740 carb 24mm bore CFM

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IBAJIM

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I have been doing some calculations ( actually been using the CFM calculator at a carb site ).

A 24 mm bore ( the 740 primary bore ) will flow plenty good on a 1100 c.c. engine with a 83% volumetric efficiency up to 7,000 RPM !!!

I also read a very interesting post on another website about a dirt track racer who installed a half 500 CFM carb on a 2.3 liter Pinto engine and got black-flagged because it was so fast, compared to running both barrels. Apparently too much CFM is as bad as not enough .......

SO, What I am thinking of doing now with my 740 carb is to modify the secondary linkage so the secondary doesn't start opening until about 85 % throttle opening. Need to stay on that primary mostly, up to about 7,000 RPM.
 
So is the goal to get best mpg or speed or power? Seems to me if using only one barrel is better then maybe one barrel carb is all that's needed?
 
[url=https://classicgoldwings.com/forum/viewtopic.php?p=139436#p139436:40r3j25j said:
slabghost » Mon Jan 19, 2015 7:59 pm[/url]":40r3j25j]
So is the goal to get best mpg or speed or power? Seems to me if using only one barrel is better then maybe one barrel carb is all that's needed?

The goal is to have the best of everything.

Originally I was going to use a 30 PICT single barrel carb, but I think the smaller 24 mm primary will be better at low & mid speeds and should give better mileage, since the smaller the bore, the higher the air velocity through the venturi, the better the "signal" and fuel/air atomization. But for above about 7,000 RPM, the second barrel will give the required extra CFM. But the 1100 c.c. motor doesn't need ( or want ? ) all the CFM the 740 can deliver.

In other words, the 30 mm carb theoretically won't work as well at low speeds and is limited at the top end. So the progressive 2 barrel is superior. It is a cheap version or approximation of a modern CV carb ( variable venturii carb).

I just got finished doing a little filing on the 740 linkage. The secondary is starting to open a little later ( wanted more delay, but not ideally possible ) and the secondary isn't opening fully, now. That's OK - Still have plenty of CFM available.
 
i only run the primary bbl on my carb, i unhooked the second bbl when i removed my heated intake. mabey that is why i get 40mpg avg? i don't need any more power as it will pull hard to 7k and go to 90 real quick and that is as fast as i want to go on my beater!!
 
[url=https://classicgoldwings.com/forum/viewtopic.php?p=139443#p139443:32oguavy said:
sgq700 » Mon Jan 19, 2015 9:47 pm[/url]":32oguavy]
i only run the primary bbl on my carb, i unhooked the second bbl when i removed my heated intake. mabey that is why i get 40mpg avg? i don't need any more power as it will pull hard to 7k and go to 90 real quick and that is as fast as i want to go on my beater!!

Makes perfect sense to me !! What mileage did you get with the stock rack ??
 
never rode one with a stock rack!! That is why it was in the junk pile, P.O. was not going to fix the carb's again. I get these old wings from back yards and never pay over 200 bux for them(i have 4) and they all seem to be the same reason for parking,bad carbs! the major downfall with these old bikes. i did a single carb on my v65 magna with a harley side draft zenith and am gettin ready (in the spring) to do a carter/weber on my st1100. i just like the single carb deal on any of them.. not saying they are better for everyone, but the only way to go for me.
 
[url=https://classicgoldwings.com/forum/viewtopic.php?p=139448#p139448:cxilx5qw said:
sgq700 » Mon Jan 19, 2015 11:14 pm[/url]":cxilx5qw]
never rode one with a stock rack!! That is why it was in the junk pile, P.O. was not going to fix the carb's again. I get these old wings from back yards and never pay over 200 bux for them(i have 4) and they all seem to be the same reason for parking,bad carbs! the major downfall with these old bikes. i did a single carb on my v65 magna with a harley side draft zenith and am gettin ready (in the spring) to do a carter/weber on my st1100. i just like the single carb deal on any of them.. not saying they are better for everyone, but the only way to go for me.

" and they all seem to be the same reason for parking, bad carbs! the major downfall with these old bikes. "


Interesting !!! Way to go, hate to see an old Wing just parked and not running !!!
 
i agree that one can run a oldwing on primary barrel only ...but be careful in reading to much ... pass a certain point words have more filling the page than real world meaning of any kind ......truth of the matter is ...graphs claiming what a certain cubic inch motor will flow at any givin rpm ...is asking a lot without cam ...bore and stroke ...valve and port size ... just silly to me ....the oldwing motor get huge flow ..well mine dose anyway being a modded 1200 for maxium flow....and paul from c5 showed a dyno result ....where a stock rack actually leans out at higher rpm ...so i bet it also dose run lean at rpm running off of primary barrel only ....not saying any of this is a problem ....

i will say ....i will say for a lot of people sgq700 is right ... my bike has way more than needed in power and torque and always has to be accounted for ....

and yes it terrible that the biggest flaws in the oldwings were poor carb engineering and same for ignition ...but it dose make for some great buys ...and here on this forum you get good advice on what to do ...either with stock carbs or the custom single carb builds ....
 
An engine is an air pump, plain & simple. By using the CFM calculator ( or a pencil and/or a calculator ) you can determine the MAXIMUM CFM required of a carb, assuming 100% volumetric efficiency. The MAXIMUM CFM has nothing to do with the cam, heads, or other specs of the engine. OF course, the CFM requirement is less than the MAXIMUM because the volumetric efficiency of a normally aspirated engine has to be less than 100 %. But the calculation is useful because you will know if the carb is too big for the job. Bigger ( too big ) isn't better when it comes to the carb, for sure.
 
[url=https://www.classicgoldwings.com/forum/viewtopic.php?p=139458#p139458:14xj8q8u said:
IBAJIM » Tue Jan 20, 2015 10:12 pm[/url]":14xj8q8u]
An engine is an air pump, plain & simple. By using the CFM calculator ( or a pencil and/or a calculator ) you can determine the MAXIMUM CFM required of a carb, assuming 100% volumetric efficiency. The MAXIMUM CFM has nothing to do with the cam, heads, or other specs of the engine. OF course, the CFM requirement is less than the MAXIMUM because the volumetric efficiency of a normally aspirated engine has to be less than 100 %. But the calculation is useful because you will know if the carb is too big for the job. Bigger ( too big ) isn't better when it comes to the carb, for sure.
+1 :good:
 
A correctly sized carb with incorrect jets either way will never be right. An oversized carb correctly jetted will do just fine.
 
well i hate to bust your bubble on this ..but ive got way to much experience to know that blanlket graph table means nothing .... try to get a flow rate with cam not hooked up ... on milder way just change cams ... like changing a jet on a carb it will be entirely different... with all the experience i have with these cams heads and port sizes ...it makes a huge difference ... sheesh 1100 cam shut down flow big time at higher rpm ....and gose flat ...carb wont help that ..but a better cam dose ...because it flows more and it not peaked out .....in general your flow chart has some merit ...but its at the lowest of level and should not be considered something to go on as deciding factor ...

but i try to keep things real here ... that flow chart gets way more credit than it can back up with so few numbers to go on ...

statements like flow not needed and carb to big is bad ...when neither of these lables are even comes close to the this dft series carb ...statement like this from me slants a different way but is more true... yes if you want to run like stock bike .. then just run on the primary barrel ....but if you want to get everything the motor has hook up the secondary and find the rest of the horses in the oldwing motor ...one that cover all the peaks of all the rpm from 0 to redline ..... :BigGrin:

sorry but to me that chart knows nothing of the oldwing motor flow needs ...people who read me posts will get my opinion on such charts :cheeky: :mrgreen: :mrgreen:
 
The volumetric efficiency charts are great to get you in the ball park but they lack in real world settings. The actual displacement of the motor is only part of the equation. Manifold volume and runner length is a part of it also along with plenum size and configuration. Each will affect how the carb responds and how it will need adjusted. A wide variety of intakes have proven to work well but each has required unique carb adjustments even if the same carb is used from one application to another. Not saying you can't have a well performing bike with mostly just one barrel used but other factors must also be considered.
 
Just to add a little more fuel to the fire, last night I found the Weber carb manual in the gallery that Dan uploaded there - Thanks Dan, it has a wealth of information !! Right from the ( Weber ) horses mouth !

One sentence really caught my eye.
It was in Part 1, Chapter 5, Section 2 : ( quote) :

"It is always most important to choose a choke size ( venturii ) which is the smallest consistent with the maximum horsepower of the engine. "

BTW, I didn't add the italicization of the word "smallest" .

So, it looks like my assertion that it is possible to have too big of a carb , is correct.

BTW, there is also a section on Calibration that describes in detail how to determine if the idle jet is the correct size, besides only checking the position of the idle mixture screw at idle speed. Cool !!
 
And max horsepower is a relative thing. It's possible to get 500hp out of stock vw inline 4 rabbit motor.
 
most of us here and the ones who use this carb already know the carb is close in size for oldwings many thousands of miles put on bikes in this group ...there are some having troubles and some who are not ...the carb your talking about is the one i use ... using paper page info as fact is probably number one reason that fails most projects ... there good guideline in info ...but spec size dose not determine carb size for motor ...this weber has many circuits inside one barrel ..and parts of it are to big and other parts kind of small .... we like info here ..thats why we collect it ...but very seldom do we use it to discount what someone says .....

as for your case if you info on 24-25 carb on a oldwing ..you would listening to me instead of answering with paper info that useless to the thread .....you have got a carb as close as you can get that i know of ...you ask in the beginning what carb now its to big .... its not ...what fails people is there work most of the time ...the carb has always been guilty... just not so ...i watched for yrs and yrs people acting like they know something and there bike are perfect and there an expert ..to find out they dont know anything .....

as someone who watches things here ... statements of carb to big to be read by people want good solid info from people who done it ...not to be deluted by words on a page with no experience at all or even talking about the carb and motor in ?
 
Joedrum and others..........We are all learning and experimenting what works with what, along with what effects various manifolds and runners have to jetting etc.
For this reason I would ask that everyone respects each others input and opinions and build on that.
There is nothing to be gained by a disrespectful attitude toward anyone and will not be tolerated here.
 
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