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6 carbs???


kmack

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Originally posted by St.stephen

see now this has me foaming to hop on my bike!! Stage three ported, turbo crank, shaved head Cam 2 fuel powered KX 125.

Its a real scalded ape thats for sure. :geek:

Now you're making me miss my old RD.

'72 Yamaha RD250 w/ 350cc motor punched 1 over, cylinders ported, heads shaved .020", Factory (brand) expansion pipe race exhaust, ported carbs, Accell Super-Stock coils (hidden), 8mm plug wires, modified K&N air filter, cafe-style seat, club-man drop bars (like clip-ons), re-valved forks. It also had a modified, but stock-looking drum brake set-up from the original bike. Total sleeper!!:devious:

1st gear was only for show! Wheelies only!:classic: It would actually pull the front wheel off the ground all the way through 3rd. The bike looked totally stock except for the exhaust. I used to terrorize all the guys on 600cc & 750cc bikes. They had a bit more top-end, but nothing but a big-bore bike could beat me off the line. In the twisties, I could keep up with anything. Power on that little buzz-bomb hit hard right before 7k rpm and pulled like a train all the way to 11k!!

I miss that bike...:(

EDIT: This what the bike looked like before I made the engine mods and put the exhaust on.

http://www.geocities.com/kenshobnob/RD_page.html

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You've got me started too.

I miss my Suzuki RGV250. V twin 250cc production racing rice burner pocket rocket. Evil bike turned quiet owner into racing dork everytime I rode it.

Problem is with bikes is that they hate being riden slow. Especially a 2-stroke. I always carried 2 spare spark plugs around as they regularly crapped out.

Sold the bike as I lost my licence on it. But what fun.

More fun than a Zed................nah!

Have you thought about using some mukuni single trottle bodies. Pretty much a single side draft, no slide just a butterfly. High and low speed jets are real easy to adjust. I ran one on my Methanol powered Yamaha YZ125 gokart. That was more fun than the Zed!!!

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Originally posted by brianglawson

that would be great, other than synchronizing(sp?)

Ok, for you guys that have never worked on early-style motorcycle carbs, here is a brief explanation on why the synchronizing will not be difficult at all:

Most multiple-cylinder motorcycle carbs have an air port that allows you to hook up a very basic, but extremely effective synchro tool. What this consists of is (up to) 4 separate tubes filled with mercury. When connected to the carbs, each carb will pull a certain amount of vacuum on the mercury. There is a scale behind each mercury tube (which, btw, is vented to atmosphere, so you need to be very careful). All you do is adjust each carb until all the tubes are reading at the same point. Doesn't matter where they read on the scale, just as long as they read the same.

In fact this same type of setup can be used on the SU's as long as you hook it to one of the vacuum ports on the intakes. Just make sure you don't suck the mercury into the engine! :finger:

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  • 8 months later...

I am working on a plan which involves a 6 carb L28.

digging this thread back up.... i've been searching on 6 carb setups and can't find much info. Pic's are great but numbers are what I need.

After using a carburetor calculator, I determined an L28 requires less than 80cfm max per cylinder (and that's an over estimate). The flow rates of 44mm Mikunis are somewhere around 150 - 160 CFM's!?!?!? Isn't that WAY more than enough airflow?

For budget sake, and tunning as well, I would like to get the right sized carbs for each cylinder. I've seen 26mm Mikuni's for $80. Still more than I want to pay ($480 for six) but just an example. However, doesn't 26mm (about 1" dia) seem small for an intake port? What are the intake runner sizes on an L28 intake mani?

Here's the carb calc;

http://www.corvetteactioncenter.com/tech/tools/carb.html

Ideas anyone!?!?

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Kmack, that bike is like cheating, being a 2 stroke. What about that RZ350, the red/white 2 stroke in the mid 80s...that would be cool too.

I would think that the only advantage to using 6 carbs would be that each cylinder can be tuned individually. A 3 weber side draft configuration already is like 6 carbs, with 2 carbs fused together and not tuneable indivually...that's right isn't it? I mean the webers have 6 air horns, so....

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I would think that the only advantage to using 6 carbs would be that each cylinder can be tuned individually. A 3 weber side draft configuration already is like 6 carbs, with 2 carbs fused together and not tuneable indivually...that's right isn't it? I mean the webers have 6 air horns, so....

Yes and no. There are more advantages of having 6 carbs. A three carb setup is actually inherently faulty. Sure it works, but not the most effiecient. Actually, a 2 carb setup is supposedly more balanced. Here's why;

3 carb setup;

Carb 1, feeds cyl 1 & 2

Carb 2, feeds cyl 3 & 4

Carb 3, feeds cyl 5 & 6

Firing Order: 1-5-3-6-2-4

So the carbs are working in this order;

Carb 1

Carb 3

Carb 2

Carb 3

Carb 1

Carb 2

or

1,3,2,3,1,2,1,3,2,3,1,2,1,3,2,3,1,2,1,3,2,3,1,2...

Uh oh! Only Carb 2 is working at a steady rate!!!

After Carb 1 feeds fuel to cylinder 1, it has to wait through three firings (6 revolutions!) before feeding fuel again. Then it has to quickly fire again after only one more firing (2 revolutions). Carb 3 is doing the same thing!

So, carbs 1 and 3 may run rich or lean in different stages of the firing order while carb 2 is working steady. The carbs will never be in sync.

2 carb setup;

Carb 1, feeds cyl 1, 2, & 3

Carb 2, feeds cyl 4, 5, & 6

Firing Order: 1-5-3-6-2-4

So the carbs are working in this order;

Carb 1

Carb 2

Carb 1

Carb 2

Carb 1

Carb 2

Bingo! Balanced!

With a 6 carb setup the same is obviously true. Syncing the carbs is not hard. The carbs will also be doing half the work than a 3 carb setup, and 1/3 the work of a two carb setup. This means more precise metering; Especiallyat high RPM's!

Also, smaller carbs can be used since they are doing much less work. Each cylinder only requires about 70cfm's in an L28. A 44mm Mikuni flows ~165cfm's!! Holy cow, talk about overkill. Not efficient! Not to mention they cost an arm and a leg. Even the 26mm will flow almost 100cfm's.

So, I need to find the smallest Mikuni carbs that will be able to flow just enough air/fuel and NOT be smaller than the intake runner, so as not to restrict air flow into the motor.

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John, your model would apply if we were discussing 3 SU carburetors. Just as you second example proves the efficiency of the dual SU set up.

The DCOE/PHH type are really 2 carbs, sharing a common float bowl They work independent from each other, and are completely individually tunable. They work precisely as if you had installed six separate carbs.

As for 44mm being too big, well, it is, for a stock engine. Nissan race prepared engines used 50mm Mikunis. Trust me, the engineers at Nissan would not have used them if they were inefficient.

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Thanks for the info Jcraig....that makes a lot of sense.

I would think that miniature SUs (6 of them) would be best...since they only allow into the motor what the motor needs....I wonder if motorcycle carbs could be modified, to have a butterfly (that the throttle controls) and the piston moves on its own, like an SU? probably tooo much work, but that would be best, then you could go with the 44mm Mukinis no problem.

Thoughts welcome!!!!

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