going nuts

[79-GS550-L]

Ok, if I give throttle *slowly*, I can give it 1/2-3/4 throttle. If I twist the throttle hard, it bogs down and will die if I don't ease up. Last night I took it out to see it's top speed... at full throttle in 6th gear I got an impressive 60mph or so :shock:. Pathetic!

I've replaced carb boot rings, sealed the airbox, replaced the petcock, battery, coils, plugs, sealed the exhaust, adjusted valves, set the timing, points gap, cleaned the carbs thoroughly in gunk dip, etc.

Today I warmed up the engine and then ran it on a single plug to do a chop test. At first it wouldn't let me give it more than half throttle, but after holding it there (around 4500 rpm) for a while, it let me give a little more, then a little more, finally I got it to full throttle for a while, cut it off and looked at the plug. It was perfectly clean, with a bright white insulator, and a silvery electrode. Looked like it was new out of the box. So I guess that's a lean condition.

I'm assuming I have no air leaks anymore as I don't have any hanging idle and I have gone over everything a hundred times. So does this mean my main jets are too small? I don't want to pull the carbs until I know I have to, as it is a REAL PAIN to get them back on without trashing the airbox boots.

Any ideas?

[Nessism]

What kind of exhaust do you have? If it's stock I can't imagine how it could be lean enough to cause the problem you describe.

[Steve]

How are you running it on one plug to do a plug chop? :shock:

Plug chops have to be done at particular throttle openings, not engine speeds, therefore must be done under load.

If you are holding the throttle wide open to check the main jet, there is no way to load it enough to keep from redlining, even if 3 plugs are not firing. Connect all the plug wires, find a mild uphill road, run the bike about 40 mph in fifth or sixth gear with the throttle wide open for about 30 seconds. If you are starting with relatively new, white plugs, you should be able to get a color read for the main jets.

Repeat the process at lesser throttle openings for the needles, then again just above idle openings to check the pilot circuit, but do it with all four cylinders operating. Check all four plugs, as one cylinder can be different from the rest and need different tuning.


.

[mark]

Do you know what your pilot air screws are set at? Pilot fuel screws? Needles? Floats? Carb synched?

[79-GS550-L]

Nessism:
AFAIK the exhaust is stock. And it's not too loud so I don't think any baffles are missing.

Steve:
I'm using the technique described here:

http://www.thegsresources.com/_forum/showpost.php?p=680831&postcount=8

So I AM holding the throttle all the way open. It does not redline, it maxes out around 5k rpm.

Mark:
Pilot screws are set at 2 turns out for air, 1 turn out for fuel... although I don't think that will matter much, as I'm trying to test the wide-open-throttle circuit only right now. It actually runs rich at idle, I haven't adjusted that yet. Needles are at the middle position (clip on 2nd notch I think). Floats were set to spec, I don't remember off the top of my head what that was. The carbs have been bench synched but not vacuum synched yet... although again I don't think that would affect anything with wide open throttle, especially if I'm running just one cylinder

I don't know what to do next

[79-GS550-L]

Whoo hoo! Fixed the problem! \\/

What was it? The ignition timing. But I just adjusted the points about a month ago. I don't know why it would have changed.

Oh well, I'm happy now. It's finally ridable (sorta). I used to only be able to reach 50 mph in 5th gear, after a long and slow acceleration. Now I can reach 50 in 2nd gear no problem :-D