1978 KZ650 wont accelerate past 70 MPH

I am a new member to this Forum, but I have spent a lot of time on it reading other posts. Thanks to everyone who has been so helpful.

I was trying to avoid writing a new post but I am completely stumped on my bike. I bought the bike not running and I have recently put it back together and got it running and it runs GREAT...until I hit about 65-70 mph, then it has nothing left. The bike acts like it has a governor on it.

I have recently pulled and cleaned the petcock, replaced fuel lines and inline fuel filter, installed new spark plugs, brand new battery. I have cleaned the carbs multiple times. The previous owner removed the carb boots from the air box, so I decided to install individual air filter pods. I know the bike needs to be re-jetted when I do this. So I re-jetted the carbs...about 8 times. :sick: This got the bike running/driving/idling great until I hit 70 mph. I have installed main jet sizes 102.5, 105, 107.5, 110, 137.5, 130, and 120 and with the pods it ran great with the 130 and 120 jet sizes, but still would not go past 70 mph.

So I decided to throw the towel in on the air pods and installed the original air box and found some tubes that would connect to the carbs. I installed the original pilot and main jets(the 102.5 main and I think 15 pilot) thinking this would fix the problem for sure. The bike ran and sounded great until again I hit 70 mph and it refuses to go faster. I don't know where to go from here.

Does anybody have any advice? I am ready to blow it up.

Thanks in advance.

Assuming the carbs really are clean - check the operation of the timing advancer and lube it. It could be sticking. Ed

Open up the gas cap, on the bottom there should be screws holding it together. Look for a blockage in the air vent. Have you tried running the engine at the same rpm on the center, or side stand to see if it's doing the same thing? You'll probably need a fan for engine cooling.

Welcome to the KZ Rider Forum. Good tips so far. I am with the others here. Also, I am unclear if when running the bike to 70 mph, you are in the same gear. Have you tried different gears such as 3rd, 4th then 5th? Each gear should be at a different RPM when hitting 70 mph. I don't really see how you could hit 70 mph in each gear and have the problem. I did not see where you described in any detail how the bike is behaving except that the bike wont go past 70 mph. Does this mean the engine bogs down? Does this mean your bike behaves as if it runs out of gas? More details can help. I bet the wall you are hitting is at a certain RPM and your bike just appears to quit running as if it is out of gas. The advancer or air/fuel lines are a good source for the culprit.

HERE IS MY $0.02 Have you checked to see if any vacuum lines or fuel lines are kinked? This could lead to a loss of fuel or vacuum sufficient to run at higher throttle openings. If the vacuum lines are kinked, try rerouting before shortening. If the fuel line is kinked then get a stiff spring that will fit inside your fuel line. This will keep the fuel line from kinking and therefore allow fuel to continue to pass at higher draw rates as well as keep from starving your engine of fuel. I have black fuel line and a see through fuel filter. The see through fuel filter allows me to visually see if there is fuel coming into the filter from the fuel line. If you do not see any fuel coming into the filter or carb bowls then you have a restriction, likely a kinked fuel line. If no kink, then you can still check to see if you have any fuel flow by draining your carb bowls and then checking once you prime and begin your engine starting/running process. AND, yes you should be able to check on your center stand. I hesitate to recommend running the bike up to 70 mph on your center stand. Just seen bad things happen to good people that run bikes in gear like that. I would simply run your RPM up in neutral to the level that seems to affect your bike. Good Luck!

Thanks Ed for the tip. I will look into that.

I don't believe this bike came with a fan to cool the engine. The strange thing about the RMS's is that when the bike is in neutral it will rev all the way up. Same thing in 1st, 2nd, and 3rd. I can get 4th to rev all the way up and 5th gear will only go up to 5000 rmps (or 70 mph).

I will definitely look under the gas cap for a blockage, but I don't think that is the issue. I already tested to see if that was the problem by leaving the gas cap open and riding the bike down the road with the same results.

I can get the bike to rev up to 8000+ in 1st, 2nd, and 3rd. 4th and 5th wont because I cant get the bike to go fast enough. I have been in 4th or 5th gear going 60-70 mph and squeezed the clutch and then twisted the throttle and the bike revved up to 8000+ rpms, but the bike wouldn't go faster because I was basically in neutral.

When I hit 70 mph in 5th gear I still am at maybe 1/2 throttle. If I give it more throttle the engine RPM's and speed stay the same. It doesn't bog down (I think, the engine will sound a little different though)and I maintain the same speed. It does slow down a little bit when I do this. Also, there is no backfiring of any kind.

One other thing that might help is this: The bike does run really good and picks up speed decent, but not as fast as I think a bike like this should. I also have a 1979 Honda CB750 and that thing will move! Through all the gears it feels like I am not getting the fast acceleration that I should be getting, especially when compared to my Honda.

I have checked for kinks and there are none. I have clear fuel lines and a clear filter that I can see is getting plenty of fuel.

I have tried seeing how fast the bike will go on the center stand as well, but the speedo cable is running to the front tire, which isn't moving. But I will try again tomorrow and see if it will rev up fast 5000 rpm in 5th gear.

Betting my nickel on a frozen timing advancer.

Good Fortune!

ok guys. I like the idea of this timing advancer. Ive never heard of it though. Where is it? Is it behind my points? also can you describe what I need to do? thanks.

As shown in the quoted post below, it may be feasible to lube the weight arm pivots via access through the timing inspection window.

Depending on the extent of rust and corrosion, each of the two weight arm pivots may simply need a drop or two of oil, or it might be necessary to also remove and clean the advancer.

Where the advancer is removed, be careful when re-installing it to assure it isn't fitted 180 degrees off.

Patton wrote: If not already done, would use a timing light to assure proper functioning of the timing advancer by watching for it to quickly move back and forth as rpm is varied by blipping the throttle.

The advancer may require lubing or both cleaning and lubing.

Likely be able to apply a drop of oil onto the weight arm pivots by access through the timing inspection window.

Make sure the advancer does NOT look like this:

Good Fortune!

Thank you Patton and Ed! It was definately the timing advance. It was rusted shut. What a relief to finally have the bike running like it should! This Kawasaki is fast!

Glad to hear that was the problem! The KZ650 is surprisingly fast when running right. Ed