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Post by motorhead on Feb 19, 2011 22:00:19 GMT -5
Hello everyone at 49cc scoot. Well, once again my SR50 broke down. This time the piston seized. This comes 70 miles into it working again since installing the racing crank (which works great). I had just pulled off the highway going 50mph and was going down a side street and stopped at a stop sign and the engine cut off dead. I opened the engine up to find the piston in the condition you see in the pic. the cylinder has a gouge, and the head is banged up but still useable.
The bike call for an 8 plug and a 9 plug when tuned, could the NGK 7 plug I was running have overheated the piston?
or
Is it possible that the expansion chamber shot metal from a previous seizure into the cylinder?
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Post by Fox on Feb 19, 2011 23:58:00 GMT -5
That gouge looks like a wrist bearing roller got in there. If your current wrist bearing looks okay then I would think that you may be onto something thinking that there was metal left in there somewhere that fouled up the works.
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Post by 90GTVert on Feb 20, 2011 7:52:37 GMT -5
I don't think I'd be pushing it with a hotter plug than recommended unless the 8 -9 series plugs were being fouled. Not sure that really had anything to do with it though.
I think you need to figure out where that debris came from. Check the crankcases over, flush 'em out. Any damage or missing bits to anything else? I'm not really sure it's likely for the pipe to wait around 70 miles to push something into the cylinder if it were lying in there?
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Post by motorhead on Feb 20, 2011 7:56:02 GMT -5
I have tested all the moving parts and no bearings or reeds are broken or seem have too much play in them. Fox: I've seen that same wrist pin damage before and I bet some piece of roller did this. Just not from anything in action.
So my next step is to clean out the exhaust. I think I will find the problem there. On most gasoline engines the exhaust is exit only. But on tuned pipes the exhaust has 2 way flow. And I am convinced something shot back up into the cylinder on the resonance charge.
Has anyone heard of this happening before?
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Post by lshigham on Feb 20, 2011 8:08:01 GMT -5
Yes, I have seen the damage it does.
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Post by 90GTVert on Feb 20, 2011 9:27:29 GMT -5
I know it can happen, and I'm definitely not saying I don't feel it's possible. I cleaned out the pipe on T2 after an engine failure for that very reason. Just surprises me that something would wait 70 miles to get into the cylinder. I've seen enough stuff in engines to know crazier things have happened though.
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Post by bigkahuna427 on Feb 20, 2011 13:46:17 GMT -5
I think you may be barking up the wrong tree by looking for debris. The marks that look like someone took an ice pick to the top of you piston could certtainly be do to "preignition". The little round part missing looks like a roller got in there I would bet looks much more like it was burned like a cutting torch cuts through metal. Your ring is certainly stuck in the ring land and it is obvious to me that the engine had some blow by which would actually reduce engine compression and could reduce preignition making me think preignition was happening before damage to the ring land. Do some research on preignition it is often misunderstood as to the causes and prevention. A properly designed and tuned engine is on the verge of preignition. Most of the performance things we tend to do can increase the chances of preignition like increased compresion for instance or ignition timing adjustments. I have read some scooters actually use EGR systems. Might not make sense to the average person kind of like eating your own poop. However an EGR (Exhaust Gas Recirculator) helps to keep cylinder tempuratures cool. Because exhaust gas has already been burnt it is an inert gas (can't be burnt again). An introduction of a small amount of inert gas helps to slow the flame expansion rate creating a more controlled burn and expansion of cylinder gases. I am not saying put the EGR back on if you had one I am just using this example of how an egr can be effective in controlling detonation AKA preignintion. High octane fuels are often misunderstood as well. Most people associate high octane with high performance and then think they are creating a bigger or faster explosion by using those fuels. Octane additves actually help to create a slower more controlled burn. In an air cooled engine that has been modified I would look to the fuel system and look to see if it is running too lean. It does not matter if you have some high performance carb on there or not and adjusting the idle screw does nothing it needs to be delivering the correct fuel air ratio at cruise speeds and an enrichened fuel air mixture under load when accelerating. Other areas to look are the cooling system, timing, and too much compression to run on pump gas. I hope this helps........  |
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Post by motorhead on Feb 20, 2011 14:11:28 GMT -5
Could it just be that the pistons structural integrity was low since since it survived 2 major previous engine failures.
1. engine got hot enough to melt connecting rod.
and
2. second connecting rod bearing broke loose.
Because everything else in the engine is accounted for and undamaged, and spins without noise and or wobble. After both those failures i used the same piston and rings. Was it just the piston playing a game of final destination after cheating death twice?
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Post by stepthrutuner on Feb 20, 2011 16:38:11 GMT -5
detonation AKA preignintion I hate to be disagreeable on this forum but detonation and preignition are two different phenomenon. Detonation is a secondary combustion event occurring in remote areas of the combustion chamber. It results from pressure waves from the initial intended ignition event meeting with overheated remote 'end gases' near the periphery of the combustion chamber. It is spontaneous compression ignition as occurs in diesel engines. This local remote combustion sets off it's own pressure front which collides with the original pressure front thereby creating the severe knock which we hear as a ticking sort of sound. The result is extreme heat production and consequently more detonation. Preignition, on the other hand, is combustion set off before the intended spark ignition event. It is caused by glowing objects such as a too hot spark plug or a sharp point on a piston or head. The result of preignition is power loss and severe stress to the engine due to the reverse work of the too early exploding mixture trying to drive the ascending piston backwards and additional heat produced since the engine is compressing an already hot exploding mixture. Either phenomenon can bring about the other due to the excessive heat each produces and an engine will collapse or hole a piston in short order. |
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Post by bigkahuna427 on Feb 20, 2011 17:12:33 GMT -5
detonation AKA preignintion I hate to be disagreeable on this forum but detonation and preignition are two different phenomenon. Detonation is a secondary combustion event occurring in remote areas of the combustion chamber. It results from pressure waves from the initial intended ignition event meeting with overheated remote 'end gases' near the periphery of the combustion chamber. It is spontaneous compression ignition as occurs in diesel engines. This local remote combustion sets off it's own pressure front which collides with the original pressure front thereby creating the severe knock which we hear as a ticking sort of sound. The result is extreme heat production and consequently more detonation. Preignition, on the other hand, is combustion set off before the intended spark ignition event. It is caused by glowing objects such as a too hot spark plug or a sharp point on a piston or head. The result of preignition is power loss and severe stress to the engine due to the reverse work of the too early exploding mixture trying to drive the ascending piston backwards and additional heat produced since the engine is compressing an already hot exploding mixture. Either phenomenon can bring about the other due to the excessive heat each produces and an engine will collapse or hole a piston in short order. Detonation and preignition are the same thing in my book. Your descriptions are nearly the same as I read them just worded differently. In any case it is ignition before the plug fires and not good. You don't have to feel disagreeable it's just debate.  |
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Post by lshigham on Feb 20, 2011 17:15:41 GMT -5
''In any case it is ignition before the plug fires and not good''
That's preignition. Detonation occurs after ignition.
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Post by bigkahuna427 on Feb 20, 2011 17:19:58 GMT -5
Could it just be that the pistons structural integrity was low since since it survived 2 major previous engine failures. 1. engine got hot enough to melt connecting rod. and 2. second connecting rod bearing broke loose. Because everything else in the engine is accounted for and undamaged, and spins without noise and or wobble. After both those failures i used the same piston and rings. Was it just the piston playing a game of final destination after cheating death twice? The connecting rod melted? Did you park this thing on top of a volcano? This has been apart and together a couple of times without replacing the rings?? HMM? Could the ring have been stuck when you put it back together the last time? Are you hearing pinging from the engine when you accelerate? |
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Post by motorhead on Feb 20, 2011 19:13:21 GMT -5
Firstly. I bought the bike with the rod already melted. The guy I bought it from claimed he got it up to 70mph. Wether its true or not the bike was ran hard and the big rod end melted into its bearing.
Secondly. I could have preignition like stepthrutuner defined it. The bike was fouling 8 plugs. So I dropped from an 80 jet to 75. And went from an 8 plug to 7 plug. The 7 plug could could have burned the fuel too soon.
Third. The bike is liquid cooled and was running on a 55-60 degree day, with premium gas 40:1 premix castrol 2t.
Next. I think I should have changed the ring and cleaned the ring groove. The ring wasn't stuck but could have been easier to slide. I didn't hear anything out of the ordinary while it did run. The only funny thing about the bike was. It was hard to start and needed starting fluid if i didn't want to crank it for 5 minutes. And after a long coast the engine popped and spat if I gave it throttle too quickly.
Once I get my piston in the mail. I will switch from castrol 2t to motul 600..
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Post by bigkahuna427 on Feb 20, 2011 19:59:07 GMT -5
Third. The bike is liquid cooled and was running on a 55-60 degree day, with premium gas 40:1 premix castrol 2t. The only funny thing about the bike was. It was hard to start and needed starting fluid if i didn't want to crank it for 5 minutes. And after a long coast the engine popped and spat if I gave it throttle too quickly. Once I get my piston in the mail. I will switch from castrol 2t to motul 600.. I myself would not put that bike back together in hopes that a different premix will do it. If the bike is liquid cooled and you have a temp gauge so you know it's not overheating than I would focus on getting it running right. You symptoms of having to start it with starting fluid and popping upon accelertion sounds like carb problems. I guess if you bought this thing already modded you do not know what someone else might have done to create this problem. However I would start with cleaning the carb unless it is fuel injected of course. All it would take is the leg of a house fly sitting in your main jet to create a lean condition which is what your symtoms sound like. All of that being said I am not a scooter pro my back ground is automotive driveability electronics and teaching. I hope i have helped in some way good luck.. |
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jimh
Scoot Enthusiast
Posts: 198
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Post by jimh on Feb 20, 2011 21:05:01 GMT -5
The 70 main jet could be too lean...
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