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Keeping Spree Elite Aero 50cc and Gyro Alive!
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PostPosted: Wed Jun 13, 2018 4:28 am 
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Spree
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Location: Lincolnshire, England
When I took my '97 AE 50 for it's UK MoT test the tester told me the crank bearings had gone and were in danger of collapse so I replaced them. Now the bearings are quiet but there's a rattle somewhere else in the motor that I can't track down. I've changed the small end bearing and can't find any movement in the big end but there's a click at tdc as you turn the motor over by hand with the head off. There's no piston slap and the rings are not loose. With the engine running it sounds like a small end, but that's been changed and has no play. Only thing that comes to mind is perhaps a flat spot on the crank pin, although the big end span freely when I had the crank out.

I'm reluctant to change the whole crank for something that could just be a normal noise for a 20 year old motor.

Thoughts appreciated.

Thanks


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PostPosted: Wed Jun 13, 2018 6:00 am 
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CB900F
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Before you hastily diagnose a bad crank pin, have you eliminated all other possibilities? While the crank spins, what else is also spinning that could be the cause of a click? Try removing one part at a time and check for the click each time....CVT, Stator, Oil pump, head and ultimately the jug. If youve removed every possible component that rotates with the crank and it STILL makes a click....its gotta be crank/bearing related.

Also, every 2-stroke scooter “clicks” to a certain degree. No matter if new or thousands of miles (Km’s). Sounds alot like a vehicle with rattling valves or lifter tick. Your noise could be completely normal...or it could be an issue that needs immediate repair.

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PostPosted: Wed Jun 13, 2018 11:28 am 
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Spree
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Location: Lincolnshire, England
Thanks for the ideas Meat, will check them out. Yeah, it could also be normal but it's really annoying & bordering on embarrassing!
Something happened today that may be the cause, or may be yet another rattle in addition to the first.. I decided that 100+ miles was enough for things to bed in, so decided to try a bit of WOT to see how it went. Well, first the little cover / heat shield on the side of the exhaust can fell off while I was going along, fortunately I found it later in the grass by the road after walking for about 1/2 mile. When I stopped after about a mile of WOT, I discovered that the paint had blistered on the exhaust can right at the end near the small pipe that lets the exhaust out. Now, I can hear a different exhaust note plus a loud rattle from the can, so I'm thinking there's a loose baffle inside.


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PostPosted: Wed Jun 13, 2018 12:53 pm 
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In the past, I have also been puzzled by the annoying buzz of an exhaust shield.
I thought I was hearing internal interference with the flywheel / fan...but no.
Hope the demon reveals itself before detonation...mm

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PostPosted: Fri Jun 15, 2018 1:52 am 
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Spree
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Location: Lincolnshire, England
OK, following Meat's suggestions I took off the variator & the magneto flywheel / stator yesterday. With the flywheel back on minus the stator & no drag from the coils / magnets & drive belt it's quite easy to feel some movement on the crank without any movement on the piston. The piston movement can be judged by putting a screwdriver on top of the piston via the spark plug hole.
So, it looks like the big end has some play and that'll mean a new crank. It's odd that I could not see or feel any play last time I had the motor on the bench, but the engine noise is the same so it must have been there.


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PostPosted: Fri Jun 15, 2018 2:25 am 
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CB900F
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Im not too clear on your exact method to determine your play is in the big end. Keep in mind that with the jug removed, a bit of side-to-side play on the rod is normal for a GOOD big end needle bearing. However, there should be ZERO play up n down. If you DO have play up n down, be sure to have a close look at the small end bearing, piston pin and piston pin housing (in piston itself) before you determine a lower end problem. Eliminate a possible “upper” issue before going deeper.

Also, if you pull the rod up n down with your fingers and you have slop...take a close look to see if the crank counterweight isnt moving also. This would indicate a bad crank bearing. The crank bearings can also be checked on both sides when the variator and flywheel is removed. Simply grab the crank’s shafts and shake them around in all directions. There should be zero slop whatsoever.

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PostPosted: Sat Jun 16, 2018 1:05 am 
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Spree
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Location: Lincolnshire, England
I've guessed the movement is in the bottom end as the crank & small end bearings have been changed so hope that they're ok at least for now. The piston pin is a snug fit in the piston & bearing.
I checked the play by slightly rotating the magneto flywheel back and forwards. I get about 1-2mm of rotational play measured at the outside of the flywheel plus a definite metallic tick noise but no movement on the piston so far as I can tell. Piston movement is measured by a screwdriver through the spark plug hole.


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PostPosted: Sat Jun 16, 2018 1:46 am 
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Someone please correct me if I'm wrong, but
I think there is by design a 1-2mm degree of rotation
on the flywheel while the piston is stationary at
top-dead center, and again at bottom. During that 1-2mm of rotation,
the wrist pin is rotating inside the piston to allow
the piston rod to swing from one side of the cylinder to the other.
I didn't look anything up, just an observation from watching engines spin over.
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PostPosted: Sat Jun 16, 2018 3:01 am 
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Spree
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Location: Lincolnshire, England
Sorry, good point. I should have mentioned that you can feel the slack and a get a metallic tick noise with the piston halfway up or down.
You're right, there will always be a 'dead spot' at the top & bottom where the crank rotates but the piston does not move.

I'll get the motor on the bench when I'm back after a few days of R&R & should then be able to tell if it's top or bottom play.


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PostPosted: Fri Jun 22, 2018 2:00 am 
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Spree
Spree

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Location: Lincolnshire, England
The noise from the crank was getting me down so I put the motor on the bench and had a good look. I can now see that the con rod has about 1-2mm of up&down play at the big end, the top / small end is fine. It's strange that I couldn't see it before, so can only guess that it was caked with carbon that's now come loose. There's certainly a lot of black in the oil that wasn't there the last time I had it apart.
At some point in the scoot's history a new piston has been put in, the crank bearings were encrusted with carbon, must have been the same with the big end.
I guess I already know the answer, but am am wondering about how wise it would be to put a used crank in? There's one on ebay, that the seller tells me has no big end play, but it has to be 20 years old...


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