1982 GPz 750 running poorly when hot

Hi All!

This is my first post (well, second, after the newbie intro forum). I've been reading the boards here and searching for a while now, and I've learned a LOT from you guys. Unfortunately, I'm also stuck, and I need your expertise.

The bike: Very stock 1982 GPz 750. The only aftermarket part is the Vance and Hines 4-into-1 exhaust. I bought the bike a couple of years ago, it ran badly on the way home, and it got stuffed in the corner of the garage while I worked on other bikes. The problem is that this bike really speaks to me, and I'd probably sell all four of my 70's Hondas if I could get this old girl to behave. Her time has come, and I'm prepared to chase this problem to the ends of the earth.

Here's the problem: Cold starts and idles just fine, let it warm up a bit and take it for a spin. About 5 minutes into a ride, it starts surging mildly at or near WOT. At the 10 minute mark, it starts to run rough at low throttle cruise, but if I twist it to about 1/2 throttle, it smooths back out. Of course, i have to lift, because now I'm going too fast, and it goes back to rough. At about the 15 minute mark, it starts bucking and losing power, sounds terrible, and won't rev or maintain idle. If I try to continue to ride, it gets so bad that it won't even sustain 30 mph. My only option is to pull over, let it cool for 20 minutes or so, and then limp it as far as I can before it gets too bad, stop, let it cool. Rinse and repeat until back home.

Here's what I've done so far:

• Checked valve clearance, found intake valve on cyl 2 too tight, replaced shim and brought back into spec. All other valves in-spec.
• Replaced battery with known good.
• Completely stripped and cleaned carbs, reassembled and set float levels (verified actual fuel level with clear tube).
• Replaced airbox-to-carb boots.
• Cleaned intake ports and sealed carb holders to head with Yamabond 5.
• Replaced plugs, wires, and even coils.
• Put TrailTech Cylinder Head Temp Gauge on it to see if it was getting ridiculously hot (cruises at about 325, highest I've seen was 375 after a 3rd gear pull to redline).
• Checked compression when bike is hot and running poorly, reads 130 across the board. Not great, but it doesn't tell me anything, either.
• Checked each cylinder with inline spark light (both cold and when hot and running poorly), and all 4 seem to be getting strong spark.

The bike acts like it is losing one or more cylinders. If I had to guess, I would say that cyl 2 is dropping out. I say that because pulling the #2 plug wire when it is running poorly makes far less difference than pulling any other plug wire. It still runs worse with #2 unplugged, but it simply dies if any other is pulled. Also, the chrome on the header pipe for #2 is barely discolored, while the other 3 show normal discoloration

I cannot see any marks on the jets that are currently on the bike, so I have no idea what size they are. I've got a K&N stock airbox replacement filter and dynojet stage 1 jet kit on the way from Z1, so that I know that the jetting is at least somewhere in the right ballpark.

Thoughts? Suggestions? Tomatoes? I'm sorry for the long post, but I've been chasing this issue for months and I'm just about at the end of what I can think of, and I wanted to try to give you as much info as I can.

Thank you for any help you might be able to offer!

It's hard to imagine that I forgot to write something in the ridiculous novel above, but I also synced the carbs. Nothing has made any difference whatsoever.

Sounds like your carbs are still dirty. I know you said you cleaned them but these carbs need a very detailed cleaning. Not the kind of cleaning you do with an automotive carb. Also check the mechanical advance under under the pulse coils.

If not already done, would perform the clear tube test on #2 carb to determine whether fuel level is within spec, because too high fuel level could be causing the lack of combustion by providing a too rich mixture.

Is the #2 carb's pilot mixture screw at about the same setting as the other carbs?
Could try leaning the pilot mixture on #2, and test ride.

Is there a vacuum nipple on #2 carb that's supplying vacuum to a vacuum operated petcock?
If so, could disconnect the tube at the carb, plug/cap the nipple (to prevent a vacuum leak), and test ride the bike with petcock set in PRIme position. This should eliminate possibility of a leaking petcock allowing gasoline down through the vacuum hose and into the mixture being furnished to #2 carb.

Good Fortune!

It runs for a few minutes, then it shuts off, and after twenty minutes you can re-start it.

Classic symptoms of a clogged fuel tank vent. Easy check, when it starts running bad, open the gas cap. if it runs better then you've found the problem.


'

Great thoughts! Thank you for the feedback!

The timing weights move freely, and the timing is spot-on, both at idle and fully-advanced.

I checked the fuel level on each carb with the clear tube method when I cleaned them, and again a few nights ago, but I've got new needle valves and seats on their way from Z1 (with the air filter and jet kit), just to eliminate a potentially frustrating variable.

The pilot mixture screws are each 1.5 turns out, which produced the lean best idle before I sync'd them.

The fuel tank vent will be my first test tomorrow. That's an easy one, thanks!

Patton, I hadn't even thought about the vacuum line to the petcock on #2. That's definitely something unique to cyl 2. I'll plug both ends and run in prime tomorrow. I'll try leaning out the pilot, too.

I felt like the carbs were pretty clean. Each one got stripped down, aircraft stripper to remove that black paint before they each went into the carb dip overnight. When I got them out, I flushed them with water and then blew-out all of the passages with compressed air. I made a point to verify the pilot circuits by alternating which orifice I plugged, and ensuring that the air came out of the only open passage (pilot screw hole, pilot air jet hole, main/emulsifier passage, the small orifices where the fuel feeds, etc). Either way, they will get cleaned again when the new jets and needle valves and seat show up next week. Do I need to invest in an ultrasonic cleaner? Please? Lol. I'm told they are good for cleaning jewelry, too. Might be a good justification angle there...

Your fuel mixture screws, which you are calling pilot mixture screws, should start at 2.5 turns out. If it runs better at 1.5 turns out, that indicates a rich condition in that circuit. If the pilot jets are stock, which they should be, that indicates the fuel level in your bowls is too high. The setting for the floats is a +/- setting, in that you can adjust them a tad to tune the levels as required.
I have attached a file, which I wrote while syncing the carbs on my GPz. You may wish to peruse it to insure that you synched your carbs correctly.
The Mikuni carbs that your GPz has are a completely different animal from the Keihin carbs most Kaws run.
Best of luck.

Edit Hmmm, I cannot seem to get the Add File function to work properly. Mods, how do I do this?

Fuel Mixture Screws Tuning Not sure why I cannot just use the Add File button.

Great write-up Koolaid, thanks!

I haven't forgotten about this, and this won't be another thread with no helpful resolution for posterity's sake.

I'm waiting on parts from Z1. New needle valves, new needle valve seats, K&N drop-in filter, Dynojet Stage 1 jet kit and a petcock rebuild kit. Obviously the carbs will be getting a colonoscopy while they are off and apart, and the gas tank is full of TSP and water right now.

That should replace all of the unknowns with knowns. Spark is good, coils wires and plugs are new, timing is correct, valve lash is in-spec, compression is consistent across all 4 holes, the gas tank vent is clear and there are no intake/vacuum leaks. Once it is running again, I'll test the charging system, but it doesn't seem to be faulty. I can't imagine why it would still have a problem, but there's a part of me that fully expects it to behave in exactly the same way, even after all of this, haha.

Thanks, WhiskeyB. If I knew how to get it in our archives here, I would do so.
Best of luck.

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Good Fortune!

Get out your meter, check out your magnetic pick up coil.

Heat it up, until it starts messing up, then check it again.

I had one on my '79 Kz1000 that would act up when hot.