Curing 2-cycle Engine Problems

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.

Comments

[ThisGuyLovesToBuild]

I have an old two stroke Echo engine that won't fire at all, it has no build up, has a new spark plug, and I just cleaned the carb and replaced the fuel line, what could be my issue,(the carb works, it was tested on a similar engine)

[Phil B]

I am not a mechanic, nor have I ever been. I tried to list some things in this Instructable that are not usually mentioned in the available troubleshooting guides, but I encountered them by experience. Those may be the solution to your problem, or not. For an engine to run it needs the correct mixture of air with clean fuel, compression, and spark. All sorts of things can interfere, like a blocked vent in the gas tank cap, a dirty fuel filter or air filter, gasket leaks that allow too much air into the mixture. In response to some of the comments below I did list a link to a troubleshooting guide. If your engine is old, there is a strong chance the crankcase gaskets are not sealing and are allowing too much air into the fuel mixture.

[Frederick C]

To your compression point - could the engine been run with the wrong fuel/oil mixture or regular motor oil used as a mixing agent with the fuel so that the piston walls are scorched due to things heating up and not being properly lubricated? Removing the plug and/or muffler to view the piston walls should give you a clue, Good luck.

[KenA3]

I have Craftsman 26 cc engine on a small rototiller .It worked for one year and stopped I checked fuel, spark ,but compression is only 30 psi.How do I incease the compression, The gasket on the crankcase is a o ring in a plastic cover.I don't see how it can stand 100psi> How can I get the pressure up to 100psi ?

[Phil B]

I looked around the Internet quickly before composing this response. More than one fault can cause the problem you are having. Did you check the screws or bolts that hold the halves of the crankcase together to see if any are a little loose? The crankshaft has a seal that should not leak. Some sites suggest removing the muffler and using a light to look for score marks on the cylinder walls. Does the compression reading rise if you crank the engine with a few drops of oil in the spark plug hole? (That could indicate a problem with the piston rings or wear in the cylinder walls.) Search the Internet for “Troubleshooting 2-cycle engines.”

[rbarrie154]

i have a 43 cc 2 stroke motor that starts up and runs fine from a cold start and will continue to run without problem, however if i shut it off it will not start back up until it cools down. there is no vapor lock and i have spark. why will it not start back up when its hot?

[Phil B]

I am simply offering information about a couple of problems not often mentioned in articles about 2-cycle starting problems. I am not a small engine mechanic. I have had very frew hot start problems, but there are articles you can easily find that discuss various causes. Those range from an improper oil-to-gasoline mixture to dirty passageways in the carburetor to a bad condenser on a points and condenser ignition.

[rbarrie154]

As it turns out Phil B it is a condenser ignition problem, i purchased an in-line spark checker and have only a low level glow when i pull the starter cord and i compared it to another working machine which gave me a bright red-white light on the in-line spark checker. thank you in any event for your time my friend.

[rbarrie154]

the section you posted about carbon build up on the exhaust port may have a bearing on why my 43 cc motor will not start when its hot, i will try cleaning the exhaust port and see if that solves the problem, however i would appreciate any other suggestions you might have.

[Phil B]

What I said about carbon build-up in the exhaust ports was not related to a hot start problem, but to restricted air flow that keeps the engine from developing full power. I did mention a problem with old fuel in which the oil did not lubricate as expected and the engine seized from friction after warming up.

[1919a4mg]

I have a Tecumseh 3,5 hp 2 cycle engine that wont start. I put in new plug and new carb. Getting good spark. it appears now that the gas is exiting the exhaust without any firing. Im not familiar with the valving on these engines. could it have a non working valve?
Thanks,
Bill

[rbarrie154]

it could be that you sheared off your flywheel key and your not firing at the right time

[Phil B]

My guess would have been a float that does not cause the carburetor needle valve to seal. A new carburetor should preclude that, unless you mean a used carburetor new to this machine. As concerns valves in 2-cycle engines, there is a reed valve that is a flap of spring steel over a port. The fuel mixture can be pulled through it into the crankcase, but the valve closes when the pressure in the crankcase rises as the piston is on the downward stroke. Otherwise, the engine has ports without valves. If the float is older, it can get heavy and it does not float. Does the gas run through the engine only when you are trying to start it, or whenever there is gas in the tank? How much gas runs through? A stream or a little moisture?

[KGU]

Hello Phil. I’m new here and I just now became a member due to this issue I have with an MTD Yard Machine 25cc 2 stroke tiller, model Y125. It’s in immaculate shape and it’s only been used in soft flower beds a couple of times, but it has also set for a long time without being started. The very first thing I did before any of the rest of this, is I pulled the plug and screwed in my compression tester and I got 125lbs compression. I’m thinking we’re good at that point so proceed with it I will. I drained all the old gas and put in fresh 91 octane gas mixed at 40:1. I’ve put on a brand new carb, new fuel lines (I tested the new feed/suction line for leaks after instal), new fuel filter, spark plug, and the air filter is in great shape and clean.
My problem. I go to do the new carb/first start/adjust, and I turn both mix screws out to the norm of around a full turn +- (I’ve found it seems, that everyone’s norm on that will vary by about a 1/4 turn) and it will flood every time I try to start it wether it’s trying it at wot or idle, regardless of choke on or off, or air filter in or out. I finally got it to start, but only after I opened the high side about 3/4 turn and left the low closed. I got it to run at idle and wot, but I can only turn both screws out about 1/2 turn to get it to do so. At wot it will spit and miss very hard and even backfire through the carb and at idle it’s not running to bad and will set idling on it’s own. I’ve checked the crank case o-ring seal and it looks good with no problems that I can see there, although looks don’t always mean good.
I haven’t checked the head and think it should be good due to the compression. That little voice has already whispered it to me and is now telling me I’m going to hear someone say “vacuum leak”. Yes, I have thought that. But I will admit, I’m not seeing it as the problem due to where the mix screws are having to be set at about a 1/2 turn out, which is less fuel going in to make it run ?? A vacuum leak is air coming in after the Venturi and mix screws, which “should” mean you’d need to open the mix screws more to allow more fuel to compensate the more air, not cutting the fuel back like I’m having to do. So, any thoughts, suggestions, hopefully cures ??
And by all means, please do....
Point out the right in front of my face “obvious” that for some reason I’m simply not seeing and missing here. Lol. Thanks in advance for any help on this.

[Phil B]

Welcome to Instructables. I hope you will publish some things as time goes on, You tend to make some friends whom you will likely never meet in person, and they can be from other parts of the world.

You have certainly checked the right things and know what you are doing. What you describe sounds like the engine is receiving far too much fuel. We all assume a new carburetor must be working as it should. I would be tempted to run carburetor cleaner through the old carburetor as well as I am able and try the machine with the old carburetor installed. Even those little 2-cycle diaphragm carburetors have a needle valve that is supposed to keep too much fuel from flowing into the carburetor. Perhaps the needle valve fuel metering system has missing parts or is stuck. I cannot point to any examples of this happening, but, I also cannot think of another part of the system that allows too much fuel to flow through the carburetor.

When you mentioned a backfire, I thought of spark timing, I had a 4-cycle mower that backfired and the flywheel key was partially sheared, which changed the spark timing,

I wish you well. When you discover the problem, it will all make sense.

[North08]

I have an old max 2 6x6 and it has a Rockwell jlo 2 stroke motor, I have completely restored it, I put a milieu carberator on it with a brand new air filter, the motor runs amazing with out the air filter, as soon as the filter is on, it completely starved the engine for air, I even zip tied a piece of screen door over it and it still restricts the air flow enough to make the engine run poorly, I do not know how to fix my issue. Any advice?

[Phil B]

Are the carburetor screws set properly? The “H” screw should be open from fully seated approximately one turn. The “L” screw should be open from fully seated approximately 1 1/2 turns. Be careful not to use too much force when seating the screws. Be very gentle.

[RayS34]

I had a problem with an old lawnmower,could not seem to get any fuel to spark plug,no matter how much it was choked. Turned out to be a worn crankcase bearing!

[Phil B]

Another person who commented some time back had the same problem. That is very similar to a leaking gasket because it allows extra air into the crankcase. Yet, these things are not often mentioned in a list of possible causes for no starting.

[tillingtom]

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.