petrol taps

i have two petrol taps on my 1971 750 tank',is one for the reserve and one for the main ,or do i require both.I am feeding a Miknui carb

Re: pertol taps

Typically, the left side petcock has a taller "standpipe" inside the tank than the right. When the fuel level is too low to let the gas flow over the top of that pipe, the other petcock can be switched on and fuel flows over the top and into that shorter pipe, allowing you to keep moving while heading for more petrol.
Strictly speaking, you don't need both.
Also, there is little opportunity to use all the gas in the tank, as the left and right sides are "isolated" after a certain level is reached. If you were desperate, you cd try to get some fuel out of the left side and move it over to the right, where the level can be lower and still accessible. Or you cd remove the tank with a 1/2" wrench, and tip the gas over the hump. Or you cd just roll the bike over and hope for the best? :roll:

As Mr Rick says, the petrol can't flow from one side to the other once the level falls below the hump in the middle. Therefore, two taps are required in order to access all the fuel.
And for the same reason, the "reserve" tap with the tall standpipe is next to useless as it is very difficult to make use of the remaining pool of fuel on that side of the hump.
I removed the standpipe from mine. Just keep an eye on fuel levels.

My used 69 came with no standpipe, but I think it was called for, but not sure. I just always used one side as 'reserve' and the other as main. But there's not a lot of reserve using the hump. I'm sure L.A.B will straighten us out. But it's not magic to figure out how far you can drive on a tank either.

I ran out one time on my 750 will glass tank. The reserve, or other tap turned on, got me 5 miles. I am sure there are no "upstands' on either of my taps.
Dereck

Having another bike with FI I really like the reserve capability.
I have had to use it.

Put the reserve tap on the left side so you can hold the throttle while turning it on.
Old racer wisdom...

rivera said:

Put the reserve tap on the left side so you can hold the throttle while turning it on.
Old racer wisdom...

Click to expand...

+1

There are a number of tank designs that will trap fuel on one side or the other, if you run out of fuel with both taps on (reserve on the left, in this example) simply find a soft place (like tall grass), or a strong friend, and lay the motorcycle on its left side, briefly, you should now have enough available fuel to get down the road some distance, beats pushing; and the experience is so educational it's usually a single event...

There are a few manufacturers, Paioli is one, I believe, that offer petcocks with main/reserve features on each tap; if you are prone to running out of fuel they are cheap insurance.

I have OMG's I got from CNW that have reserve on both taps. I'm on my third season with no problems using 10% ethanol. When installing them in the tank look at the threaded part of the tap from inside the tank. When the threads start to appear do not screw them in any more (adjust them so they look right outside). Lock them in place with thread sealer, a dowdy washer and lock nut. That way every drop of gas will flow into the tap and they won't leak.
I had a set of BAPs with the plastic tube inside. I didn't like the diameter of the tube so I removed it, drilled a slightly larger hole and pressed in a piece of brass tubing I had laying around. I guess by using different lengths of tube the reserve capacity can be changed, but I'm not sure how that would work with the screen. I don't remember why I stopped using the BAP's. Maybe only one had reserve....?