1977 KZ650 drag bike

Hi guys. I'm a newbie, so be gentle.

I have an old KZ650B1 that was built by a local drag racer back in '78. I bought her in '80.

She's over bored 4 mm over to a 739cc. She has custom Venolia pop-up pistons, RC Engineering 400 cams, 29mm smooth bore carbs, and a RacerOne header. The original builder had ported and polished the stock head, but she still has stock valves. She was a real screamer back in the day.

Anyway, I bent a valve about 25 years ago (stock cam chain tensioner), and she's sat in the barn since.

So I've decided to resurrect her for occasional street riding, and I'm looking at a few different options here.

Now let me preface this by saying that I haven't messed with bikes in 25 years. So I am green to say the least. But I do know my way around a wrench as I have built a few car engines.

I will be doing a complete rebuild, but my question at this time is in regards to the head. On the original head, two of the exhaust studs are broken off. I tried drilling one out with a hand drill (OK, so I was was pretty dumb when I was 21). Plus the guide with the bent valve is hosed. So this head would need to be re-worked.

I've already purchased another stock KZ650 head (a '78).

Since I stared this project, I have heard that the KZ750 head was a bolt on for the '77 KZ650, true or false? Also I heard that it has 1 mm larger intake valves, 2 mm larger exhaust valves, and that the exhaust port was slightly improved. I ask because there is a local guy who has an '81 KZ750E head that I could purchase.

A GPZ head is out as the combustion chamber would not match my old Venolia pop-up pistons.

So what would be better?

1) Get the '81 KZ750 head, and port and polish it.

2) Get the KZ750 head and install the valves in one of the two KZ650 heads I currently have (I can get the head for less then the cost of one valve). Checking with dial calipers, the valve relief's in the Venolia piston are plenty big enough.

3) Or have the original head re-worked and new guides and valves installed in it. I suspect that this would be the most costly option.

Thoughts, opinions, ideas please!!

Welcome to KZR. I think I'd put the bigger valves in the second 650 head. But that's just me. I've had a hopped up 650 since 81 with Yoshimura parts and love it.
Mark

my 1.50, price of gas increase from 2 cents.
first question, what is the compression ratio of the pistons, ans are you planning on running pump gas.

next, if you are going to be doing any kind of racing, and since it has sat for so long, I would go with new 1 piece valves, and new springs and keepers.

I would do this work on either head you choose, btu my gut is telling me to reowork the head you have, unless it is damaged way too much.

Also last but not least what is the condition of the bottom end, since it has been run hard, a good check prior to going back together would be needed

Hey Mark thanks for the welcome and the suggestion. I was also kind of leaning towards the bigger valves in the 650 heads. But then I thought if the 750 was a bolt on and the exhaust ports were better, why not? Did you have a specific technical reason that you would stay with the 650 head, or was it just from a "purist" stand point?

BSKZ650, good question regarding the compression. Those pistons have a pretty substantial dome on them. I contacted Venolia, but unfortunately they never officially carried pistons for the KZ650. These were a custom job. I would guess them to be somewhere between 11:1 and 12:1. I will do an actual compression check once she's back together.

I doubt I'll do any track racing, maybe just a blast down a country road. But yes, new springs and keepers for sure. And I was leaning towards new/different valves also.

It would be nice to keep the original head if it's not cost prohibitive. I am looking around for a local machine shop that specializes in motorcycles (I'm just outside of Dayton, Ohio). I take this as two votes for a 650 head.

Yeah, I had planned on splitting the case. That's why the questions now, figure I have all winter.

Thanks for the comments gentlemen. Anyone else?

can you post some pictures of how bad the exhaust studs ended up?
maybe a good machine shop can correct any mistakes you might have made.
replacing guides is not that big a deal for a good shop.
we have one of the premire names in kz engines here as a member.
it all depends on how much you want to spend.
www.aperaceparts.com/resources/heads/DSCN2272.JPG

sounds like it is a hell of a motor and the po had it dialed in right,might as well bite the bullet and fix her right.

btw, we all want pictures.

Won't the 750 head drop compression a bit?


I remember that being posted in some threads about 810 builds (either decking a 750 head or 750 valves in a 650 head to bump the compression)....

I may be wrong on this because it hasn't been mentioned in any recent threads.

If you want to run it on pump gas you may need to drop compression a bit, 12:1 seems a bit high for old 87.

what level of budget are you dealing with?

any thing is possible so set some limiting factors for yourself.

get organized and make a plan. stick to the plan.

big valves will require clearance work on the piston crown valve pocket area.

milling the head will require a different camchain adjuster

big cams will need to be degreed for valve to valve clearance AND valve to piston clearance

GPZ 750 pistons head cams cylinders carbs slide on with stock gaskets and will overdrive the 650 clutch plenty easy.

you'll be putting in many hours the more exotic you go.

Yeah, the original builder was able to knock down a mid-11 at the track (with a slick and NO wheelie bar). He was a big guy too, 200lb plus.

I have one old picture from back in the day (see attached). Currently she's in pieces scattered about the garage, so not much to look at. All the chrome has flaked really and somebody dumped brake fluid on my lacquer paint job before I parked her 25 years ago.

I'll try and get a photo of how badly I butchered the stud this evening.

If the 750 head would drop the compression a bit, that might not be a bad thing. Then I could run a little bit more initial timing. We do have Sunoco 94 here in Dayton, and I wouldn't be afraid to run octane booster on top of that. Or maybe 50/50 race fuel and Sunoco 94 if need be. It won't be a daily commuter, just a take it out for some fun bike.

Budget is kind of open ended. If it cost more, it will just take me longer. My mental budget for the engine/drive train is about $2,500.00. Of course if I can get by for less, then I will be able to start on the other stuff (tires, suspension, paint, chrome, etc.) that much sooner.

wow that was a nice looking bike!

if you dont drop the compression, then you will be needing to run fuel or a combo of both.
Forget the octane booster, that will give you nothing but a hole in the piston, it dosent mix even,,, been there done that.
Sounds like you have a pretty good budget for the bike, and I would think it would come in a lot less than that.

BSKZ650 wrote:

Forget the octane booster, that will give you nothing but a hole in the piston, it dosent mix even,,, been there done that.

Thanks a lot for the octane booster tip!!

PLUMMEN wrote:

wow that was a nice looking bike!

Thanks.

Thought it was about time to restore the ol' gal to her former glory.