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Author Topic: saving a 1980 KZ750 twin  (Read 85580 times)

Offline doc_rot

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Re: saving a 1980 KZ750 twin
« Reply #590 on: Jul 29, 2019, 12:00:34 »
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 didnít hear it knocking, and so far I havenít 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 doesnít explain the ticking sound.

Offline der_nanno

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Re: saving a 1980 KZ750 twin
« Reply #591 on: Jul 30, 2019, 02:18:49 »
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...)
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Offline doc_rot

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Re: saving a 1980 KZ750 twin
« Reply #592 on: Jul 30, 2019, 03:09:27 »
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.

Offline irk miller

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Re: saving a 1980 KZ750 twin
« Reply #593 on: Jul 30, 2019, 11:28:39 »
220psi is high. Everything youíre describing suggests to me high compression and pre-ignition. Thatís 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 youíre describing. The ringing says it all.


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Offline doc_rot

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Re: saving a 1980 KZ750 twin
« Reply #594 on: Jul 30, 2019, 13:17:53 »
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
« Last Edit: Jul 30, 2019, 13:33:33 by doc_rot »

Offline doc_rot

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Re: saving a 1980 KZ750 twin
« Reply #595 on: Jul 30, 2019, 13:31:40 »
I found a video of the sound. preignition?

« Last Edit: Jul 30, 2019, 13:39:30 by doc_rot »

Offline teazer

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Re: saving a 1980 KZ750 twin
« Reply #596 on: Jul 30, 2019, 20:13:29 »
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?

Offline doc_rot

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Re: saving a 1980 KZ750 twin
« Reply #597 on: Jul 31, 2019, 02:53:58 »
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.

Offline der_nanno

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Re: saving a 1980 KZ750 twin
« Reply #598 on: Jul 31, 2019, 07:26:08 »
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
Real freedom starts with the freedom of thought.

My Blog:
http://greasygreg.blogspot.co.at

My Mule TR1.1 build here on DTT:
http://www.dotheton.com/forum/index.php?topic=75458.0

My Turbo TR1.1 (XV1000) build here on DTT:
http://www.dotheton.com/forum/index.php?topic=72571

My XS Triple Sidecar build here on DTT:
http://www.dotheton.com/forum/index.php?topic=74211

Offline doc_rot

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Re: saving a 1980 KZ750 twin
« Reply #599 on: Jul 31, 2019, 11:37:19 »
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?