The Science of Carburetion

[G_YamTech_314]

I want to start a discussion with the hope that EVERYONE will learn something from this.

I'll start with a tidbit that I've been researching to try and dial in a bike for a side job I'm fighting to get running right.

The customer bike is a 2004 Honda rebel 250. Originally a single carb, but they "upgraded" to dual carbs. While I feel this is a real potential upgrade, I fear the $50 set of eBay carbs may be leaving a lot on the table.

I was researching how to calculate the proper size carb for the engine it's being used on and I found this formula for determining the CFM (Cubic Feet per Minute) your choice of carb needs to have.

It's as follows:

Engine Displacement X Max RPM ÷ 3,456...

So we have the CFM's now... Cool. How does that help me? How can one determine carburetor bore size by calculating CFM?

 

[650Skull]

650×7500÷3456=1410.6 CFM

 

[G_YamTech_314]

650×7500÷3456=1410.6 CFM

250x8250÷3456= 596.788 CFM. But how do I take this info and use it to calculate carburetor size?

 

[GLJ]

250x8250÷3456= 596.788 CFM. But how do I take this info and use it to calculate carburetor size?

That formula would be for a single cylinder engine. A twin would be half as only 1 cylinder would be sucking at a time.
I don't know how it would relate to bikes, back in the 70s-80s the rule of thumb for race car engines was 2 CFM per cubic inch. That was mostly on V8s.
I do know if the carbs are too big it will create drive-ability problems.
Yamaha used 38mm carbs the first 7 years on the 650s then went with 34s. You could use that as a baseline and do the math for a 250 twin.

 

[G_YamTech_314]

That formula would be for a single cylinder engine. A twin would be half as only 1 cylinder would be sucking at a time.
I don't know how it would relate to bikes, back in the 70s-80s the rule of thumb for race car engines was 2 CFM per cubic inch. That was mostly on V8s.
I do know if the carbs are too big it will create drive-ability problems.
Yamaha used 38mm carbs the first 7 years on the 650s then went with 34s. You could use that as a baseline and do the math for a 250 twin.

That's what I thought!!! So my CFM per cyl would be 298.

This is what I'm figuring... But it cannot be right. (My math skills are TERRIBLE)

650 ÷ 38 = 17.1

SO... If I work kinda backwards and din
650 ÷ 250 I get 2.6...

38 ÷ 2.6 = 14.6

If I check my math by doing

250 ÷ 14.6 I get 17.1 so theoretically I found the right ratio if both get me to 17.1

Again, my math skills are AWFUL so don't judge me too harshly.

I'm thinking a 15mm (14.6 rounded up) would be way too small for that application.

They made a rebel 250 in Australia that had dual carbs using a Keihin VE26F. The main jet size was 95 and the pilot was 35.

Obviously there's my answer for a baseline, and I already KNOW that. But I don't understand it lol.

Not to mention, the aftermarket carbs on the rebel are Mikunis... And I know that jet sizes can differ between manufacturers.

 

[GLJ]

You are right 15mm would be too small. CFM is not directly proportional to size. Example a 1 inch water line will flow a lot more water than 2 1/2 inch water lines.
My math skills are not what they used to be. Really need someone like @TwoManyXS1Bs to help you.

 

[G_YamTech_314]

Just confuses me a lil... I'm pretty sure the single cylinder rebels ALSO use a 26MM carb. I suppose we will all get to the bottom of this as a team hahaha..

Also, I recognize that this isn't related to XS650's directly. But I believe I recall some people switching to a single carb set up. I also just think it's good to know how carburetion works in general.

 

[Wingedwheel]

That formula would be for a single cylinder engine. A twin would be half as only 1 cylinder would be sucking at a time.

So does that mean you halve the cc’s for a twin? At 325(Thereabouts) you would still be using a 34/38 mm carb for each side. So given my powers of blindly guessing it just seems logical in my mind that a 26/28mm (or two) carb would be needed for that 250. Just a disclaimer, total speculation on my part as I have zero knowledge of the math of carbs

 

[GLJ]

So does that mean you halve the cc’s for a twin? At 325(Thereabouts) you would still be using a 34/38 mm carb for each side. So given my powers of blindly guessing it just seems logical in my mind that a 26/28mm (or two) carb would be needed for that 250. Just a disclaimer, total speculation on my part as I have zero knowledge of the math of carbs

In theory a 125 single and a 250 twin could use the same size carbs. You have to keep in mind a twin is only using the carb during the intake stroke of each cylinder. So the cylinders alternate using the carb. Most people that put a single carb on a XS650 use a 34 to 38 mm carb.

 

[Wingedwheel]

Right, my point being the # of cylinders changing appears to have no bearing on carb size.

 

[G_YamTech_314]

In theory a 125 single and a 250 twin could use the same size carbs. You have to keep in mind a twin is only using the carb during the intake stroke of each cylinder. So the cylinders alternate using the carb. Most people that put a single carb on a XS650 use a 34 to 38 mm carb.

The intake stroke being shared is a really good point. I am sure the science of it all is insane. I'm just trying to learn.

 

[GLJ]

Right, my point being the # of cylinders changing appears to have no bearing on carb size.

You are correct on carb size based on cylinder size.
There is a lot of things that go into sizing carbs. A lot depends on intended use. A good example is a 250cc trials bike uses a lot smaller carb than a 250cc MX bike. Smaller carb better low end response.
Another thing to keep in mind is a CV carb does not flow as much as a TM or VM. That's due to the fact a CV has a throttle shaft and butterfly. That's why people that get rid of CVs go to 32/34 TMs or VMs.
I think in sizing a carb use what has worked for other people for the intended use of your engine. Reinventing the wheel usually causes headaches.

 

[drgonzo]

I always found it odd that automobile carburetors are sized by cfm rating and motorcycle carburetors by nominal size. I made the argument with a fellow racer once that a 30mm Amal from Triumph 650 flowed comparable to a 38mm solex Mikuni from an XS650. I have no data to back that up but the Amal has open hole at WOT and the Yamaha a butterfly valve in the flow path.
It's pretty impossible to just quote the size number given and it not be apples to oranges or even bananas.
Not so oddly, a 26mm Keihin from a Honda 305 Super hawk runs very well on my 145cc XL125. Apple to apple. That is all.

 

[650Skull]

My GS650, (672 factory), has 4 32mm carbs. 166cc per cylinder.

Would have to relate to inlet and outlet manifold shape and size as well I would imagine

 

[drgonzo]

My GS650, (672 factory), has 4 32mm carbs. 166cc per cylinder.

Would have to relate to inlet and outlet manifold shape and size as well I would imagine

I understand but the numbers quoted for all these carbs just don't tell the whole story. A 1976 Kawasaki KZ900 (903cc) has 4 smaller 26mm carburetors.per 226cc cylinder.
Flow numbers are more comparable but not readily available for motorcycles.
Maybe it's like a friend of mine told me in high school, "carburetor and derailleur are French words that mean; don't fuck with it."
Never stopped me though.

 

[G_YamTech_314]

"carburetor and derailleur are French words that mean; don't fuck with it."

I love it.

I'll likely not even mess with that stuff on a customer bike. But I can't say I'll never try a different carb on my XS. I see the round slide carbs advertised a lot. I'm not sure when that'll be... But ya never know.

Ive always been intimidated by carb tuning and jetting. I'm tired of being scared and this was my first step outta the hole to get my head wrapped around it.

It'd be awesome to have someone in my circle with a flow bench that was willing to teach me a thing or two.