poor gas mileage, no reserve

[aujax]

Hi,

I have a '79 gs750E. It is getting about 80 miles to the tank driving in the city (about a 3.5 gallon tank I think) with a fair amount of hard accelerating - this seems to me to be pretty poor. When I worked on the carbs I had to push out the idle screws with a metal tool which enlarged one or more of the idle holes. It idles fine but I can smell some gas. I think this might be part of the problem. Also, may just be running too rich. And my bike doesn't like it when I put the fuel petcock to the regular setting - only fully open in the 9 o'clock position, so I don't really have a reserve.

Anybody have any thoughts on these two problems?

Thanks,
Aujax

[Nessism]

Replace the petcock.

[chef1366]

If you think you buggered the carbs than get some replacements off Ebay or ask in the parts wanted section of this site.

[Steve]

When I worked on the carbs I had to push out the idle screws with a metal tool which enlarged one or more of the idle holes.

Let's see ... pushing on screws instead of turning them. And,...just what 'idle screws' are we talking about?
By chance do you mean 'idle jets'? But still, which ones are we talking about?

If you have pushed on something that allows fuel through it and enlarged the hole by an indeterminate amount,
that part needs to be replaced.

...enlarged one or more of the idle holes. ... I can smell some gas. I think this might be part of the problem. Also, may just be running too rich.

Sorry, there is just a big "DUH" here. That is not just part of your problem, that is your problem.
Yes, enlarging the fuel holes will make it run rich.

And my bike doesn't like it when I put the fuel petcock to the regular setting - only fully open in the 9 o'clock position, so I don't really have a reserve.

How are you measuring this "9 o'clock position"? The bottom of the petcock lever is the pointing device. On the two petcocks I have here, 3 o'clock is PRI, 6 o'clock is ON, 9 o'clock is RES. If yours will only run in the PRIME position, the valve is stuck. The vacuum diaphragm is not strong enough to pull it open, so you need to open it up and carefully see what is stuck and free it up. However, based on your experience with the carbs, maybe you had better just order a new petcock and play with this one while you are waiting for the new one.

Oh yeah, "a fair amount of hard accelerating" can easily drag your gas mileage down to the 25-30 mpg range, so 80 miles per tank sounds fair. If you want to go farther on a tank, lay off the "hard accelerating".


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[Dave8338]

Let's see ... Sorry, there is just a big "DUH" here. That is not just part of your problem, that is your problem. Yes, enlarging the fuel holes will make it run rich.



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Steve...that was exactly my first thought. :-D

Buy a new petcock and Fleabay a set of carbs if you need them. These bikes are very touchy with their fuel issues.

[aujax]

mileage

Sorry, i should have been more specific. I meant to say that I had to push the tips of the idle adjustment screws out of the idle circuit holes after I foolishly screwed them in too tight (not enough studying of the manual before I tinkered). I tested the holes by spraying carb cleaner through them afterward to see if they still atomized the fuel, and they all did except this one, which just kind of spits out the fuel.

The petcock will not go to the 3 o'clock position. It only goes back to about 4 or 5. I opened it up once before and didn't notice anything wrong. I guess I have to check it out again...

Thanks,
aujax

[Steve]

Well, if you had to push the tips out of the holes, that means the idle mixture screws are,...well...screwed, and need to be replaced. Depending on how hard you had to push to get the tips out and how hard you landed in the hole when the tip gave way, you may have destroyed the holes, as well. Those holes are not just a precise size, there is a precise shape above the hole, which needs to match the taper of the needle that broke. You may have deformed the bottom of the taper, in which case you will need new carbs.

Spraying carb cleaner will not verify that they will still atomize fuel. It will only verify that the hole is clean and open.
The one that just spits probably still has something in it that is partially plugging it.

About the petcock...earlier you said that it will only run in PRIME, but now you say that it won't even go into that position. :shock:
Remember, it's the bottom of the petcock 'pointer' that determines the setting. Rotating the lever counter-clockwise will put the pointer to the 3 o'clock position for PRIME. Rotating it clockwise from there will take you to 6 o'clock for ON, then 9 o'clock for RESERVE. If the lever does not move to all of these positions rather easily and flow fuel at the proper times, you need to disassemble the petcock to find out why.


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[aujax]

screws

Steve, thanks for the info - well, I did replace the idle screws, and, as i said, it idles fine. The only problem with the idle is the smell of gas and that the exhaust pipe from that cylinder doesn't get hot when it's idling, only when it's running with throttle.

I was thinking of somehow using some sort of epoxy or compound that is fuel resistant to make that idle hole smaller, but, as you said, the shape of the hole is also important. P)robably wouldn't work.

I originally said that the bike would only run with the petcock at the 9 o'clock position (reserve). At prime it doesn't run and it will only run for a few minutes at the run position. I think I just need to disassemble it and figue out what's wrong...

[brveagle]

Keep in mind that not all suzuki petcocks are the same. On my gs550, 3 o'clock is reserve, 6 o'clock is run and 9 o'clock is fuel off.

[Steve]

Keep in mind that not all suzuki petcocks are the same. On my gs550, 3 o'clock is reserve, 6 o'clock is run and 9 o'clock is fuel off.

Good point, but as I mentioned in post #4, I am basing that on the two bikes I have in my garage at the moment, a 650L and an 850L, both of them from '82.

I have found that the vacuum valve has a tendency to stick if the bike has sat for a long time. Remove the diaphragm chamber carefully. Gently pry the diaphragm away from the valve assembly. The last on I worked on came apart rather hard. There is a post in the center of the diaphragm that is supposed to have an o-ring around the end that seats into the valve body. Mine had the o-ring stuck in place and the post pulled out the middle of it. Fortunately, the o-ring was still useable when I coaxed it from where it was stuck.

While you have the diaphragm off, remove the selector valve and see why it won't move through its entire range. It might have something stuck in one of the holes preventing proper movement. While you have it apart, clean the sides of the selector and the bore it sits in so the pieces move freely. You will appreciate this touch later, as most petcocks get harder to turn when they are warm.


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