Help needed w/new MAC Adaptor

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I was trying to put the new MAC conversion part for the Bob Violet needle on my CMB carb for my .21 green head. It's not a perfect fit but pretty close. A visual observation shows the original fuel intake on the carb protruds furthur into the carb then the new part. Is that okay?? I know it was designed specifically for the MAC but others said it would work on the CMB. I'm gonna attach pictures for visualize aids. The first is of the stock piece on the left and the after market on the right.

Please feel free to comment:

 

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Second picture shows the stock part in the carb. Notice how deep in goes inside?

 

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Here's the aftermarket part. Notice it doesn't stick in as far as the stock. I guess the purpose is to find out if this really matters or not. Will performance change??

 

[Bob Kensill]

On the Mac carb,the spraybar does not reach in as far as the the stock one in the CMB carb. It performs better.

 

[Don Ferrette]

Here's the aftermarket part. Notice it doesn't stick in as far as the stock. I guess the purpose is to find out if this really matters or not. Will performance change??

The spraybar should protrude 1/3 to 1/2 into the bore diameter. You need a good draw past the end & the fuel will atomize better. B)

 

[Bob Kensill]

just remember that the air flows fastest the farther out you get from the center,so fuel draw will be the lowest at the center. That is one of the reasons we use pipe pressure.

 

[TimD]

Rodney - I have the same drama with the adaptor. I have the red head CMB carb.

Can someone save me a lot of running about and tell me the thread size on the BVM needle end????? PLEEEEAAASE! Trying to salvage a pile of incompatible parts now!

 

[Don Ferrette]

just remember that the air flows fastest the farther out you get from the center,so fuel draw will be the lowest at the center. That is one of the reasons we use pipe pressure.

Fuel atomization will suffer a great deal if spraybar does not protrude far enough. Found this one out the hard way.......... :huh:

 

[Roxyflash]

Looks like your using the low speed needle i would ditch it as it wont work with the mac adapter its to far away i would try it see how it reacts you could leave the low needle in there just back it off if it works out then i would chop it off i know most people like the low needle for heavier scale boats and the likes but i dont use them.Other than to have someone make the right adapter for the cmb carb.Its a good idea but some boats theres not enough room for all that needle hanging out the side.

 

[DJT]

Wouldn't the air flow faster in the center due to less friction?

 

[Don Ferrette]

Wouldn't the air flow faster in the center due to less friction?

Well going back to the car racing days the BEST performing carbs always had center discharge venturis. As an engine guru told me back then the signal is most accurate in the center of the carb venturis. Holley even went to the point of designing a discharge nozzle that would straighten the airflow. B)

 

[Bob Kensill]

My best racing Holley carb was one of their "jet ring" carbs,much like what Carl Brey was building for model boats. I was running a turbo charger and the "center discharge" carbs would have hesitation and flat spots.

 

[AWaddle322]

Bob Violet mounting threads are 1/4" x 32

Sincerely,

Allen Waddle

 

[Chris Attebery]

Rodney,

I can't see any of the spray bar in the picture with my adapter on the carb. Is it sticking out into the bore at all?

I can't say if it will work or not. Unfortunately you might just have to try it. I have ZERO time to make parts right now, putting in 7 days a week. My lathe is down too, which means I'm not making parts for a while.

Sorry I can't be of more help. BTW, the needle should screw down flush to the adapter, is yours loose?

Chris

 

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Chris,

No, it doesn't fit all the way in, about the thickness of a penny difference.

 

[GTR]

Hi Guys

The carbs you are talking about are 'Annular Discharge' and rely on airflow to work. They have less restriction to the airflow (smoother at WOT) and promote more even atomisation. If the end of the spraybar extends at all into the venturi try filing the end at 45deg and it will have no problems drawing off fuel. As long as there is a low pressure area behind the spraybar it will draw.

 

[TimD]

Bob Violet mounting threads are 1/4" x 32Sincerely,

Allen Waddle

Allen,

Thank you VERY much! Appreciate your help!

Tim.

 

[Roxyflash]

Ok i think you need to send me the carb and needle one problem is the mac carbs are a barrel that justs rotates no low needle the cmb rotates and goes side to side for the low needle so if you want the low needle to work the spray bar has to be at the right lenght.Will need all the parts to measure and make the right adapter.I think its not worth the trouble my self but thats my opinion im sure i can make one but dont have that style of carb to try.Email me if your interested.