'82 GS550L Can't Get Past 5,000 RPM

[mikeydidit98]

I'm a fairly good 2 stroke dirt bike mechanic but have little experience with 4 cylinder 4 strokes. I am good at carbs and logic-driven diagnostics.

I have an '82 GS550L that sat for a few years in a barn. I have done the following:
1) emptied tank (was not rusted), cleaned filter and petcock (auto on/off diaphragm was intact and operations - verified while running) and verified flow, as well as clear passages on the vacuum line and static tube on tank. All tests conducted while on over-ride petcock setting (flows whether it is has vacuum or not)
2) pulled carb rack (have cleaned 4 stroker carb sets/racks before), put carb kits in all 4 (hi and low jets, o-ring, etc), removed float needle clips and left them off, used single wire from a spare clutch cable to clean/ream out primer circuit (both bowl side and brass post, including side pick-up holes on post) and verified good solid flow on this circuit (common cold and hot start issue on these old Mikuni diaphragm carbs) and cleaned every circuit on the carb, verifying flow on each one, inspected diaphragms (supple and no cracks), cleared main fuel intake line and verified uniform flow to all bowls, verified the correct seating of the main needle seats (they have to click into the correct notched position, thereby setting the correct position of the upper tang in the throat of the carb), cleaned slide throat well, as well as slide - seems smooth but of course not fast acting like a direct cable carb (speed inhibited by vacuum on diaphragm).
3) new air filter, plugs and plug wires (the good stuff, not cheap ones).

It now starts and idles correctly. It is firing with uniform heat on all four cylinders (spit sizzle test only - do not have heat gun... reminder to self - break down and buy a good $300 laser gun). I have verified there are no blockages in the exhaust system - no wasp mud nests, no million dollars stashed inside... It has appropriate power off the line, gets to 55mph fast and then bogs. It will not bust 5,000rpm or so and not go any further than the 60mph/5Krpm mark. At that point, I still have 1/4 or more throttle turn left. When forced, it bogs, not sputtering. Backing off, it immediately comes back to power, without backfiring. To me, it seems like it is hitting lean mixture at the 5k throttle position and goes further lean as you open the throat valve further (to full/horizontal position). It seems like it is not getting enough ram air to the large intake air throat, which pressurizes the underside of the diaphragm thereby elevating the slide and needle... thereby keeping enough fuel to air mixture at the high RPM end. Could the deformation of the old air box boots be causing some sort of air flow restriction to the diaphragm circuit ram air intake, thereby not pressurizing the underside of the diaphragm to completely elevate the needle?

Does this sound logical? Are there other possible issues that I am missing. This is a CDI bike, correct? It does not have a mechanical auto timing advance system does it? There is no rev limiter system on this bike is there? The CDI's on these models don't have an issue with failing and causing this limited rev condition, do they?

I am going to see if there is a way to raise the high end needles in the slides, to test the one theory. Should I replace the slides/diaphragms and slide springs? Should i look elsewhere for the problem?

Feel free to call me anytime 8am until 11pm central US time. I am bad about checking emails.

Thanks in advance for the assistance. This is driving me crazy... crazier.

Mike
913-757-4717

It is not me that I worry about. It is the guy inside my head that keeps insisting that I crack the throttle further open.

 

[mikeydidit98]

Forgot

Forgot

Also, I have verified that the vent lines are not blocked or kinked (one issue that would cause this condition). They are routed to their correct securing loop on the top of the air filter cover.

 

[mike_of_bbg]

No ideas really why it's bogging at 3/4 throttle, but no, it's not CDI. It is a transistorized electric ignition system on that bike. And yes, it does have a mechanical advance. It's not impossible that the igniter might be breaking down at higher RPMs. Check primary and secondary ohms on coils, make sure plug caps are not too high in resistance.

 

[gustovh]

Pull the cam cover and check the cams, the intake could be one pin retarded, don't ask me how I would know something like that.
V

 

[eil]

Well, I will say, you've included just about all the information anyone could think to ask for. Thanks!

put carb kits in all 4

Hmm... Do you mean an aftermarket carb kit with new jets and all that?

If so, those parts are generally regarded as inferior to the factory originals. All you really need to rebuild those are a set of o-rings from http://cycleorings.com, and possibly new float bowl gaskets if the existing ones are wrecked.

(spit sizzle test only - do not have heat gun... reminder to self - break down and buy a good $300 laser gun)

Didn't know those could cost so much! I bought a Sears cheapie and it's not terribly accurate, but it is consistent.

Could the deformation of the old air box boots be causing some sort of air flow restriction to the diaphragm circuit ram air intake, thereby not pressurizing the underside of the diaphragm to completely elevate the needle?

I'm not super familiar with the 550's, but one thing you have to do is make sure there are no air leaks anywhere. This usually means sealing up the airbox, replacing carb boots, replacing intake boots, and replacing the o-rings under the intake boots, if yours has them. I can't say whether the symptoms you've described could be caused by air leaks, but you didn't talk about any of these items in your post, so I thought I would mention them.

 

[tom203]

Check operation of mechanical advance, maybe it's stuck after barn sitting and not advancing to 40 degrees BTDC