Cylinder Replacement Issue

Dead66 · Oct 17, 2018

I just replaced the cylinder on my PK80 cause the muffler got knocked and cracked the old one. I bought a 66/80cc cylinder. It looks very close to identical to the old one. I installed using the original gaskets and the piston bottomed out into the cylinder head. So I removed the cylinder and placed two more gaskets at the bottom (the cylinder shipped with 3 gaskets actually) and reinstalled. Now it operates correctly. I definitely have spark, and I definitely have fuel, both making it to the chamber. But I get no combustion AT ALL!

Is there some magic I need to perform to be sure that the space between the piston a head is correct? I assume that would change timing slightly, no?

Any advice appreciated!

Dan · Oct 17, 2018

Hmmm, D. Got 2 outa 3. Only air, left?

Dead66 · Oct 17, 2018

Dan said:
Hmmm, D. Got 2 outa 3. Only air, left?

Right... I assume you mean compression? I have no way of measuring that.

Or do you mean just air in the fuel mix? I changed nothing on the carb. I see no reason it would not be functioning as it did before?

Suggestions?

crassius · Oct 17, 2018

so many diff cylinders for these - with H2 cyls, one had to add an extra head gasket to keep piston from hitting while keeping port mapping right

HepCJosh · Oct 18, 2018

the problem is that there are many cylinder, piston and connecting rod combos. a cylinder from a short rod and high wrist pin engine wont work with a long rod and low pin engine. and a low pin short rod, or high/long. theres a lot of different combos

Dan · Oct 18, 2018

Dead66 said:
Right... I assume you mean compression? I have no way of measuring that.

Or do you mean just air in the fuel mix? I changed nothing on the carb. I see no reason it would not be functioning as it did before?

Suggestions?

If you have fuel & spark. The only thing left is air. Could be compression. But as it's the last possibility. To much or to little, there is only those 3 things. Fuel, air ignition.

Have had visible spark that didn't lite her off. Have had fuel flowing but a bit of schmutz stopping atomization.(spray so it could explode)

I have changed things that had nothing to do with it not starting later.

Ya just gotta rule out stuff in the order of easy (iest first) to check first and work yer way down.

There are some great posts here to check compression. I'd start there.

Please do post on your progress!

You'll get it!

MotorBicycleRacing · Oct 18, 2018

Dead66 said:
I just replaced the cylinder on my PK80 cause the muffler got knocked and cracked the old one.
I bought a 66/80cc cylinder. It looks very close to identical to the old one.
Any advice appreciated!

Measure the heights of the two cylinders.

Venice Motor Bikes · Oct 18, 2018

I remember a customer a few years ago came to me with a similar problem... (He replaced the cylinder & it wouldn't run).

He ordered a stock cylinder, but the seller shipped him one with a 3rd port cut into it. (for a reed valve conversion).

The customer didn't notice the difference before installing it.

Dead66 · Oct 19, 2018

MotorBicycleRacing said:
Measure the heights of the two cylinders.

OK. The old cylinder sleeve is 81mm. The new cylinder sleeve is 79.5mm.

The part of the sleeve that extends from the bottom down into the crank case is 10mm on both cylinders.

Might it work if I were to place 1.5mm of additional gaskets between the cylinder and the head?

Am I beating my head against a wall and I need to get a different cylinder?

HepCJosh · Oct 19, 2018

Dead66 said:
OK. The old cylinder sleeve is 81mm. The new cylinder sleeve is 79.5mm.

There's your problem. You need to either order the 81mm jug, or get the con rod or piston that goes with the 79.5

Dead66 · Oct 19, 2018

Would it work to add additional washers between the head and the cylinder?

Also, how do you find an 81mm jug? I can not find any place that lists those kind of numbers on their cylinders for sale.

crassius · Oct 20, 2018

I usually measure deck height (distance between head gasket surface and base gasket surface) - on my PK80s (which are really A80s and recently renamed as superhawks) this is 60.1mm - I get them from mzmiami cheap

see thread: https://motorbicycling.com/threads/replacement-cyl-piston.64338/#post-670015

EDIT: 60.1 SHOULD HAVE BEEN 69.1mm

Dead66 · Oct 22, 2018

crassius said:
I usually measure deck height (distance between head gasket surface and base gasket surface) - on my PK80s (which are really A80s and recently renamed as superhawks) this is 60.1mm - I get them from mzmiami cheap

see thread: https://motorbicycling.com/threads/replacement-cyl-piston.64338/#post-670015

Thanks for the help! I went to the other thread and followed the link to eBay. Here is where I ended up:

https://www.ebay.com/itm/283000556709?ul_noapp=true

I sent a message on Saturday asking if they could give me any measurements. No response as of this evening.

Any advice?

crassius · Oct 22, 2018

takes a while as answers come back thru china - if your deck height is also 69.1mm, that link is the one I used (looking back in thread, I see I had a typo in first measurement 60.1 is wrong)

Dead66 · Oct 22, 2018

crassius said:
takes a while as answers come back thru china

Not to argue... Are you sure I have the right people? This member is stated to be located in Miami Florida?

And again, thanks for your time and help!

Larry Trotter · Oct 22, 2018

Cold be that by rising the cylinder you are not allowing the piston skirt to uncover the transfer port into the crankcase for the fuel air mixture. As mentioned above there are lots of piston/rod/cylinder combinations. I would be measuring the distances to the ports in both cylinders and also the pistons to make sure they uncover the ports at the proper time. Larry

Larry Trotter · Oct 22, 2018

Remove the intake and look to see if the port opens when the piston is at the top of the stroke.

Dead66 · Oct 23, 2018

Larry Trotter said:
Remove the intake and look to see if the port opens when the piston is at the top of the stroke.

With two gaskets at the bottom of the cylinder and two head gaskets, the piston completely clears the intake at top of stroke, but I can hear the piston touch the head as it moves up and down. I can check to see what happens with three gaskets at the bottom of the cylinder (it will be a close call as to whether it COMPLETELY clears the intake. I already tried running it with 3 gaskets at bottom and one at the top. No combustion whatsoever...

Dead66 · Oct 23, 2018

I just put a third gasket on the bottom. The intake clears completely this way as well. I already tested with three on bottom and on on top. I will try testing with three on bottom and two on top as soon as I can.

Any thoughts based on this info?

DRK · Oct 23, 2018

Not sure if this helps, but I would try to find the correct cylinder and use the same amount (and thickness) of gaskets that were originally used on the motor. The last picture says it is 81mm. https://www.amazon.com/JCMOTO-Cylinder-Motorised-Motorized-Bicycle/dp/B01M3SNR9H

crassius · Oct 23, 2018

Dead66 said:
Not to argue... Are you sure I have the right people? This member is stated to be located in Miami Florida?

And again, thanks for your time and help!

never had a problem getting info from them, I think some tech questions get forwarded back home

the link I mentioned is what I use with 40mm stroke CDH motors - didn't save the email I got from them

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