saving a 1980 KZ750 twin

doc_rot

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Parts back from machinist. Assembly continues.

I went with these 1mm over pistons from Scheuerlein in Germany over NOS pistons/rings because they use a 3 piece oil ring design where stock is a single. The pistons are coated which allows for a very tight piston/cylinder fit. The quality is high, they are faithful copies of the OEM pistons.

I usually bench shim the valves if the head is off so i don't have to mess with it on the bike. it looks like i have alot of shims but they are all in the same sizes from bikes i have parted out, luckily I had exactly what i needed from the last head rebuild.

Anytime you have the head or cylinder block decked you need to check and make there the oil passage is de-burred and has a chamfer, otherwise you can cut the o-rings as you press in the oil restrictors.

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doc_rot

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Annnnndddd its a runner. seems to be pretty close on jetting, runs great. put about 10 miles on it. The new rotor surface totally removed the chatter from braking. The only thing that seems to need to be addressed is it needs a new starter clutch, bad. The stock engine is reminding me why i hot rodded it - leaves me wanting more power. Its a fun rider though.

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Tanshanomi

New Member
I love it!!!

EDIT: Perhaps I missed this earlier in the thread, but are those Z2 side cover emblems?
 
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doc_rot

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I love it!!!

EDIT: Perhaps I missed this earlier in the thread, but are those Z2 side cover emblems?

thanks! Yes they are the z2 emblems. I liked them better than the stock kz750 emblems
 

doc_rot

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Changed out the starter clutch this weekend. This really is a crap design from Kawasaki. I pulled out an old starter clutch that had only 1000 miles on it since full rebuild and one of the springs was already deformed. and the engagement surface already had some roller marks.

Instead of the mickey mouse crap i have been doing to hold the rotor I finally made proper rotor holder. Makes it so much easier to remove and install the rotor.

I also swapped the foot pegs out for these rubber Tarrozis, dropped a jet size on the main, and did the full break-in service including re-tightening the head studs which is always a PITA because you have to remove the blind plugs under the valve cover to get to the internal studs.

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doc_rot

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The straight through free flowing megaphone muffler i made is pretty loud, I was OK with that when this bike had a performance oriented engine, now that it is a very mild hop up I want to tame the exhaust sound a bit.

To try apply some science to this completely non scientific venture I bought the cheapest dB meter off ebay to measure results. I doubt it is accurate, but it is probably adequate for testing the differences between the options.

The two options I wanted to try to reduce sound are just inserts so I would not have to modify the muffler. The muffler has an 15" baffle with a 1.75" 16GA perforated tube. It is wrapped with a layer of stainless steel mesh, followed by stainless wool, and then fiberglass.

Option 1: The classic "dB killer" style inserts that i have seen for years. I used a 1.25" tube - 4" long - (20) 3/8" holes. I added some neoprene foam around the flange so i could just tape it in the exhaust and get a good seal.

Option 2: "laminar flow cone" style muffler from Flowmaster. I know the Flowmaster has engineers sciencing out all the parameters for their cone designs to get optimal results, I created mine based on the existing tube diameter, and an arbitrary length of 4.5" I used a cone calculator to draw a template on 20GA stainless perforated sheet with 1/8" holes - 50% open area. I then used a tapered punch and a hammer to form the sheet into the cone before welding the seam and adding a flange. I then riveted the cones on either end of the existing baffle pointing in.

The results were:

Existing muffler - 84 at idle 102 at 4k RPM
Db killer - 81 at idle 99 at 4k RPM
Laminar Cones - 84 at idle 100 at 4k RPM

The Db killer does make a noticeable reduction in sound, however it modifies the tone and sounds tinny and cheap.

The laminar cones also make a noticeable reduction in sound, but retain a very low tone. it actually sounds better than the straight core, more of a rumble.

Both the inserts took away the sharp percussive nature of the straight muffler so I consider that a win, however neither of them are significantly quieter. Its OK for now but i probably will return to this to see if i can quiet this down further. Unfortunately I think this means a major revision to the internals, something i was hoping to avoid, as adding them will probably mean cutting the muffler open which i really don't want to do. I cant seem to find any good info on how to design muffler baffles, so it will probably be mostly trial and error. I think i may try a stock style baffle system.




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Jimbonaut

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Proper nice bike man. I can't remember if you modded the engine or not - I overbored my CB750 to 823 with CB900 overhead cams and pretty sure that combination blew out my starter clutch, or at least contributed to it. I've since rebuilt the thing and while it's better it still huffs and puffs to turn the engine over, especially cold. Cool what you're going with that exhaust too - I'll be watching closely as I have little to no understanding of how baffles work and what can be done to modify them.
 

doc_rot

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Thanks. This engine is pretty mild. Mostly stock save for a light port and polish, 1mm over pistons, flatslide carbs, and custom exhaust.

Compression should be lower when cold so in theory it should be easier to turn over. have you checked your starter motor?
 

Jimbonaut

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Thanks. This engine is pretty mild. Mostly stock save for a light port and polish, 1mm over pistons, flatslide carbs, and custom exhaust.

Compression should be lower when cold so in theory it should be easier to turn over. have you checked your starter motor?
Yeah, rebuilt that too. It works well, in fact the turning starter clutch may have more to do with the thicker engine oil I'm now using. Still, a world away from where it was before I rebuilt the starter clutch - the springs looked like yours only way longer.

I was going to ask you what the science is behind the baffles you're making, but just re-read your post and saw it's a grip it and rip it type situation. Nice :cool:
 

3DogNate

"You Meet the Nicest People on a Honda"
Whelp, I finally pulled my finger out and finished the header. It came together pretty nice, but I did have to fudge the fitup a little bit where the second bend meets the straight. You can't see it when its on the bike and I doubt it will affect flow very much. Eric made this sweet tool for putting a uniform finish on headers and it worked great, started with 80 grit and then a polishing belt for a uniform brushed finish before welding it all together. My welds came out OK, still a bit too hot, I'm undecided if I want to polish the heat marks off or not. I also need to start thinking about what I want to do about the muffler, thinking about a kerker-esque stainless can for that vintage performance vibe.

Dude.... nice... I need to make a set of headers for my KZ750 Twin. this gives me ideas.... really bugs me that there are no aftermarket exhausts available. I'm still torn between 2-1 or 2-2 with a a cross over.
 

doc_rot

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Thanks! building exhausts is fun, and the weight reduction alone is worth it.
 

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