Pk80/68.5

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truckd

truckd

Well-Known Member
Oct 19, 2010
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palmdale calif
I purchased the PK80/68.5 cc from Zoom and got a ZB8os-sr after argue'ing with Zoom,they finnally sent what I think is the Pk80/68.5 jet, but I have my concerns First what does the arrow and # 3 indicate on the piston and since they removed all identification tags from the engine how do I tell that what I have is actually the PK80/68.5 and not 49cc or other brand of lesser quality and if it is the right engine what are some tweek tricks to improove performance?
Will some one shed some light on my situation
Thanks!
 
Goat Herder

Goat Herder

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Apr 28, 2008
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I can help on the piston. The arrow should be facing forward when installed in the engine towards the front of the bike. This insures that the piston rings do not hang up on any of the ports in the cylinder jug.

As far as motor identification I feel like I am loosing track. The smaller china has a shorter stroke and the bigger china a longer one. The smaller china will rev more the bigger china seams powerful all the way around from what I hear. http://motorbicycling.com/f52/basics-port-polish-port-matching-16646.html This might help shed light on the subject. Consider getting a tuned pipe too.
 
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MotorBicycleRacing

MotorBicycleRacing

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Jul 28, 2010
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SoCal Baby!!!
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Goat Herder said:
I can help on the piston.

As far as motor identification I feel like I am loosing track. The smaller china has a shorter stroke and the bigger china a longer one. The smaller china will rev more the bigger china seams powerful all the way around from what I hear
Click to expand...
The 48cc has a 40 mm piston and bore
The 66 cc has a 47 mm piston and bore
therefore they have the same stroke
 
Goat Herder

Goat Herder

Gutter Rider
Apr 28, 2008
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BiMoPed said:
The 48cc has a 40 mm piston and bore
The 66 cc has a 47 mm piston and bore
therefore they have the same stroke
Click to expand...
I think there is still two different stroke lengths floating around. I had re jugged the bigger of the two engines before piston wise and came up with having to shim a jug to get the piston to clear the top and make the ports work. Meaning that there was a short stroke version out there? I nick named my motor a stroker motor after that lol.
 
truckd

truckd

Well-Known Member
Oct 19, 2010
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palmdale calif
I haven't fired up the engine yet as I am waiting for some after markert performance parts
Fat Cat expansion chamber
CNS Carburetor
Manic mechanic Sprocket adapter and 40 tooth sproket
HMB123 front motor mount
Billet clutch cover with clutch cable hole
Lucky 7-36 tooth crank sprocket
custom machined chain tensioner
10-tooth replacement Idler sprocket
#415-heavy duty chain
I'm a little lost with the terminology you used"op has #3,yours has #4"
I've also ordered another PK80/68.5 and pictures were sent before the purchase was made ( then sent to some one who is fabricating a BTR for me)
and it looks different then the one I have in hand
I can use all the help in this situation also any tweek tricks for engine would be helpful as well
Thanks!
 
O

oldtimer54

Member
May 15, 2010
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Iam wondering what the numbers stamped on our pistons mean? Your piston is stamped with a 3 mine is stamped with a 4? Is it a size thing? Whats up with the numbers on these pistions?
 
truckd

truckd

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Oct 19, 2010
2,837
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palmdale calif
Do you or any one else know the amount of torque LBS. needed to re-istall head and piston cylinder and or any other bolt, nut or screw that requires torque'ing and sheming the head, does that just require another head gasket on top the other ????? I want to do this right with the best results possible, can't screw it up!!
Thanks!
 
B

Black_Moons

New Member
Oct 25, 2010
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80/68CC engines all have 8mm studs for holding the head on (And larger nuts too of course, but I forget the exact size.. Bigger then 10MM anyway)

AFAIK there is no 50cc/68cc jet diffrence. They run the exact same carb and jet. Needle settings will make any diffrence needed.
 
truckd

truckd

Well-Known Member
Oct 19, 2010
2,837
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palmdale calif
What is the torque lbs setting for re-installing head, I do see 8.8mm stamped on side bolts at bottom of cylinder, but sliding an open end wrench on it a takes 10mm, top head bolts (acorn nuts) are 13mm . What does that tell ya?
The engine came from Zoom and I just can't trust those shaddy guys but I'll get in to that after my head ache goe's away.
 
B

Black_Moons

New Member
Oct 25, 2010
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Yea, if it seems fuel starved, even with the E clip at the bottom groove of the needle, you have an air leak. Most likey where the manifold goes into the carb, or manifold to engine. Very common.
Alternately, you might need to drill a hole in your gascap because it isent venting properly. Smallest drill you can find, theres allready a 'dent' along the rim you can drill into, but you gotta drill into that and into the next chunk of metal behind that, to vent it.
 
bluenosegoat

bluenosegoat

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Dec 29, 2009
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For the torque on the 66cc engines with the larger 8mm studs you dont want to exceed 14 ftlbs and on engines with the 6mm head bolts (49cc) you dont want to go over 10 ftlbs and be sure to check your torque a few times during breakin. They do losen up until they are settled. Another good idea is to replace the "acorn" nuts most kits come with for a regular nut. Sometimes the acorn nut bottoms out on itself before really tightening the head down giving you a bad torque reading and leaking head gasket.
ps dont tighten them when the engine is hot- only when cold.
 
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