticman]

I was in the middle of installing my new carbs/manifolds when I notice there was a LOT of aluminum reed cage material in the air flow path.


So I marked the area that was exposed, removed the manifold and took another picture to show how much of the reed cage was blocking the air stream.


Is this normal, or can I grind away at the upper/lower areas.  I looked at the sides, there isn't enough aluminum to really grind away in that area without creating daylight.

Rob

[m in sc]

I've modded mine in the past like you are suggesting. with 38s you might want to go to some v-force reeds,. thats a LOT of air flow.

[Plasticman]

I've modded mine in the past like you are suggesting. with 38s you might want to go to some v-force reeds,. thats a LOT of air flow.

I was thinking along those lines.  Trying to do this incrementally. 

Rob

[m in sc]

id do the reeds at the same time TBH.

[Dvsrd]

I've modded mine in the past like you are suggesting. with 38s you might want to go to some v-force reeds,. thats a LOT of air flow.

FWIW, a reduction in cross section along any flow path is actually less of a restriction than an increase in cross section. However, any abrupt step or mismatch is a bad thing, and should be eliminated.
Wouldn't RZ reeds be better suited than RD reeds for big carbs? After all the RZ reeds are as big as TZ 750 reeds, just with an RD sized mounting flange. And the TZ had 34 mm oem carbs, I guess often replaced by 36 or 38 mm Lectrons.

[m in sc]

you can fit rm100 v force reeds in rd cycls without modifying the cyls, have double the tip area and will outflow rz reeds.

[1976RD400C]

Back before RZs and V Force I fitted TZ750 reeds (same as '78 IT175). An adapter plate had to be welded on and intake hogged out. Since TZ manifolds seem hard to get and I wanted to upgrade with a cross over tube I found Tri-Z manifolds fit the larger bolt pattern. The intake tract has no obstructions. I have 34mm carbs with single petal TDR reeds.



Some spare parts to get the idea



TZ manifold


Tri-Z manifold