Intake manifold balance tube location top or bottom?

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Schwany

Schwany

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I got some cheap Commando MkII spigot intake manifolds I modified for a Keihin FCR install on a straight up Norton motor. These manifolds have the balance tube ports on the bottom. Every time I have removed the balance tube, it has fuel in it, so I know fuel gets in there and hangs out at the bottom of the loop. Apparently getting sucked back and forth between intake manifolds. Seems to me balance would be inconsistent, or non-existent with the fuel in the tube.

Has anyone that had the intake manifold balance tube at the bottom capped those lower ports and installed ports near the top, and seen any improvement in how the motor runs at idle and lower speeds? I'm not trying to fix a problem, just screwing around with an idea.

I'm going to do it regardless, but wanted to know if anyone has any experience trying it, and what the results were.

I won't be capping the lower tubes. I'll be cutting them off and filling them with epoxy. Then using the pieces I cut off for the raised ports.

I'm probably underthinking the whole thing, but thought I'd ask anyway.

I tried a search. Nothing
 
My thoughts are.... What the hell can go wrong with fixing what's obviously , known to be, not correct? I'd go for it and make the proclamation of salvation afterwards.... The balance tube wasn't meant to be a fuel trap ever, so save the world...., Via con Dios Caballero. You walk amongst giants this day indeed.

Pain meds kicked in and topsy turvy thing caught me in a ... What were they thinking moment....
 
Many thousands of BSA and Triumph twins had the balance tube below without it causing any problems that I'm aware of.
 
Theoretically at least, have small volume of fluid trapped in the balance tube should not prevent pressure pulses equalizing across the manifolds. The fluid will just pulse back and forth in the tube with the pulses, so no biggie.

I suspect the fluid is a mixture of condensed fuel and water vapour. Blow back of combustion products from cylinders is a mix of unburnt fuel, carbon and water (which is a by product of burning fuels).
 
jbruney,
Saving the world by typing in a message box is what the internet is for. I'm on it for sures. ;)

OK, so far no one responding has done it, or would do it.

Jim > JSMotorsports has put together Commando and vertical mounted motor intake manifolds with the balance tube above the centerline of the intakes for his Keihin CRS carburetor kits. He may have a reason for putting the balance tubes above the centerline.
 
Liquid in the tube would drive me bonkers whether it was okay or not... Stuff like that wakes me up in the night.
 
Excess fuel there has only one cause, too much fuel being sent to cylinder(s)
Most as far as I know, BSA and Triumph using Amal carbs don't suffer because they don't fit pumpers. Even with my 750, I removed the tubes and blocked the holes up because I found I couldn't get correct carburation.
 
The balance tube always had some fuel in it with the twin Mikuni carburetors as well. I'm talking a very small amount. I can't remember if it did it with the Amal carburetors and the short intakes. My plugs don't show an excessively rich tune.

I would not be that surprised if the Keihin FCR pump shot literally made a hole in one right into the balance tube hole before the motor started. The hole in the tube is located right under the pump shot flight path. Once the engine is running the pump shot probably makes it to the cylinders without issue. I can't see through the motor though, so I'm not positive.

Off to the garage to see what else I can fix that aint broke.
 
I finally got around to going out in the garage.

I pulled off the balance tube and this time it was completely dry. Much ado about nothing.

I did do some tuning since the last time I had the balance tube off, but really didn't think it would make that much difference since it did the same thing with the Mikuni carburetors. Apparently the tune changes did make a difference. I should have checked the balance tube before I touched my keyboard today.

Excuse my wasting bandwidth.

I'm going to do some other things to the manifolds, since they are sitting on the bench now, but I promise I'm not going to post anything about it. :)
 
I would think intake vacuum would clear it, except at shut down.
I don't see how it can linger. Maybe I'm wrong.........
 
Nobody is wrong except me for posting what I was thinking about doing in the first place. Sheesh :rolleyes:
 
Schwany said:
jbruney,
Saving the world by typing in a message box is what the internet is for. I'm on it for sures. ;)

OK, so far no one responding has done it, or would do it.

Jim > JSMotorsports has put together Commando and vertical mounted motor intake manifolds with the balance tube above the centerline of the intakes for his Keihin CRS carburetor kits. He may have a reason for putting the balance tubes above the centerline.
Click to expand...
Simply makes more sense.

Easier to install from the top.
No chance of falling off.
No crud build up in the tube.
Easier too inspect.


Intake manifold balance tube location top or bottom?
 
Torontonian said:
Never understood why to fit/run the manifold tiny balance tube.
Click to expand...

With lots of simile faces, I think Norton envisioned the invention of the Twin-Max, or the use of the manometers available at the time to set dynamic balancing after the mechanical sync.

With a freshly broken-in engine, in my experience, the Twin-Max and the mechanical sync are so close that only a crusty, old curmudgeon like me would shot for the 10 ring...

Best.
 
RoadScholar said:
With lots of simile faces, I think Norton envisioned the invention of the Twin-Max, or the use of the manometers available at the time to set dynamic balancing after the mechanical sync.

With a freshly broken-in engine, in my experience, the Twin-Max and the mechanical sync are so close that only a crusty, old curmudgeon like me would shot for the 10 ring...

Best.
Click to expand...
Could not have explained that clearer.
Kinda like toast n' jam falls jam side down ?
 
jseng1 said:
Simply makes more sense.

Easier to install from the top.
No chance of falling off.
No crud build up in the tube.
Easier too inspect.


Intake manifold balance tube location top or bottom?
Click to expand...
My manifolds aren't nearly as pretty as those, but I installed the balance tube ports today in just about the same location as yours. My tube is black vacuum tube though. I've been thinking about doing it about 3 months, but didn't folllow through until today.

Are your straight manifolds 32mm, 33mm I.D.? I'm tapering from 35mm at the carburetor down to 31mm at the head.

If I would have said "simply makes more sense" the blowback from the Norton stalwarts would have been entertaining.
 
Last edited:
Schwany said:
My manifolds aren't nearly as pretty as those, but I installed the balance tube ports today in just about the same location as yours. My tube is black vacuum tube though. I've been thinking about doing it about 3 months, but didn't folllow through until today.

Are your straight manifolds 32mm, 33mm I.D.? I'm tapering from 35mm at the carburetor down to 31mm at the head.

If I would have said "simply makes more sense" the blowback from the Norton stalwarts would have been entertaining.
Click to expand...

I have both curved for Commando and straight for Atlas.

32mm at the head end and 33mm at the carb end to match the 33m Kiehin smooth bore CRS.
 
Moving the balance tube ports up above centerline in the intake manifolds, then plugging the old balance tube ports and smoothing the intake floors has had a placebo effect. In addition to making more sense, the motor is easier to start, responds really well to throttle, and when coming to a hard stop the idle doesn't dip down and come back up. I suppose it could all be a coincidence, but in my opinion it was well worth moving the balance tube port location for these pumpers. If you haven't tried it, don't knock it.

Intake manifold balance tube location top or bottom?
 
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