1981 gs 650 gas mileage problem

[Guest]

I have a 1981 gs 650. shaft drive, 18.160 miles and just had it tuned up. It has a little bit of light backfiring when it first starts it but then it runs smooth. I burnt 3/4 of a tank while driving a 30 mile highway stretch and I kept it under 60 MPH. I installed new plugs wires, changed the oil and filter, air filter and tried sea foam, and it made no difference. Any ideas would be appreciated.......Please advise. Thx

[Gorminrider]

30 miles on 3/4 tank? Test the obvious... Maybe your fuel gauge is broken. Maybe your petcock is leaking into your oil sump.(smell the oil!) and No smoking-this bike must stink of gas....Maybe there's a pool of gas under your bike kidding, I'm sure you'd notice but that's really record-breaking bad mileage.

the startup backfiring might be the engine choking on gas due to stuck floats in the carb bowls.... I wonder if you need choke/enricher to start this bike? That'd be a clue per stuck floats, bad float needle..

[tom203]

The backfiring does sound like excess fuel being dumped into exhaust

Was the tune up intended to correct this problem? Is this a new to you bike?
driving modestly, this model will get 50 mpg easily

[johne]

the original oil i changed didn't smell of gas and there is no gas leaking and i have had several bikes in the past and never encountered this. Is there a ? adjustment screw i can try the make it more lean than rich.

[johne]

Is there a adjustment screw i can use to the make it more lean than rich if I'm getting to much fuel?

[tom203]

Do you understand proper use of this style petcock? I'll ask again... Is this a new to you bike? If so , it might need more than a tune up and seafoam... Previous owners can be very creative with maintenance.

[johne]

its very new to me, can you tell me about the style of this petcock? and if i need to do a rebuild on it. Any ideas would help.thx

[tom203]

Assuming it's still the stock petcock...... This has 3 positions: "pr ", "on " ,"res" : in "on" and a "res" ,fuel should only flow when engine is running (engine vacuum is applied to the petcock letting fuel flow). The "pr" spot lets fuel flow at all times - this is used after bike has sat idle and fuel has evaporated from carbs this spot quickly refills carb. But you don't want to leave it in " pr" when bike is not running,as carbs will likely overfill - if this happens, fuel will run into crankcase diluting the oil.

Test the petcock to make sure it is working correctly.

[johne]

I looked at a few clips on how to check the petcock valve and I'm going do it asap. Thanks for the heads up on this.

[Nessism]

Is the bike stock? Detail all the maintenance you have done? Valve adjustment? Full carb tear down and replacement of all the O-rings?