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TR2/3/3A Is it possible to drill vacuum advance port in SU H6 carb?

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TuffTR250

Jedi Warrior
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I acquired a couple of SU H6 carbs (AUC878) but unfortunately they are both back carbs and therefore neither of them has a vacuum advance port. Is it possible to drill a hole and tap the hole in one of the carbs and mount a vacuum port? If possible, does anyone have detailed specs on where the hole should be drilled and detailed instructions on how to go about doing this? Also, where would I get the proper fitting to screw into the carb? Thanks for the help!!
Regards,
Bob
 
Then you'll want the fast idle cam; and the choke arm; and a longer throttle shaft ...

I believe the casting boss is there, but I've never tried drilling for the advance port. You'll need to drill all the way through with a small drill, to create the port into the carb throat. It needs to be carefully aimed so the opening is just in front of the throttle plate when the throttle is fully closed. Then you'll have to drill partway with a larger drill, and tap for the fitting. The factory fitting appears to be available from TRF, their PN SUAUC4490. But it used a strange thread into the carb body and I'm not sure where you'd find a tap for that.
 
Thanks Randall!! And thanks for the TRF p/n! Any idea what size holes needs to be drilled? Would 1/8 be right for the hole that goes all the way through? If no one knows the size hole for the fitting, I may be able to call TRF Tech Support and find that hole size. I hope TRF Tech Support would also know what the thread size and spacing is.
Regards,
Bob
 
The hole for the port is quite small, about .041" (#59 drill), at least on the HS6 carb I measured. It may be easier to find a front carb body.
Berry
 
I just checked an H6 body, and it appears to be between .040 (#60) and .041. The #60 went through easily, the #59 wouldn't go without force. I agree about finding a front body.
 
Any ideas as to why the port hole is so small? My guess is that it acts as a restriction to dampen the movement of vac advance unit.
Berry
 
My guess would be more along the lines of not disrupting throttle operation, but that's just a WAG. A bigger hole would allow air to flow into the hole on one side of the throttle plate, and out on the other.
 
I don't think the hole, large or small would allow the carb to suck air into the mixture on a continuous basis...unless there were a hole in the Advance (or Retard) module's diaphragm or a loose fitting on the end of the vacuum hose. As to the size of the hole in the carb's throat, it must have been figured out by taking the Bernoulli principle into consideration..since the speed is greater in the narrower pipe, the kinetic energy of that volume is greater.

Read more: Bernoulli's principle | Infoplease.com https://www.infoplease.com/encyclopedia/science/bernoulli-principle.html#ixzz2ovsZEvHf
 
poolboy said:
I don't think the hole, large or small would allow the carb to suck air into the mixture on a continuous basis.
Click to expand...
Ok, sorry, I should have written "would allow fuel/air mixture to flow into the hole on one side, and out on the other". In other words, it would form a throttle bypass, only when the plate was over the hole.

And since the air, er, fuel/air mixture in the small tube is essentially stationary (for a given throttle position and intake vacuum), I don't think Bernoulli applies.
 
Yeah, you got a point there about the movement or lack thereof of air in the tube, Randall.
There must have been some consideration of some Law of Physics in determining the diameter of that hole, though.
 
Randall, could you please do me a favor and measure exactly how far the center of the small hole is from the very back edge of the carb? i.e. the back being the face of where the carb mounts to the manifold. Thank you!
Regards,
Bob
 
I get .940" to the edge, so .960" to the center. Give or take a thousandth or two, I just measured with calipers.

BTW, check out eBay auction 161183567311 It'll probably get run up later on, but right now it looks like a deal.
 
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