1981 GS550 - Bogging at WOT and I'm stumped.

[ShawnR]

** I didn't know if this should belong in the Fuel or Ignition forum since I don't know the cause yet....

1981 Suzuki GS550

I consider myself to be pretty good at bikes, but this one has me stumped.

It starts fine and will free rev up to 11K rpm with no issues.

When you actually try to ride it, or run it on the center stand with a load from the rear brake, it tops out around 5500 – 6000 rpm and has a horrible bog at WOT even after the load is removed. It only "resets" after letting off the throttle and letting it recover when not under any load.

Carbs have been cleaned & rebuilt multiple times. I have 2 identical racks of carbs with stock jets and they both behave exactly the same way. Main needle jet is fixed (not adjustable ).

Compression and leak-down tests both are excellent.
Cam timing and valve clearances have been verified twice.

It has a Dyna S ignition module, new coils, wires, caps & plugs. It has good spark and I can even see it in the daytime.

I wondered about a plugged exhaust so I cut it off before the mufflers – no difference, just louder.
The timing check with light does advance with RPM increase.
Original air filter, not pods

It acts like it is running rich because it does run better with the air filter removed from the airbox. When it is at WOT and bogging, the diaphragms that pull the needles up in the carbs are all the way up even at lower RPMs while it is bogging. I don't know if they should be or not.


Note the white exhaust smoke while it is doing its “thing”. Yes, running like this on the center stand is a bad idea, I know and will deal with it if something happens.

and

This was supposed to be a cheap easy flip, but has now become my nemesis with easily 200 hours into it.

[rphillips]

Seems something is way off, I would have thought it shouldn't run at all with air filter removed...

[Rob S.]

So it's only under load that it misbehaves, correct?

Electrical? What say you, group mind?

[rphillips]

Well, thinking 6000 rpm "under load" is on a totally different carb circuit than when "not under load". Also will something in an electronic ign. system ("81" GS550), know if under load or not? He said 11000 rpm. not under load only 6000 under load. He said runs better with air filter removed, to me that should be an interesting clue.

[ShawnR]

So it's only under load that it misbehaves, correct?

Electrical? What say you, group mind?

Correct. In neutral or on the center stand in gear it starts and appears to run just fine. When you actually "ride" it, or run in gear on the center stand and bold the rear brake, then it does "the thing" and won't go past 6000 rpm at all, just bogs until you let off the throttle.

Even if the load is removed, it won't return to normal until the throttle is released.

I bought a cheap Chinese "magnetic" pickup that was complete junk. The rotor had a piece of metal that wasn't magnetic at all. The Dyna S does seem to have a good spark and advance like it should. I did only check cylinder #1 for timing.

[first timer]

The 550 is very sensitive to air flow, especially at stock jetting. If you are going to run stock jetting then you need everything stock. That means fix the exhaust, put the air box on with the OEM style oiled foam filter, and make sure you have fresh intake boots with fresh o-rings.

You don't go into much detail about your carb rebuilt, rebuild means a lot of different things to different people. Did you just hose the carbs off with carb clean or did you meticulously take them all apart and chase every orifice with fishing line or wire? Did you replace all the o-ring in the carbs with fresh o-rings, did you replace the dried out rubber plugs that plug one of the jets in the bottom of the carb in the float bowl, did you use new float bowl gaskets, did you check the slide diaphragms for holes and the slides for free up and down operations and if they can hold a vacuum?

A lot of time folks get cheap and reuse old dried out parts and they will create small issues you keep chasing your tail on. The 550 especially is very fussy with its carbs and it sucks to take them on and off in a stock setup so people get lazy.

First thing is to go back to base and make sure you got it all set up per the book. Then start looking at outside influences, but like i said 99% of the time poor running is carb related and the only way to check is go back to a know baseline, aka stock.

My guess after watching your video, one of the carb slides is hangin in the wide open position after you rev it up and build lots of vacuum, then you back down the engine speed and the slide doesn't drop to match vacuum causing a rich condition. But without you answering the questions above it's anyone guess. I assume all your slides have springs to return them to the bottom position when there is no vacuum?

FYI I had a 81 550 in 2002 and spent much time working on the damn thing and learning so i know your pain. I joined the GSR back in 2002 basically asking the same questions you are.

[ShawnR]

The 550 is very sensitive to air flow, especially at stock jetting. If you are going to run stock jetting then you need everything stock. That means fix the exhaust, put the air box on with the OEM style oiled foam filter, and make sure you have fresh intake boots with fresh o-rings.

You don't go into much detail about your carb rebuilt, rebuild means a lot of different things to different people. Did you just hose the carbs off with carb clean or did you meticulously take them all apart and chase every orifice with fishing line or wire?

I have an ultrasonic cleaning tank and follow up with dental picks and compressed air to blow out passages.


Did you replace all the o-ring in the carbs with fresh o-rings,

I did the float bowl gasket and float valve, but did not replace the O-rings between the carbs.


did you replace the dried out rubber plugs that plug one of the jets in the bottom of the carb in the float bowl,

No, never thought of that and didn't know it even existed. I'll look in the spare rack.

did you use new float bowl gasket

Yes, on both carb racks

did you check the slide diaphragms for holes and the slides for free up and down operations and if they can hold a vacuum?

Yes, I did replace diaphragms and use a shop vac on the head side to confirm operation. The suck up under vacuum and drop when vacuum removed.

A lot of time folks get cheap and reuse old dried out parts and they will create small issues you keep chasing your tail on. The 550 especially is very fussy with its carbs and it sucks to take them on and off in a stock setup so people get lazy.

I have 2 complete carb racks and it behaves the same with both racks. Always rich by looking at the plugs.


First thing is to go back to base and make sure you got it all set up per the book. Then start looking at outside influences, but like i said 99% of the time poor running is carb related and the only way to check is go back to a know baseline, aka stock.

My guess after watching your video, one of the carb slides is hangin in the wide open position after you rev it up and build lots of vacuum, then you back down the engine speed and the slide doesn't drop to match vacuum causing a rich condition. But without you answering the questions above it's anyone guess. I assume all your slides have springs to return them to the bottom position when there is no vacuum?

Correct. When I ran it without the intake boots on, the slides do stay up while it is bogging and the RPMs drop. I confirmed that they are the correct diaphragm springs. I even added spacers on top of the slide to increase spring pressure and that made no difference.

[KiwiAlfa156]

The rubber plugs over the pilot jets in the Mikuni BS are important. With the plugs in place the pilot draws fuel though the main jet (i.e all fuel entering the engine -apart from the choke circuit- passes through the main jet first). With out the plugs when the throttle is opened up fuel to the main circuit can by-pass the main jet as it flows past where the plug should be, reverses back down the gallery that connects the main to pilot, and whole shooting match runs super rich. I know this from experience when I forgot to install the plugs (duh) and the bike would run perfectly on low throttle opening as and then not go over about 5,000 rpm winding on to WFO throttle. Plug chop in that condition and plugs were sooty.



Also if the ignition system runs the OEM mechanical advance, make sure you set the timing with a strobe at full advance (above 3000 rpm or whatever your manual says). Although the ATU units all interchange across various models, the range of degrees over which they operate varies from model to model by as much as 5°, with different part numbers for each. If its been replaced over the bikes long and illustrious life, and if its ever crashed on its LH side, this is a real possibility, you may have a unit off another motor and if you set the ignition timing at idle, it might be wrong at full advance.

My 750 has exactly this. And with the timing set correctly at full advance (the F1.4 marks align) the timing at idle is a degree or two advanced (i.e. the marks F 1.4 don't align). The only issue I have is that the cranking at start up is a little sluggish as the starter has to work against the pressure of an early lit mixture expanding before TDC.