'84 GPZ 750 Carb Questions

I picked up an '84 GPZ 750 yesterday. It had been sitting in a shed for a couple years. I have a feeling it was sitting because there was too much monkeying around and butchery by the previous two owners and they couldn't get the carbs set up right. It has a 4 into 1 header with a Kerker muffler, stock airbox, 37.5 pilots and 117.5 mains installed in the carbs.

I put a new battery in it and got it running. Outside two cylinders were running, inside were cold. After verifying spark on all cylinders, I pulled the carbs. They were way out of balance with the outer two cylinders open while the inner two were closed... I did a visual sync on the butterflies and put the carbs back in. It was firing on all four cylinders at that point and I took it for a short ride. It seemed to pull good through the mid-range, but was running pretty hot. After it warmed up, it developed a racing idle like it was running lean with the throttle hanging at 4-5K. I went through it looking for vacuum leaks and in my explorations and head scratching I realized that somebody had butchered the air box cover and that there was supposed to be a baffle tube that went down through the air filter. That tube had been cut off of the air box cover. I pulled the carbs again to look inside and found that the non-adjustable jet needles had been shimmed up with a thin washer and a machine nut.

I just need somebody to verify my thinking on this. Without the air box baffle, the low end is not going to tunable at all with the stock pilot jet, nor is the 117.5 main going to be appropriate. Which way should I go with this? My thought is to fab a new baffle out of plastic tubing to get the air box back to stock flow rates and drop the jet needle back to its stock position. (Can anybody tell me how many and what size holes were in the stock air box cover / baffle tube?). Otherwise where can I start to get this straightened out?

I’m not the welcoming committee, but first off welcome. Usually the request is that you fill out the profile information sufficiently that people can tell generally where you are located and what bike(s) you have so they can be more helpful to you.

Re:your problem, I believe you are overthinking it (this from a guy who is frequently guilty of it too). The cut off internal baffle was an old-school mod to improve airflow while maintaining street-ability. It doesn’t have the negative side effects that you are theorizing.

The usual advice is to first start by performing a valve adjustment (do your homework first though, this is not trivial and it is easy to do more harm than good). There is an online copy of the Factory Service Manual for your bike (I believe on this site); it’s actually the KZ750 Four manual, and there is a specific Appendix for the 82-85 GPz variants. Be careful of the published torque value for the cam cap bolts; at least some manuals have a typo in the value and folks have stripped out the aluminum threads. I think the consensus here is tighten until resistance is felt, then about another 1/6 turn or so. Enough that the bolts won’t back out but not so tight that the threads strip. This adjustment needs to be done before anything else, otherwise you will be just chasing your tail. Previous owners usually skipped it because it isn’t all that simple (will need a new valve cover gasket, the camshafts have to come out, and the automatic cam chain tensioner disabled during the process, plus the actual shims needed) and that results in very poorly running (or dead) GPz 750s. If you have to, swallow your pride and pay a shop that is familiar with “shim under bucket” Kawasaki valve trains. Expect to pay over $100. Maybe have them do a compression check while they are at it to make sure your engine is healthy enough to support this kind of activity.

Once that is taken care of, then clean the carbs but do not replace any of the jets with those that come in whatever kit you buy. They are always (I hesitate to use that word) inferior to the OEM Mikuni parts. Be sure to press out and clean the needle jet (not to be confused with the jet needle) too. That one frequently gets overlooked.

From the sound of it you do have an intake boot leak (you didn’t describe how you convinced yourself there were no leaks, but this is textbook with the symptoms you describe). Most of us will recommend using only OEM Kawasaki items for this, but they won’t be super cheap. This is probably a “bite the bullet” sort of item though. Figure it will set you back about $70.

The good news is that it sounds like you’ve got a really good starting point. You’ve still got the stock air box, the jets and needles are pretty much perfect for your setup, and you’ve got a classic and almost unobtanium exhaust. Please consider keeping it as close to stock as practical; I think you will enjoy it a lot more that way. It was a pretty close to optimum design right out of the factory.

Oh, and we always like to see pictures!

Forgot to mention: assuming that it doesn’t start running perfectly after you’ve checked the valve adjustment and replaced the carb boots then at some point you’ll need to become familiar with what they call “the clear tube method” around here. This will be [subjectively] needed to adjust the float levels in your BS34 carburetors. You can use other techniques, but this is the most authoritative. What sometimes isn’t mentioned is a reminder that the float valve acts as a check valve, so make sure you are measuring level only during filling of the bowl. Another thing that usually isn’t mentioned is that you need to purge any air bubbles out of the tube first, since that will screw up the measurement. You’d think that any air would just bubble on up and outa there, but it gets hung up on the internal passages of the drain screw of this design.

Thanks for the reply.

Your comments on the valve adjustment are well received.

I did order a new set of intake boots yesterday, but in the meantime I pulled them, inspected them, and touched the back of them with some black RTV. They didn't seem too bad. I softened them a touch with some belt dressing and made sure the carbs were well seated. There was no change.

I did check the float height, they needed a little adjustment between them, but weren't too far out. Carbs looked pretty clean, bowls were clean, no crap in the jets. I have them apart for a soak today. (I accomplished a lot for only having the bike a day).

A lot of recommendations I have seen on the racing idle were to make sure there are no vaccuum leaks, to include the boots on the airbox side, as the carbs are sensitive to airbox pressure. I looked up baseline jetting recommendations for the 4 into 1 exhaust with a stock airbox and it is jetted along those lines (37.5 and 117.5). I also looked up suggested jetting for a bike with 4 into 1 exhaust and individual pod filters, and the recommendations were larger jets. (40 and 140).

It makes no sense to me to be checking the airbox boots for a tight seal when the airbox is wide open, nor does the jetting seem to be in-line with an unrestricted intake.

I should ignore the airbox modification and search elsewhere for the cause of the racing idle?

mmccarty wrote: Thanks for the reply.
I should ignore the airbox modification and search elsewhere for the cause of the racing idle?

Yes. Mine is modified the same way and I don't have a racing idle. My jetting is a little richer than yours but my needles are not shimmed. A machine nut would be quite a large shim and that does not sound right. How many miles? What are your idle screws set at? What do the plugs look like. Considering the shims it might not be a good idea to assume the jets are OEM and could be replacements from a rebuild kit.

mmccarty wrote:
I did order a new set of intake boots yesterday, but in the meantime I pulled them, inspected them, and touched the back of them with some black RTV. They didn't seem too bad. I softened them a touch with some belt dressing and made sure the carbs were well seated. There was no change.

It's highly possible that the leak is due to a radial crack at the flange rather than at the joint between the cylinder head and the boot aka "carb holder". They have a way of being virtually undetectable by eye. Did you try the propane test? Probably doesn't matter at this point since you've got new ones on the way.

It makes no sense to me to be checking the airbox boots for a tight seal when the airbox is wide open

I'm with you on that one - a minor leak upstream would have almost no effect. On the other hand, if the airbox were removed completely and replaced with pod filters then that's a "known bad" configuration that should be avoided.

You do have an air filter installed within the airbox, right? That would make a difference too. I personally run the OEM ones, and the correct internal "cage" although K&N seems to still be making drop-in replacements that eliminate the need for the cage (which frequently gets lost or thrown out).

I also looked up suggested jetting for a bike with 4 into 1 exhaust and individual pod filters, and the recommendations were larger jets. (40 and 140).

I couldn't even get mine to run with the stock airbox and 42.5 jets. I imagine 40 would be OK, but I'm running a little on the rich side with 37.5/115 already. I sorta attribute it to very little backpressure (I've got a Kerker with the largest racing baffle they made). I've not tried to run pods on the BS34 personally but the conventional wisdom is that it's nearly impossible to find a jetting combination that works properly. I ran pods on my bigger KZs in the past, but swapping back to the original airboxes was a net improvement (especially in windy areas like where I live).

I should ignore the airbox modification and search elsewhere for the cause of the racing idle?

That's my suggestion, but I could easily be wrong.

I checked the valve clearances. #1 intake was a little tight at .003 inches. There was no measurable clearance on the #4 exhaust. #4 cylinder had the best compression though. The compression readings (cold) were 155, 125, 135, 160.