saving a 1980 KZ750 twin

doc_rot

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cb360j said:
Wow! This is fantastic... Is the headlight/speedo a stock unit that came on these bikes?
The tach/speedo is by speedhut. I fit a screw in retainer ring into a universal headlight to hold it. The details are on page9-14
 

doc_rot

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All this prep was to take the bike on a road trip with a good friend. Him on his 1978 650 B we rebuilt together a couple years ago. The bikes performed great. The only issue being my exhaust hangar bolt vibrating loose at one point, but a field fix was easily had. We did 900 miles in 5 days through the Cascade mountains of Washington state. Beautiful riding crisscrossing through the mountains. We strictly stayed on secondary highways the whole way and even got a good amount of forest service gravel road riding in. We camped out in the national forests every night, went to a different brewery every evening to fill growlers up to take with us and cooked out over a fire every night. Washington state has an incredible amount of ecosystem diversity, the riding was awesome. Good weather most of the way, We did ride through a small squall but other than that it was lovely.
 

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ridesolo

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Your bike looks excellent and that sounds like a great trip. Did you happen to cruise through a town called Concrete while you were crisscrossing through the mountains?
 

doc_rot

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So now we get to the bad news. Just before I left on the trip I noticed a sound coming from the top end. It was a ticking noise that had a ring to it, almost like a rattle. It sounded like it was coming form the valve cover and only on the right side. I checked all the valve clearances, and and checked the cam journals with plastiguage and everything seemed to be fine, cam chain looked ok. I thought it might be the cam sprockets making extra noise since i removed the rubber dampers from them in order to slot the sprockets to time the cams. The sound got worse over the course of the trip and I noticed the oil level was low, and dark. Indicating some blow by. Compression was still good though - 220 psi on both cylinders. Blow by got so bad that riding around in LA after the trip I burned through 1.5L in about 50 miles, which ended up running low on oil without realizing it. ::) I tore the top end off to find the wrist pins galling and nearly seized in the pistons again this may have been from the low oil levels, but I cannot be sure. Also an incredible amount of carbon on the piston domes and in the chambers. I guess the cheap piston rings weren't a good bet. either that or the 1 piece oil ring was doomed from the start. The wrist pin bore on the pistons are trashed so I have been exploring other piston options. I dunno if it was my poor assembly, the one accidental 11k over rev I had on the fresh build or something else that caused the problem but either way it will not be a cheap mistake to remedy.

I have been doing a bunch of research on piston options to move forward with. As far as I can tell there are 4 choices to move forward.

1) find some OEM overbore pistons - about $350-400 with rings and wrist pins depending on what deals I can find. I'm not really considering this because I want MORE POWER!!

2) Custom JE pistons at 80mm bore - 783cc . custom pistons w/ pins and rings - $400 for two - min order of 4 pistons, I could probably sell the other set. This allows me to use the stock sleeves and stock head gasket.

Anything bigger than 80mm bore will require new sleeves. the cost of new sleeves is 65-150 each depending on what needs to be done, plus an additional $65 to bore the cylinder for new sleeves. It will also require the cases bored for the bigger sleeves - Something I can safely do myself either on a friends mill or with a die grinder. Cometic already has a 85mm gasket for this bike for $55 each. That is about what I was paying for the OEM green gaskets.

3) Other platform piston -ie. Raptor 350 84mm piston - 864cc $260 for two. May or may not work with what i have, probably less than ideal for the combustion chamber design I have.

4) Custom JE piston at 85cc - 884cc again I end up with a second set


The total price breakdown with boring and a gasket will be

1)OEM $570-620

2)custom 80mm $620 + $400 in pistons that i may or may not be able to sell

3)Raptor 84mm $735-915

4)Custom 85mm $855-1055 + $400 in pistons that i may or may not be able to sell

The benefits of getting custom pistons are they will be optimized for the combustion chamber design, and I can have custom features included to help with wrist pin galling, as this is the second time its happend on this bike. for example - forced pin oiling - where a hole is machined from the oil ring to the wrist pin so as the piston drops in the cylinder it forces oil directly to the wrist pin bore.

I'm going to drop my cylinders off at LA Sleeve which is like 1/4 mile from my work. When I get a hard number on the cost of the sleeves, this will give me a concrete budget to look at.

Any thoughts or suggestions?
 

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ridesolo

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doc_rot said:
Yes. We rode out on highway 20 day 1
Been there many times, my brother lives in Concrete. Really pretty country for sure.

Really sorry to hear about your need for a rebuild. Hope you are able to come up w/ a workable and reliable solution that doesn't bust the bank!
 

Jimbonaut

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Ah, bollocks. Sorry to read this man, I got nothing. Other than good vibes and a beer with your name on it.
 

der_nanno

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Interesting damage to the engine...

Did you check the AFR and maybe even for knock to see what your ignition advance is like?
 

doc_rot

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I had the bike on a dyno, AFR was pretty spot on just a tiny bit lean in the midrange, I got new needles but before I could put them in the bike came apart. I didn’t hear it knocking, and so far I haven’t seen any evidence of that. The crank vent was connected to the air box and the air filter was saturated with oil indicating quite a bit of blow by. I think the rings were not up for the job. But that still doesn’t explain the ticking sound.
 

der_nanno

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The last bit is where I am coming from: Ticking sound and judging by how white the oil-deposits are, extreme combustion chamber temps, that could quite well be indicative of some sort of detonation. What's your squish clearance and do you have a built in squish area to keep things cooler? (By the looks of it, the chamber appears to be a hemisphere...)
 

doc_rot

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der_nanno said:
The last bit is where I am coming from: Ticking sound and judging by how white the oil-deposits are, extreme combustion chamber temps, that could quite well be indicative of some sort of detonation. What's your squish clearance and do you have a built in squish area to keep things cooler? (By the looks of it, the chamber appears to be a hemisphere...)
squish at the edge was about .040" The ticking had a ring to it, like a rattle almost. it didn't sound like detonation. It also was only coming from one side.
 

irk miller

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220psi is high. Everything you’re describing suggests to me high compression and pre-ignition. That’s going to produce blow by, or oil burning off. It would explain the galling. High compression produces high heat. Could also be too much advance, but with that high of compression, seems like pre-ignition. Even the sound you’re describing. The ringing says it all.


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doc_rot

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why would it only be in one cylinder though? and why would it get worse over time? i guess it couldnt hurt to add a knock sensor. Nanno - can you recommend a knock sensor to retro fit? any tips on doing so? thanks
 

doc_rot

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I found a video of the sound. preignition?

https://www.youtube.com/watch?v=GdvnSl0y9Vs
 

teazer

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Some pin galling but no signs of detonation on pistons or the head.

Are you sure about that 220 psi number? That's really high - as in 15:1 type high. I would get a set of rings, clean up the carbon and find a way to get the compression down or arrange for a truck with 112 Octane gas to follow you around. I would also go richer on the midrange which is where the motor spends most of its time.

You might get away with retarding the timing but I'm not a big fan of that. What do the plugs look like? do they show signs of overheating on either center or side electrodes and what heat range are they?
 

doc_rot

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teazer said:
Some pin galling but no signs of detonation on pistons or the head.

Are you sure about that 220 psi number? That's really high - as in 15:1 type high. I would get a set of rings, clean up the carbon and find a way to get the compression down or arrange for a truck with 112 Octane gas to follow you around. I would also go richer on the midrange which is where the motor spends most of its time.

You might get away with retarding the timing but I'm not a big fan of that. What do the plugs look like? do they show signs of overheating on either center or side electrodes and what heat range are they?
isn't cranking compression more related to the intake valve closing? for example you could have a 15:1 engine with a long cam duration and a very late closing intake valve that would crank the same compression as a 8:1 with a very small duration and a very early intake closing event.
I read 200-220 was OK for a air cooled performance engine with iron liners.

that being said i could advance the intake cam 1 degree to lower the cranking compression.

After looking at the pugs more carefully it does look like they are little hot. Stock calls for B6ES I was running a BR7ES. I could easily go up to a BR8ES.

sorry for the weird photos I was using a magnifying glass to get a better close up with my phone.
 

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der_nanno

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doc_rot said:
why would it only be in one cylinder though? and why would it get worse over time? i guess it couldnt hurt to add a knock sensor. Nanno - can you recommend a knock sensor to retro fit? any tips on doing so? thanks
The reason why it's only on one cylinder can be quite a few, the most harmless being slight differences in piston height or ever so slightly twisted crank (not sure if the 750 is a pressed roller crank like on the 900s and 1000s/1100s).

With regards to a knock sensor, ages ago I built this, using (I think) a Nissan knock sensor, but pretty much any sensor will work, it might be tuned to slightly different frequencies, depending on which car you take it from.

http://greasygreg.blogspot.com/2016/03/one-from-workshop-diy-knocksensor.html

Also judging by your last post's pictures: you have some engraining and perhaps even slight erosion on the exhaust valve, suggesting that combustion chamber is THROUGH THE ROOF. Your 0.040" squish clearance (roughly 1.016mm), is right on the limit of where a squishband becomes effective. So tightening that up to 0.90 or 0.85 mm (if it's a plane-bearing crank) will cool down the ignition notably. My TR1 lost about 20 to 25 degrees of oil-temperature due to improved squish (from 1.5 to 1.6mm down to 0.95 and 1.00), just from the more effective mix in the combustion chamber.

Cheers,
Greg
 

doc_rot

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der_nanno said:
The reason why it's only on one cylinder can be quite a few, the most harmless being slight differences in piston height or ever so slightly twisted crank (not sure if the 750 is a pressed roller crank like on the 900s and 1000s/1100s).

With regards to a knock sensor, ages ago I built this, using (I think) a Nissan knock sensor, but pretty much any sensor will work, it might be tuned to slightly different frequencies, depending on which car you take it from.

http://greasygreg.blogspot.com/2016/03/one-from-workshop-diy-knocksensor.html

Also judging by your last post's pictures: you have some engraining and perhaps even slight erosion on the exhaust valve, suggesting that combustion chamber is THROUGH THE ROOF. Your 0.040" squish clearance (roughly 1.016mm), is right on the limit of where a squishband becomes effective. So tightening that up to 0.90 or 0.85 mm (if it's a plane-bearing crank) will cool down the ignition notably. My TR1 lost about 20 to 25 degrees of oil-temperature due to improved squish (from 1.5 to 1.6mm down to 0.95 and 1.00), just from the more effective mix in the combustion chamber.

Cheers,
Greg
what do you mean by through the roof? That squish was as tight as I could go given the pistons I had. I decked the head .050" and that was a much as i could take off before getting into the valves

moving forward as I likely am going to go the custom piston route; would it make more sense to use the stock head with a taller piston to get more squish area and a 10.5/1 comp, rather than the decked head with a smaller squish area and a 10.5/1 comp?
 

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