40 miles to a tank? Help!

[hwb120240]

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

[J_C]

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

[hwb120240]

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

[DanTheMan]

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

[Mojoe]

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.

[hwb120240]

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.

[SqDancerLynn1]

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.

[fgh]

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?

[JHawkins]

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

[flyingace]

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.

[t3rmin]

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

[5RON5]

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....

[hwb120240]

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.

[flyingace]

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.

[t3rmin]

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