Homemade single carb manifold; any good?

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Fast Eddie said:
You have a small carb, feeding a big chamber, feeding small ports. I am NOT a flow expert like other posters, but that seems to me that its likely to cause undesirable effects on gas velocity and pressure waves.
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That makes you just as much an expert as most here. ?

Analysing manifold flow and effects is a mix of science and black arts.
Almost all the folks who produced commercial manifolds did it by trial and error (?),
seeing what worked and what didn't (?).
 
snail said:
Yep, the SU dashpot was chromed too!
My manifold was a "Y" shaped construction made from tube.In reality it was pretty much a plumbing fitting. Not sure about the air flow, but I got the impression that it was the inertia of the fuel droplets that was the problem.
No problem with an air filter, it didn't have one. It did have parts of an alarm clock as a throttle linkage!

At the time, it seemed like a good idea ( to a 19 yr old) but there are better ways of doing things today. Why didn't I have my machine shop and knowledge when I had that bike and a drag bike?
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Haha, great story, weren´t we the best back in the day! No knowledge but the "try, it might work" attitude was the rule :lol: . I had that chrome frenzy as far back as 76, when I built the first "chopper", chromed half the bike. My chromed Amals actually caused me a broken leg once, was trying to get some kind of regular idle, didn´t work, got p-d off and twisted the throttle too fast on the gravel in front of the club house. The bike twisted around and fell on one side, my foot got stuck under and twisted with the bike, no good.
Tommy
 
Rohan said:
fiatfan said:
Ok, this was unexpected, didn´t mean to start a fight.... :shock: . Don´t like it, but we´re all different people, so "sh-t happens" I guess.
Back to my question;
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Don't worry about it, this forum has its share of trolls that take offence with anything.
Nit pickers too, all of us...

I have a short stubby little 2-1 manifold for a Commando, could just make a version like that ?

Jim Comstock showed a large 2-1 that he made to suit a larger single carb, to keep much of the the performance (and more)
Not sure if he showed it fully, or said if they were for sale.
Getting air cleaners on them is always an issue, not much room in a C'do.
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Yes I read something from Jim before, I think he said the intake that worked very good with a single big carb was 8" long? I wonder where to put the carb :? . But that was the only way he got the same effect with a single big carb as with twin carbs on both low and high revs.
Tommy
 
V8's fed from one carb very rarely get even performance from all cylinders.-Rohan


Wonder how they get all that bhp out of 300 cubic inch NASCAR V8s with only one (sometimes restricted) carb?
 
Very clever manifold design....

Look how much they get when they have individual injector trumpets.
 
Rohan said:
fiatfan said:
Ok, this was unexpected, didn´t mean to start a fight.... :shock: . Don´t like it, but we´re all different people, so "sh-t happens" I guess.
Back to my question;
Click to expand...

Don't worry about it, this forum has its share of trolls that take offence with anything.
Click to expand...
Homemade single carb manifold; any good?


And so many wonder why you don't see hobot on here anymore!
 
OK, being a dimwit can someone explain to me when the intake flow wouldn't favour the cylinder that has it's valve open?
 
Nater_Potater said:
And so many wonder why you don't see hobot on here anymore!
Click to expand...

Because Steve was the master at steering threads off in weird and wonderful directions ??
Anna and her Manxman and Steve and his Trixie/M Appeal come to mind....
 
brxpb said:
OK, being a dimwit can someone explain to me when the intake flow wouldn't favour the cylinder that has it's valve open?
Click to expand...

Not dimwit. It will favor the port that's open, but how the airflow + fuel flow changes between the straighter runner (in this case, the right side) and the almost 90 degree bend that's required by the left port is where the problems start. If it was just air, it would be easier to correct by manifold massaging. However, when you throw the partially atomized fuel into the mix with its greater density, the air and fuel will take somewhat diverging paths as you force it to make corners. This is what will cause the right port to run richer than the left (presumably). Modern fuel-injected engines can have intake manifolds that play upon the intake air's resonance and ability to wrap around some pretty tight corners, then inject the fuel just before entering the cylinder. This just wasn't possible back in the day of the carburetor because of the fuel separation issue.

Nathan
 
brxpb said:
OK, being a dimwit can someone explain to me when the intake flow wouldn't favour the cylinder that has it's valve open?
Click to expand...

Of course the flow will 'favour' the cylinder that has a valve open.

But if the flow has to veer around corners, or impinge on port walls or lumps or bumps, or keep flowing into closed off ports along the way and then change course again, or have sudden expanded or constricted port areas, all of these impede flow, throw suspended fuel droplets out of the airflow, and limit how much fuel/air gets into the cylinder doing the breathing.

Commandos have a lovely port design, whereby the fuel/air flows near straight down into the cylinder, out of each dedicated carb.
While the design isn't the very best by modern standards, or even at the time, its not bad for a mass produced motorcycle of the 1970s.

Inspect the inlet arrangements sometime, if you get the opportunity, of a manx norton - or a ferrari.
NOTHING gets in the way of having an about perfect port design = straight in.
Perfect tuned lengths, nothing in the way of interference with that ideal....

Road bikes need things like air cleaners and rider comfort, so tend not to be so single purpose designs in this area.
V8s with single carb design is (theoretically) a horrible idea, but perfected along the way to still be quite strong with a well suited manifold design.
 
Nater_Potater said:
However, when you throw the partially atomized fuel into the mix with its greater density, the air and fuel will take somewhat diverging paths as you force it to make corners. This is what will cause the right port to run richer than the left (presumably). Modern fuel-injected engines can have intake manifolds that play upon the intake air's resonance and ability to wrap around some pretty tight corners, then inject the fuel just before entering the cylinder. This just wasn't possible back in the day of the carburetor because of the fuel separation issue.
Click to expand...

If you can get the fuel to all vapourize into the airflow, rather than just atomise and be dragged along with the airflow,
then a lot of the flow problems/fuel separation are minimised.
V8s on road vehicles (and other engine type too) used to have that hotspot in the floor of the manifold, that encouraged the fuel to vapourize
early on in the induction process. Not so good for efficiency...

A lot of carburetted manifolds still used pulse tuning of tuned length induction systems.
Look at manx nortons, where the inlet tuned length of about 14 inches was almost written in stone.
It may not look it at a glance, but V8 manifolds with their meandering passages were also aimed at specific lengths of flow paths to the ports.
Thats precisely how some tuners got so much mumbo out of them....
 
Danno said:
V8's fed from one carb very rarely get even performance from all cylinders.-Rohan


Wonder how they get all that bhp out of 300 cubic inch NASCAR V8s with only one (sometimes restricted) carb?
Click to expand...

NASCAR engines aren't about making the most hp, they are about making the most hp within the rules (single carb, pushrod engine).
 
any info on 2 in 1 out the side intake for norton? just saw a manifold for a triumph & the xs650 folks have been all over it,

despite the usual equal length/distribution blah blah

https://www.accessnorton.com/NortonCommando/homemade-single-carb-manifold-any-good.20370/

http://forkco.com/category/select/cid/2/pid/8936/language/en

looks like stuff in this thread has been covered but easy enuff to start another

scroll down for double weber setup
http://www.xs650chopper.com/2009/08/get-the-lowdown-hans-from-holland/
 
Last edited:
Danno said:
V8's fed from one carb very rarely get even performance from all cylinders.-Rohan

Wonder how they get all that bhp out of 300 cubic inch NASCAR V8s with only one (sometimes restricted) carb?
Click to expand...


Both simple and complex. Its done with the camshaft. competitive NASCAR teams spend a fortune developing camshafts that "level the playing field" to enhance the distribution of the A/F mixture from a single, centrally mounted carb to all cylinders.

I saw a Velocity segment that had the camera crew touring the Penske engine development facility for their NASCAR team; they showed how the cranks, rods and pistons were "manufactured", they even showed a bit about the heads, but they refused entry into the camshaft development area...
 
RoadScholar said:
Both simple and complex. Its done with the camshaft. competitive NASCAR teams spend a fortune developing camshafts that "level the playing field" to enhance the distribution of the A/F mixture from a single, centrally mounted carb to all cylinders.

I saw a Velocity segment that had the camera crew touring the Penske engine development facility for their NASCAR team; they showed how the cranks, rods and pistons were "manufactured", they even showed a bit about the heads, but they refused entry into the camshaft development area...
Click to expand...
I friend of mine is building a hotrod MG Midget, but the intake port configuration is very ummmm uneven. He is getting a cam that is supposed to compensate for the intake issues he referred to it as a scatter cam.
 
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