High Revving GPz750?

New to me 82 GPz750 with stock Mikuni BS34 carbs. As described in prior posts, I fixed electrical, cleaned tank and carbs twice.

when cold, she turns right over with choke on, and goes straight to 4K. I push the choke in and throttle it to warm up. Once I'm riding for a couple minutes, and the bike is really warmed up, the engine revs high and won't drop down between shifts. When I come to a stop, I have to feather the clutch to drop the RPMs. When I get home and turn it off, if I start it again it will generally rev really high. When it does idle, I use the throttle adjuster to try and set idle at 1K, but she runs rough and will stall.

Today I sprayed WD-40 around the air box boots looking for air leaks, and it didn't do anything different. I hooked my timing light up and with high idle, I saw the II mark. When I stopped the motor, I checked the advance mechanism and it was normal, and wasn't stuck. I looked at the throttle cable, there was a little slack so it didn't look like it was causing the problem.

I'm looking to get a carb sync tool, so I haven't synced them yet. Anyone have suggestions or insight?
Thanks
Nick

Well did you do a "Bench Sync" where you set the butterflies to the same opening with a small drill bit as a feeler gauge?
I think maybe take the throttle cable out of the carb rack and set the Idle speed and mixtures. Then try to give it some with your finger, so the cable is still disconnected. It should return to the same setting each time, or really close. Then you can let it sit and try a cold start and see what happens, it should only increase to around 1500 or so as it warms and you slowly push the choke back in. My bikes with these carbs start on full choke nut quickly go to 1/2 and Idle for a few minutes this way. Building up to around 1500 before I turn the choke off.

Motorhead, thanks for the reply. No, looks like I overlooked syncing the butterflys in my manual. I'll try your suggestion tomorrow regarding the throttle and setting idle and mixtures. I'm assuming you just move the tank a little to the side to get to the mixture screws?
Nick

Well I take the tank off, then put it up on my cart, and with a long fuel line and filter, hook it to the carb's, then pinch off or plug the vacuum line to the petcock. Now you have full access.
By the way you can build a cheap carb sync for around $6.

I'm not familiar with your bike, but carbs are carbs. Being an 82 it is probably has CV carbs correct?

If so, I'd search around more for a vacuum leak, are all the carb vacuum nipples properly capped? Even cracked caps can do that, and loose ones can fall off or be blown off with a slight backfire. Do you maybe have a leak at the petcock vacuum line?
Other than a vacuum leak on the intake side on CV carbs with a stock air box the only other things I've seen cause what you're describing are weak springs in the sliders, or a sticky slider, or ripped velocity stacks, or diaphragm. All of which you probably checked during your carb cleaning.

Thanks guys. Some observations and questions.

I reviewed my Clymer manual and something hit me. There was a picture of a carb rack that looked different than mine as they were leaning forward. Mine were more level to the ground and therefore they weren't flush against the holders. I'm thinking they were like that because I had such a hard time with the airbox boots. On that subject, mine are hard like I've heard they get. If the prevailing thought is that there are air leak(s) causing my problem, these may be an issue. Mine tend to seal but not snug on both sides.

My air filter also needs to be replaced since its torn, but would that make a difference considering that airbox isn't sealed up tightly anyways by design?

Questions on bench sync:

As for the throttle plates, I took a maglite and shined behind each carb. There was no light coming through the intake side, indicating they were all closed. Should they be closed at rest or a little bit open? It looks like there is a single adjustment for the inner carbs 2 and 3, and the outside carbs each have their own.

Thanks
Nick

Going off the diagram pics I was just looking at, at kawasaki, your air box should be a pretty sealed up unit like all CV carb boxes are.. it should only be getting air through the main port. Are you sure you have all the pieces?

tinyurl.com/266nth7 click on air cleaner, just to check yours against it.

I'm going to say YES on two accounts. Yes your boots not seating up properly and allowing air to bypass your box could cause you the grief you spoke of, and Yes even a ripped air filter could cause that as it would alter vacuum pressure.

CV carbs require a specific vacuum pressure to operate properly, too little, or too much, vacuum will screw them up.

If you boots are really stiff, and the little springs won't hold them tight. You could get some (thin hose clamps) and put those on them to get a tight seal.. I forget what they used to sell them for, to point you in the right direction. Maybe someone else will know of them. These clamps came in all diameters and look/work the same as rad hose type clamps, but were half the width. Sorry I can't be more help.

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As for the butterflies, all of them are opened in unison with your idle screw. Unless your carbs are completely different than all the inline 4s I've worked on. :blink:

There should be a very tiny opening at rest, though I'm not sure you'd see light through it unless you were in absolute darkness and were holding the sliders up out of the way. The only real major concern with those is that they're all opening evenly.

If you still have them off just screw your idle screw in until you see them start to open a bit so you can see under them, and check to see that they're all pretty even.

They would have to be pretty drastically out of sinc to produce the problem you've described though. If they're not fairly even we'll go into the whole bench balancing act.. chuckle.

Best of luck.

Captain,
Thanks. I have been running without the springs you mention (I'm assuming you mean these):



Do these hold well?

My throttle plates are definitely all shut pretty tight because I looked at them this morning in a dark garage. They do all seem to move in unison. I'll adust them to open a crack. I'll also check the fuel levels in the bowl when I mount them back up.

Nick

Big progress tonight. I believe its fixed. I put the rack back on and really made sure the carbs were fitted snugly in the holders and tightened the clamps. The rack was definitely tilted more forward then they were before.

The air filter cover was torn, but I managed to make it cover the entir outside of the cage. Then I made sure the airbox was bolted to the battery cage on one side (something I hadn't done before). This made it sit higher in the frame. I heated up the airbox ducts and installed them, making sure they were snug. They were much easier going on - probably because the rack and the box were aligned better. Once I popped each duct through from the inside of the box, I turned it to get the best fit. On carb #1, I noticed a gap at the bottom looking from inside the box. I twisted it around unti it closed up. After the ducts were in, I made sure there was no light coming through the duct holes in the airbox.

I started the bike with choke on and for the first time, it didn't race. Once I got the bike warmed up, I turned the mixture screws out a 1/2 turn and combined with the idle adjuster, she's idling around 1,100 right now. When I open the throttle, the rpms go up and return to idle. There is a little fine tuning left I think, and I still have to road test it, but alot better!

Thanks to everyone for the help.
Nick

Well that's good news. :woohoo:
Glad to hear you got it pretty much straightened out. They're hard to enjoy in the lane way aren't they... heh btdt. I hope it keeps going your way. So your carbs weren't in their proper seats in the head intakes and threw everything else off on ya, as well as the other minor issues you corrected? Do I got that right?

From your earlier post: Yeah, them silly boot springs don't hold them very tight, but I always put them on since imo, they do keep the shape uniform when the boots get older, and do make the boot grab better when they're warmed and a bit flexible. A pure mechanic/customer convenience idea that one, I figure.. easy to leave on the boot and just roll back and forward on/off the inline 4 pain in the rump boxes.. chuckle

nicksorg wrote:

New to me 82 GPz750 with stock Mikuni BS34 carbs. As described in prior posts, I fixed electrical, cleaned tank and carbs twice.

when cold, she turns right over with choke on, and goes straight to 4K. I push the choke in and throttle it to warm up. Once I'm riding for a couple minutes, and the bike is really warmed up, the engine revs high and won't drop down between shifts. When I come to a stop, I have to feather the clutch to drop the RPMs. When I get home and turn it off, if I start it again it will generally rev really high. When it does idle, I use the throttle adjuster to try and set idle at 1K, but she runs rough and will stall.

Today I sprayed WD-40 around the air box boots looking for air leaks, and it didn't do anything different. I hooked my timing light up and with high idle, I saw the II mark. When I stopped the motor, I checked the advance mechanism and it was normal, and wasn't stuck. I looked at the throttle cable, there was a little slack so it didn't look like it was causing the problem.

I'm looking to get a carb sync tool, so I haven't synced them yet. Anyone have suggestions or insight?
Thanks
Nick

A hanging high idle is almost always an air leak. Usually carbs are not "out of sync" so badly as to cause a 4K idle, but it's a possibility. Did you check all the emissions vacume hoses, and the vacume feed for the fuel petcock ?

Thanks for the help so far. Went to work on the bike the other day, and once it started running, gas started dumping out of the #1/2 carb tube. I pulled the carbs and decided to replace the seats and needles since I had them from the rebuild kits. I put the carbs back and the high idle returned. I pulled the carb holders and inspected them. I took one of the holders and dry fitted it on a carb and realized that I've never seated the carbs as deep as they are supposed to go! So I reinstalled them correctly and sure enough the high idle is gone.

However, now the bike is hard starting with the choke and will not idle on its own. It will run but only with assistance from the throttle.

The idle mixture screws are 2 turns out from seated. Compression test numbers are:

#1 - 150
#2 - 130
#3 - 150
#4 - 160

Carb float bowl levels are all about 1MM with in spec.
Nick