40 miles to a tank? Help!

Just got a '79 GS750L running after it sat for 5 yrs. I cleaned the carbs and tank up, put a new air filter in, sealed up the airbox and she runs. Can't see any fuel spilling out: Where do I start? I know she'll get better milage than this...
Thanks guys,
Wayne

You probably want to check the plugs first. If the spark plugs are all black then its running way too rich. At least its a start

Thanks for the quick get back. I'm going to my son's house now to start looking...

Hey, Wayne,
My 850L started with really poor mileage right after I got it too. As the carbs start sealing up and things run better it gets better. I've got a 3.2 gal fuel tank, so the best I've ever had with it has been 48 mpg, 140 miles on the the tank. All "L" models came out with a small tank, but with some more riding without slamming open the throttle it should get better. :-D

40 miles to a tank is EXTREMELY low.
In fact, I think it is pretty much impossible that the engine is actually "burning" it. If you have no leaks anywhere, that means it is definately going in the engine. Are you sure your floats are all adjusted correctly? All it takes is one to not be working properly and dump way too much gas into the cyclinder. be careful of that because it washes away the oil on the cylinder wall. It could lead to a broken ring. Check your plugs. If you notice one stands out much more than the rest, that is the carb that needs attention. Or it could be all of them.

Thanks Dan. I know it'll get better, I just felt like it was way low and something is probably wrong.

Then Mojoe hit me with a good place to continue too.
'preciate it.

Did a copy paste of one of Keiths old posts A lot of this pertains to rejetting but the info is still very usefull

Keith?s post
Stock carbs (U.S.) are VM26SS. I don't know if Canada bikes got 26mm or possibly 28mm? I do know 33mm are not stock anywhere. Sounds like your bike is starving for fuel above 6,000 rpm's. The jet-needles need to be raised and the main jets increased and probably the pilot jets increased too as I posted.
Your jet-needles are inside your slides.
As for the screws...the PILOT screws are reached from underneath and are engine side of the floatbowls. Use a mirror too see and adjust them. Turning them OUT (counter-clockwise) richens the mixture. They can be tight if never moved before and it's easy to strip the head off. Use a good fitting tool. They are sensitive to adjustments and even 1/8 turn can change the sparkplug color. For a starting point regarding the pilot circuit, I would adjust each pilot screw out 1/4 turn and test. When plug reading for the pilot circuit, ride the bike a few miles in 4th or 5th gear at about 40 mph. Just have the throttle open just past "idle". Do what the plugs/performance tells you. If you find yourself adjusting these screws more than a full turn out further from where they are initially, then you most likely need a step up (2.5) in pilot jet size and of course turn the pilots back in to where you started. You may even have to try larger pilots and turning the pilots out a little. It's suggested you start with fresh plugs or at least clean yours well.
For the SIDE AIR screws...adjust as follows. Put the bike on the centerstand and warm it up first. Turn the screws on both carbs facing you in 1/4 to 1/2 turn increments until you achieve the highest possible rpm's. Fine tune these two screws. Do the other two screws the same way, until you achieve the highest rpm. Now re-set the idle to 1,100 rpm with the idle screwknob. They usually end up about 1 1/2 to 1 3/4 turns out. Turning these screws OUT leans the mixture. Careful, these screws strip easily too if never moved.
Do the pilot screws first and then the side air screws. Jet needle adjustments must be followed with a vacuum tool synch. Then test.
A good carb synch is important with these carbs and re-jetting is too difficult without a good synch. Raising the jet-needles is the most work.
Test for the jet-needles at 1/2 throttle and the mains at 7/8 or wide open throttle.
I forgot to mention, remove the two float bowl vent tubes so the bowls vent better. The increased air intake creates a vortex in the tubes and this cause fuel starvation. Removing the tubes helps the jets draw fuel easier. This is recommended by jet kit makers regarding stage 3 kits, especially with VM carbs.

I've heard of mileage that bad but never seen it. If you're sure your not leaking fuel check the plugs first. If it has a vacuum actuated petcock it could be leaking into the cylinder it taps for vacuum. That plug will be sooty. Other than that it almost has to be bad float settings that are allowing the overflow to be pulled into the cylinders. Also check the oil level. It can be getting into the crankcase. Has the oil level risen or does it smell like gas?

Just curious, when you say 40miles to the tank, how much fuel are you putting in when you fill up?

As suggesed, look at your oil. Your gas may be in there.

If not, your plugs are your next stop. Such bad mileage may indicate that 1 or 2 plugs aren't firing. If they aren't, they will be fuel wet when you take them out. If the problem is an overly rich mixture, they will be heavily caked with black soot.

Check your timing. I was getting crapppy mileage while my timing was 20 degrees off. ;-)

Poor mileage!

Poor mileage!

I too get the same bad mileage as the man w/40 miles to a tank! I also have a 750L-'79; w/16,900 on it. I too get about 19 to 21 MPG. The book says that new, they were getting I think, some 42MPG then. I do not know what to do either about it. I do have 1 plug w/a somewhat black sooty plug. But my problem may be w/the baffle being spitted out of my right side muffler, which I have had no luck finding these past 3 weeks! I am at a loose as to what to do about it. I can not afford to buy new. So I can't even find a baffle to go into it, w/o knowing the internal size I could get. So w/o that, I can't even get a baffle & somehow make that work. So if anybody knows of the size I can use for a baffle, I saw some on ebay pretty cheap. That hopefully will work on mine to prevent me from ever getting new ones. Sorry to interrupt the thread, but it is on the same lines as the gentleman who started it. I am waiting to do a carb adjust & sink till after I get my homemade sync made. Has anybody else made one that was posted awhile back? If so, please let me know how it worked, will ya? Or has anybody know what the I.D. of the baffles are so I can get one as close as the one that Suzuki used, that it may just do the trick. I know I would have to have it welded back in. But I think that would be about all I could afford to do. It does not run as well as it can. It pops all the time & just is not right! Please get back to me or please post some numbers for the baffle size I can use to get another. Thanks...Ron....

SqDancerLynn1: Thank you for the base line settings! My manual does not give them and I never found them on here.
FGH: I had the float problem on my Madura and filled the crankcase, so far that is not happening here. All the plugs were too dark and had soot. I will try to adjust the carbs tomorrow night.
JHawkins: Three gallons!
Flyingace: I did have one bad plug after we got her running. Changed the plugs across the board and all were sooty to some extent. Now that I have a starting point for the carbs maybe I can gett a little further along on this resurrection!
Thank you to all,
Wayne

BTW: I'll post results as soon as I get over to his house and make some changes.

4 sooty plugs indicate an over-rich mixture that could explain your bad mileage. Look at your air-filter and make sure it's cleaned up. Next, your carb float levels, then the integrity of your floats, needles and seats. No gas should leak out when you push the needles into the seats.

I had sooty nasty plugs when my timing was off even though I wasn't running rich or using oil.

t3rmin said:

I had sooty nasty plugs when my timing was off even though I wasn't running rich or using oil.

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Well, then don't ignore this possibility!

'79GS750Lman said:

JHawkins: Three gallons!

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HOLY FU*&^%*&^(*&!!!!!!!!

Dude, that's a lot of fuel!

On the moosehead ride we were going between 80 and 110mph pretty much constantly and my 79 750L got 33mpg!!!!

I have my doubts that the jets could even flow that much fuel. and if it did, the bike would be so rich I can't imagine it running. Even the petcock would have trouble with those flow rates I would think!

'79GS750Lman said:

Just got a '79 GS750L running after it sat for 5 yrs. I cleaned the carbs and tank up, put a new air filter in, sealed up the airbox and she runs. Can't see any fuel spilling out: Where do I start? I know she'll get better milage than this...
Thanks guys,
Wayne

Click to expand...

If the petcock is not opening properly then you will feel like you are running out of gas,
Is there fuel left in the tank when you go to fill it up?

'79GS750Lman, that old post Lynn put up is in response to another members problem. We were progressing and I suggested he make some additional adjustments on his pilot fuel screws, among other things.
The factory sets the pilot fuel and side air screws with expensive equipment. You can re-set these screws to near factory with some patience. If your bike is stock, the pilot fuel screws (underneath) can vary from about 1/2 to 1 1/4 turns out straight from the factory. If you have no idea where yours are at I'd try 1 turn out from LIGHTLY seated for starters. Use a good fitting tool to avoid stripping the head of any tight screw. Initially set the side air screws at 1 1/2 and then set them using the highest rpm method. They generally end up about 1 1/2 to 1 3/4 turns out. Again, be careful of stripping. If never moved, the air screws can be very tight. These adjustments will at least get you in the ballpark and any serious poor fuel mileage will not be the fault of these initial adjustments. You can fine tune the screws as necessary when the bike is running well which will probably require other tune up work.
Other areas in the carbs to check regarding poor mileage is the needle jet/bleedpipe (for clogged holes), dirty air jets at the carb throat/filter side, incorrect float levels (should be about .94"), loose jets, incorrect jet needle e-clip position (if PO changed?), yours should be at position 3 if stock, jets changed and now too large? Worn float needle valves can cause rich problems. Needle jet o-ring failing, etc.
After the carbs are set up correctly, at least bench synch the slides. I made a detailed post awhile back how to do that if you search? Once the bike runs better, vacuum synch with a vacuum tool.
You also must check the spark/ignition. Correct plugs? NGK B8ES. Gapped correctly.
Are the points clean and closing squarely? No pitting and uneven contact? Dwell must be set and then the timing. Check timing advancer action. Check all plug cap connections and coil terminals too. Check ALL connections for clean and tight. Check battery voltage and charge/fill with distilled water if needed.
Let us know how it goes.