saving a 1980 KZ750 twin

doc_rot

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I had a chance to degrees the cams. When I set the intake to 110* lobe center I'm showing that its "closing" at 42* ABC @0 .050"
using the calculator i found that puts me at 8.933/1 which i believe is too high. To get me under 8.5 i need to retard it 7 degrees further putting the lobe center at 117ATC
this will change the lobe separation from 110 to 113.5.

does this seem correct?
 

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teazer

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I would set the cams where you want them to be for valve timing events. Don't change cam timing to achieve a preconceived CR number. What is the static CR and are you OK with that number? If so, you are good to go at 108-110 degrees.

Later inlet valve events will in theory allow the motor to peak at higher revs but that's not really what you are trying to achieve with this motor. You want as much low end compression as practical to get more low speed torque.
 

Nebr_Rex

Member
What is your cranking compression?
While knowing what your dynamic compression is nice to know.
I am more old school and prefer a compression gauge.
How do people come up with these dynamic compression ratio limits?
There are different combustion chambers, ignition systems, grades and blends of fuel.
I have a 10.1/1 cr V8 that pumps 185 p.s.i. and a 464cc KZ440 with 10.2/1 that pumps 195p.s.i.
and they both run fine on 91 octain.


.
 

doc_rot

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hmm it never occurred to me to try cranking it to see what compression it was making on the bench. I will have to finish fixing up my starter clutch.
 

doc_rot

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Some info of interest to KZ750 twin owners. guy used Ducati 750 2 valve pistons (88mm) for 948cc twin

https://translate.google.com/translate?sl=auto&tl=en&u=www.z750twin.de%2FManfreds_LTD.htm
 

datadavid

New Member
I Hope you are aware that with a piston stop you need to note where the piston stops when rotating both cw an ccw, then you have true tdc exactly in between your markings.
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.

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.
 

beachcomber

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Hi Doc - excuse the intrusion, but I know a few of the KZ750 Twin peeps will be here from time to time.

Here in the UK / EU there's a big problem with lack of new advance / retard springs - unobtainable.

A lot of ignition / tickover problems have been traced down to weak springs - they don't last forever.

My tame spring man [ Uprated KZ750 twin - and 60 odd other - clutch springs] is prepared to make a run of 100 [ 50 bikes ] if he can get new or as new samples. Cost based on 100 would be entirely reasonable, and of course would be a readily available source.

Pix of uprated clutch springs and the RAM Hi-Po studs / nuts - in stock.
 

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doc_rot

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Finally some updates here. I took a little break and saved up for some goodies. RS34 carbs all the way from Germany, and a custom grind from Web camshafts!
 

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JadusMotorcycleParts

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Now that's some trick stuff! Pumper carbs will be awesome 8)

But from the photo, I only see a pump on one of the carbs? So only one cylinder sorts the lean throttle condition and carries the other until fueling is stable?
 

doc_rot

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$500 with tax and shipping. Megacycle wanted 750 before tax and shipping
 

doc_rot

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JadusMotorcycleParts said:
Now that's some trick stuff! Pumper carbs will be awesome 8)

But from the photo, I only see a pump on one of the carbs? So only one cylinder sorts the lean throttle condition and carries the other until fueling is stable?
yes, this is how the racks of 4 are set up as well
 

irk miller

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JadusMotorcycleParts said:
Now that's some trick stuff! Pumper carbs will be awesome 8)

But from the photo, I only see a pump on one of the carbs? So only one cylinder sorts the lean throttle condition and carries the other until fueling is stable?
There's a circuit of tubes that feeds nozzles in each carb. The pump is always on #2, which also usually doesn't have a sync adjust. You sync the other three to that carb.
 

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