GCB: Many years ago I ran a shiny black and chrome chopped '49 Panhead up and down I-5. I know all about blue loctite and of course real Harley nectar: the Red flavor. I don't want to strip or break off tank screws or anything else. I use silicon adhesive/sealer.

Good question about the air intake. Back plates on the Cox reed valves come in a number of different flavors. The earliest were pot-metal with two vents, one on either side of the needle valve. I've never noticed any difference in these, but I've never looked either. Next came pot metal back plates that were made for their engines. Some have no vent at all; these were for the Golden Bees and Black Widows. Their tanks were vented and as a result the back plate didn't need vents. The intakes on these appear as large as 1/8" and some small as 3/32". I had two of these in my stash that are about 1/16". I don't know why this is. The next type of pot-metal back plates had a single tall vent and a short one at the surface. I can only guess these are a newer Baby Bee back plate.

Then, Cox started making their black plastic back plates. These are my favorite back plates because they are noticeably lighter than their pot metal counter parts. If you're trying to save some weight this is a good place to do it. I have specimens (junk in my parts box) from Black Widows (no vents), Baby Bee's and Texaco's. Again there are variations in the air intake openings from 1/8" or 3/32". The pot metal and plastic back plates have (relatively speaking) a course ill fitting needle valve that loves to screw in or out due to vibration when they are in the air.

The product backs plates are yet another type, very often have screens over the intake and vary often have a fine thread needle valve and seal (though not always). These are nice. The needle valve is easy to adjust and the seal locks them in place. They also have one other thing; they have a large air intake, and the closest thing to a venturi is the reduction at the needle valve seat, which is hard to see because of the screen and needle valve seat. Current sure starts (which do have the large air intake but have crappy needle valve and no seal) are the exception - sorry.

Is there any rhyme or reason for all of this (he asks rhetorically)? Not that I am aware of.

Having said all this the intake opening in the tank back isn't the venturi. The venturi is in the tank. It's the tapered tube that runs from its seat in the back plate to the reed valve opening.

If you are concerned about the air intake fit the tank against it and look at the actual venturi. It will be smaller than the air intake (except product engines) the venture in the tanks vary. The Black Widow starts out at about 3/32". The Baby Bee starts out about 3/32" and the Texaco starts out a bit smaller than 3/32". The exit port behind the reed valve varies also. So... if your going to using a Cox reed valve tank your stuck unless your very clever and paitent. You could use small taper pin reamers, you could JB Weld the hole up and come at it with taper pin reamers or... You could junk the hold mess and use a production back plate and external tank.

I suspect that as long as the air intake is larger than the venturi what ever you have at hand will work. Oh, one more thing, that stupid bronze screen can be poked out - it's just an air obstruction anyway.

HTH - Steve B.

PS Bruce as always feel free to edit or reorginize as you see fit.