Boring out the spray bar?

[BobBonahoom]

I have seen some posts in the past where the dicussion talks about boring out spray bars. I think it was related to CMB 101s and if I remember right it was something Andy was doing. I am confused as to why this would matter. Since the needle valve is in series with the spray bar and the flow resistance of the spray bar (even a small bore one) is much less then the needle valve, wouldn't the needle valve control flow with almost no impact from the spray bar bore size? Or is it that there is so much fuel flowing into these big engines that the spray bar restiction is not small compared to the needle valve restriction?

Thanks in advance for helping me understand this.

 

[Samuel Hagan JR]

They where boring out the spray bar in the 101 carbs to keep the piston from burning holes in them from what I read.

 

[Tom Foley]

It seems the popular recipe is around .095 " for easier flow at less vacuum . Andy , Jeff or Joe will probably chime in . Gene may have a setup on this as well .

 

[BobBonahoom]

What I don't understand is why it matters. If the engine isn't getting enough fuel, why not just open the needle valve?

 

[dwilfong]

The diameter of the hole creates a vacuum signal to the needle valve.

More vacuum on the front side of the needle will give a different flow rate of fuel across the needle.

The needle will still flow the same with the air pump flow meter. But that is not the same as a running eng with fuel involved.

Think of it like a straw in a glass the bigger the straw the more you get...........

I run a .125 size straw in my 1.01............

Unless it is highly moded as in case and liner port size .095 / .100 should be good.

The bigger you go the harder it is to find and hold the needle setting.

Do it one step at a time. when it gets hard to needle go back 1 size.

All eng and setup are different you need to test and test some more.

Also remember the more nitro % the bigger it needs to be.

Higher nitro = more fuel flow needed

A 40% will not need as much as a %60 mix

 

[BobBonahoom]

That makes sense! The signal must be non-linear with air flow, so at high RPM you get a disproportionally bigger signal increase.

 

[dwilfong]

If you look at a spray gun it works the same way. Blows the air across a orifice. The needle is just for fine adjustment the orifice is the key to the whole thing.

There is a balance of pressure in the tank and the vacuum from the tube in the carb. The needle just changes this balance.

The flow meter is just for reference nothing more.

Now on the same carb and eng you can swap readings with the same meter.

Why do you think most struggle with a good custom pipe? It changes the whole game plan from a off the shelf consumer pipe.

Pipe pressure changes and the draw of the pipe change the intake flow. The pipe has to draw more air to slam more back in.

Unless you have endless hours to test a conservative set up or a proven set up is the best way to go.

 

[DickJones]

The diameter of the hole creates a vacuum signal to the needle valve.

More vacuum on the front side of the needle will give a different flow rate of fuel across the needle.

The needle will still flow the same with the air pump flow meter. But that is not the same as a running eng with fuel involved.

Think of it like a straw in a glass the bigger the straw the more you get...........

I run a .125 size straw in my 1.01............

Unless it is highly moded as in case and liner port size .095 / .100 should be good.

The bigger you go the harder it is to find and hold the needle setting.

Do it one step at a time. when it gets hard to needle go back 1 size.

All eng and setup are different you need to test and test some more.

Also remember the more nitro % the bigger it needs to be.

Higher nitro = more fuel flow needed

A 40% will not need as much as a %60 mix

I wonder where u get all that crazy weird info from?

Mobydickk

 

[Andy Brown]

The diameter of the hole creates a vacuum signal to the needle valve.

More vacuum on the front side of the needle will give a different flow rate of fuel across the needle.

The needle will still flow the same with the air pump flow meter. But that is not the same as a running eng with fuel involved.

Think of it like a straw in a glass the bigger the straw the more you get...........

I run a .125 size straw in my 1.01............

Unless it is highly moded as in case and liner port size .095 / .100 should be good.

The bigger you go the harder it is to find and hold the needle setting.

Do it one step at a time. when it gets hard to needle go back 1 size.

All eng and setup are different you need to test and test some more.

Also remember the more nitro % the bigger it needs to be.

Higher nitro = more fuel flow needed

A 40% will not need as much as a %60 mix

I wonder where u get all that crazy weird info from?

Mobydickk

From us Dick! hehehe! Yup! David pretty much covered all the details. Pipe stinger bore adds to the equation also.

 

[DickJones]

The diameter of the hole creates a vacuum signal to the needle valve.

More vacuum on the front side of the needle will give a different flow rate of fuel across the needle.

The needle will still flow the same with the air pump flow meter. But that is not the same as a running eng with fuel involved.

Think of it like a straw in a glass the bigger the straw the more you get...........

I run a .125 size straw in my 1.01............

Unless it is highly moded as in case and liner port size .095 / .100 should be good.

The bigger you go the harder it is to find and hold the needle setting.

Do it one step at a time. when it gets hard to needle go back 1 size.

All eng and setup are different you need to test and test some more.

Also remember the more nitro % the bigger it needs to be.

Higher nitro = more fuel flow needed

A 40% will not need as much as a %60 mix

David,

U posted "the bigger U go, the harder it is find and hold the needle setting.", If U are having trouble regulating the needle setting using larger spray bars, use the "CPT" to alleviate needle fluctuations, and please don't forget to cross your eyes when U start attacking the poor buoys'.

JM2CW

MOBYDICKK

 

[dwilfong]

You two where the best at helping me understand what was what.

Heck 7 years ago I never even seen a nitro boat or eng.

I miss are talks at the pond Dick..........

Done crossing my eyes it gives me a head ach.....LOL

I'll be back just have a few bigger engs to build first.

Doing the LS2 in my TBSS right now. Then the 2.5 Merc out board to finish up.

The VAC 1.05 is all ready to go. Gust need the time to play with it again.