saving a 1980 KZ750 twin

doc_rot

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Got the head back from the machinist. removed .050" which is as much as is possible considering the last pass ever so slightly skimmed the edge of the intake valve. I think it is only possible to remove this much because I had a 3 angle valve job done, which sunk the valves in a bit.

New combustion chambers are 47.7cc and 47.8cc which brings the compression ratio to about 10.6/1 which is a little higher than I was aiming for but totally fine.

I clayed the head and the combustion-chamber-to-piston clearance is now about .030-.040" around the edge which is much better. The clay i used was too sticky so it was hard to be super accurate, however it looks like there is plenty of room.

The only problem now is that because the head sits closer to the pistons the exhaust valve has pretty much a zero tolerance fit on the edge of the valve pocket, however there is ton of room for valve lift, 0.220"! I'm thinking about taking a dremel with a small sanding drum and just whisking the edge of the valve pocket to make room.

thoughts?
 

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irk miller

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Would it hurt to take a burr bit and dremel some of that edge away? Or, wouldn't going slightly thicker on a copper head gasket give you the ability to go back to the compression ratio you want?
 

Nebr_Rex

Coast to Coast
doc_rot said:
Got the head back from the machinist. removed .050" which is as much as is possible considering the last pass ever so slightly skimmed the edge of the intake valve. I think it is only possible to remove this much because I had a 3 angle valve job done, which sunk the valves in a bit.

New combustion chambers are 47.7cc and 47.8cc which brings the compression ratio to about 10.6/1 which is a little higher than I was aiming for but totally fine.

I clayed the head and the combustion-chamber-to-piston clearance is now about .030-.040" around the edge which is much better. The clay i used was too sticky so it was hard to be super accurate, however it looks like there is plenty of room.

The only problem now is that because the head sits closer to the pistons the exhaust valve has pretty much a zero tolerance fit on the edge of the valve pocket, however there is ton of room for valve lift, 0.220"! I'm thinking about taking a dremel with a small sanding drum and just whisking the edge of the valve pocket to make room.

thoughts?
Did you disassemble the head to have it machined?
The valve pocket will need to be extended out towards the edge of the piston.
It is best to be done on a mill.
During valve overlap both valves will be open. My experience is the intake valve
likes to push the piston down the bore. This is most prevalent at 10 to 15 degrees
ATDC on the intake stroke. How does the intake side look?
Will you be using an exhaust cam on the intake?


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doc_rot

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irk miller said:
Would it hurt to take a burr bit and dremel some of that edge away? Or, wouldn't going slightly thicker on a copper head gasket give you the ability to go back to the compression ratio you want?
The compression ratio is a little high but still under 11/1 so i think its fine. a copper base gasket would be more trouble than its worth.

Nebr_Rex said:
Did you disassemble the head to have it machined?
The valve pocket will need to be extended out towards the edge of the piston.
It is best to be done on a mill.
During valve overlap both valves will be open. My experience is the intake valve
likes to push the piston down the bore. This is most prevalent at 10 to 15 degrees
ATDC on the intake stroke. How does the intake side look?
Will you be using an exhaust cam on the intake?
I did not disassemble the head to have it machined, it didn't seem like it was necessary.

I know its best to cut the pockets on the mill but since the pockets are so close to fitting I'm wondering if i can just lightly grind them back by hand and check with clay. I have a steady hand and i'm pretty sure I can do this cleanly.

The intake is close but it looks like it will clear

I was thinking about using an exhaust cam on the intake as it has more lift but after looking closely at the profiles the exhaust looks like it has less duration but more lift than the intake. how would I compare them other than lift?
 

irk miller

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doc_rot said:
The compression ratio is a little high but still under 11/1 so i think its fine. a copper base gasket would be more trouble than its worth.
I said copper head gasket. I use them anyway, so getting one thicker or thinner to make a fine adjustment like that makes sense to me.
 

jpmobius

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I wouldn't hesitate to take a die grinder to the piston crown. Obviously, this will add some chamber volume, but likely negligible depending on how much material needs to be removed. Remember, you actually need a bit more clearance than what you measure - whirly bits tend to stretch a bit at high accelerations!
 

Nebr_Rex

Coast to Coast
KZ750 specs......

http://biquetoast.com/assortedDocs/750manual/008a-Specifications.pdf

The rest of the Factory manual.

http://biquetoast.com/cgi-bin/750ManualList.cgi



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doc_rot

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How much gap around the valve perimeter do I need on the piston crown?


Derp, didn’t even think to check the manual for that. Thanks

My plan is to find tdc with a piston stop, then find lobe center on the intake, then drop the exhaust in and set it to the same position.
 

teazer

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I do it by rotating the engine slowly from say 30 BTDC to 30 ATDC with single vale springs and check clearance every 5 degrees and double check where clearance is least.

2mm should be enough radially.
 

doc_rot

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I dremeled out the valve pockets and there is plenty of room now. I also cut the tach gear off an extra exhaust cam so I can use that cam on the intake.

intake base circle is 33.1mm with 8.1mm lift the exhaust has a 32.3 base and 9.84mm lift

my stock intake cam was shimmed at the very end of the shim range at 340. once i put in the exhaust cam i needed another .4 mm to make up the difference so I had to disassemble the head to put in some different vavles to get me in the shim range. I had the new valve faces ground and lapped them before reassembly. I realized i never took an photos the port and polish my buddy helped me with. We didn't go too crazy, just removed the casting flash and smoothed out the ridge where the valve seats were cut.

I measured the stock intake lobe center and found it to be at 79.5 BBDC, the manual states it is 80 BBDC. I plan on timing the exhaust cam on the intake the same as stock.

Interesting to note that the TDC mark on the advancer was 2 degrees off actual TDC which I found with a piston stop.
 

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Nebr_Rex

Coast to Coast
doc_rot said:
I dremeled out the valve pockets and there is plenty of room now. I also cut the tach gear off an extra exhaust cam so I can use that cam on the intake. I measured the stock intake lobe center and found it to be at 79.5 BBDC, the manual states it is 80 BBDC. I plan on timing the exhaust cam on the intake the same as stock.

couple photos of the port and polish my buddy helped me with and my cam timing setup. Interesting to note that the TDC mark on the advancer was 2 degrees off the actual TDC which I found with a piston stop.
Intake lobe center is measured from TDC. The numbers in that link I posted figure out to be 100* ATDC which is 80* BBDC.
Watch your cranking/dynamic compression. With a 100* lobe center the intake will close at 60* ABDC, 10* sooner than a KZ900/1000.


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doc_rot

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thanks for the heads up on that. This is why I am posting this on here, i would have never known to look at that. My Dynamic is 8.74 with the valve closing at 60* if i retard it 10* to 70* dynamic drops to 8.08

from what I have read a 8/1 dynamic is about as high as you can go safely on pump gas


out of curiosity i plugged in the stock engine numbers, at 8.5 static compression at 50* ABDC the stock engine is making a 7.49 dynamic

for future reference; the piston rod length is 132mm (center to center)
 

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doc_rot

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the cam sprockets are 34 tooth which means moving it one tooth would retard the cam 10.6 degrees. If i slightly oval the holes I should be able to adjust tit right where i want to without slotting the sprockets

should i also adjust the timing on the exhaust cam?
 

Nebr_Rex

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720 crank degrees on the cam sprocket.
720* / 34 = 21.17* per tooth.
I would install the cam using the factory marks first.
That way you know for sure how much of a change you need.



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doc_rot

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Doh! forgot the cams spin once for every two crank rotations. ::) I swapped the intake sprocket to the exhaust and the lobe center is at 99 ATDC. which is pretty much stock. I guess I will have to slot the sprocket since i cant change a tooth, if i move the slot over 5* on the cam sprocket that will change the closing position by 10 degrees, correct?
 

Nebr_Rex

Coast to Coast
doc_rot said:
If i slightly oval the holes I should be able to adjust tit right where i want to without slotting the sprockets

should i also adjust the timing on the exhaust cam?
That is slotting the sprockets.

Degree the cams where ever you want.
110* lobe center on either cam is considered straight up as I remember.


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Nebr_Rex

Coast to Coast
doc_rot said:
Doh! forgot the cams spin once for every two crank rotations. ::) I swapped the intake sprocket to the exhaust and the lobe center is at 99 ATDC. which is pretty much stock. I guess I will have to slot the sprocket since i cant change a tooth, if i move the slot over 5* on the cam sprocket that will change the closing position by 10 degrees, correct?
According to my math, yep.


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doc_rot

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True that is slotting the sprocket but i meant cutting it on the mill. Since i think i only need to change it 5* i may be able to do it with a file.

I would like to get the most out of my setup without making it go boom. seems like 110- 108 is the sweet spot as that is where mega cycle has their lobe centers at for the kz750 as well.
 

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doc_rot

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looks like the exhaust is set at 110 BTDC from the factory. but since my head was decked i will have to check it.
 

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