The Chopper Compendium

Thinking about building a Manifold to run a single Carb.Is there a Formula to determine what size I will need?

Page 10

https://www.dropbox.com/s/1xc1ux8hgnn12 ... %20101.pdf

But really as long as there is equal runners no cyl will starve then set your carb to that!

unsure about formulas but food for thought if your switching to single carb maybe a carb with accelerator pump . I been thinking about it for my triumph i remember back in the day used to run a mikuni was thinking if I could still get a manifold modern sporty carbs have the same mount and an 883 is close to 750 so jetting should be available an accelerator pump should have many benefits no bog off the line jumping on it too hard , easier starting no fuel pouring on the engine from hitting the ticklers . only thing I worry bout is the squirt being too much for the twin but a triple might handle it better but I bet playing with the length of the rod for the pump could control the squirt . I am going to try no matter what but your a lot closer than I am and its just food for thought

Been thinking....(no my Brain doesn't hurt)...Since only one Cylinder is firing at any given time,seems like I wouldn't really need a Carb bigger than what's on each Cylinder now.The amals are 26mm..I'm gonna try a 30mm. mikuni.

It will need jetted up but one carb will work from stock.

the needle will needed shimed.

I have also seen manifolds made not equel runners work ever seen one on a striaght six engine? Same concept!

I had a whole response typed up aboout CFM needed, and such, based on my own experiance, and then accidentally closed the window.

But then I stumbled oon this, and it would help you tons.

http://www.csgnetwork.com/cfmcalc.html

The only number you may not know is the volumetric effiency, Usually I crunch the numbers based on about 70%, 99% of motors dont get over 90 unless they're boosted(turbo or super.) It will at least get you in the ball park for cfm.

For the sake of argument, If I run the numbers on a 750 3 cylinder running stock 30mm carbs, the carbs each flow approx 70cfm.

At 70% effiency and a 7000rpm ref limt, the engine requires 64cfm and you';d be fine. At 100% effiency though, you'd 92 and the motor woruld starve. Raise the rev limiter, and you wouldnt have enoough to hit redline under load.

I pulled the cfm #s off the net, and just used that as an example, but you get the idea. Its worth crunching the numbers before hand to know what to expect, never know what you might come across.

That's great info, thanks for that.

The bike I'm playing with at the moment is a Viagra XV1000 (previous generation to the XVS1100 donk in your chop), and one of the cafe boys found that it's possible to fit the XVS1100 barrels, heads and internals to the XV1000/XV1100 cases as a simple bolt in operation. I've been hunting a donor XVS1100 to do this, and want to do pretty much the same PCS upgrade as you're doing, but I've got a Mikuni HSR42 on a single carb manifold on at the moment, it was interesting to see that even if I got 95% efficiency and ran an 8000 redline, the numbers still come well inside what the HSR42 will deliver.

Cool, ta ;^)

Neale

Thanks Spidr...I plugged in 8000rpm @ 80% ( digging around found Numbers that state most stock Engines run between 75 -85% efficiency)& got 85cfm.At 100% the cfm required is very close to the rating for the 34mm.(The only ratings I could find had the 34mm Mikuni flowing at 109cfm)..Not sure if later down the Road I'll put bigger Cams in or not,It will probably be better to be over Carbed than under Carbed.I'm thinking this might be a good Size to start...thoughts????

Just make sure you leave yourself a bit of a buffer. Something as simple as a wet filter in the rain could screw you up. Your intake needs to flow free enough to evenly distribute the air, etc.

Last thing would be to make sure that you can jet that carb biig enough to work. depending on the model, jets only go so big without drilling them custom, if you cant put enough fuel into that air soo that the ratioo is right, you'rre still up the creek.
Jet numbers are based on MM, a 100 jet is 1 mm. You need to consider square area of the jet, if it was running an 80 main on each carb, you would need too step up to a 140 main to triple the amount of fuel. Tuning is always a bitch, but its just one mroe thing to consider.

I put a call into Mikuni & was trsnsfered to a Co.that dustributes for them (guess they don't have Tech support...odd).I told the Guy what I was planning to do & using a 34mm.Carb.He told me there was a single Carb convertion for the Norton 750 that uses the 34mm.Carb,but because the Bore on my Engine is smaller His suggestion was stepping down to a 30mm. Or 32mm. Carb.

Jeff, I'm no expert by any means, but as Spidr said, you should allow yourself a buffer, this comparison that I dug up when searching for the CFM of my carb (HSR42) shows that pretty well.

http://www.nightrider.com/backup2/biket ... irflow.htm

FWIW

Neale

I wish they would publish Tests like that for all thier Carbs.I could only find cfm ratings for 34-38mm.Carbs.Unfortunately I have to start somewhere.I picked up a 32mm. cheap, so I can start on the Intake.If it turns out it's not big enough,I'll step up...time will tell.