Anyone run a single carb?

[kyle.quamme]

For ease of tuning, I was thinking of making a new intake manifold and using a Mikuni VM44 and Holley Red Electric Pump to feed my '80 GS50. Has anyone done anything like this? Is a single VM44 going to flow enough fuel w/ an electric fuel pump ahead of it? Will it flow too much fuel?

[salty_monk]

I'm no expert on bike carburation but I know that bike carbs on Kit Cars are generally seen as an upgrade over single carbs... not sure why you'd want to do this mod.

Dan

[Commodus]

For ease of tuning, I was thinking of making a new intake manifold and using a Mikuni VM44 and Holley Red Electric Pump to feed my '80 GS50. Has anyone done anything like this? Is a single VM44 going to flow enough fuel w/ an electric fuel pump ahead of it? Will it flow too much fuel?

I don't know the answer, but for ease of tunability like you say it's often crossed my mind as well. I don't think you would need a fuel pump, the gravity feed will work just fine I would imagine. The amount of fuel flowing is not changing, after all. But I'm not a carburetor expert.

[pan]

The only valid reason for running a single carb on a 4 cylinder bike is a draw through turbo application.

Mark...

[kyle.quamme]

I don't know the answer, but for ease of tunability like you say it's often crossed my mind as well. I don't think you would need a fuel pump, the gravity feed will work just fine I would imagine. The amount of fuel flowing is not changing, after all. But I'm not a carburetor expert.

Yea, I was unsure of the fuel pump part, I just figured that with one bowl instead of four that it might be harder to keep enough in there with just gravity feed.

The only valid reason for running a single carb on a 4 cylinder bike is a draw through turbo application.

Mark...

Can you give reasoning? I guess I've seen it done on other naturally aspirated bikes and it seems so much easier to adjust than a quad carb setup.

[Commodus]

The only valid reason for running a single carb on a 4 cylinder bike is a draw through turbo application.

Mark...

Or simplicity/ease of tuneability...

[kyle.quamme]

My research shows that 42mm are used on Sportster 883s, so maybe a 38 or 40 would be a better size for a 550?

[salty_monk]

Oh dear... now we're talking about Harley's in a "Performance Mods" thread. That can't be right!!

[kyle.quamme]

Oh dear... now we're talking about Harley's in a "Performance Mods" thread. That can't be right!!

HAHA, more or less for a CC reference, since most HDs run a single carb setup from the factory, heck I dunno of an HD model that doesn't come with anything but a single carb.

[Commodus]

Oh dear... now we're talking about Harley's in a "Performance Mods" thread. That can't be right!!

Heh, it doesn't say anything about performance improvement!

"Mods" is a two way street :-D

As to the question, I don't think a Harley sportster head would necessarily flow better than a GS head. The 42 would probably be OK. Maybe a good idea to call Sudco and check.

[drhermanstein]

The big deal with a carb per cyl is to help keep intake velocities up by using the intake pulses at higher revs to haul in the next intake charge. Harleys make intake velocity by big displacement. We make intake velocity with higher revs. The common upgrade on smaller displacement engines is a butterfly or carb per cyl not the other way around. If the single carb is the way you wish to go, measure CFM not cc's for your carb size requirements. For example a 1000cc engine turning at 5000 revs pulls approx 175 cfm, while a 1000cc engine turning at 13,000 revs pulls in 450 cfm.

[kyle.quamme]

The big deal with a carb per cyl is to help keep intake velocities up by using the intake pulses at higher revs to haul in the next intake charge. Harleys make intake velocity by big displacement. We make intake velocity with higher revs. The common upgrade on smaller displacement engines is a butterfly or carb per cyl not the other way around. If the single carb is the way you wish to go, measure CFM not cc's for your carb size requirements. For example a 1000cc engine turning at 5000 revs pulls approx 175 cfm, while a 1000cc engine turning at 13,000 revs pulls in 450 cfm.

ooooo, just the info I was looking for. Is there an online calculator to find CFM for different configs?

[lecroy]

For ease of tuning, I was thinking of making a new intake manifold and using a Mikuni VM44 and Holley Red Electric Pump to feed my '80 GS50.

A lot of my old twin 2-stroke engines were this way. Two of my 4-strokes were also this way.

[kyle.quamme]

A lot of my old twin 2-stroke engines were this way. Two of my 4-strokes were also this way.

Did that seem to work pretty well, or would you recommend something different?

[lecroy]

They all came this way from the factory. Most were snowmobiles, but I had a Triumph (650cc) and my Harley (1000cc) are also this way. Yes they all ran pretty good for what they were. Nothing by todays standards. Most of your carb'ed cars were this way as well.

I currently use a single 42mm Mikuni on my race bike.