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Unread 08-15-2018, 04:18 PM   #1
Crazy Ole Man
 
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Default 300 CFM Holley

I have looked at the V Twins for a long time and have seen an intake problem I would like to learn about. Please correct me if I'm wrong. There are 2 -- 90 degree elbows or turns in the intake tract of a V configuration engine. One for the air filter into the carburetor body ---- and one that mounts the carburetor to the intake manifold.
Has any one ever thought of using a small Holley 2 barrel carburetor with an accelerator pump on there Briggs or 670 Predator. 300 CFM is as small as Holly goes. https://www.holley.com/products/fuel...arts/0-80320-1
Gravity goes a long ways when used properly. You would correct the limitations or restrictions in the intake tract and add an accelerator pump to the system. It also reduces the length of the intake tract from the choke plate to the intake valve.
What's your thoughts on this subject, height may be a problem.
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Unread 08-15-2018, 04:36 PM   #2
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Default Re: 300 CFM Holley

300 cfm sounds like a lot for that small engine.

I understand the accelerator pump for starting off.

Would that small engine have enough suck to pull, make that Holly work??
If it did would it over fuel the engine?

My Harley V-twin has two 90's between carb and head, but its also has 1340 cc's of suction
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Unread 08-15-2018, 04:51 PM   #3
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Default Re: 300 CFM Holley

How many CFM are the carburetors used on the Briggs or Predators?
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Unread 08-15-2018, 05:35 PM   #4
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Default Re: 300 CFM Holley

I don't know.

but if you compare the size of the carb openings.

I would think it would flow so much more air, there would be no way the engine could handle the amount of gas it would take to get a proper air-fuel ratio. just guessing *** if I know. LOL
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Unread 08-15-2018, 07:33 PM   #5
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Default Re: 300 CFM Holley

Assuming your engine is stock and still 670 CCs and you are allowing it to rev to 5500 or so, and about 80% volumetric efficiency [which would be excellent in an stock industrial engine, you need about 60 cfms at 3 inches of vaccuum. A 300 cfm holley will supply that at probably 1/4 throttle. Open it further and velocity through the venturi also vacuum will drop to nothing and after the pump shot is gone, no fuel flow. Do not be worried about bends/curves in the intake tract yet. The biggest impediment to airflow is generally through and around the valves. In fact, in order to create torque [which is FAR more needed to move these buggies than horsepower], small intake runners create it in spades by increasing velocity which creates a small compressed charge behind the intake valve while it is closed ready to "charge" into the cylinder the instant the valve unseats.
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Unread 08-15-2018, 07:39 PM   #6
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Default Re: 300 CFM Holley

I would look at something like an S&S carb for a Harley Sportster,that would be more comparable in displacement.I think that anymore than that just wont run,and if it did,it would be a nitemare to tune right.The motor just doesnt have enough vacuum and intake pulse for a bigger carb,along with the short stroke,and small cams they run.Old 650cc English bikes ran a single 28mm carb,or 2 on twin carb models,and the only difference was a little more aggressive cam.Those are way less sophisticated than todays carbs,so If you were to put a pair of 34-38mm Mikunis on,It would be a huge difference over the stock carb,or even the Briggs 2-bbl.
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Unread 08-16-2018, 06:13 AM   #7
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Default Re: 300 CFM Holley

Quote:
Originally Posted by dundeebarnbuggy View Post
Assuming your engine is stock and still 670 CCs and you are allowing it to rev to 5500 or so, and about 80% volumetric efficiency [which would be excellent in an stock industrial engine, you need about 60 cfms at 3 inches of vaccuum. A 300 cfm holley will supply that at probably 1/4 throttle. Open it further and velocity through the venturi also vacuum will drop to nothing and after the pump shot is gone, no fuel flow. Do not be worried about bends/curves in the intake tract yet. The biggest impediment to airflow is generally through and around the valves. In fact, in order to create torque [which is FAR more needed to move these buggies than horsepower], small intake runners create it in spades by increasing velocity which creates a small compressed charge behind the intake valve while it is closed ready to "charge" into the cylinder the instant the valve unseats.
YEA, what he said! LOL, Smart guy there dundeebarnbuggy
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Unread 08-16-2018, 07:10 AM   #8
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Default Re: 300 CFM Holley

The bore on the 300 cfm Holley is 1.5" -- what is the bore on a Briggs carburetor
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Unread 08-16-2018, 07:33 AM   #9
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Default Re: 300 CFM Holley

Quote:
Originally Posted by Shepard View Post
The bore on the 300 cfm Holley is 1.5" -- what is the bore on a Briggs carburetor
Think that's going to be X 2 = 3

Holley showing the main jet as a 61, that has to be a lot larger then the B+G
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Unread 08-16-2018, 07:42 AM   #10
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Default Re: 300 CFM Holley

I have never understood why someone hasn't fabbed up an adapter to use a Mikuni on the V twin vanguard like many of us have on our harleys. Even performance 670 has made one up for the 22 hp predator.
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