KZ650 Bore Kit need Cams?

Hi Guys, 

I am new here and just getting into bikes. Just bought a 1977 KZ650 which the previous owner did a ton of work on. The top end of the engine was apart and I am reassembling. His setup was 717cc Action Fours bore kit 10.5:1 compression, 0.41 high lift cams, and 29mm smooth bore carbs. Apparently the bike was great on top end but rough on idle and low rpm. I want to make it more simple and drivable then upgrade single components in future if I want. I am looking at keeping the 717cc kit, but going back to stock cam and carbs. Should I have any concerns about detonation with the increased compression? Any other concerns with a bore kit on an otherwise stock engine? 

Thanks! 

Was that a race bike?  It has some serious parts on it.  The Dymag wheels only really show up on race bikes.  Plus the CALFAB swingarm, the Fox shocks, and the 29mm smooth bores are all period race parts.  That's quite a find.

As for cams, my impression is using stock cams with 10.5:1 pistons may result in very high cylinder pressure.  Maybe consider some mid-range cams?  It would certainly be more money than finding a stock set of 650 cams, but the mid-range cams should have a broader power range than big cams you started with, but still work well with the high-comp pistons.

As for carbs, you could go back to the stock 24mm carbs or maybe find a set of 26mm carbs from a KZ1000.  But you might just try tuning the 29mm with milder cams.  If that doesn't work it's easy to swap another set on at any time in the future.

Just in case you weren't aware, the 29mm are worth a fair bit now, over $1000 often.  The Dymag wheels are probably worth a fair amount too.  Not sure how desirable the 650 CALFAB swingarm is, but it's a cool piece.  Don't let any of those parts go. 

 

Lot of cool info! I think the gentleman who built it was into racing, but didn't race that I know of. I already have a set of stock cams. Is there any cam supplier you recommend? I only found Webcam, but they require sending cores. Is upgraded cams considered a requirement for running high compression pistons? The bike has electronic ignition and I planned to run high octane regardless. 

My goal is to get the bike running and get comfortable with it. Then I can add components and play with performance at my own pace. 

You probably don’t want to hear this but it sounds like that bike was well put together, it should have stayed that way. Period items like that don’t come up very often. Lots of 650 on the market today for still a good price. I completely understand it’s your money and you build what you want, just a shame to see a well built bike devolved. 

The previous owner who built the bike said it was very rough at low RPM. He said he was constantly playing around with the carbs to try and get it to run better, but never found the right setup. Do you think that could be attributed to too large of cams (0.41" lift) or too large of carbs (29mm)? I'm open to using the high performance parts but want it to be drivable around town. I bought the bike with the engine disassembled as he planned to replace the valve guides, so I am that deep into it anyways. 

Appreciate the advice, as this is all new territory to me! 

I rode around with stock 900 cams after I bored it out to 1015cc. It had more grunt and ran well overall. Got my Andrews 1X cams then 33's later. Every time it gets better. With the 1075 kit I have in it now I'll be getting some larger midrange cams soon.
Steve

wrote:

Is there any cam supplier you recommend? I only found Webcam, but they require sending cores.

 

You could also try Megacycle:   www.megacyclecams.com/

But this thread suggests that no one has blanks, and the only way to get performance kz650 cams is to have cores welded.
kzrider.com/forum/2-engine/386350-kz650-choice-of-cams

If you can afford it, it would be straightforward to buy a set of cheap kz650 cams on ebay and have them welded.

Like this set for $3.99
ebay.com/itm/264522992602

There are later cams that are higher performance than the KZ650, like the GPz750 cams (and maybe some models of KZ750?).  But it would take time and effort to find a good set.

Is upgraded cams considered a requirement for running high compression pistons? The bike has electronic ignition and I planned to run high octane regardless. 

I'm definitely not an expert, but my impression is that longer duration cams essentially let some of the charge escape before the valve closes.  This reduces the effective compression ratio (dynamic CR vs static CR).  So long duration cams require high compression pistons to make up for the loss in cylinder pressures.  Folks don't really tend to run the opposite combination (high CR with very mild cams).  But it seems like you might end up with cylinder pressures higher than you'd like, which may lead to detonation.  These old KZ have a very simple cylinder head design that doesn't have any of the fancy features of modern engines that allow modern bikes to run very high compression ratios.  They are also air cooled.  Detonation and heat are both risks if cylinder pressures are too high.

You could potentially play with the cam timing which adjustable cam sprockets, but that's pretty advanced stuff.

From this article:  www.motortrend.com/how-to/engine-detonation-knock-explained/

A performance camshaft with higher duration and lift is one of our favorite ways to gain performance from an engine, but sometimes the cam choice can cause unintended problems. The more common problem is a cam that is too big for the compression, whereby the valve timing is too aggressive for the static compression ratio and a loss of low end occurs. Sometimes, however, a camshaft will have a wide lobe separation angle that can trap too much charge in the cylinder. How will you know if the overlap is too short? A quick test of the cranking pressure will tell you: Anything over 180 psi is an indication that you're in the realm of a high-compression race engine that may need race fuel. A good, safe number to shoot for on a traditional pump-gas street engine is between 150 and 170 psi. If the cranking pressure is too high, you need a cam that traps less air/fuel charge in the combustion chamber.

wrote:

Do you think that could be attributed to too large of cams (0.41" lift) or too large of carbs (29mm)? I'm open to using the high performance parts but want it to be drivable around town. 

Do you know what brand and model/grind of camshaft the .410 you have is?  Megacycle lists a .410 with a pretty modest duration (250°).  Definitely on the bigger side, but many folks get away with it in a street bike. 

The 29mm are also on the bigger side, but folks ran those on KZ650s and GS and CB750s back when so it's not totally ridiculous.  At the same time, the KZ1000 (1015cc) came with 26mm carbs for several years, so the 29s are probably more than you need.

The reality is that no matter what you do, if you stick with the 717cc kit at 10.5:1 you have no choice but to become and expert on carb tuning.  You're already outside of what would work with the stock jets, and I'm guessing you don't have an air box on that bike either.  You're in for an adventure no matter which carbs you select.  The most likely reason it didn't run well in the past is that carb tuning is tricky, and the last guy didn't manage to get it quite right.  Just going back to the stock carbs and cams isn't going to save you. 

Oh, what exhust system did it come with?  I'm curious if that's another cool vintage piece.

It was rough at idle and the low end because of the cams and the carbs. Those cams probably do not give too much vacuum at idle, and the way the slide ends are on the VM29'S (hollow bottoms) would create a bog at low end throttle pulls. You may be able to set the cams up at a low lobe center (104deg) and that may help the idle but there is no way around the way the slides are designed on the 29's. There is nothing wrong with those carbs, they were designed as a race carb not a street carb - They run great over half throttle. You can tune the carbs to be better but it takes time and patience.

If you have .410" cams in it then I would suspect you may have some port work done and possibly some bigger valves installed. A bigger port will effect jetting and the way it runs at idle. Too big a port reduces velocity, there is a narrow window where its correct to cam lift and carb venturi. Stock 650 valves are 28mm EX/33mm IN, oversize would be 29/34 or 30/35. You should also check to see if its a 650 head, a 750 head, or a 750 GPZ head.  A 650 head uses a tach cable, some 750 heads do not have a tach cable, and the GPZ does not use one at all.

There is no harm in running 10.5:1 pistons on the street, they may require you to run premium gas if your in a hot location. I don't think you need to worry about detonation. That is the biggest piston you can run in a stock liner so I would suggest keeping the oil cooler.

There is no issue running a bore kit with stock cam or carbs. That piston kit has been superseded with the Wiseco 700cc kit (10.25:1 comp) which generally can be run without any other engine changes.

I would be careful to check that engine to see what year it is.  It has no kicker, so it looks like a later engine.  Knowing exactly what year engine it is is very important because Kawasaki made some important changes after 1977. Below is a link to a site that shows the engine serial number / date.   Ed

models.kz650.info/index.htm

That is a early clutch cover, kicker hole is using a block off plug or cap. The later motors with no kick starter have a cast boss but no hole. The 750's are flat where the kick start hole would be.

I think 29 mm carbs are a bit oversize for that engine.  They probably don't get enough air flow to run properly.  The original 24 mm should be OK but might struggle a bit at the top end.  A set of 26mm off a '76 KZ900 would solve that issue if you ride that fast.

At the higher compression with stock cams you should not have detonation problems but I would use high test gas instead of the stock recommendation of regular.