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Low gas mileage, low power in high gears

  • Thread starter Thread starter klylor10
  • Start date Start date
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klylor10

Guest
Im in the midst of a long trip and am experiencing a recent drop in gas mileage and what I think is a decrease in power in the higher gears and I'm hoping one of you may have some suggestions.

some context: I have an 82 GS850G with very low mileage that I rebuilt (see my thread in the GS stories about a big rebuild and big trip, no ability to link to it right now, sorry). When I first did the rebuild I was getting around 36 mpg, and felt a good deal of power in the higher gears.

i've gone 3,000 miles, changing both drive oils 1,000 miles ago and the engine oil just a few hundred ago. I'm currently in southeastern Utah, where the elevation is around 4,000-7,000 feet, and I'm doing a bit of mountain climbing/falling. I'm running 87 regular, but no ethanol when I can find it, which hasn't been for quite a while. In the past few days I've noticed a what seems to be a decrease in power in gears 4 and 5, and I'm only getting about 30-33 mpg, possibly less on this most recent tank. From what I've been reading this gas mileage is pretty low for this bike, I'm seeing numbers from y'all around 40 or so.

any ideas what could be going on and how I may remedy this while away from home? Dirty air filter? Running too rich?

thanks!!
 
Suspect the petcock first - fuel might be dribbling down its vacuum line causing power loss. Stick some vinyl tubing on petcock vacuum fitting and suck- no fuel allowed here. If so , you can always plug vacuum fitting on #2 carb and run bike on "PR". Of course, you would need to remember it back to "ON" when engine wasn't running.
Functioning CV carbs should be able to cope with the elevation.
 
The elevation in Utah shouldn't effect anything except power, it should still run correctly. You have CV carburetors which compensate for elevation changes very well as long as the intake system is either stock or if it's been modified it's been tuned very well. There will be less power since there is less air to burn but the CVs should reduce the fuel to go with that less air, it should run smooth and nice with proper throttle response. If there is any missing or popping or obviously incorrect carburetion going on something isn't working right. At 4,000 - 7,000 it will be very noticeably less powerful than at sea level. At 7,000 an 850 will be about like a 650, there's no way around that with a normally aspirated engine. You will need to adjust the idle if you care about that as the idle circuits are not compensated for elevation, the higher you go the slower it will idle unless it was way too lean to begin with.
Fuel economy if you are just cruising should be the same or even slightly better that at sea level. The exception is if you are whipping the heck out of the engine to make it accelerate like it does at sea level, this will suck a little more gas. if your mileage is in the thirties you have a problem.
But is this problem new or has it been there a while and you are just noticing it because the engine is making less power?
What was your gas mileage before you got to Utah? A well tuned 850 will be mid to high 40s while cruising at moderate speeds, some riders maybe 50 mpg. If you are cruising at 100 mph it will be a lot less. Again whipping the heck out of the engine uses more fuel, but most people don't do that too much on road trips.....

Tatu may be onto something with the shims, did you adjust the valve clearances recently? Synch the carbs too? What other maintenance?
 
I would pull the plugs first and see what it going on. It sounds like it was possibly running a bit rich to begin with at 36mpg. It might be as simple as a mixture screw adjustment. If the plugs are a black as night I'd throw in a new set and do a plug chop to see what's going on in there.
 
Thanks for the info.

Tkent: I checked the valve clearances 3,000 miles ago, and everything was between 0.002 and 0.0015 inches, in the sweet spot from what the manual recommends. As far as the problem goes, I suspect that it has been going on since I got it. I have never gotten above 40 mpg on this machine, and it has very low mileage on it.

i synchronized the carbs and did some airbox work 3,000 miles ago. See the list the page that Larry posted (Thanks, Larry!).

azr: I did have some trouble figuring out exactly how far in/out to turn my mixture screws when I balanced the carbs, so it could be that they're too rich. I took a peek at one of my plugs and it looks pretty dark. Any suggestions on where to get plugs other than from a dealer? It might be a bit before I can get new ones.
 
Use the old plugs, they will clean right up once the mixture is set correctly. If they are old and wasted any auto parts place carries them, NGK B8ES is used on some cars. They won't have a cross reference and I can't remember which cars, but they have them. Quite often poor fuel economy is the result of a lot of little things acting together, brake drag, oversized or underinflated tires, improper timing of ignition or cams, weak spark, worn drive chain or sprockets, partially blocked exhaust, bad wheel bearings, improper jetting or even idle mixture, there are literally dozens of factors that can have some detrimental effect, their effects seem to multiply, not add.
 
klylor10 said:
I checked the valve clearances 3,000 miles ago, and everything was between 0.002 and 0.0015 inches, in the sweet spot from what the manual recommends.
Click to expand...

I have not seen any manual that has a "recommended sweet spot". :-k

Loosen your valves by one shim size all around. Yep, run them on the LOOSE side, not the tight side. As they pile on the miles, they only get tighter, so you might be flirting with disaster there. This is especially true of low-mileage engines. Although the book says 0.03-0.08mm (close to 0.001-0.003"), many of us will run up to 0.10mm (0.004") on clearance with great results.

On a side note, is the bike getting a bit harder to start when it's cold? Another symptom of tight valves.

.
 
Any tips on adjusting the idle mixture screws? I initially did it using the method on bikecliff's website (I've forgotten the name, but it involved listening for engine sounds while backing the screws out). My hunch is that I made them a bit too rich, which would make me think the screws need to be turned out slightly to allow more air in. Does this sound right?
 
Steve, thanks for the info. I got the "sweet spot" from the Clymer manual that recommended 0.0012 to 0.003.

Hearing that they get tighter with wear is a bit unnerving, as I was told by a different forum member that they only get looser with wear. I'm 1,000 miles from a place where I can do work on this unless I take it to a dealer.

Do you think I should take them to a dealer ASAP to get the valves done and do the rest of my tune ups when I have a shop in 1,000 miles? I have no idea what the cost would be for that...

oh, it doesn't seem to be getting harder to start when cold.
 
klylor10 said:
Any tips on adjusting the idle mixture screws? I initially did it using the method on bikecliff's website (I've forgotten the name, but it involved listening for engine sounds while backing the screws out). My hunch is that I made them a bit too rich, which would make me think the screws need to be turned out slightly to allow more air in. Does this sound right?
Click to expand...

Nope, it's the other way around. They are not air screws, but rather A/F mix screws and turning them out allows more gas in.

Your stock settings on this screw are around 1 1/2 turns out from lightly seated. For many, that proved to be way too lean.
 
Yep, fresh from the factory, most of the carbs were in the 1 1/2 to 1 3/4 turn range on the mixture screws, but over time and with the different gas formulation today (and the desire to run properly, not necessarily to meet EPA standards), most of us have ended up between 2 and 3 turns out. Staring with a richer setting (3 turns) then leaning it out is eaiser than starting lean.

If the engine is still starting easily, don't worry about the valves until you get home. Most dealers won't even let your bike in the door, let alone know how to work on it, so I hope nothing major happens on the rest of your trip.

.
 
klylor10 said:
Any tips on adjusting the idle mixture screws? I initially did it using the method on bikecliff's website (I've forgotten the name, but it involved listening for engine sounds while backing the screws out). My hunch is that I made them a bit too rich, which would make me think the screws need to be turned out slightly to allow more air in. Does this sound right?
Click to expand...

No, on your bike with CV carbs, the more you turn those screws out, the more fuel/air mix goes in, so you are making it richer. Starting point is 2.5 to 3 turns out from lightly seated. Turn in slowly in small increments, pausing between each small turn, and look for highest idle speed. You may end up around 1 3/4 to 2 1/4 turns out.

However, idle mixture screws only affect engine operation to about 1/8 to 1/4 throttle opening. Thereafter the needle position affects midrange throttle position, and finally only main jet circuit around 3/4 to full throttle.

You are actually better off running rich rather than lean. Although running rich could result in poor gas mileage, your engine will actually run cooler compared to a lean mixture, which could lead to engine damage.

Incorrect float height setting could affect fuel consumption, as will non-standard jets. Unfortunately these are not really practical to attend to while you are on your road trip.

Good luck and enjoy your adventure!
 
No, turning them in give less fuel in the mixture, turning them out gives more fuel in the mixture.

They are not airscrews, they are mixture screws.

This backwards thinking could be a lot of your mileage problem, but it will have gotten richer at the higher elevations anyway. Remember the pilot circuit is not elevation compensated like the others. If you had it a little bit too rich at home it would be a LOT too rich now. Suzuki set them very lean which doesn't hurt anything, it's only used at very low throttle openings, as long as it idles well and has good throttle response just off idle it's rich enough. Nothing to be gained by going richer, unlike the circuits that are in use when the throttle is open. Those circuits being too lean will burn stuff up like valves or even pistons.
 
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I turned in the mixture screws 1/4 turn each, which puts them at around 2 turns out total. I'll see if this has an impact on my MPG.

As far as valves go: that's good to hear that I don't need to worry so much right now. The challenge is that I won't be back home for another 3,000-5,000 miles, so all this maintenance is going to have to happen somewhere on the road. I think I've got a place and tools lined up in TX, and time to hang out while I wait to order gaskets or other parts.

I'm headed back out into the woods, but I'll check back in soon. Thanks again for all the help everyone.
 
No, screw each of them in until it starts to slow down a little, then back out a tiny bit. Tune each cylinder separately to idle correctly. Do them all individually, don't worry about the number of turns out you end up with, it does not matter. The number of turns is just a ballpark starting point to get it to idle well enough to adjust it.
 
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