couple fun questions

[khardrunner14]

The bike: 78 gs400
The story: carbs have been disassembled, cleaned and everything has been gone over measured and set. Dyna-S ignition with new coils, new plugs, new plug wires. Timing is perfect (or as close to it as possible).

Question 1: Whenever the choke is on, the left cylinder cuts out until about 4000 rpms, then it fires sporadically. What's the deal?

Question 2: The bike doesn't want to idle, even when it's warm, unless I have the air screws turned at least 4 full turns out (no matter where I had the idle adjustment set). I tried to set the number of turns using the highest idle method and it seems to work ok there. The interesting thing is, i could keep turning the left air screw out even more and the idle would keep rising (though not much after 4 turns) even to the point of pulling it out. I checked the plugs several times and finally they seem ok (not lean). Should I just leave it, or do I have a bigger problem somewhere?

Question 3: The bike hesitates at full throttle, especially at lower revs. Then, it seems to jump a bit as it catches up to where I turned the throttle. Any ideas on this one?

Sorry for the long post, but I tried to sepparate it out for you. Any ideas??
Thanks!

[uudfourty]

Question 1:
I dunno.

Question 2: Valve adjustment?

Question 3: Mine does that too. I'd be interested to see what someone else says about it. I just kinda thought it was the power band since I had no GS basis for comparison. Mine gets a lot better after it's warmed up a lot. Like 30 minutes of riding warmed up.

Good luck.

[catbed]

your bike is running really rich. four turns out for the air screw is a lot. try seating the fuel screw and then back it out 1 turn and then seat the air screw and back it out 1.25 to 1.5 turns. it should help the stumbling when you get on the throttle too.

[trippivot]

Float height adjustment is off allowing too much fuel in the bowl
start there. because you have more than 1 problem to cure. when youy get the fuel under control message with update on new symptoms

[khardrunner14]

Fuel screw?? Are you talking about the idle screw? I'm not sure which one you are talking about.

Also, I pulled the plugs several times at idle (actually just after the bike died since it wouldn't idle) and found that the plugs were stark white. When I backed the air screws out I got back to a nice tan color.

Rich at high RPMs definitly fits the symptoms. Is that a float bowl problem or a needle height problem?

[Nessism]

your bike is running really rich. four turns out for the air screw is a lot. try seating the fuel screw and then back it out 1 turn and then seat the air screw and back it out 1.25 to 1.5 turns. it should help the stumbling when you get on the throttle too.

GS400's use BS/CV carbs; no fuel screw.

Regarding the origional issue, sounds like something is still wrong with the carbs. Did you pull out the pilot jets and hold them up to the light to see if they are open?

[khardrunner14]

Yeah, I tore the whole thing apart, stripped, dipped, and poked every last hole.

[trippivot]

when dealing with a fuel problem start at the tank. I know it sounds stupid ....

fuel flow is it satisfactory? no such thing as too much unless the shut off does not work.

shut off/on operate right? ok the tank is done or should be before moving on.

filter? clear or plugged??

next fuel height. at or 2~3 mm from mating surface bowl to body on carb.
approx. too little inside or overflowing the bowl when removed??

carefully remove the bowl to not disturb the fuel level in there. takes practice or try another method. many ways to do this none are perfect.

clear hose on the float drain spigot will work too- hold it up to the side and verify where the fluid level is.

ok more to go..

when you clean out the pilot circuit need to see spray or bubbles all 3 directions.

1-out of the pilot jet area
2-out of the mixture screw area
3-out of the intake side air bleed
4 transfer holes need to be clear too

check all throttle plates have same clearance in the venturi bore and controlled by the idle speed screw.

go from there because something is out of adjustment or no fuel flow

float vents needs to be clear

[khardrunner14]

Petcock leaks a bit when in the on and reserve positions...could be it. I'll save the pulling of the carbs again for after I replace the petcock.

Sounds like I have some carb adjusting to do. I feel like I do that more than I ride it!! Oh well...back to the beginning.

Any ideas on the choke question?

[catbed]

GS400's use BS/CV carbs; no fuel screw.

Regarding the origional issue, sounds like something is still wrong with the carbs. Did you pull out the pilot jets and hold them up to the light to see if they are open?

my bad, i thought since it was a '78 that it had VM carbs. sorry

[DimitriT]

Petcock leaks a bit when in the on and reserve positions...could be it. I'll save the pulling of the carbs again for after I replace the petcock.

Sounds like I have some carb adjusting to do. I feel like I do that more than I ride it!! Oh well...back to the beginning.

Any ideas on the choke question?

Maybe there's air leaking into the intake somewhere. That's would explain why your plugs are white. You need to make sure those intake boots seal up real good. Did you remove the intake boots and replace the o-rings?

It's hard to really diagnose a problem where there are so many variables. Did you sync the carbs after you put them back on? Have you adjusted the valve clearances on the motor?

[Steve]

my bad, i thought since it was a '78 that it had VM carbs. sorry

It seems that most manufacturers started using CV carbs on the smaller bikes. Not sure when Suzuki started using them, but my '76 KZ400 had them. My '77 KZ650 did not. My '79 KZ1300 did, and, like the later GS550s, they were two-barrel carbs. :shock:


.

[khardrunner14]

I bench synced the carbs, so they should be close. I'll check for leaks again tonight when I get home from work. I don't remember if I replaced the o rings on this bike or not (did them on another not too long ago).

It seems to be rideable as long as I have the air screws out 4 turns. It isn't perfect though, and I know I'm compensating for something there.

[khardrunner14]

UPDATED couple fun questions

Ran a couple tests on the bike this morning...
- warmed her up, then sprayed carb cleaner all over around the seals for the intake and....nothing. They aren't leaking.
- drove her around a bit and did a couple plug chops after giving her WOT from 3000-6000 rpms and...they were white (lean). She also popped a bit, also suggesting a lean mixture. Overall it was rideable, but it just wasn't quite right as it seemed to hesitate a bit...especially at WOT.

My thoughts...
First, lean midrange and top end at WOT would suggest I change the needle heights (I can't remember which direction) and the floats (again raise or lower?? I'd have to figure that out). My petcock is old, so it may be causing some of the trouble...though it seems to flow freely enough.

[Steve]

You definitely seem to be running lean if the plugs are still that white. Assuming you have the proper (stock) main jets, you can try raising the float height. Before you change the float height, make sure you are measuring the proper location on the float. Measure to the top of the rounded part, not the mount.


If your measurement is indeed in the correct location, lower your measurement about 1 or 2 mm. Remember that since you are working upside down, lowering your measurement will actually raise the floats when you turn them back over.

Are your needles actually adjustable? Early ones were, but later models only had one groove for the clip. Stock location in the adjustable ones is the third (middle) position. Get the main mixture running right before tweaking the needles. If you have to richen the needles (mid-throttle positions), lower the clip, which raises the needle.

Most important part of troubleshooting ... only change one thing at a time. If you change more than one, you won't know which one fixed it. :shock:



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