Terrible Gas Mileage (<20 mpg) - Possible Causes?

Still say you should pull the petcocks. See if both sets of tubes are equal (high and low) and the strainers if they're installed - are clean

An ST with the standard tank should hit reserve around 125 miles...and around 15 -25 miles on reserve.

If you are actually burning 3 gallons of fuel in 60 miles through a 1L engine, those plugs should be near fouling and wet. Power would be as dismal as the range. If it's not leaking out then something else is amiss here. It's not the fuel tap tubes, IMO.

Very true...just want to start with the easiest parts of the system to check. Checking the tank might take 15 minutes unless the gas tank is full.

Had a similar situation in the shop last week..lousy mileage and died at idle. checked the usual suspects but no joy. Then loosen the carb clamps and rotated the carbs out to check the cables and saw the right carb choke cover was hanging off by the one remaining screw. MASSIVE air leak. New gasket and 4 new screws and runs perfectly.

Have you checked the chokes for proper assembly and function like I mentioned earlier? That plume of black on your plug pictures looks like a very rich condition. The obvious ways of getting that are the needle at about 2 positions off, and the chokes not being correct. I do not think any amount of wear of the needle or jet could drop you to <20 mpg. Not. At. All. I have seen mileage go from near 50 down to about 43 with worn needles and jets.
A friend of mine put the choke lever on upside down on one side when he rebuilt his R80RT carbs, which meant it was at about half mast when 'off'. His mileage was in the low 30s, from just one side being off. He didn't notice it, it was obvious to me.
You should be getting at least low 40s, maybe up to 50mpg, depending on your style.

Because this is my first time doing this, I’m taking it super slow, reading the manuals and looking at videos.

I pulled the carbs and will take the next few days to disassemble and reassemble. And will post my findings as I make my way through.

Thanks, all!

Are you in the Bay Area? Can you post a picture or two of the bike?

If you don't have the rebuild kit, bring them down to the shop and you'll watch me build one while you do the other.

I'm late to the game, but seems to me for a beemer to get 20 MPG, it would be running poorly, blowing smoke, stink, and burn your eyes to be near it.

I know when it's cold out and I have to start with the choke on, full choke will only run for a few minutes before it gets too rich and dies. Half choke I can ride off with, but after a few miles and warmed up a bit more, it will run really poorly until I flip it all the way off. It will die at idle if I sit too long.

Does it get 20MPG while parked???? Meaning I wonder if it leaks and evaporates before hitting the ground.

Makes me wonder if maybe someone is stealing the gas when he isn't looking?

The usual problem in some SF neighborhoods is broken spark plugs in the morning to make crack pipes..and missing gas. Mostly spark plugs though. I tell my customers to always have an extra plug and tool ready...

In good condition the bike should get 21 km/l @ 80 km/h .. on hard packed dirt, loaded.

In american ~52 MPG @ 50 mph.

For 20 MPG .. deep soft sand? @ 20 mph?

Do one carb at a time! This way you can be certain not to mix the parts from one side to the other.

And renew both the jet needle and needle jet.

I wish you lived closer to me, that’s a very generous offer.

When I was young my oldest sister bought a Jetta and bragged about the fuel mileage. So my other sister and I started adding fuel to her car at night. Keep in mind gas was cheap in the early 90's. She went most of a summer on one tank of gas. Then she went to university and couldn't figure out why her car was getting such shitty mileage.
~T

Do you still get Christmas cards from her;-)

Thanks again for all your comments and ideas. I took the carbs apart, and it's clear that a general refresh is needed. Many of the o-rings were bad/squished/disintegrating. Some of the jets looked worn, so I went ahead and ordered new needles, needle jets (they were mismatched, 2.76 and 2.66), main jets (the manual has 132, but the carbs had 135 - probably not much difference, but I figure why not), floats/pins and float jets. After I get everything put back together, I'm going to take it to a local inmate who has graciously offered to help me with the carb balance and timing. If things haven't improved after that, I will head down to Redwood City to seek Greg's expertise.

As for some other guesses--- I also thought maybe gas was evaporating somewhere but that's not the case. Still have a full tank of gas since my last fill up. (And this is why I haven't checked the petcock. But I do have a plastic inline fuel filter downstream from the petcock and it seems too be fine....)

Best, Greg

What does a worn jet look like.? I was about to stuff one of my high mileage ones under the microscope and have a look.

Sometimes you can see the ovaled out jet. The needle under a scope is very telling.

Gee all the parts I have on the shelf. Didn't even use them all when I rebuilt two pairs of carbs today ;-)