gas mpg dropped significantly

[gogi]

1987 with 36k with sidecar was averaging 38-40 all of a sudden dropped to 21, 26.2, 31.7, 28.6 , 29 no long idling no excessive speed rig coasts fine no brake drag i thought if there is a return line do to fuel pump maybe it broke or was leaking but there is no gas smell or residue. is there a return line line and any suggestions to look for for milage drop could it be clutch?doesnt seem to be slipping.

[flyday58]

I had a slide diaphragm come loose from the slide. Was able to reglue it and it's been fine since.

 

Most here will suggest a fresh set of plugs to begin with. I've also come to realize it's such a complex motor that just 'cos it's running well now doesn't mean it'll fire up the same tomorrow. Lots to go wrong.

[Flyinfool]

First thing is see if it is running on all 4.

Start it up cold and see if all 4 exhaust pipes get warm at the same time. These bikes run amazingly well on 3 cylinders. The gas mileage is the first hint that you lost a cylinder. At least you narrow things down if you know which cylinder is the one that is nor running.

[cowpuc]

If you find she isnt running on all four by hot checking the pipes as Fool mentions, ya might wanna check your plug on the cold one first.. Got good advice going here IMHO.. These motors really will surprise ya at how well they run even when they aint feeling so good..

Nope,, no return line in these old girls, the fuel pump moves fuel till the float needles close, then the pump shuts off.. Actually a pretty low pressure system compared to an EFI..

Sounds to me like you may have lost a diaphram too but, just for kicks you might wanna double check your over flow lines coming from your carbs for dampness,, those sneeky buggers can be running fuel out under the bike while your moving without you even noticing.. Also glance at the choke levers on the carbs and make sure they are closing all the way.

Pull the cover over the air box, remove filter, start the bike, watch the slides as they respond to throttle, if you see one (or two) not responding - take a screwdriver and carefully push the non-responder open a couple times just to see if is stuck - it should open very easily and close on its own due to its spring.. If its sticky but stll moveable you can sometimes clean em without removing them by squirting them with carb cleaner and working em back n forth... Best to go ahead and pull em for a good cleaning and a diaphram check though..

If they are not stuck and not moving its a good chance you have indeed lost a diaphram but, take a real good look around the vacuum ports/lines associated with the dead slide also,, maybe got a vac leak..

[KIC]

I agree with CowPuc. He knows his crap ( I have been dying to say that)

 

Check the slides. I was battling a constant mileage problem and then finally checked the slides while running. The bike runs really bad with the air cleaner off, but they should all pull together. When I checked mine I found ONE moving freely. I had to pull the diaphragm cover and make sure the diaphragms were correctly sealing. One at a time I got the the sliders to work together. The bike almost threw me off on the test ride after that.

 

Check plug wires and caps. Keep us informed and we will give you the next step until you are back up and running happy.

[greg_in_london]

I don't know if this would apply if the change was sudden and I'm not sure that I'm the best to comment as your mpg have dropped to not much worse than I get most of the time (though my sidecar might be bigger), but...

 

Only think think about this after you have looked at the other stuff above, but...

 

The carbs will pass more fuel if the o-ring on the emulsion tube has perished - it serves to makes sure that fuel only goes along the prescribed path and some leakage will let more petrol in.

The carburation will be upset if the rubber bungs that seal off the ends of the jet block (no - I've never seen those on another bike either) have shrunk and petrol shortcircuits it too.

Finally - and I have never really worked this one out - there used to be posts on Venturers.org (showing my age) about the vacuum chambers (above/in front of the cam boxes) leaking air - and any pipes that have worked loose would damage economy.

 

Other than that, you could try using proper English gallons of petrol, which should get you 10-12% better mpg straightaway.