200x larger carb 28mm?

[200xkwit]

the xr carb is 26 or 27 mm and are known to run a 200x, what im asking is will a 28mm carb run a 200x engien efficently? or is it to big

[Studytime]

Hmmm...

I can't say yes it is too large or not, but I can talk theory. As the carburetor's size increases (increase in cross section area) without a change in engine displacement the velocity through it is going to decrease. If you go too large on your carb then you will not have enough velocity through it to supply sufficient enough vacuum for a good mixture. This is why a 600 cfm carb with provide better off idle response than a 750 double pumper on a SBC.

So, for our case you may sacrifice some off idle perfomance for better topend pull. Now, you said the bike in question is a 200x. You may have to feather the clutch more to get the bike going- i.e. bigger stall converter to make up for the loss in low-end torque. Again, if you experience a loss in low-end torque it's from decreased fuel atomization. Torque will pick back up once velocity through your carb is sufficient enough to provide a good mixture. Once you're feeding your hungry 200cc motor properly there will be less restriction (pressure differential) on the intake side so your motor will allow more air to enter it.

So, if you're talking about a performance bike and you drag race it mostly it won't hurt you. If this is your trail bike, you may be best going with the xr carb. If you want the joy of experimentation- i say go for it!

Keep this in mind, on a 350 chevy;

Holley 600 CFM carb,
best milage (high venturi velocity), crisp throttle response, least top end power

Holley 750 CFM carb,
deminished efficiency (velocity is now decreasing -moving the same volume of air through a larger orificie), throttle response is still relatively good, increased top end power traded for less power below ~3000 rpm.

Holley 850 CFM carb,
milage has gone to crap, as engine doesn't move enough air through the carb, throttle response? about the same as gas milage, top end power- will perhaps go down except at all power levels about 5000 rpm.


Hope this parallel information about car carburetors helps some.

Good luck. Also, you will not get a valid answer to your question as it is really a matter of opinion/ application.

Studytime

[200xkwit]

lol studytime always coming through with the knowledge yea I think I got it im running enough performance parts I can spare 1mm and go with the xr carb and boot

[OldSchoolin86]

Keep this in mind, on a 350 chevy;

Holley 600 CFM carb,
best milage (high venturi velocity), crisp throttle response, least top end power

Holley 750 CFM carb,
deminished efficiency (velocity is now decreasing -moving the same volume of air through a larger orificie), throttle response is still relatively good, increased top end power traded for less power below ~3000 rpm.

Holley 850 CFM carb,
milage has gone to crap, as engine doesn't move enough air through the carb, throttle response? about the same as gas milage, top end power- will perhaps go down except at all power levels about 5000 rpm.


Hope this parallel information about car carburetors helps some.

Good luck. Also, you will not get a valid answer to your question as it is really a matter of opinion/ application.

Studytime

Your close on the 1st example, the other 2 are pretty off and they are all missing one major part of the formula for figuring out carb sizes. Opinion really has nothing to do with it. It is a direct science that can be calculated. It is hard getting some of the info on these smaller carbs but on the cars it's pretty easy. Keep an eye on my cfm question in the open forum. I'll post the formula and the link to a great tech article explaining it all.


Edit: LOL, oh yeah the question. That 28mm should be fine on there.

[Studytime]

Your close on the 1st example, the other 2 are pretty off and they are all missing one major part of the formula for figuring out carb sizes. Opinion really has nothing to do with it. It is a direct science that can be calculated. It is hard getting some of the info on these smaller carbs but on the cars it's pretty easy. Keep an eye on my cfm question in the open forum. I'll post the formula and the link to a great tech article explaining it all.


Edit: LOL, oh yeah the question. That 28mm should be fine on there.

Close on the first one, good. The second one I also believe is close. The third, even David Vizzard says to avoid the 850cfm holley for street use. The reason is there is very little venturi effect between the diameter of the small part of the carb body and the base plate. There is somewhat of a venturi effect on the 750 & smaller as the mainbody venturi is smaller than the baseplate diameter.

This is the reason why people just rework 750 Holleys and why the 750 mainbody on the 800 cfm base plate is good race carb conversion.

Yes, I still contend that opinion has to do a lot with it.

THE ENTIRE PROCESS of dialing in a carburetor is all about opinion. My opinion of how it is to be done; NEVER plug the power valve circuit. Enlarge the PVRC's, jet leaner, enlarge the low-speed air-bleeds, and increase squirter size. Other's opinions may be to do away with the powervalve, jet 10 sizes richer and have their squirter only come-in to drive around the pit area because they launch on a trans brake well into their carb's main circuitry.

Yes, it's CERTAINLY all about personal preference (opinion).

Two people can both tune carbs for the same car. The two tuners can have the exact same WOT performance, idle quality, and throttle response with way different parameters.

Studytime

[OldSchoolin86]

CFM is NOT an opinion. How you tune that cfm can be though.

[Studytime]

CFM is NOT an opinion. How you tune that cfm can be though.

I 'somewhat' agree with this statement.

Yes, the air-flow that an engine needs IS NOT an opinion. Too little of a carb and you'll build vacuum on the top end of the track (i.e. carb doesn't flow enough air).

However, the opinion part is what do you want to compromise. Would you rather give up 10 ft-lbs below 3000 RPM for an extra 12 horsepower or would you rather have a better low-end torque curve with better fuel atomization and throttle response but loose 10 hp on the top end. That is the opinion part.

Also, I really didn't understand your response- the second sentence. Also, please try again.

Additionally oldschoolin' validate why you siad this, "other 2 are pretty off and they are all missing one major part of the formula".

These are not off in my opinion.

Studytime

[OldSchoolin86]

Additionally oldschoolin' validate why you siad this, "other 2 are pretty off and they are all missing one major part of the formula".

These are not off in my opinion.

Studytime

Sence we both agree that volume is not an opinion, why don't we stop hyjacking this thread and finish it over in the cfm thread. I'll answer this sentence over there.

[greenhuman]

What a bunch of slappers. The XR carb is 30mm. Put it on . It will work just fine. More power everywhere. This is from experience.

[StangLX302]

Question for the original poster:

If you do end up getting a bigger carb, I might be interested in your 200x carb if it is in good working condition. I have a 185s, so it would be an upgrade for me. Let me know.