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#113405 25/11/21 04:05 PM
Joined: Jan 2017
Posts: 349
Likes: 4
Apprentice level 4
Hi,

A 139 cc Chonda has found its way to my workshop. It was running very well but my grandson run it at full throttle into block of solid concrete. It stopped it of course. But now it seems to have very little compression. It still turns over and the crankshaft is not broken. Seems like the valve gear has suffered catastrophic failure. Still fires but seemingly when exhaust valve is open.

I think it's a lost cause. Anyone have any idea. By the way it has a cutter bar not a disc.

Jeff

Joined: Sep 2015
Posts: 2,059
Likes: 146
SENIOR TECHNICIAN & HISTORIAN
Hi Jeff,

I would check the flywheel key to see if it's sheared and possibly the timing of the spark is out if
the flywheel has moved also check the blade hub hasn't broken the key on the crank.


Unlikely but I guess the cam gear could have stripped a tooth.

Cheers
Max.

Joined: Jan 2017
Posts: 349
Likes: 4
Apprentice level 4
Thanks Max,

I guess that's worth checking. Might save the motor.Such a pity it was a really sweet running little unit.

It came from hard rubbish a few years ago. Only ever cleaned the carby & adjusted the governer.

Jeff

Joined: Feb 2006
Posts: 4,675
Likes: 164
SENIOR TECHNICIAN
I hope you won't be loaning him anything again.

Last edited by Mowerfreak; 25/11/21 08:01 PM.

Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!
Joined: Jan 2015
Posts: 1,487
Likes: 23
AVB Offline
SENIOR TECHNICIAN
When it comes to compression problems a leak down is invaluable in pointing you in the right direction. But when something a solid as concrete with a rigid bar blade setup all bets are off as just about anything can happen. Usually you get at least a bend blade and a sheared flywheel key but worst can happen from the sudden stop.

It at least worth a tear down as it is still at least a good learning project. The last GCV160 I tore down here that had a leaky exhaust valve turned out to be a move exhaust valve guide. At least the leak down pointed me in the right direction.

Joined: Jan 2020
Posts: 171
Likes: 9
Apprentice level 2
Originally Posted by Mowerfreak
I hope you won't be loaning him anything again.
Whats the old expression of things not to lend others? Chainsaws, sleeping bags and wives? Might have to add mowers to it.

1 member likes this: Mowerfreak
N1KK0 #113416 26/11/21 09:54 AM
Joined: Feb 2006
Posts: 4,675
Likes: 164
SENIOR TECHNICIAN
Originally Posted by N1KK0
Originally Posted by Mowerfreak
I hope you won't be loaning him anything again.
Whats the old expression of things not to lend others? Chainsaws, sleeping bags and wives? Might have to add mowers to it.
Especially mowers with unforgiving bar blades!!
I used to loan my rather heavy handed father my Victa Powertorque Mustang and while I knew he wouldn't straight fuel it (he taught me mixing fuel), blades wouldn't last a day with him, which is why I would always loan him that one and only change them when they resembled popsicle sticks!
That mower still runs today, years later. With a bar blade set up, it would have been all over before the fat lady sings!


Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!
Joined: Jan 2017
Posts: 349
Likes: 4
Apprentice level 4
Hi Max and AVB,

I had a look and yes you are right Max. The flywheel key is completely sheared. Must have been completely gone perhaps explaining the seeming lack of compression. I checked the bar blade key and its OK. It seems much bigger. No obvious bends or dents either. Ill get some keys, they seem about 4 mm thick. then Ill see if it goes and hopefully runs smoothly.

i wonder why the flywheel key shears? Seems like it almost the furtherest think awau from the impact. I guess the sudden stop combined with the weights on the flywheel.

jeff

Joined: Sep 2015
Posts: 2,059
Likes: 146
SENIOR TECHNICIAN & HISTORIAN
G'day Jeff and all

Originally Posted by jefffrombrisbane
I wonder why the flywheel key shears? Seems like it's almost the furthest thing away from the impact. I guess the sudden stop combined with the weight of the flywheel.

Yes it's because of Inertia and Mass, Mass and speed determines how much Inertia an object has ,The blade, crankshaft, and flywheel are spinning at about 3000 rpm, so when the blade hits an immovable object, the crankshaft decelerates suddenly but the flywheel doesn't want to stop so fast. Thus it shears the flywheel key. Since the flywheel continues to rotate relative to the crankshaft, the ignition timing is advanced.

If the key didn't break the crankshaft could twist or break.

Inertia: the resistance an object has to a change in its state of motion. .

Cheers
Max.

Joined: Jan 2015
Posts: 1,487
Likes: 23
AVB Offline
SENIOR TECHNICIAN
Yes as Max explains it is there to prevent major most times. But in this case it may or may not the compression lost. The only way to know for sure it is to replace the key and try starting as It could be that the flywheel was just spinning and the crankshaft was not. Or could be an additional problem too.

AVB #113495 07/12/21 04:53 PM
Joined: Jan 2017
Posts: 349
Likes: 4
Apprentice level 4
Thanks Max and AVB,

I replaced the key and tried to start. Now it will actually run very poorly for a few revolutions. Backfiring through the air filter quite often.Compression seems normal now.

Looks like we have damage to the valve train. Is there another key there or is it likely to have jumped a tooth on the can gear? If there is no key and the camshaft gear is damaged not much point going any further. Are they a metal or a fibre gear?
No way getting any of those parts economically.

Probably end up dumping this one
Keep the carby and starter etc perhaps.

Thanks for help
Jeff

Joined: Sep 2015
Posts: 2,059
Likes: 146
SENIOR TECHNICIAN & HISTORIAN
I'd made sure the spark plug is a good working plug first Jeff as they can fail anytime , then you could
take the rocker cover off and make sure a push rod isn't bent ,check the rockers aren't damaged.

Sometimes when the flywheel key breaks the crankshaft needs to be cleaned with a file around the key way
slot otherwise the flywheel won't sit down properly on the tapered crank end thus causing the new key
to shear again ,you could just remove the flywheel nut to make sure the key is still lined up with the
flywheel and crank key way then put the nut back on.

When the key sheared the first time the engine could have backfired because of advanced timing
so something may have got damaged internally ,so remove the rocker cover and check rockers and
push rods.

Some motors use metal timing gears and some are plastic like ,difficult to tell until apart. If there is a key on
the cam gear I wouldn't have thought it would have broken from a sudden stop or backfire.

With these Chinese motors it comes down to how much time do you want to spend on them ,I've
thrown out good motors that just needed a lower crank seal as I can get a good mower at the tip for $10.

Cheers
Max.

Joined: Jan 2017
Posts: 349
Likes: 4
Apprentice level 4
Hi Max and AVB,

I decided to have one more go before I sent it for recycling. On your advice Max I changed the spark plug to one from a good running Chonda I have here.
After a few pulls with the fuel tank topped up away she went. I had pulled the carby apart to check the main jet previously so I just needed to get all the springs in the right place and adjusted the choke and I mowed my yard with it.

Alls well that ends well I suppose. I wouldn't have tried the plug if you had not suggested it Max. I think it's only fouled from trying to start without spark. I'll clean it and see how it goes in the other machine.

Thanks again for your help. My grandson will be delighted. He will get it back with a little advice.

Jeff

Joined: Sep 2015
Posts: 2,059
Likes: 146
SENIOR TECHNICIAN & HISTORIAN
Hi Jeff and all,

Good outcome Jeff ,this mower was definitively worth fixing ,I had one of these mowers years ago that I was given,
then after a service and a few repairs I sold it for $100 ,the guy who bought the mower used it for a few years didn't
maintain the mower then gives it back to me as the coil had failed ,so I sold him another mower then a neighbour
asks for a cheap mower so I clean the carby ,replace the coil ,give the mower a tune and say he can have the mower
for $50 ,he uses the mower for a year then was moving and gives the mower away, this next owner uses the mower for
another year then gives the mower to someone else because they don't like pushing mowers around ,this last
owner of the mower bought 2 new shop mowers that failed straight away so he took them back for a refund and was asking around
if anyone had a mower he could use, after he uses this mower he says how great this mower runs and cuts.

That's not bad ,over 5 years for a cheap mower and never having to buy blades as it has a bar blade that is easily
sharpened.

Originally Posted by maxwestern
I'd make sure the spark plug is a good working plug first Jeff as they can fail anytime , then you could
take the rocker cover off and make sure a push rod isn't bent ,check the rockers aren't damaged.

I somehow used a (d) in make in my last post LOL .

Cheers
Max.


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