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Engine dies under throttle ('79 gs550)

  • Thread starter Thread starter cheebah
  • Start date Start date
C

cheebah

Guest
Hello folks,

new to this forum but not at all new to motorcycling. My previous bike was a 87 honda magna and she gave me enough headaches to learn me most regular maintenance tasks. Now I have this ol 79 gs550L that is puttin me through the ringer again.

Skip to bottom for long story short

I got her before winter and though she ran on only 3/4 cylinders she managed to lope around town well enough. I only rode her a handful of times before puttin her up for the season. I then began working to achieve a sound running motorcycle. I cleaned the carbs and assessed the ignition system. Ignition system was good and after re installing the well adjusted carbs (floats, screws and bench synch) number 2 cylinder was brought back to life. The ol gs idled like a new born kitten but there was a catch. Under approximately one quarter throttle the engine immediately would cut out. Throttle response improved as the pilot air screws were backed out leading me to believe the mixture was extremely rich. I pulled the carbs again to confirm float settings and pilot fuel screw settings. No improvement. Any idea why I would have such an excellent idle and yet such an sudden loss of power at about a quarter throttle?

Long story short:
-great idle
-all cylinders go
-complete loss of power at one quarter throttle
-pilot fuel screws: 1 turn out
-pilot air screws: 1.5 turns out
-floats: 24mm

Many thanks in advance!!!!!
-Doug
 
How did you clean the carbs? Did you just spray the jets down or did you fully strip each one down, soak it and the metal bits for 24 hours, and then put back together with new orings (after spraying carb cleaner and compressed air again through all orifices)? If not, go to Basscliff's site (he's away for Lent right now), http://members.dslextreme.com/users/bikecliff, and read through his information and then do some searching here on other topics you want answers to.

Make sure you also replace the orings behind the intake boots as well as the boots themselves if they are getting pretty hard.
 
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Carbs were soaked and thoroughly cleaned with spray and compressed air. Boots are good. Have not checked manifold o-rings, however, even if they were bad I don't think it would cause this kind of issue. Again the engine instantaneously dies when throttle is approx. one quarter open. It is not a gradual death it is sudden and violent. I thought perhaps the main jets had been clogged somehow but a double check proved them to be clean as a whistle. Not to mention the plugs are plenty wet. The only think I can think of is that the sudden influx of fuel as the main jet is opened leads to an overwhelmingly rich mixture and that I may not be getting enough air. I have a brand new lightly oiled k&n installed so no filter problems.
 
yes it's oem style. For the record I have attempted running her without the air filter. Just runs worse or not at all. Interestingly it ran b4 winter with no air filter, missing boot clamps, no. 3 cylinder not firing at all, unclean carbs, a broken pilot fuel screw, and worn o-rings all around. Explain that haha!
 
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cheebah said:
Carbs were soaked and thoroughly cleaned with spray and compressed air. Boots are good. Have not checked manifold o-rings, however, even if they were bad I don't think it would cause this kind of issue. Again the engine instantaneously dies when throttle is approx. one quarter open. It is not a gradual death it is sudden and violent. I thought perhaps the main jets had been clogged somehow but a double check proved them to be clean as a whistle. Not to mention the plugs are plenty wet. The only think I can think of is that the sudden influx of fuel as the main jet is opened leads to an overwhelmingly rich mixture and that I may not be getting enough air. I have a brand new lightly oiled k&n installed so no filter problems.
Click to expand...

2 things

The main jet doesn't "open" . The needle moves out of the way

The main jet isn't operating at quarter throttle, you're on pilots and cutaway

No new manifold O rings? How many of the Top Ten have you skipped?
 
Thank you. I understand how the main jet operates, open just seemed like an easier way to describe it for instance you wouldn't say "could you please move that door out of the way? Thanks". I guess semantics are important in text based communications though. Wasn't sure when the main jet was being used, just speculation based on the symptoms.

As for the top ten: The Bold is what I've done or what I've checked and found to be operable.

1. Intake O-rings (install NEW OEM or Viton only - common nitrile O-rings will quickly deteriorate from heat)
2. Intake Boots (install NEW -- these cannot be repaired)
3. Valve clearances (more important than most people think)
4. Carb/airbox boots
5. Airbox sealing
6. Air filter sealing
7. Petcock (install a NEW one)
8. On '79 models, install new points or Dyna electronic ignition (or at least verify that the old points are working correctly)
9. On all models, it's fairly common to have problems with the spark plug caps. These are $3 or $4 each, and often worth replacing if you're keeping the stock coils/wires.
10. Stock exhaust with NO leaks or holes -- good seals at the head and at the junctions underneath.

The things I have not addressed on this list don't seem likely to be the culprit. If any one of these things could cause this issue I would love for someone to elaborate so that I may further my understanding of the operation of this motorcycle.

Thank you,
Doug
 
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Looks like you're on your way to having it all sorted out, based on what you've done so far.

Adjust the valves and replace the intake o-rings. PM me if you need help sourcing them, since Robert Barr doesn't stock them. I replaced the o-rings on my '79 550 with some Viton pieces I found online.

I'm thinking that whatever issue you're having came on slowly, and the previous owner just kept making minor adjustments to the carbs to make up for it. You know, you could just drag race it. Full throttle, all the time. No problems then.
 
I think I will do a valve adjustment today. Even if it isn't my issue It definitely needs done. I think you hit the nail on the head with the previous owner. Weird thing was that all the pilot screws were in factory position, I think he was using black magic to get this thing to run!

11) Pay way too much for some neglected old junker, spending a few months and paychecks learning how to work on a few simple old mechanical things.

I may have made this mistake!! I am officially spending my food money on this thing! I'll have to hit you up later for them o-rings as I simply have NO money for them.
 
Just checked my email. You need a -122 o-ring. Pronounced "dash 122". Dimensions are 28.2mm I.D. with a 2.6mm or slightly larger thickness.
 
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