49cc Engine...Unknown Problem

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  #1  
Old 07-02-06, 10:25 AM
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49cc Engine...Unknown Problem

OK, I have a Dirtdog scooter with a 49cc engine on it. I spoke all about it in the chats and wines forum. I was riding it yesterday and I was going back and forth from 50% throttle to 100%. All of the sudden the engine was getting louder from the muffler. The engine would die, I would start it again and be able to go just a little ways, almost as if it was struggling, then it would die again. I would start it again and it died even faster, so I pushed it home.

This is a new scooter that I have already broken in, everything is clean. But I took the carb off and cleaned it anyway, reinstalled it and when I pushed the primer, gas was in the bulb. Then I took the sparkplug off which is of course also new and I cleaned it and gapped it, reinstalled it. I went to pull start it and it just won't start. I know the engine is getting fire cause I put a sparkplug tester on it and it lights up when I pull the rope. To rule out a bad sparkplug, I put a brand new sparkplug that is for my Stihl trimmer and it would not even fire on that!

For an extra test, I put some gas into the cylinder through the sparkplug hole, and guess what? It won't start with even that! All engines getting fire should start if you do that. However, when you pull the rope, it feels like its not burning the gas inside the cylinder. Do any of you know what the problem is with my 49cc 2-cycle engine?
 
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Old 07-02-06, 11:15 AM
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You mentioned it got louder just before giving you fits, right?

2 cycle engine are especially susceptible to lack of free air flow through the exhaust. If a baffle within a muffler broke loose and is causing a restriction, this may be the culprit.
I would take off the exhaust and try to fire the thing.

WARNING: do not run it without an exhaust pipe for too long. 2 cycle engines don;t like too free of an exhaust either and they usually will not run well with absolutely no back pressure.
 
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Old 07-02-06, 12:51 PM
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While you have the muffler off, check for any marks on the piston.
 
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Old 07-02-06, 02:10 PM
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OK, I took the muffler off and the engine would not start. If there is gas in the cylinder, the only time it feels like its burning gas is when you go full throttle, but still no go. So I used a drill and a carbon remover bit and removed noticible carben buildup in the muffler hole and the muffler bore on the engine. I reinstalled the muffler and I still could not get the engine started.

Just to let you know, before I put the muffler back on, I put my palm to the exaust bore and pulled the recoil. It feels like its trying to suck my palm into the hole, is this normal? I looked for marks on the piston and I do believe I see some scoring on there. A piston is supposed to look completely smooth right?
 
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Old 07-02-06, 04:48 PM
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I just have to ask the obvious. Did you have 2 cycle oil mixed in the gas?
Mike
 
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Old 07-02-06, 05:03 PM
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Originally Posted by mla2ofus
I just have to ask the obvious. Did you have 2 cycle oil mixed in the gas?
Mike
Good question Mike.
-------------------------
Some scoring is normal in engines but it should be very minimal and your machine seems pretty new to be having wear indications such as this yet. This would tend to indicate foreign matter has gone through your cylinder. You didn't take off the air filter did you??

The sucking thing; it is normal within limits. As the piston moves upward in the bore, air/fuel mix is drawn into the crankcase. The piston is not a perfect seal in the cylinder so when the piston rings are above the exhaust port and the piston is still moving upward, you can feel some sucking when putting your hand over the exhaust port. If there is less restriction to draw it in around the piston, it will and that is probably what you are experiencing.
A 2 cycle engine requires the dynamics of a fast moving piston, such as when it is running, for everything to work as designed.
 
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Old 07-02-06, 05:10 PM
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Yes, of course I had oil mixed with the gas at the proper ratio. Well, it looks like I will be popping the head off and taking a look in the cylinder cause right now, thats the only thing I can do considering I have done everything else.

I had a feeling the suction was normal, I am not expert on engines by far but I do know allot about them considering I have been working on them for years. I got a question for ya, I know 2-cycle engines use reed valves. Could a faulty reed valve cause the no start issue?
 
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Old 07-02-06, 05:15 PM
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Yes it could although not all 2 cycle engines use a reed valve. There is also a rotary valve and a piston port design. Yours is probably not a rotary valve but could be a piston port design.

If the reed valve was stuck open, it would allow the fuel/air to flow backwards out of the carb as the piston starts its' downward travel. This would cause a lack of pressure build up below the piston that is the mechanism used to push the fuel/air into the cylinder when it exposes the inlet port.

If it is stuck closed (not very likely) it wouldn;t allow air/fuel to enter the engine.
 
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Old 07-02-06, 05:43 PM
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The piston should be shiny with no marks. Scoring is "not" normal but it`s nothing unusual. As recently mention "minimal" scoring does not usually interfere with the performance of the engine.
The main issue on 2cycle engines is "pressure". The gas tank is pressurized, the carburetor is pressurized and the crankcase is pressurized, if pressure is not maintained it will result in loss of power and eventually engine failure. If pressure, spark, and compression is good. You can put a drop of gas in the cylinder and the engie will start even without the carburetor or muffler installed.
That`s one way I test Chain saws for blown seals. (if equipped), also if it runs fine at WOT but dies at idle, thats another indication. It was mentioned that gas was put in the cylinder and it did`nt start. So I believe theres a pressure leak somewhere. And if I`m not mistaken this engine has crankshaft seals. This would be the first place I would check, especially if it was ran without mixture. Striaght gas can blow the seals out fast. This is something I learned years ago from the Echo PB400 blowers.
 
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Old 07-02-06, 05:49 PM
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Thanks for your help Nap, not only are you a knowledgable person on small engines, you are helping me out allot! I just wanted to make sure you knew that...

OK, I popped off the head and I saw what I thought I was going to see. The piston is damaged, there is a small crack on one side of it and on that same side the bottome edge of the piston is chewed up. Don't ask me how this happened on a new engine but it did. Well, if I can get a new piston for this thing, I will do that cause that should be allot cheaper than buying a new engine.

Also, the same side the piston is damaged, the head where the carb goes on is also chewed up. So this means I am going to have to replace the piston and the head. Well, I will see what it costs to do this, heck, this thing should still be under warrentee. But keep in mind, if they want me to pay shipping, I will rather doitmyself.
 
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Old 07-02-06, 06:13 PM
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Originally Posted by mikesmalleng
The piston should be shiny with no marks. Scoring is "not" normal but it`s nothing unusual. As recently mention "minimal" scoring does not usually interfere with the performance of the engine.
The main issue on 2cycle engines is "pressure". The gas tank is pressurized, the carburetor is pressurized and the crankcase is pressurized, if pressure is not maintained it will result in loss of power and eventually engine failure. If pressure, spark, and compression is good. You can put a drop of gas in the cylinder and the engie will start even without the carburetor or muffler installed.
Not to start an argument mike but with all the riceburners I have dissassembled scoring is very common i.e. normal. Definately not desirable but it is almost always there to some degree. The amount and depth of the scoring would determine if it is excessive or not. And I have not seen a shiny piston since using Wiseco pistons. Pistons are generally not shiny on the skirts for a reason and the head is not shiny because it costs more to make it that way. They usually have a dull sheen at best.

Now to the pressurized fuel tank and carb. While this may be normal in an engine that is going to be inverted or run in positions other than upright, e.g.chainsaws, weedeaters, airplanes, etc., it is not common in engines that are designed to remain upright e.g. motorcycles. Actually since the fuel to the carb is gravity feed, a vented tank is required.


Terminator:

There are other things you need to check as well. I ask again; did you take off the air cleaner?? It sounds like the engine ate something that didn;t agree with it. It could have been something like a screw from the carb or other areas as well. If there is a reed valve, this may have self destructed and caused the damage you incured. It is possible the piston self destructed also. Sometimes it is hard to determine.

Now you have to determine the extent of the damage.

Cylinder scoring can be no more than "light" (boy is that a definative term, huh?) or you will not have acceptable compression. The rings are more than likely trashed as well.

If something in the inlet came apart and caused this, you would need to find out what came apart as well.
 
  #12  
Old 07-02-06, 06:26 PM
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Well, the damage is quite extensive. I think I am just going to buy a better scooter. You get what you pay for and its not like I never read somwhere that somebody had their scooter for not only a month and it craped out just like mine. A guy I spoke to said these Chinese made scooters don't last more than a year, I guess I should have heeded his advice.
 
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Old 07-02-06, 06:53 PM
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I'd say you're lucky if you even get a full year out of the ones I've seen. On these cheapo imports, I havent seen reed valves used in any of the engines, so that probably isn't what came apart on this one. I'd tend to think that either the engine overheated and galled the piston/cylinder, or it ran too lean, or both. It may have picked up some tiny piece of foreign matter also, especially since the air filter covers on those things don't even close well, and the filter is a very open-pored sponge that doesn't even fit right.
 
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Old 07-02-06, 07:14 PM
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I didn't mean to step on your toes,Terminator. You'd be surprised to find how many 2 cyc engines are ruined because someone didn't use mixed gas OR grabbed the can of gas for a 4 cyc engine.
The symptoms you gave sounded like it was starved for oil.
As for the chinese engines, I've been hearing the Chondas,exact copies of Hondas,have a very short life compared to real Hondas.
Mike
 
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Old 07-02-06, 07:52 PM
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What you found may have been a needle bearing and that may be what caused the damage you described.

Nap
I was talking general terms. I guess I was a little too specific on OEM parts and too far in detail on some procedures. Anyway, it`s not easy for anyone to determine the exact cause of a problem sight unseen. (with some exceptions)

We`re all in this together.
 
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Old 07-02-06, 07:54 PM
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Originally Posted by mikesmalleng
What you found may have been a needle bearing and that may be what caused the damage you described.

Nap
I was talking general terms. I guess I was a little too specific on OEM parts and too far in detail on some procedures. Anyway, it`s not easy for anyone to determine the exact cause of a problem sight unseen. (with some exceptions)

We`re all in this together.
I'm cool with it all. No problems here.
 
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Old 07-03-06, 02:33 PM
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I needed the experiance so I changed my decision and decided to rebuild it...

I have ordered a piston pin puller tool, a cylinder head, a piston, and rings. I have spent a total of $116.00 Wish Me Luck...
 
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Old 07-03-06, 02:53 PM
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Good luck. Let us know how it turns out.
 
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