I need help locating a missing gallon of tank capacity.

[88bmwJeff]

I'm missing a gallon (of capacity) in my gas tank (1988 R100 RT). I ran out of gas the other day, and it took 4.6 gallons of gas. My tank should be 5.5 gallons per the manual--granted this is the maximum capacity.

I drained the tank and checked both petcocks. There was very little debris on the petcock screens and all tubes in the petcocks were open (not blocked). So this begs the question--What happened to the missing gallon?

I will concede that there will be some unattainable amount of fuel between the bottom of the tank and the top of the reserve straw, but I doubt it would be a half gallon on each side of the tank. The flap on the filler neck of gas tank does limit the amount of gas I can pour in it during fill ups. It seems a little much that these two items could result in a loss of a gallon reducing the tank to ±4.5 usable gallons.

[20774]

How were you or how do you typically use your petcocks? Have you confirmed that both petcocks flow fuel in both on and reserve positions?

You said it took 4.6 gallons to fill. Is this after you ran out of gas and got to a gas station or is this after the complete drain and refill? If it was the former, it might be possible that one of the petcocks wasn't working or wasn't used correctly. You think it was on reserve but in reality it wasn't actually passing fuel.

One of the books I have says the '88R100RT tank is 22 liters, or about 5.8 gallons. That makes the situation even worse. Based upon the measurements on my 6.3 gallon tank, there's about 0.1 gallons below the reserve straws in my /7 tank.

[88bmwJeff]

How were you or how do you typically use your petcocks? Have you confirmed that both petcocks flow fuel in both on and reserve positions?

Yes both reserve and on positions on both petcocks flow properly. I checked when I had them removed.

You said it took 4.6 gallons to fill. Is this after you ran out of gas and got to a gas station or is this after the complete drain and refill? If it was the former, it might be possible that one of the petcocks wasn't working or wasn't used correctly. You think it was on reserve but in reality it wasn't actually passing fuel.

It was 4.6 gallons after running out of gas. I honestly believe both petcocks were properly switched to the reserve position. However, that could be possible since I can't go back and check.

One of the books I have says the '88R100RT tank is 22 liters, or about 5.8 gallons. That makes the situation even worse. Based upon the measurements on my 6.3 gallon tank, there's about 0.1 gallons below the reserve straws in my /7 tank.

The 5.8 gallons could be the non US version. The BMW owner's manual says 21 liters or 5.5 gallons.

[20774]

Well, I'm doing my best Stan Laurel impression and scratching my head... I guess the only way to know for sure is to redrain the tank and measure the amount of gas that is actually in the tank.

Do you keep track of your gas mileage and how many miles it takes on average to hit reserve? If you do, then you could back calculate the amount of gas above the main straws. You could also push it and count the number of miles from the main tank to the 1st reserve, depending on how you use your petcocks. I do this and know that it is about 4.6 gallons on "main" and 0.7 gallons on the first reserve, and about the same 0.7 gallons on second reserve, leaving the small amount below the reserve straws.

[swall]

What were the circumstances that lead you to run out of gas? Could this have a bearing on the condition of the reserve position of the petcock?

[lkchris]

Actually, if your fillup only required 4.6 gallons, you really weren't out of gas. You could have had a feed problem due to plugged tank vent, for example.

Do you still have the SHED system connected on your bike?

Unlike the earlier bikes, these later tanks are not symmetrical, i.e. right side is smaller internally than left side. Don't know how that might make a difference, but it's a clue of sorts--especially if right-side petcock was only one working or especially if left-side reserve straw is high.

[39987]

Hey guys. The problem is that tank has a neck with a flapper valve at the bottom. I goes so far down in the tank that it will not let you fill it up to the top. You will have to cut the valve out or get a older tank. I have the same problem with my 1988 R100RS.

[pedrocasper]

If you are just going by the gas station pump, that may be the issue. Some pumps, intentionally or not, are not calibrated that well and may not provide you with an acurate gallon reading. If you have not drained the tank and checked the volume, I'd do that to make sure it wasn't the pump.