KZ650C runs great up to 5000 rpm

Hello,

This forum is great, I've read many good tips.

I have a new to me 78 KZ650 that starts and runs good up to 5000 rpm then really strugles to get to 6000, it goes no higher than that.

I should have my repair manual in the mail in the next day or so, with this in hand I'll start checking the electrical side of things to see if I can find a problem befor I tear into the carbs.

I've taken the gas tank off and checked out the petcock valve. I had a tiny bit of rust on the metal screen, I clead the petcock up and re-installed the tank, and my fuel line is not kinked, so I'm guessing the fuel is making it to the carbs ok. Riding after the petcock cleaning changed nothing. I have a stock airbox and pipes, new airfilter in place, and spraying wd-40 around the outside of the airbox by carbs while motor is running doesn't change anything(no air being sucked in where carbs meet the airbox).

Spark plugs do show black carbon fouling, I have a new set in hand and am ready for a chop plug run. I have a question though, I can only run up to 6000rpms, and it runs rough there. DO I just run upto 6000 rpms for 30 sec and cut motor (kill switch or Key?), or do I need more rpms for it to work.

After the chop test, I'll do the tests in the repair manual to see if its spark related. Then post back here to see if the carbs need work. The bike starts fine, and purrs really nice upto 5000 rpms, so I'm guessing this is more spark related than the carbs.

Thanks in advance for all your tips and help.

Good day

Brad
Salt Lake City, Utah

Post edited by: BradL45, at: 2006/09/01 17:27

First, the plug chop is a major waste of time if you can't get the bike over 6K rpm.

If you have an inline fuel filter, change it. If it is new, change it anyway. If you bought one with an insufficient gas flow, it will restrict gas flow at the top. You have a stock airbox/filter and pipes. Unless the carbs were rejetted lean, I can't imagine why they would run out of gas on top (main jets). It does sound like the main jet is too small. Find out what jets you have. Last, make sure you have vacuum caps on the carb holder vacuum ports and they are not cracked or damaged. Make sure your vent lines are not obstructed nor you gas link KINKED or has a real sharp bend. It really sounds like you are gas starved on top.

del

Hello,

I'll check the vac lines, and vent tubes for kinks, I don't have an inline fuel filter and the fuel line isn't kinked (I'll add an inline fuel filter when this is fixed). I did remove the fuel line to the carbs and it flows fuel just fine. I'll also check the caps on the carbs, they are in place, but i'll check to see they are not cracked.

Thanks for stopping the plug chop.

Can I ID the jets without pulling the carbs from the bike?

The bike is a 1978 KZ650H, this is what is on the title, but those are not always correct. Is there a post detailing the difference between models?

Thanks for the tips.

Good day

Brad

Check the VIN number stamped on the headstock, it should also be on a white tag on the headstock.

Do you have points or electronic ignition?

If it pulls to 5000 rpm and just sits there, check the timing advancer, it's probably stuck in the retarded position.

If it goes to 5000 rpm and starts missing out or surging, then it's running out of fuel.

The timing advancer is located under the small round cover on the right front side on the engine cover, it's the thing that opens and closes the points (I'm assuming it's points since it's a 78). It should rotate easily in the counterclockwise position and return to it's original position when you let go of it.

Check condition and gap on the points while you are there

Post edited by: steell, at: 2006/09/01 11:10

Hello,

All plugs and lines to carbs are ok, no kinks or leaks.

Looks like the bike is a KZ650C, thanks for the tip to look on the frame.

I'm not sure I have points, but took pictures of my parts under the cap on the right side of the engine. These parts look brand new, I'm getting a bit worried, I just bought the bike knowing it needed adjustment to get the bike running smoothly. I got the title with the sale as well as 5 bill of sales from previous buyers that never registered the bike in the last 18 months. I'm thinking they couldn't fix the problem and keep selling the bike to another victim.

My manual didn't come in the mail today, I hope I can get it tomorrow, so I can work on the bike this weekend, including looks at the jets installed.

Thanks for all your help.

Brad

Post edited by: BradL45, at: 2006/09/01 16:53

Picture of Ignition parts, points??

Carb shot, what kind of carbs are these??

VM24's I think, but I'm no carb expert (just ask around )

Now what I want you to do is grab hold of the round thing behind the two hex (one bolt and one nut) in the center between the two sets of points in the picture, and see if you can turn it counterclockwise and if it returns to it's original position when you let go.

Here is the part I'm trying to rotate. I can NOT rotate counterclockwise, I can rotate it clockwise for a short amount (hard to grab w/ fingers), and it does rotate back counterclockwise on its own. This part is hard to turn, and also isn't square with the 2 bolts that hold it in place.

Should it rotate freely? Maybe we have found or problem, my fingers are crossed.

Thanks for your help.

Brad

Welcome Brad.

Here's a few links on the 650s: www.kz650.info/
and Kz650s .

The cam lobe (where your arrow is pointing) is not "square with the two bolts that hold it in place". It is a cam with a high point (lobe). The lobe opens the points at the right time to fire your spark plugs.

Like Steel mentioned, your advancer could be stuck and in need of some cleaning and lubrication.

I'd suggest you remove the points plate (first mark it so you know the exact position it's in, or you'll need to re-time it, although you should check the timing at some point soon). Then remove the advancer and give it a once over.

A service manual would help with this. I'll see if I can scan the related section for you.

Post edited by: rstnick, at: 2006/09/01 22:00

My fault, it should rotate "Clockwise", I'm to used to the 750 twins (they turn backwards compared to the fours)

It should rotate around 1/8 of a turn or so, and return on it's own, so it appears to be working. To take it off and clean/lube it, you need to remove the points mounting plate and the small bolt head (13mm I think) in the middle.

How about my other question on how the bike is acting when it hits 5000-6000 rpm?
Does it just run out of power or does it start missing/surging/bucking/or what?

The correct procedure is to do a tune up and check the entire ignition system before proceeding in any case. If you do a search on the site this has been covered many times.

Here is one

And another

Post edited by: steell, at: 2006/09/01 22:03