What's wrong with this picture? 83 Gpz550

So my buddy and I picked up an 83 Gpz550 with CV carbs. It would not idle. It would either run at about 4000 RPM or it would die, depending on where you set the idle knob. There was no in-between. I did manage to ride it a few blocks to my buddy's garage by oscillating the throttle between the two extremes.

Normally that symptom means the idle circuit is clogged. So I took it apart and cleaned everything, but really I didn't find much in the way of clogged pilot jets. When we put it back together, it would do the same thing again.

When I had the carbs apart I took a photo of the jets for reference.


After fooling with the carbs for a few hours... yes the fricken pain-in-the-ass airbox was in place, but its days are numbered... I decided to do some googling because something bothered my about the pilot jet.

Sure enough, here is what the actual pilot should look like.


After some more googling, it looks like some previous owner, or more likely the seller of the used carbs, swapped in some pilots from a Yamaha. We found paint-marker writing on the carb as if it was from ebay or a breaker/junkyard.

The only difference i see are the missing bores in the pilot jet

Yes. No air enters the pilot system, so it just floods the motor with fuel. At first we thought the floats were failing because of all the excess fuel in the intake. But we replaced the fuel valves as part of the rebuild and they proved to be working fine.

I mapped out the carb ports and it bothered me that there was a path for air to get to the pilot, but obviously the pilot would seal out the air. Now I see it shouldn't.

I think some ebay seller just threw in whatever pilots he had. On ebay the pilots for the Yamaha are much cheaper, but Jets-R-Us has them for around $11. They also come in some of the rebuild kits, so we ordered them from Z1.

To the untrained eye.. a screw-up like this would definitely cause swearing to kingdom-come and hours of troubleshooting. :ohmy:

Maybe someone installed aftermarket "carb kits" in the carbs at one point?

Nessism wrote: Maybe someone installed aftermarket "carb kits" in the carbs at one point?

I wondered that too, but I googled a bunch of kits and the ones that had jets appeared to have the perforated bleed pipe.

Mcren wrote: To the untrained eye.. a screw-up like this would definitely cause swearing to kingdom-come and hours of troubleshooting. :ohmy:

Well, yes, it normally would, but my buddy and I met working in a backyard shop in the city and we got really used to expecting something to be screwed up like this. We found someone would screw something up, then sell the bike cheap, then the new owner would bring it to us. So we didn't spend too much time puzzling over it. We knew we'd have to pull the carbs to find the bug. (We had already checked compression and ignition etc.)

We did have to spend some time, though, syncing the carbs. It wasn't easy since it wouldn't idle. You'd get about half a second to check the sync then it would die or you'd have to rev it. But we did get it synced, at least temporarily. We needed to do this becuase someone stripped the threads out of one of the syncing linkages. I tapped a steel backing plate so the sync procedure would still work the same using the same tools etc. I needed to test this before we yanked the carbs in case changes needed to be made to the plate. It worked out fine.

So now we wait for parts and hopefully will get a nice day to work. We still need to clean the outside of the carbs, but we might wait until we convert the carbs. The season is running out fast, especially to do any tuning, so we may have to wait.

Note, to anyone interested, this is the repair to the sync linkage. It's M5 x .5 thread (metric fine). The excess of the plate on the right side, as seen in the photo, had to be cut off so it not interfere with the intake manifold when the throttle moved. This photo was taken before it was cut and installed on the bike. The bell-crank coming in from the left pushes up on the bottom of the set screw, so it holds the threaded plate up against the linkage. The plate is wide enough so it won't spin. So it adjusts the same as it would have come from the factory.

I guess one moral to this thread is take a lot of photos! Taking good photos can easily double the time spent, but it sure was nice to diagnose this in front of a computer instead of freezing in the garage getting high on gas fumes.

We should have a banner that tells newbies to take a lot of photos, as one of them may hold the answer, and it may take several people to spot it. That cracked 650 head comes to mind as a recent example.

Update: We swapped the pilot jets, ditched the airbox, installed the Dynojet kit, and did a very quick sync, and now the bike idles like it should and the bike runs pretty good. We only had a few minutes left to ride it around the block. I'm sure the neighbors hate us now . We still need to change oil, check valves and ignition, just out of due diligence, but right now nothing is really amiss. We may move on to tires, suspension, steering damper, but the season is pretty much over. We were installing the carbs in the cold with a propane heater... not really fun. That was part of the reason we decided to leap-frog over the step of going totally stock carbs with airbox... no time and too cold. Luckily the jet kit worked right out of the box and nothing else was messed up, though we might try to locate intake rubbers since ours are not very fresh looking. Next year we might slap a wideband on it to fine tune the needle and jets for optimum mpg etc.

The airbox on this bike actually has two filters. The main one and a little supplemental one on the other side. I guess they couldn't get the airflow even to the #3 and #4 carbs with just the one filter. Seems weird to have it so asymmetrical.

The repair to the stripped sync linkage worked perfectly.

While doing the carb work, I was taking photos and making notes for a TK26 website. It's posted now, but probably needs some proof reading etc.
s3.amazonaws.com/gpzweb/TK26mainPage/TK26mainPage.html

32 and 34 refer to jet size, not numbers of holes.