Gas Mileage/Performance

[Guest]

So I'm inquiring as to why my bike is getting such poor mileage. 25 to 30 at best. All my driving is very short distances. Longest I've got in one stretch is 10 miles tops. I've searched several of the threads regarding this issue and I haven't really gotten any strong answers yet. All other's issues seem to stem from the bike running poorly, idling bad, etc. My bike runs great! I have a GS750L. Everything on it is stock with the exception of the exhaust. It's still a stock exhaust, but it's off a 750E. It's basically the same thing, except a bit longer and slightly bigger mufflers. All carb settings should be stock. I am in the process of making sure the jets are stock. Contacted the po and he is getting back to me currently. I am assuming they are. No leaks anywhere boots, airbox, etc. All that has been doublechecked. New petcock. Seems there is fuel vapor (so to speak) seeping from the vacuum line on the petcock to carb 2 where it connects. Dunno if that is normal. Carbs were completely disassembled and cleaned. replaced all the o rings. Valve clearance was in spec. Carbs were synced. I will post a pic of my plugs as soon as I can. From what I am seeing, I think they are running rich, but I could be way off base. Currently my mixture screws are set to 2. I did fix one problem the po missed. I noticed that exhaust pipe#1 was not heating up nearly as fast as the others. The hole in the float bowl where the pickup tube goes was totally clogged. Tried to clean it and soak it. No amount of cleaner in the world would work. Replaced the bowl with another I had. Heats up great now. I'm guessing that wouldn't have messed up the mileage. Any ideas based on what I have written? Suggestions on where to go?

[1980GS1000E]

I would check the vacuum line from the petcock again. It should not have any gas at all in it. If gas is leaking past the petcock diaphragm, that could affect mileage. Also, ensure that the carb vent tubes are not too long and end pointing rearward under the back of the tank.

[platinum2]

I would check the vacuum line from the petcock again. It should not have any gas at all in it. If gas is leaking past the petcock diaphragm, that could affect mileage. Also, ensure that the carb vent tubes are not too long and end pointing rearward under the back of the tank.

this is true! also if the diaphragm is leaking,it will get worse and the motor will flood and become hard to start, i went through a couple of these on a old bike i had, i hate the prime/diaphragms i dont know why the factory ididnt put in a pressure relief valve on the tanks, the pressure builds up ,and blows the diaphragms, i see bikes in racing have a relief valve on the tanks,but not a street bike you use daily

[DanTheMan]

this is true! also if the diaphragm is leaking,it will get worse and the motor will flood and become hard to start, i went through a couple of these on a old bike i had, i hate the prime/diaphragms i dont know why the factory ididnt put in a pressure relief valve on the tanks, the pressure builds up ,and blows the diaphragms, i see bikes in racing have a relief valve on the tanks,but not a street bike you use daily

The pressure shouldn't build up in your tank if the tank vent (in the cap) is working properly. These tanks aren't sealed pressure vessels, there's a built in vacuum/pressure relief in the caps. make sure the vent hole in the filler neck is open and the ball check valve in the cap is intact and working.

[kawfeedave]

Just a thought - if all of your trips are less than 10 miles, and you live in chilly MN, think about the amount of time you spend with your bike warming up compared to the amount of time you spend riding.

All the gas you burn during warm up will (seemingly) reduce your milage, and the shorter your ride, the bigger the reduction will seem. If you run at high idle for 2 minuntes to warm up everytime you go 5 or 6 miles down the road, it's going to add up.

Also, I can only imagine that your short rides aren't all freeway, so every stop and go will also knock your milage down.

If you really want to 'know' your milage, you need to fill up the tank next to the freeway, hop on and ride for an hour (or close), then fill it up again and compare the distance you rode to the amount of gas needed to refill the tank.

[DanTheMan]

Just a thought - if all of your trips are less than 10 miles, and you live in chilly MN, think about the amount of time you spend with your bike warming up compared to the amount of time you spend riding.

All the gas you burn during warm up will (seemingly) reduce your milage, and the shorter your ride, the bigger the reduction will seem. If you run at high idle for 2 minuntes to warm up everytime you go 5 or 6 miles down the road, it's going to add up.

Also, I can only imagine that your short rides aren't all freeway, so every stop and go will also knock your milage down.

If you really want to 'know' your milage, you need to fill up the tank next to the freeway, hop on and ride for an hour (or close), then fill it up again and compare the distance you rode to the amount of gas needed to refill the tank.

There aren't any freeways even remotely close to International Falls, MN. However, I agree the short rides destroy gas mileage. If all I'm doing is riding to the next town 9 miles away, my mileage sucks, but then again, if I'm riding into 25mph headwinds at 70 mph, my mileage sucks. On a good day, with no wind and very little idle time and if I don't twist my wrist too much I can actually push the 40 mpg average.
Generally though, when I figure my mileage up gas fill-gas fill it's somewhere in the 30s, high or low.

[thkbaron]

Yeah I think I'm going to take it on the highway, head south for a bit and then turn around and see what I can manage and then go from there. I'll take the cap apart and make sure that's in good shape too. can't hurt. As far as the vent tubes, they are routed according to the book and should be stock. Yeah I don't know why the petcock would be bad since it's brand new. I'll pull it off when I run the tank low and check it out.

[shinzon1]

Most of my driving is done in the city and I only average about 25mpg but out on they hwy I average between 40 and 45mpg.

[thkbaron]

I was leaning towards that thought too. I'll hop on the highway when the sun starts shining again. I hope it's that simple!

[tkent02]

There is a lot of difference in mileage between using fuel with no alcohol and using fuel with a lot of it. You can test your alcohol content yourself at home, it is often much higher than what is listed on the pump.

[trippivot]

I was reading thru this thread and would like to add a couple factors beyond the carb realm because you may have the best running 750 on the planet but if the frame has resistance to movement you are wasting gas to overcome friction.

incorrect tire pressures

Old wheel bearings and crusty hard grease inside the axle areas. colder equals more restriction when it comes to grease.

slightly dragging brakes on any of the rotors will also eat your economy

slightly slipping clutch

wind resistance and rider weight are last on my list

smaller engines have to work harder - and more work takes more fuel to do the same job.

[thkbaron]

all of those are going to be a non issue for me. new wheel bearings. all rebuilt brakes. 160 lbs and short. haha

[gearhead13]

I guess I dont feel too bad about the 22mpg I got with my last tank. Most all city riding and lots of throttle at times

[thkbaron]

I guess it's good to hear that a few other people are getting mileage like that with similar riding. This bike has been sitting for quite some time too so I'm sure things will improve a bit the more I ride it.

[koolaid_kid]

As a counterpoint, just checked my GPz 750 (a GS750 8-valve motor is a copy of this moter). Fully tricked out, 810 kit, pipe, pods, Stage 3 kit. Around town 36 mpg. Not too shabby.