Shade tree modified Mikuni intake manifolds get a balance tube.

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This is a story based on what I do with my P11, not a plea for advice, or an example of how to do anything with a sound mind. It is just stuff I do. LOL

I believe the intake manifolds I have for my Mikuni 34mm carburetors came from MAP, but to be honest, I'm not sure. At the time all that was available were twin Mikuni carburetor setups for the Commando. Having a P11 with an upright motor required a little modification to the intake manifolds. If I knew how to weld, I could have made a better set, but I never have taught myself how to weld.

The intake manifolds were originally designed for the crappy flange to spigot style rubber mounts for the carburetors. I wanted to make a spigot mount and have the option to change the length of the intake by using automotive fuel filler hose for the section between the intake manifold and the carburetor. So what I did was hacksaw chunks of the flange off to get a rough roundish looking spigot end, and then hand file what you see below for the spigot mount. It looks out of round but that is the outside diameter not being perfect. I also smoothed out the inside of casting with a Dremel, a little cutter, and some hand sanding, but did not polish the interior. The intake manifolds go from 34mm to 31mm and the intake runners start at 31mm on my FBS ported head. Nuff about that.

Today 27 years later... I used some drip irrigation fittings, that's right, drip irrigation fittings to install hose barbs for the balance tube. I'll not bore you with the details, but they are installed with a press fit. I did that because the only small brass hose barb fittings I had that I could thread into the manifolds did not match. I tested a drip fitting in a small bowl of fuel to see if they would dissolve. They don't.

Result was that the balance tube actually did do what people say it does. It smoothed out the idle from what was like a lumpy big cam, to a relatively smooth mild performance cam idle. I did not have to do a thing to the tune. It just worked. YMMV

I only posted this because this P11 forum is slow.... and I have nothing better to do before I put the burgers on. If this mod has been done a hundred times already opps.

 
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lcrken

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Nice job of hand filing. I'm impressed. I need to do something similar to slightly shorten a set of Maney CNC manifolds, but I don't trust myself to get them right by hand work. I'm thinking I'll have to make up a holding fixture to do them in the lathe, and I suspect it will take a lot longer than your method.

Ken
 
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Shade tree engineering. I forget how long it took to do that, but I figure the better part of a day. I was working on the inside of them a little yesterday before I put the hose barbs in them. The manifolds I have are really poor castings, which according to Manley is solved with his manifolds. That stuff wasn't around when I did what I did, besides I don't think the curve is right for an Atlas head. I had to cut a different angle through the remodeled flange so that my carburetors would be horizontal once mounted. Otherwise, the carburetors would have been angled down at the filter end. A shade tree engineering tour deforce.
 

lcrken

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Shade tree engineering. I forget how long it took to do that, but I figure the better part of a day. I was working on the inside of them a little yesterday before I put the hose barbs in them. The manifolds I have are really poor castings, which according to Manley is solved with his manifolds. That stuff wasn't around when I did what I did, besides I don't think the curve is right for an Atlas head. I had to cut a different angle through the remodeled flange so that my carburetors would be horizontal once mounted. Otherwise, the carburetors would have been angled down at the filter end. A shade tree engineering tour deforce.
Yeah, Steve's manifolds are works of art, knocked out of a billet of alloy with his CNC machinery, but they are designed for the slanted forward Commando, so probably no use on P11 or Atlas. They wouldn't have worked for you anyway, being parallel, not splayed out to fit the Mikunis. Not sure if he's still making them now that he's semi-retired, but I did get a set from him not too long ago, so might still be available.

Ken
 
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They look like a nice tidy job and best of all, they work! Keep it coming, enjoying reading about all developments!
 
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SixtySeven

Glad somebody gets something out of my mad science. Well, actually lack of science. I do it because I think it will work based on nothing but my gut and my inner delusional tuning whisperer.
 
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SixtySeven

Glad somebody gets something out of my mad science. Well, actually lack of science. I do it because I think it will work based on nothing but my gut and my inner delusional tuning whisperer.
That’s a voice most of us probably have! Mines now telling me I need to get back in the shed and do more on the bike! :)
 

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