TOPIC:

So I've spent the last few weeks riding around on my '83 KZ750N and I have been enjoying it immensely, especially now that I managed to fit the Daytona Sportbike bars. Man, those really changed how the bike handles and rides! Should have had those from the factory, IMO.

Anyway, something I've noticed is what seems to be a lack of power in the upper RPM band. Maybe it's just my imagination and I need to get used too it, my last 750 was a GSXR and that had considerably more power (scarily more, at times). However, there is an 8% grade I have to travel to get to work in the morning, and I'm WOT going up it just to maintain 55 in 5th gear, 4th gear is only slightly better. Now, that being said, the bike runs great! It starts right up, purrs at just over 1000RPM idle, and doesn't pop or carry on when riding. It has decent low end grunt and it never really feels like I've been lugging the engine, it just smoothly goes, just not so much above say, 6000 RPM.

Here's what I've got:
1983 KZ750N Spectre, 4200 original miles
Carburetors were overhauled three years ago by the previous owner's mechanic.
Valves were adjusted three years ago by the previous owner's mechanic.
All new vacuum line
All new fuel line with filter
New K&N filter (had an Emgo filter up until today, and I've not ridden it yet)
New NGK B8ES spark plugs
Verified fat blue spark on all 4 plugs.
New LiPO battery pack
165, 160, 150, 160 compression tested while warm at WOT
I've run two tanks of premium ethanol free gas through it, seeing 38.5mpg
Fresh Mobile 1 10w40 Racing 4T oil

I'm a little concerned about cylinder 3 being 15PSI out of whack from cylinder 1. The manual states this is within spec, if just barely. Plugs for cylinders 1 and 3 have turned tan on the tips, while 2 and 4 are darker. All 4 header pipes get hot and, like I said, it doesn't seem to be missing or anything. Heck, it's a joy to ride and pulled me up Brockway Mountain Drive in 3rd gear at 35mph with no trouble, it's just the high RPM stuff that it seems to be having issues with.

One thing I should note ,is that I haven't ridden it with the new K&N filter. I just installed it this morning and, at the same time, removed about 90% of the sticky-down bit on the air filter cap because it was crushed/melted. Sadly, while finishing the compression test, I dropped a spark plug. It bounced twice and did a hole-in-one in the floor drain. It's totally gone, I'm actually on my way to Auto Zone to get new ones right now.

Is there anything I should be looking at? A vacuum advance on the ignition that I should check, or something? My gut feeling is to look at the carbs, but I'd really prefer to not have to pull those off. I'm not a huge fan of working on multi-carburetor setups as I can never seem to get them synced correctly.

You might want verify the choke plates work properly and don't hang up closed, which will restrict airflow. Just thinking out loud here, I'm not even sure if it's possible for the chokes to hang closed, I think they usually get stuck open. Anyway, it's something to check just for kicks. Checking spark advance would be worth doing too. Your bike should be able to rip up that hill you describe easily.

Something is definitely wrong. My 1977 KZ650 really wakes up above 6000 rpm and it screams from there to redline. Years ago my friend had a KZ750 and it would out-accelerate my KZ650 at high rpm. His bike had one of those silly 85mph speedometers, but I know for a fact his bike would easily exceed 120 mph because he did exactly that while we were riding home from some races. I was running flat out at right around 120 mph and he kept pulling away from me and slowing back down so I could catch up. We did this for 10 miles or so. He didn't realize he was going more than 120 mph until we stopped and I told him why I wasn't keeping up with him. Ed

Might be too rich on top if you're Ridin' at higher elevations.

www.jetsrus.com/FAQs/FAQ_rejet_elevation_and_temperature.htm


Good Ridin'
slmjim & Z1BEBE

On one hand, it's nice to be validated and know I'm not crazy. On the other, dang it, that means my bike is broke.

Good thought on the rejet, when I pull the carbs I'll have to see what it's got in it. I'm not really at high altitude, we just have some pretty steep hills they call mountains around here. Maybe 1400' or so above sea level. I think they highest point I may be able to reach is right around 2000' above sea level. Still, better than Florida where I used to live!

It may be running rich, the old plugs were pretty black and the new ones haven't been in long enough to get a good read. The previous owner was running it without an air filter, so I'm wondering if this has been a persistent problem.

Is there a trick to pulling the carbs off, or just unbolt them and pull them free from the boots on both sides? It doesn't look like I can remove the airbox without removing the carbs, unless there is a trick I'm missing.

What kind of carbs? CV or VM?
Steve

650ed wrote: Something is definitely wrong. My 1977 KZ650 really wakes up above 6000 rpm and it screams from there to redline. Years ago my friend had a KZ750 and it would out-accelerate my KZ650 at high rpm. His bike had one of those silly 85mph speedometers, but I know for a fact his bike would easily exceed 120 mph because he did exactly that while we were riding home from some races. I was running flat out at right around 120 mph and he kept pulling away from me and slowing back down so I could catch up. We did this for 10 miles or so. He didn't realize he was going more than 120 mph until we stopped and I told him why I wasn't keeping up with him. Ed

Maybe 120 mph indicated.


.

Nebr_Rex wrote:

650ed wrote: Something is definitely wrong. My 1977 KZ650 really wakes up above 6000 rpm and it screams from there to redline. Years ago my friend had a KZ750 and it would out-accelerate my KZ650 at high rpm. His bike had one of those silly 85mph speedometers, but I know for a fact his bike would easily exceed 120 mph because he did exactly that while we were riding home from some races. I was running flat out at right around 120 mph and he kept pulling away from me and slowing back down so I could catch up. We did this for 10 miles or so. He didn't realize he was going more than 120 mph until we stopped and I told him why I wasn't keeping up with him. Ed

Maybe 120 mph indicated.

.

Correct. The actual top speed was probably 119 mph. Keep in mind that the bike was only a couple years old and truly running great. For more info, including the measured top speed of the stock 1977 KZ650 please see the Feb 1977 Cycle World road test at the link below. Ed

www.kz650.info/articles/documents/KZ650%...World%20Roadtest.pdf

NEVER trust what a PO tells you. I would just assume no work had ever been done would verify valve clearances, carb jetting, re-clean the carbs and make sure everything is to stock specs. I would bet the PO had bigger main jets installed.
Like Ed said something isn't right my 650 is also fast and pulls real hard all the way to redline. I wouldn't worry about the 150lb compression.

Yeah, I'm not just blindly trusting the PO on this one, although considering he dug it out of a barn, I know he had some of the work he said done, done. That said, I don't know if it was done right. Honestly can't imagine the valves being out of adjustment unless someone did something to them, since it's only got 4200 miles on it. Still, couldn't hurt to check.

I've got rebuild kits and a jet kit coming in the mail so I can play around with it, as well as a new valve cover gasket. Hopefully later this week or weekend I'll get a chance to start tearing things down. Sadly today was somewhat of a wash, didn't even get a chance to go into the garage. I'm really hoping the vacuum actuated slider valves aren't bad, I've yet to find any place that actually sells those anymore.

Ok, haven't had much time to work on things as I've been house hunting and working. That said, I did get the new clutch cable and speedo cable on, so those are working again. I also removed the pickup coils and cleaned/lubricated the timing advance mechanism. It seemed fine, but since I had the cover off, why not? Re-assembled it with the adjustable screw at the middle point. The shop manual didn't say where it should be, and it was rotated all the way forward.

Since it was a nice evening, I hopped on and took her for a quick jaunt up the route to work (and the 8% grade) and back. I'm not sure if it was the swap to the K&N or the slight adjustment of the timing plate, but things are different. The idle is silky smooth, as usual, but she has a slight stumble if you whack it open below 3k RPM before she leaps forward. From there to 6K she goes like a scalded cat, although it starts dropping off around 5K and is pretty flat above 7K and doesn't really accelerate past that point. It still requires near full throttle to maintain 60MPH up that hill in 5th.

I've also tried the WD40 on the carb boots trick, which did nothing. All the parts for the carburetor rebuild have arrived, as did a DynoJet kit (was figuring I'd slip the Stage 1 on it). After reading the instructions, I went out and checked the carburetors and confirmed my suspicion that someone may have tinkered in there. The cap over the pilot adjustment screw has been removed on all 4 carburetors.

I think the PO possible had someone try to tune it, and they screwed it up. So this weekend the carbs are coming apart and I'm overhauling them with the DynoJet Stage 1 setup which is supposed to be good for stock/near stock bikes. Hopefully that straightens things out.

Man, she's fun from below 6K though. Put a grin on my face.

Final note, just in case anyone is curious.

Before ordering a bunch of carburetor rebuild parts and a jet kit, make sure your throttle cable isn't kinked and you are actually getting more than 2/3rds throttle actuation out of it. I wasn't, that was the problem. Rerouted the cable and lubricated it, now it goes like stink!

I'll just hang on to those rebuild kits, I guess. I'll need them eventually.