Huge fuel usage. 40 MPG on a GS400?

[MrZig]

How come I'm getting 40 MPG at best on a 1982 GS400 engine?

With stock jets, 70% highway, and 30% backroads. 5000 RPM cruising speed, 5-7k rpm on backroads.

I think that I should be getting 50-60 mpg, shouldn't I? My pilot screws are turned out 2 turns each. I think my jet needles may be too rich. The book (Haynes) calls for them to be at the fourth notch down, but that might be wrong? When I'm cruising at a steady low throttle position and roll on the gas, I also get a slight bog before it picks up. Too rich?

[BassCliff]

Hi Mr. MrZig,

Yeah, that's not too good. How's your air filter? Not over-oiled I hope. What do the plugs look like?


Thank you for your indulgence,

BassCliff

[sv650]

Due to my inexperience with carb tuning, I purchased a colortune plug. It made the determination of settings and adjustments clear enough that even I could understand the effects adjustments made. I ended up replacing the jets to finally get my carbs balanced, as there was no way to adjust them to get a clean burn. New jets -- easy adjustment.

Apparently a previous owner cleaned the grundge off with a wire, rather than carb cleaner!

[MrZig]

My air filter is fine. What could cause such a huge drop in mileage?

I havn't synced the carbs, could that do it? The pilot screw is 2 turns out like I said - that couldn't effect the mileage THAT bad could it?

The only thing I know I changed to make it richer was the jet needle. I made it use the 4th clip down which haynes says to use and changed the holder to match a diagram.

If it's running rich on a circuit, could I tell from a plug chop?

[Dave8338]

My air filter is fine. What could cause such a huge drop in mileage?

I havn't synced the carbs, could that do it? The pilot screw is 2 turns out like I said - that couldn't effect the mileage THAT bad could it?

The only thing I know I changed to make it richer was the jet needle. I made it use the 4th clip down which haynes says to use and changed the holder to match a diagram.

If it's running rich on a circuit, could I tell from a plug chop?

Yes a plug chop will tell you where it is running rich. Keep the RPM's to the 3500 range and run it a mile or two. Hit the kill switch and pull the plugs. This will tell you what the pilot circuit is doing. Next, run the RPMs up to your crusing speed hold it there for a few miles and kill it and check them. I would suggest a real good plug cleaning first so you can see the change in the plug color. New plugs would be even better yet.

Why did you bump the jet needles?

[srsupertrap]

I would synch the carbs first and see if that changes anything, probably won't make that big a difference in mpg. I would also set the needles to the factory position per the sticky thread "Carbs specs per OEM specs" unless you have made some performance changes. Last spinning in 5-7k will increase consumption. The GS400 redlines to 10K or higher?

[MrZig]

I set the jet needle to the fourth clip down because Haynes says this:




Is it wrong?

I also turned my jet needle holder upside down from what it was (which raised it) to match the carb diagram. Look at this thread for that: (used position A)

http://www.thegsresources.com/_forum/showpost.php?p=1065452&postcount=5

[Nessism]

That bike should get 50 mph easy. Sounds like something is wrong. Did you adjust the valves and check compression? What do the plugs look like? Dark brown? Maybe the petcock is leaking? Not sure but I'd check all the basics including verifying the jets are correct for the application.

Good luck

[MrZig]

That bike should get 50 mph easy. Sounds like something is wrong. Did you adjust the valves and check compression? What do the plugs look like? Dark brown? Maybe the petcock is leaking? Not sure but I'd check all the basics including verifying the jets are correct for the application.

Good luck

Valves are adjusted. Compression is unknown. Plugs depend on throttle position, petcock is brand new, all jets are correct. The only thing thats iffy is the jet needle.

[Nessism]

Valves are adjusted. Compression is unknown. Plugs depend on throttle position, petcock is brand new, all jets are correct. The only thing thats iffy is the jet needle.

Well, lean out the jet needle then and see how it runs and check the compression. If compression is low all bets are off.

[67fire]

Eric

[MrZig]

Since I don't have a compression tester (yet) I'm going to change the jet needle holding plate back to what it was (position B). It seems more natural in that position, it also makes it stick out farther and in effect be leaner.

Also, one of the holes in the plate lines up with the screw hole to keep it in place, and the other hole should line up with the hole in the bottom of the slide, correct?

I got something to report now though. When I took the slide covers off, the top of the vacuum diaphrams were all shiny and smelled of gas. Is that normal? What does it mean?

[MrZig]

Okay so I put the needle holding plate in the other way to lean it out and I'm still getting 38-40 MPG. I don't know what to do here.. Maybe it's running extremely lean?

[donimo]

I get 40mpg 100% city, so i am not sure if 40 mixed is all that far off. I have rebuilt and synched the carbs on mine, the only unknown is jetting due to the 2-1 exhaust...

[doctorgonzo]

Oh to get 40 MPG. I average about 25.