NO power below 6000rpm

Hello everyone,

I'm new to the board and am wondering about some problems that I'm having with a 1980 KZ750 that I recently purchased. After reading a few posts from this board, I have some ideas but would like to get your input.

For starters, it has no power below 6000rpm. In order to drive the bike, I have to rev it to 3000, slip the clutch and give it full throttle. It responds by barely moving and almost dying. Once the clutch is fully engaged, and the bike is moving, it slowly accelerates, (still at full throttle), but sounds like it's missing on at least one cylinder. When the rpm reaches somewhere around 6000 or so, the miss seems to go away, (it could just be that it's at such a high rpm that I can't hear it anymore), and the bike finally starts making a little power. At that rpm, the accleration feels the same as an older 175cc 2-stroke dirt bike; probably around 30hp or so.

I've checked the plugs, they're new but are heavily fouled. The ignition coils are Accel, along with the plug wires. The bike is definitely running rich at idle. The stock airbox has been removed and replaced with individual carb air filters. The carbs appear to be the Keihin 34CV stock units. My guess is that the problem is a carb issue, but it could be ignition.

Somewhere along the way, one of the previous owners converted the original electronic ignition to points. I don't know where he got the parts or why he would do such a thing, but that conversion might be part of my problem.

The bike runs smoothly at idle; it revs just fine. Throttle response at idle isn't very good but then again, I'm not sure how these bikes are supposed to run from the factory; I just know that it shouldn't take 3000rpm, full throttle and slipping the clutch to make it go.

Any information/advice would be greatly appreciated. Thanks from Mesa, AZ.

have you checked your valve clearances yet?
(double posts happen every now and then to everyone, a mod will zap it after awhile)

SRobertson wrote:

Sorry for posting this twice guys; I don't know what happened. Chalk it up to newb error. It doesn't look like there's any way for me to delete it either. Oh well, thanks for understanding.

Utterly no problem; there seems to be a 30 minute timeout value in the woodpile somewhere, probably the text editor module - you probably noticed a "retry" window when you tried to post...

A moderator will come along presently, notice the double-clutch, and delete the doppelganger...

BTW, welcome aboard...

Post edited by: Pterosaur, at: 2006/06/15 19:06

Have you cleaned out your carbs? I'm going to guess it's your slow jets causing the problems, like the bike doesn't get enough gas until the main jets open.

Just my guess...

I haven't checked valve clearances yet, I didn't even think about it. The book shows that the clearance should be .003-.007in. I'll confirm that they're within spec.

Something else of note, my carbs have an idle adjustment screw. Supposedly, the US models of that year had a plug covering the adjustment for emissions purposes. I guess it's possible for someone to remove the plugs but I wonder if these carbs were swapped from another bike. I was considering purchasing a set of Mikunis and getting rid of the Keihins but I wasn't sure if that would help or not.

Thanks for the welcome and the words of wisdom.

Double post zapped

Start with the basics, since you just got the bike you need to do them anyway (beats hoofing it back home).

Check/adjust/replace the points and condensers.
Check the coils (requires multimeter, but they are cheap).
Check the resistance of the plug wires and plug caps.
Replace the spark plugs.
Check/adjust the valve clearance.

Only after doing all the above do you mess with the carbs

By the way, if you don't have a manual, get one HERE

I thought about rebuilding the carbs and cleaning everything out all of the jets/ports. However, I've read that the Keihins don't like to be run w/o the airbox and are a pain to tune once the airbox is removed. Evidently, reversion seems to be a major problem. I've also read that finding jets for the Keihins is impossible, so rejetting the thing is out of the question.

As I mentioned in a previous post, I was thinking about getting a set of Mikunis from a later model, as opposed to rebuilding the Keihins. Although, it might not hurt to take the carbs off, clean them up, reuse the old gaskets and see how it runs.

get a set of 26mm or 28mm mikunis, you can get jets anywhere

Welcome to the board.
Like steell said - do all the basic stuff first. Strange that the bike has points. I wonder if the previous owner fried the ignitor and then went to points rather than get an expensive replacement. He may have screwed around with the wiring in the process.

The 750/4 makes oodles of power. At 6,000rpm you should be purring along at 120kph/75mph. From your description I wonder if perhaps you are only firing on 1 or 2 cylinders. Try spraying some water on the exhausts where they exit the cylinders. That'l tell you soon enough.

Regarding the carbs I have also heard that jets are hard to come by. I was lucky in that the bike came correctly jetted for my setup.
Good luck

Post edited by: JR, at: 2006/06/15 21:54

I'd check to base timing and make sure the advancer is working. Sounds like the timing may not be advancing correctly.
Check it with a timing light and also use a good bright shop light to watch it advance as you are slowly revving the engine to see that it is advancing smoothly.
Ride safe,
Les

Post edited by: The Milkman, at: 2006/06/15 22:36

Steve Eliot gave you the very best but I would like to amend one thing and that is the order of what is done:

1. CHECK COMPRESSION
2. CHECK VALVE CLEARANCES / CHECK CAM CHAIN TIMING
3. ADJUST CAM CHAIN TENSIONER, IF POSSIBLE
4. REPLACE PLUGS, PLUG WIRES/CAPS, CHECK COIL VOLTAGES
5. CLEAN/LUBE MECHANICAL IGNITION ADVANCE
6. SYNC CARBS AND ADJUST IDLE MIXTURE AND IDLE; INSTALL NEW FUEL FILTER AND REPLACE ANY FUEL LINES IF NEEDED, CLEAN PETCOCK AND TANK IF NECESSARY
7. DO CHASSIS RELATED STUFF BEFORE RIDING: CHANGE FLUIDS, LUBE CABLES AND INSPECT CABLE ENDS FOR FRAYING, CHECK TIRES FOR WEAR AND DRY ROT, CHECK CHAIN, CHECK BRAKE PADS

I would do the stuff above in the order I described. If you do some of the things out of order, you will be wasting your time... You must know that you have compression, good spark and of course, the proper air/fuel mix deliverd to the combustion chamber. Most folks want to jump on the bike and ride at that point but don't neglect safety items as well.

if you have the bike running on all 4 cylinders and it idles fine that is a good start. having a camshaft off 1 tooth or both is a possibility,the engine will run but pull low vacuum. I am inclined to ask questions about if the power comes on way up in the rev range, or at all. That would tell me more. But more than likely having no air box or jetting kit installed is the root of the problem due to lack of air velocity and proper fuel mix. As with all mechanical problems other factors are also possible. it is all fuel-compression-timed ignition or a combination of all three. that simple??