Fuel Economy on a GS750

[earlfor]

Re: Gas Milage

Hmmm, 168 miles sounds about right. To get milage that good, it cant be more than a "hair" out. My opinion is that its better to be getting something in the 45 to 50 range rather than trying to lean it out to get a couple more miles per gallon at the risk of cooking an engine.

Earl

I got my 78 GS750 last week and on my first full tank I hit 168 miles before I had to switch to my reserve tank, that's about 45 mpg. But it needs a tune up, prior owner told me he was getting about 50 mpg when it was properly tuned.

[Anonymous]

Yeah, I seem to remember getting about that much too -- way back when I first got the bike. Maybe I'll get that much now without the brakes dragging, but if I don't... I have no idea how to make it better. The plugs are perfect now and I'm not racing it. The difference of 20 MPG from one bike to the next has to be more than what you'd expect -- unless it's a race bike vs a commuter.

Roger Moore
27 MPG or so on the last tank

[earlfor]

Roger
After a ride, my rotors are cold. If the brakes are dragging at all, theyre going to be pretty warm to very hot.

Earl

[quote="Roger Moore"]Yeah, I seem to remember getting about that much too -- way back when I first got the bike. Maybe I'll get that much now without the brakes dragging,

[Anonymous]

Earl,

Oh, they "were" dragging. My first trip out after redoing all the brake components was like riding a whole new bike. I never felt the rotors, but just spinning the tires by hand (before and after the rebuild) showed an appreciable change.

Weather's been kind of crappy the past two days, so I've driven to work, but we're looking at about a week of sunny motorcycle weather now, so I should have a good tankful of driving to do to see how much better it'll be.

Roger Moore

[Anonymous]

Maybe not my carburetor settings...

OK, so I took Earl's suggestion and set my FUEL screw to 7/8 and my AIR screw to 1 3/4, and I am still getting below 40 mpg. I've got the stock pilot jets in it, the clip is in the second notch from the top on the needle, and the main jets are 100's (one leaner than stock). The plugs look good - ie grayish tan. Also, I know the brakes are not dragging (I recently rebuilt all three calipers).

The bike starts OK, but it won't idle without the choke in the morning (about 55 degree F air temperature). Even after a fifteen minute ride, including about 5 highway miles, I have to keep my hand on the throttle or it will die. In the afternoons, sometimes it will idle fine (about 1100 rpm) by the time I get home (temperature has been in the 70's in the afternoons). Also, it seems to pop a lot more through the exhaust than it used to, especially when cold. The bike seems to run well around town, on the highway, and it accelerates smoothly.

So I decided to try synchronizing the carburetors. #3 was waaaay out of whack. I brought it back into line with the others, but the best I could do was about 7 inches of vaccuum across the whole rack.

Next, I did a compression test. Cylinder #1 = 115 psi, #2 = 85 psi, #3 = 115 psi, and #4 = 90 psi. I repeated the measurements to make sure I didn't mess anything up, and the results were the same. With a little oil in the sparkplug hole, the compression jumped to 170 on # 1 and #3, and 140 on the other two.

So, I think I know what all this means, but I need someone to confirm my worst nightmares.... Do I need to rebore the cylinders and fit larger pistions? Any chance I'll be able to just hone the walls and replace the rings? Any idea if this will improve fuel economy and make it less cold blooded? The bike has about 30K on it.

Actually, that isn't exactly a nightmare. I love doing stuff like that, I just hate being without
a bike while I do the work. :?

[Anonymous]

Probably just worn rings.

[Anonymous]

I'm riding a 82' GS1100E and getting around 20MPG. I have only recently begun trying to track down the problem. I just wanted to thank everyone that posted here because it's given me some good ideas and a number of things to check. I'll let you all know what I find.

Thanks all..

[earlfor]

Re: Maybe not my carburetor settings...

Brian
It seems to me that its likely all you need are new rings.

Seven inches difference in vacum between the carbs is way out of sync. They should be within an inch or two.

I thinks its doubtful you will be able to tune the engine precisely with the compression readings that far apart. Your gas milage is related to the compression, so I expect that when your compression is evened out to what it should be, your milage will also improve.

Earl

[Anonymous]

Yep... Sounds like rings to me too. You may have "stuck rings" though, not necessarily bad rings. If the bike only has 30,000 miles on it, it's hard to imagine rings being that far gone (and has it sat for some period?). My bike has about the same milage, and its compression is not that low.

You might want to start off by pulling the spark plugs and pouring a half a cup of carb cleaner (something like Berryman's B-100 chemtool) into each cylinder and letting it sit there for a half hour and then turn it over a few times with the plugs re-installed, but the kill switch in the OFF position. Then do it again. Make sure the cylinder bore are as perpendicular to the ground as possible. If the rings are "glued" to the pistons, this should help free them up. Rings should be floating in their grooves, and they may not be.

You'll need to change the oil after doing this, or shortly thereafter. A little Berryman's in the crankcase shouldn't hurt, since you use the same chemical to flush an engine anyway -- but wet clutches are my only concern. Shouldn't really hurt, but change the oil -- since diluted oil isn't a good thing.

Whether you can just re-ring (assuming the above doesn't help) depends on the actual wear in the cylinder bore. This will have to be mic'd to see what it the wear actually is. If it's within tolerances, then a good hone and re-ring should make it good to go.

Good luck!

Roger Moore

[Anonymous]

Earl says: so I expect that when your compression is evened out to what it should be, your milage will also improve.

Earl, my compression is about 150 across the board, and this fill-up I got only 30 MPG. This seems very low to me for a bike that really runs well now, and has no sign of sooty plugs and is not been "got on" much at all. I'm a pretty laid back cruizer on the bike. Most of my riding is "in town" commuting, and have yet been able to (find the time to) get it on the freeway for a tankful. Hopefully soon I will. Maybe I got spiders still living in the cylinders? Haven't synched the carbs yet, but plan to. They can't be too far off for how well it runs -- I' think. But hey, a motorcycle mechanic I don't claim to be, but it's really only the carbs on the bike that I'm foggy on.

Roger Moore

[hoyterb]

Brian Did you check the valve clearances?Too tight valves can do the same thing.
Brian H.

[Anonymous]

Brian Did you check the valve clearances?Too tight valves can do the same thing.
Brian H.

Good idea. I didn't check them because they were supposedly checked just prior to the previous owner parking the bike. It is probably worth it for me to do, because who knows if it was really done or not.

Yep... Sounds like rings to me too. You may have "stuck rings" though, not necessarily bad rings. If the bike only has 30,000 miles on it, it's hard to imagine rings being that far gone (and has it sat for some period?). My bike has about the same milage, and its compression is not that low

I think I'll give the berryman's a try. The bike sat for at least two years before I bought it, and then for another year after I bought it.

Thanks for the advice.

[earlfor]

Well Roger, I donno, something is screwy here. What year is your bike? CV's or slide carbs? I have my 750 set pretty rich, (the power is good and the engine runs cool....relatively) and I still am getting 45-46 to a gallon. Just a thought, are you running in 2nd and 3rd gear up and down hills around town all the time? Also, when I initially got my bike running, I thought it was running reasonably well. When I later got around to synching the carbs, I realized what I thought was running well, was actually borderline. The difference balancing the carbs makes is like night and day if they are very far out. The first time I balanced the carbs (when i thought it was running ok), the initial readings varied from about 15 inches to nearly 50 inches. :-) Do you have stock pipes, carbs, airbox and sprockets? There has to be a simple reason why you are only getting 30 mpg.

Earl

Earl, my compression is about 150 across the board, and this fill-up I got only 30 MPG. This seems very low to me for a bike that really runs well now, and has no sign of sooty plugs and is not been "got on" much at all. I'm a pretty laid back cruizer on the bike. Most of my riding is "in town" commuting, and have yet been able to (find the time to) get it on the freeway for a tankful. Hopefully soon I will. Maybe I got spiders still living in the cylinders? Haven't synched the carbs yet, but plan to. They can't be too far off for how well it runs -- I' think. But hey, a motorcycle mechanic I don't claim to be, but it's really only the carbs on the bike that I'm foggy on.

Roger Moore

[Anonymous]

Earl,

It's bone stock. I typically run in as high a gear as I can without lugging it. Most of my commute is flat, so I'm always in at least 4th gear and accelerate smoothly through the gears. I've been meaning to synch the carbs -- and have the mercury carb stix to do it, but need the taps for the cylinders made. Just time. I'm sure they may be out some and I'm sure I'll be surprised by the improvement, which if it's running this good now -- look out Seattle!

Thanks!


Hey Brian,

Although it's true valves can cause low compression, squirting oil in the cylinder does not typically increase compression 40% if it's valves -- that's rings, my friend, or at least related to rings. And you said you saw a marked increase when you did this, by about that much. In the long run, it may be from the long sit -- especially if the previous owner didn't change the oil. Old oil can be quite destructive to an engine. It can essentially become an acid that eats away bearings and cylinders if it wasn't changed (accumulated sulfer from gas and oil mixing in the combustion chamber turning to sulferic acid :twisted: ) Not good. But let's not think that, just yet.

Roger

[Anonymous]

Roger:

I picked up a can of Berryman's on the way home tonight. It says to change the oil IMMEDIATELY after flushing the engine, so I'm going to wait to do that. Busy couple of weeks coming up. Anyway, I'll let you know if it is just stuck rings, or if it is something worse.

Thanks for the advice.