|
|
Post by wolfbane240 on Nov 12, 2018 1:21:11 GMT -5
|
|
|
|
Post by gsx600racer on Nov 12, 2018 1:33:09 GMT -5
Looks like the center electrode broke off and spent some time bouncing around in the cylinder till it wore a hole in the piston
|
|
|
|
Post by FrankenMech on Nov 12, 2018 2:38:24 GMT -5
Something was bouncing around in the combustion chamber, what is unknown.
|
|
|
|
Post by 90GTVert on Nov 12, 2018 7:58:24 GMT -5
 Any details on the engine? Had you recently changed anything around? Any sounds or symptoms prior to the failure?
|
|
lupo76
Scoot Enthusiast
Posts: 157
|
Post by lupo76 on Nov 12, 2018 8:19:59 GMT -5
Was a valve, not the plug. The valve is missing, it broke off ang got "vaporized".
|
|
|
|
Post by wolfbane240 on Nov 12, 2018 12:08:41 GMT -5
I had just reset the valves and put in a brand new spark plug. I put about 20 miles on it with no issues. Was running up to specs. Then I started going down a steep hill at idle and I heard a bang and lost power. Pulled over and tried starting it and felt no pressure. Pulled the plug and saw picture one. Got home and opened it up for pic 2 and 3. It looks like the intake valve broke off. Could that have come from misadjusting it? I gapped it at .004. Also the day before I changed carbs and replaced the air box with a mushroom air filter.
|
|
|
|
Post by gsx600racer on Nov 12, 2018 15:50:48 GMT -5
Was a valve, not the plug. The valve is missing, it broke off ang got "vaporized". I dont think its vaporized, I bet its at the bottom of the crankcase. You will need to get inside the case and find the valve. |
|
|
|
Post by FrankenMech on Nov 12, 2018 16:03:44 GMT -5
Get a new engine....
|
|
ratdog
Scoot Enthusiast
Posts: 342
|
Post by ratdog on Nov 12, 2018 22:04:18 GMT -5
No mystery there! Pre Ignition . Basicly your engine had a “knock” and ate it’s self. Happens all the time when people do big bore kit, run way too much compression, and maybe use regular gas instead of 93 octane. I’d be willing to bet you also were running way too lean. Endresult, you melted your engine
You might be able to rebuild it, but first you would have to tear it all the way down to see if the bottom end is damaged. I would guess that you have enough melted alum down there that the bearings are shot as well.
going to a cone filter will make your engine run much leaner, and no doubt you didn’t do anything to check the carb jetting
|
|
lupo76
Scoot Enthusiast
Posts: 157
|
Post by lupo76 on Nov 13, 2018 2:27:45 GMT -5
Was a valve, not the plug. The valve is missing, it broke off ang got "vaporized". I dont think its vaporized, I bet its at the bottom of the crankcase. You will need to get inside the case and find the valve. You know, what i meant. I meant not vaporized in the meaning of that word, more like it broke, shredded parts of that engine and got shredded itself... |
|
|
|
Post by GrumpyUnk on Nov 13, 2018 11:55:04 GMT -5
Sometimes valves come apart 'just because', with no apparent reason or cause. Likely parts are in the sump. They may stay at the bottom, or get jostled into movement. A magnet could attract bits'n'pieces, allowing you to drag the high majority out, and the screen 'filter' would catch a lot of the rest. But. Any small bits that got into the main bearings (ball bearings) would ruin the bearings quickly. {which may have occurred already}
You have several options. 1- Clean it up as best you can, replace the jug/piston/head with new and see what happens. Not real expensive if you go with 'generic' parts. 2- Take it apart and clean everything, check crankshaft bearings and replace as needed, re-assemble with new jug/piston/head. Cost a bit more plus time invested. 3- replace the whole thing. Cost more, but less time & effort.
If this is a toy, you might want to try to flush the crankcase, and do the repair. If you need to use it daily for work, etc, replacement is the next best.
I saw no evidence of anything melting. Pre-ignition damage is not apparent as far as I can tell. The indentations on the piston likely occurred within seconds of the BANG, until it slowed down enough to disengage the clutch and stop. At ~7-8,000 rpms, a LOT of cycles happen very quickly. The edges of the piston do not appear damaged or eroded as might happen with overheating.
tom
tom
|
|
|
|
Post by fugaziiv on Nov 13, 2018 12:06:07 GMT -5
I have to agree with @grumpyunk here. I don't see knock so much as I see a valve that let loose for whatever reason. Normally with an extra lean condition or high compression issue on these engines you'll see some definitive damage on the edges of the piston face.
Time for a full tear down or replacement IMHO.
Matt
|
|
|
|
Post by gsx600racer on Nov 13, 2018 18:08:07 GMT -5
I dont think its vaporized, I bet its at the bottom of the crankcase. You will need to get inside the case and find the valve. You know, what i meant. I meant not vaporized in the meaning of that word, more like it broke, shredded parts of that engine and got shredded itself... No worries.  You have a project on your hands now. |
|
|
|
Post by snaker on Nov 13, 2018 19:19:03 GMT -5
The mention of the OP going down a steep hill makes me wonder if the clutch never got below disengagement rpm's and over revved the engine.
I know from experience that a certain Briggs and Stratton over rev will pile a piston into the combustion chamber. Not sure what will go first on one of these.
|
|
|
|
Post by GrumpyUnk on Nov 14, 2018 16:53:31 GMT -5
Most B&S air cooled will fail the connecting rod, making it multi-piece instead of one-piece. The oil fails to get to the journal at too-high rpms, and the rod siezes to the crankshaft. All the bits keep rotating, and the rod gets 'bent' too far, and snaps. Mayhem ensues, with all the parts trying to escape. Some make their way through the sides of the block, and are never seen again. Others get jammed between the camshaft and the block, and the camshaft gets a new profile. It also wobbles a lot.
The GY6 and QMB are such that I would expect a super over-rev would cause the piston to come to pieces before the rod as the rod has pressure feed to the big end bearing. The small end might fail as it only gets splash lubed, and it may not be enough when cycle rates get to 8-9,000 cycles per minute, and it is not rotational cycles on a lubed surface but oscillating, causing no 'wave' of lube to form and protect the bearing surfaces.
If you go to an oversized piston, you add significant mass that must be accelerated/de-celerated at the top/bottom of the stroke. The forces involved are almost not believable in such a small engine.
I figure in this case, the steep downgrade and crankshaft locked to output shaft(no mention of stopping or coasting down to disengage) would result in a very high rpm situation. The weakest link was found, and in this case it was the 'tulip' of the valve that separated from the stem, and went a-bouncing, being beaten by the piston crown several thousand times before it broke out and escaped into the sump.
The engine could be rescued, but it might not be worth the effort. To be sure that things are free of debris and 'chips' and 'particles', you'd have to take it apart all the way and clean things, then inspect the bearings for smooth rotation, and fit new as needed. A set of bearings and new rod, and the labor to press apart and together. Then that of the piston/cylinder and rings. Additionally, the cylinder head has taken a slight beating, with likely damaged seats. New parts galore and empty wallet.
tom
|
|
