Curing 2-cycle Engine Problems

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About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

Many 2-cycle engines have problems and are sent to the junk yard before their time. Typical problems include hard starting, rough running, a need to adjust the carburetor during use to keep the engine from stalling, a need to rev the engine to keep it from dying, and not starting at all.

I will assume the user knows to replace the spark plug regularly, to use fresh fuel, and to replace or clean the air filter.

With time and normal vibration, the screws that seal the crankcase from air leaks loosen just a little and air gets into the engine through leaks in the crankcase gaskets.

First check the mounting screws for the carburetor and tighten them. Then go to the screws that hold the cylinder head to the crankcase body and those that cover the end where the crankshaft comes out of the engine. See the yellow circles on this photo of a weed whacker engine. If any of these screws loosen as little as a quarter of a turn air begins to leak into the engine and the fuel/air mixture either is not pushed into the engine on the piston's downstroke or it becomes too lean for the engine to run by pulling in extra air during the piston's upstroke.

After about ten years of use, no amount of tightening on these screws will make a dead engine run. Chances are the gaskets have become hardened beyond their ability to seal the engine. Dismantle the engine completely and install new gaskets. You may not be able to buy the proper gaskets, but you can buy a sheet of gasket material. Use the old gaskets or the engine castings as a pattern to cut new gaskets. Your engine will run like new again.

Step 1: Clean the Exhaust Ports, Too.

Remove the muffler. The exhaust ports are under it. In time carbon residue from the burned oil in the fuel mixture begins to close off the exhaust ports. An engine is an air pump. What cannot get out blocks what needs to get in. Move the piston to the downstroke position (most distant position away from the spark plug) and knock off carbon deposits with a screwdriver. Turn the engine over by hand a few times to blow the carbon granules out of the engine.

Once I used our small 2-cycle garden tiller. After ten minutes the engine siezed and stopped. I had mixed 2-cycle oil with the gasoline, but it was old by the time I was using it. I found advice on the Internet that said to let the engine cool. Then use a wrench with a long handle to slowly turn the engine over. Turn in the same direction, not back and forth. That engine still works very well, although I am sure it suffered a little. Lesson learned: always use freshly mixed fuel.

When a gasoline engine of any kind will sit unused for any length of time, drain the fuel from the tank and run the engine until all remaining fuel in the system has been burned through the engine. This prevents gums and varnishes from closing off small openings.

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    322 Discussions

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    tillingtom

    Question 4 weeks ago

    15yo Stihl Yard Boss cutivator. Cannot get it to start after replacing the Coil. Replaced Coil, carb, fuel lines, fuel filter, air filters, checked the coil for fire, ohmed the start switch, removed the cylinder and checked the piston. I'm using an electric drill to try to get it to crank, and this is not working either. Does anyone have any suggestions? It almost seems like it is not getting enough fuel. Oh yeah I also removed the exhaust and cleaned the screen.

    4 answers
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    Phil Btillingtom

    Answer 4 weeks ago

    Your cultivator looks like a weed trimmer engine mounted on a cultivator frame. I have a weed trimmer about the same size, and it can be started with an electric drill motor, but the electric motor must be a specified minimum speed and horsepower or the cultivator engine does not turn fast enough to start.

    I do not think your carburetor has a float, but is a diaphragm type. Still, it would be good to know fuel is flowing freely through the fuel line up to the carburetor. You also want to be certain there are no air leaks in the gaskets between the engine halves or between the carburetor and the engine. In my experience, gaskets are no longer pliable enough to seal properly after ten years. I am guessing 15yo means 15 years old? Are the high and low idle jets properly set and free of dirt in the seats of the jets?

    Does the engine start briefly when you spray ether starting fluid into the air intake?

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    tillingtomPhil B

    Reply 4 weeks ago

    Hey Phil B, thanks for the quick response. From what I have discovered, the MM55 motor was used on quite a few Stihl tools. I discovered this on a UK website, diyspareparts.com, from which, I have ordered 2 gasket sets. The website is layed out pretty well and they will give you a nice
    Parts Break Down diagram for every part of the eingine. They supply parts for many brands and even the old McCulloch chainsaws. Anyway if anyone needs engine gaskets or other parts for the MM55, I would definitely give the guys a try.
    Part # 4140 007 1050 - Stihl Set of gaskets
    Back to my problem, it was my gaskets, they were dryed out and done for. The MM55 is 15 years old yes. The carb I put on was new and was working. However, when I started to use the drill to start it, I noticed where my leaks were and there were more than one. So not only did I order two sets of gaskets, but I ordered another engine as well. So ,we shall see. If any of the instructables subscribers are tired of trying to figure out what's what on Ebay, I would highly suggest, they check out
    diyspareparts.com. my .02

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    tillingtomtillingtom

    Reply 4 weeks ago

    Here's a listing of all the machines that the MM55 gasket set will fit:
    Machines this part fits

    Stihl FC 55 Edger
    Stihl FC 55 Z Edger
    Stihl FC 55-DZ Edger
    Stihl FS 45 Brushcutter
    Stihl FS 46 Brushcutter
    Stihl FS 55 Brushcutter
    Stihl HL 45 Long Reach Hedgetrimmer
    Stihl HL 45 Z Long Reach Hedgetrimmer
    Stihl HL 45-DZ Long Reach Hedgetrimmer
    Stihl KM 55 C-E Engine
    Stihl KM 55 Engine
    Stihl KM 55 R Z Engine
    Stihl KM 55 RC-E DZ Engine
    Stihl KM 55 RC-E Engine
    Stihl KM 55 RC-E Z Engine
    Stihl KM 55 R-DZ Engine
    Stihl KM 55C Engine
    Stihl KM 55R C Engine
    Stihl KM 55R Engine
    Stihl MM 55 C-E Z Multi Tool Engine
    Stihl MM 55 Multi Tool Engine
    Stihl MM 55-Z Multi Tool Engine
    Have a happy Memorial Day!

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    Phil Btillingtom

    Reply 4 weeks ago

    Your list of engines should be very helpful to others in the future, as well as your link to parts. A few years ago we owned and used the 1989 vintage Ryan weed trimmer in my photos for this Instructable. After ten years I had air leak problems and replaced the gaskets. I did not even try to find a gasket set for that relatively unknown brand, but purchased a roll of gasket material at an auto parts store. Then I made my own gaskets. I did not use a ball peen hammer to cut them against the original parts as I once saw my father do with cast iron parts many years ago, but used an X-Acto knife and a small scissors. We used that machine for a dozen more years before we passed it on to someone else because we were moving and believed we would never again need a weed trimmer. (Last year I had to go out and buy a new weed trimmer because we moved again and now need a weed trimmer again.) Thank you for the report. I think you will be pleased at the difference new gaskets make.

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    Average Jo

    Question 5 weeks ago

    I have a Brand New small pulsar tailgate Generator using premixed oil / fuel from Lowe’s and having hard time starting the engine. I changed the spark plug and tightened visible bolts but think I’m not using the choke right. The one time I got it to start, it died after 20 minutes. Any suggestions or advice would be appreciated..:

    3 answers
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    Phil BAverage Jo

    Answer 5 weeks ago

    It sounds like a fuel system problem. I found an owner’s manual for a 900 Watt Pulsar generator at the Lowe’s web page. Did you open the air vent switch on the fuel cap before starting? Air must be able to get back into the gas tank to replace the gasoline running out of the tank through the fuel line as the engine runs. If air cannot get back into the gas tank, fuel will stop flowing out and the engine will die after running for a short time. The troubleshooting guide in the Pulsar manual also suggests a dirty fuel filter or air filter as causes of your problem. I assume you are using fresh fuel (less than six weeks old). That sounds obvious, but a friend brought his Husqvarna chain saw and wanted me to determine why it would not start. His fuel had been mixed and stored on a shelf for over a year! The saw ran great with fresh fuel. If you had old fuel in the tank, a new fuel filter might be a good idea.

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    Average JoPhil B

    Reply 5 weeks ago

    Thanks Phil, I cleaned the carb and put in another can of TruFuel (already premixed and supposedly good for 5 years). It seems to be working fine now. Everyone told me I shouldn’t have to clean the carb on a brand new generator but that seemed to work. I couldn’t find the vent switch on the fuel cap and unless I overlooked it, didn’t see it it the manual. I really appreciate your help... thanks again

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    Phil BAverage Jo

    Reply 5 weeks ago

    I am glad you are making progress. The illustration I saw in the electronic manual showed a two position “switch” on top of the fill cap for the fuel tank. An easy test for fuel tank venting is to run the engine for a few minutes, stop it, and unscrew the fill cap. If you sense a rush of sir into the tank when you crack the cap open, it is not venting properly. My son-in-law has a small generator and it is not Pulsar, but it also has a vent open/close switch on the fuel fill cap. I suppose those sre to keep vapors from getting into the atmosphere.

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    Ronald Rogers

    Question 3 months ago on Step 1

    This is a question regarding 2-cycle fuel. I made 2 mistakes as follows: Mistake #1: Without realizing what I was doing I added approximately 1 quart of pure gasoline (no ethanol) to approximately a 1 gallon mixture of 2-cycle oil and pure gas at 50:1 ratio. Mistake #2: After realizing Mistake #1 I thought "okay, I'll just add a small amount of oil to the mixture." Without thinking I picked up a bottle of SAE30 and added maybe one ounce to the above described mixture. I now have maybe a little more than a gallon of this mixture. Am I okay in running this in a 2-cycle engine or am I better advised to just discard the mixture somewhere that accepts gasoline?

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    Nick005

    8 months ago

    Hi Phil. I have a Troy Bilt TB80EC trimmer and the original carburetor was not getting prime and I could get it running after about 5 minutes of trying to prime it and start it. Nothing seems clogged and I've cleaned out every port on it. I bought a new carb and it it floods it with the HI and LOW screws open only 1/4 turn. Any ideas for either carbs? Thanks in advance

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    Phil BNick005

    Reply 7 months ago

    Nick,

    I was not getting e-mail notificstions of comments, but Instructables has fixed the problem. So, I did not see your questions earlier. Could your air cleaner be blocked? The engine would be as if the choke is fully engaged and would flood easily. Try starting the engine with the air cleaner removed and see if that makes a difference.

    A couple of moves ago we gsve away the weed trimmer in the photos, but discovered we need one again and bought a Troy-bilt TB35EC a few months ago. Any problems with it have been due to a lean condition. I have not yet seen it flood. Customer reviews on it complain about hard starting after a season, maybe sooner. I have found screws loosen on it easily, including two on the back for the JumpStart power starter feature, and the loose screws allow too much air into the engine.

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    Nick005Nick005

    Reply 8 months ago

    With the old carb, I would try to start it the next day and I couldn't get it running again.

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    LouisP34

    7 months ago

    Thanks, I was about to give up on the engine because I had replaced changed filters, carburetor, then spark plugs. I double checked and there was a damaged gasket. One that I just replaced. Fixed that and it runs. Not too well, it does need tuning up, but it runs.

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    Phil BLouisP34

    Reply 7 months ago

    Thank you, Louis, for the good news. I love it when small things make a big difference. I hope you find whatever makes it rough when it runs.

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    Myron523

    11 months ago

    Toro straight Shaft Trimmer, M/N 51932B, S/n 240000000-up Subject: Why am I getting fuel up into the combustion chamber after the trimmer has set horizontally for 5 days.It's as though it's set a hydraulic block. This has been happening for the past two times I run the trimmer. Is this a carburetor problem or a fuel cap problem? Purchased back in 2006.

    1 reply
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    Phil BMyron523

    Reply 11 months ago

    I am not a small engine mechanic by training or by trade. Is the fuel tank elevated above the combustion chamber? I am wondering if the floating needle activated by the metering valve is not sealing and fuel can flow through by gravity. That might mean you need a new metering diaphragm. A plugged vent hole in the fuel tank cap usually stops the flow of fuel because air cannot get back into the tank to replace fuel leaving the tank. I recently read that more and more repair shops do not bother with diagnosing a carburetor problem, but simply replace the carburetor with a new one. If you can match up a replacement carburetor, the orice of a new one is often less than $15 or $20.

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    ChrisM944

    Question 1 year ago

    Hi Phil,

    I have an Echo leaf blower. It runs beautifully for about 5 seconds, but then fails to develop full power. I'm new to this problem. Do you have a diagnostic flowchart? Does this sound like it is symptomatic of anything specific? I can think of a few things that might cause it:

    Restricted Air intake (No.)
    Restricted Exhaust (Just realized it from this article, so maybe.)
    Restricted Fuel flow? (I have no idea how I would diagnose this. I need to be able to measure current flow and compare it to projected flow...)

    Thanks for the help,

    Chris.

    1 answer
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    Phil BChrisM944

    Answer 1 year ago

    i listed a link to an on-line diagnostic help in my comment to 86suzuki directly below. It is close to s flow chart.

    About a month ago I worked on a chainsaw for a friend. Two repair shops told him he almost certainly needed a new metering diaphragm in his carburetor. After I got into the saw I found the diaphragm was fine. The fuel in his saw's tank was more than a year old. After mixing fresh fuel and replacing the old fuel with it, the saw ran great. I mention that because yu never know exactly what you have until you get into the machine.

    By the way, this chainsaw did not develop full power at first. Then I opened the High circuit mixture screw just a little and it was fine.

    I would check everything in a logical order, and I would make certain there are no air leaks anywhere in the crankcase.

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    86suzuki

    Question 1 year ago on Step 1

    Hello there!
    I recently purchased a 86 Suzuki lt250r 2 stroke atv now when I bought the atv it ran good but you had keep giving it gas so I research it first thing I see is start it warm it up n adjust fuel/air screw to were it idles just right so that's what I did I noticed after I did that it did start to smoke more white smoke out of the exaust also there is oil coming out of exaust but I heard that was normal for a 2 stroke so anyway went to start it one day after riding it 2 days in a row wouldn't start pulled plug it was bad got new plug started right up yesterday went to start it today won't start pulled the plug black oily / maybe burnt gas smells like fuel more than oil on the end of spark plug. Any advice on what could of caused this and white smoke from exausy would be greatly appreciated all I am completely new to 2 strokes my first one.