The new guy needs carb help

[Fl Hermit]

I just got a new project. I'm trying to find out how many CFM it takes to run a 1300. What I have now is a custom intake with a 2 barrel holley.

[cowpuc]

While I am certainly no expert and I do HIGHLY suggest that you check back to this post and see if one of our much more knowledgeable mechanics gets involved with answering your question (I KNOW I will be watching too - I LOVE to learn:big-grin-emoticon:) I am gonna guess somewhere around 125 CFM should be good. I am thinking maybe a little larger if you are planning on keeping her spun up in the R's and looking for some extra on top, maybe a little less if your planning on low R torque. I also know the V-max 1200's "v-boost" is actually a device that turns 2 carbs into 1 (increasing CFM) BUT, they also do so at high r's (above 6500 with a peek horse at 9500 - much higher than the normal Venture - different cams and valving and all that make it necessary). For a stock 1st Gen - my GUESS would be that 125 CFM would do ya.

I LOVE your question and LOVE where your headed - have always about doing exactly what your up to but just never shut my bike off long enough to play with it to that extent.

By the way, to VR FL and all the best in what sounds like a VERY interesting project!!

 

Please keep us informed about your project and any 's would be greatly appreciated!

Puc

[cecdoo]

A few years back there was a guy at the Asheville meet with a holley 4barrel setup on his first gen. Maybe someone will remember his name and be able to pass along some info.

 

Craig

[krispy]

I'm not sure I got this right, but 1300 cc engine running at 6500 rpm filling itself every other revolution will draw 149 CFM. That's calculated volume thru the engine. No idea if that really happens, but should be close.

 

Larry

[Fl Hermit]

I think the Holley is in the 250 to 350 range. (need to check the numbers) If the motor is in the 125 to 150 I should have plenty of carb. What I have is a real bad flat spot just off of idle. Then about 70 mph it over heats instantly. Not sure if the two are related? One thing at a time.

 

Thanks for all the help..... Will keep yall posted on the progress.

[MiCarl]

I believe your carburetor needs to be roughly the right size to have a reasonable expectation of making it run well.

 

I'd guess your stumble off idle is because you're lean. Your high speed overheating can be caused by it running lean. If you don't get enough air velocity through the carburetor it'll never pick up enough fuel.

 

Bigger isn't necessarily better, and may be worse.

[tlm]

I know that back in the 80's some of the goldwing guys would put a single barrel on the 1200's. Never heard of a 4bbl.

[Flyinfool]

Having a carb that is to big for the engine will always give poor performance. If the carb is to big the airflow will not be enough to get the pressure differences that the engineer designed into the carb. This will create a situation where you can tun the carb to optimize it for one airflow and it will be off at all others. Yours was probably tuned for best cruise mixture and that is why it is acting lean at idle and WOT.

The reaction that you are finding is what others that have done this mod have experienced. Now if you were to replace the carb with a small TBI system, you might just have something, since the computer and sensors will keep the fuel mixture correct at all speeds.

 

I have never heard of anyone putting a TBI on a manifold like yours, If you do PLEASE post a lot as you go through to process, we all like to learn new things. There is someone making an after market fuel injection system that modifies the existing carbs by adding an injector to each carb and necessary electronics. but the whole conversion is not cheap, IIRC it was around $1500 for the kit. Since you already have the manifold, a TBI conversion may be less costly, especially if you have a good relation with a local bone yard to get parts from.

[Fl Hermit]

TBI sounds interesting..... I think I'm going to try rejetting for now. Wish I had an exhaust gas analyzer. Cowpuc asked for pics... sorry so dark. will try again soon.

 

[Fl Hermit]

Better Pic

 

[Prairiehammer]

Is there an accelerator pump on the Holley? Does it work?

[djh3]

Here is a Holley TBI. Might be expensive, but should do the trick.

[Fl Hermit]

670 CFM I think that's a little big. Not to mention 2 Grand

[Fl Hermit]

Yes to both

[cowpuc]

I think the Holley is in the 250 to 350 range. (need to check the numbers) If the motor is in the 125 to 150 I should have plenty of carb. What I have is a real bad flat spot just off of idle. Then about 70 mph it over heats instantly. Not sure if the two are related? One thing at a time.

 

Thanks for all the help..... Will keep yall posted on the progress.

 

TBI sounds interesting..... I think I'm going to try rejetting for now. Wish I had an exhaust gas analyzer. Cowpuc asked for pics... sorry so dark. will try again soon.

 

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Better Pic

 

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1st things first,, THANK YOU for the pics and WOW ZZZZ WOW WOW WOW, BEAUTIFUL thing ya got going there FL:clap2:.. We're talking AWESOME IMHO

 

Also IMHO, what you are describing for performance ailments add up perfectly when considering the size of carb your running, just to big (in CFM). While you may be able to jet out of it running lean and causing her to run hot - I have my doubts that even running larger jets will be the answer cause it takes velocity of the air your moving threw the throat of carb to atomize the fuel (giving you a good fuel/air mixture) and it seems very likely that your engine isnt producing the kind of air movement needed to do so at the larger CFM. The flat spot your experiencing at the bottom is more then likely also caused from such a massive amount of air being available when the throttle plates open. As suggested, you might take a peek into the throat of the carb and snap it open real quickly and see if the accelerator pump is working (should see a good squirt every time you crack the throttle) but I would not be surprised to hear you say the pump is working fine. Bottom line, I really think your way over sized on the carb and thats why she's performing like she is..

Never tried this but I wonder if choking the whole system down with a small air filter or a small air box would decrease the CFM and increase the velocity of the air movement enough to make a difference.. May sound crazy but I think if I were in your shoes that is what I would try next. Even if you built it out of card board or something. Maybe just take a piece of cardboard and cut it so it fit over the top of the carb throats, then play with cutting different size openings above the throats - increasing little at a time and see how it runs.. Never know, might just be able to end up with some kind of a metal CFM reducer plate

REALLY COOL what cha got going there my friend!! Hope your enjoying this as much as I am:big-grin-emoticon:

Edited December 21, 2015 by cowpuc

[Fl Hermit]

Cowpuc,

Thanks for the kind words!!!!

I looked up the numbers and the carb is 350 CFM

Venturi diameter is 1-3/16"

Throttle plate diameter is 1-1/2"

But you gave me an idea. The restrictor plate might just work. It works for NASCAR and I'm only one wheel short of those guys.

Will keep you posted

THANKS AGAIN!

[MiCarl]

Never tried this but I wonder if choking the whole system down with a small air filter or a small air box would decrease the CFM and increase the velocity of the air movement enough to make a difference..

 

Adding restrictions will further reduce the CFM. The speed of air through the carburetor is basically CFM/throat diameter. Further limiting air into the engine will slow the air down.

 

Sprint Cup cars use fuel injection - they don't need air flow rate to pick up and atomize the fuel. That's why they can use restrictor plates to reduce power. Even so, I'll bet the teams re-tune the FI map when they run with restrictors. I"d be willing to bet that before they went to FI they used a smaller bore carburetor when using the plates.

Edited December 22, 2015 by MiCarl

[djh3]

All 3 national divisions now use a tapered spacer vs the flat plate with smaller holes than the throttle plate. It actually works much better for throttle response as it is tapered like a funnel so fuel get down it instead of the flat plate that made aire go every which way and killed throttle response.

[Fl Hermit]

I didn't get the stock carbs with it but with 1-1/2 inch throttle plates on my Holley. If my math is right that's 3 inches of carb. So the stock carbs (there were 4 of them right?) The stock carbs only have a 3/4 inch bore????

NASCAR has been using restrictor plates at Daytona since 1988 but only started FI in 2012.

[Flyinfool]

There is a lot more to it than just the diameter of the bore. The aerodynamics of the inside of the carb also play a big part in the flow capacity of the carb.

Your 300 cfm holly with 2 1.5 inch bores has about 3.5 sq in of area. Yet the stock 32mm carbs have a total area of about 5 sq in. Yet the engine at 8500 rpm will only need about 150 cfm. The stock carbs seem to work well so it means that they must flow around 150 cfm even though their swept area is a lot more than the Holly

[Fl Hermit]

There is a lot more to it than just the diameter of the bore. The aerodynamics of the inside of the carb also play a big part in the flow capacity of the carb.

Your 300 cfm holly with 2 1.5 inch bores has about 3.5 sq in of area. Yet the stock 32mm carbs have a total area of about 5 sq in. Yet the engine at 8500 rpm will only need about 150 cfm. The stock carbs seem to work well so it means that they must flow around 150 cfm even though their swept area is a lot more than the Holly

 

That's what I was afraid of the math is going to be way above my pay grade. Only thing to do now is start throwing parts at it and see what sticks. On the bright side "It ain't my ride home"

[mralex714]

Better Pic

 

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Who makes the manifold and how do I get one?

[Fl Hermit]

Who makes the manifold and how do I get one?

 

The guy I got it from said a friend of his made it. Not sure if he would make another one or not. I will check with him the next time I see him.

[cowpuc]

Who makes the manifold and how do I get one?

 

I was actually checking back here with that exact question on my mine Mralex,, good looking piece of art work there FL - would LOVE to see any more pics of your project as you progress or tinker with it!!

Still oughta try the restrictor routine FL - never hurts to try..

 

Got thinking about this thread the other night,, I used to run Festiva's years ago, had an 89 Carbed model,, they were 1.3 liter motors = 1300cc. I remember back then there was a little 2 barrel carb you could get for em that functioned like a four barrel does - rear barrel opened up with vacuum increasing.. Suppose to add some pretty good ponies to the stock 1300cc Festy.. Wonder how one would perform in this application, wonder how a stock Festy carb would do.. Sometimes I wonder to much..

Probably way to much dinkin around trying to modify the intake with an adapter plate..

Maybe a trip to an auto junk yard to see if you could find a smaller CFM carb with the bolt pattern your currently using is where this will all end FL..

[djh3]

Puc- Many of the small 4 cyl cars of the late 70's had those split bore design. A primary and a secondary. I think the Ford Fiesta, Dodge Omni, maybe the VW Rabbit. I raced a Omni on circle tracks, the problem with the particular year one I had was the carb also had a computer controlled electronic circuitry built in. Best guess I had was once I had modified it the carb went in to a "limp mode" and you could turn the mixture screws in all the way and still run. So I put a small Ford 2 bbl (holley looking knock off) on it. I had to make jets for the thing to get it to not run so rich. Even then it was rich enough when you got to the end of the straight and lifted It would bellow a big ole blue flame out the tail pipe 2ft or more.