Picture of 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.
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hi Phil

i have a issues with my water cooled 2 stroke motor ok let me list off what i have done so far

new piston ring

new ignition coil

new water pump and bearings

when i pul;l the starter i see fire come from the exhaust but the motor wont hold a idel or even turn over to complete a full cycle

whats is wrong with my motor if i have strong compression but the motor wont operate meaning it will not turn over and contune to run on its own

thank you for ur advise

Phil B (author)  DarnellP1yesterday
My first guess is your engine is off of its proper spark time setting. I do not know if your engine has adjustable timing with a distributor you can adjust. If so, find the specifications and check the setting. If there is no distributor, the engine is likely timed by a soft key that locks the flywheel into its proper orientation on the crankshaft. (The magnets for the magneto that produces the spark are in the flywheel.) These soft keys are designed to shear (partially) when the engine stops suddenly as a protection against twisting an expensive crankshaft. They do not need to shear very much to create problems. I had a Tecumseh 4-cycle engine on a lawnmower and the soft key partially sheared. When the engine did fire, it was a backfire and felt like the starter cord wanted to rip my arm off of my body.
From what I could find and read, the mixture setting on the carburetor may also be far too rich. Gasoline gets into the hot exhaust pipe without burning, but ignites there causing flames. Also look for anything else that can cause a rich mixture, like a heavy float in the carburetor.
Also mentioned in what I read was improper valve timing.
I wish you well.
ChaseW526 days ago

i have a 2000 cr125 2 stroke and i got it about a week ago and it ran fine and now it keeps turning its self off for no reason it sounds good when you first start it but about a min into the ride dies on you

Phil B (author)  ChaseW526 days ago

It sounds like fuel starvation. When the engine stops, unscrew the cap on the gasoline tank. Do you hear the sound of air rushing in? If so, the gas tank vent in the cap is not allowing air back into the tank. If that is the problem, you may need a new cap, or need to make the venting in the cap work again.

This may not be the problem, but it sounds probable.

ChaseW526 days ago

I have a Honda cr125 2 stroke and it ran fine uptill 2 days ago it runs for about a min and then dies whats wrong with it?

BobbyC32 months ago
What can cause compression loss??
Phil B (author)  BobbyC32 months ago
Problems with the cylinder walls or the piston rings or the spark plug threads could cause a loss of compression. These could be scored cylinder walls, broken or stuck rings, a cracked cylinder, stripped spark plug threads, or a plug loose in its threads, or foreign matter in the plug seat, or loose screws (if the head is a separate piece from the cylinder). Some of these problems result from using no oil or the wrong oil mixed with the gasoline. You probably need to take the engine apart and have a look.
Rupertguy2 months ago
Rupertguy2 months ago
toddk632 months ago

What about the crankshaft seal. Does that need to be replaced along with the crankcase gasket?


Todd K.

Phil B (author)  toddk632 months ago


You have done a very careful and capable gathering of data on your machine's problems and possible causes for them. One other commenter below had a leaky crankshaft seal, and his machine worked again when he replaced that. Our weed trimmer moved on to a new owner before it hit 24 years in age, and I currently have nothing with a small engine in our garage. Also, our weed trimmer was not used heavily, so our crankshaft seals may not have worn out as quickly as yours. I did find I needed to replace crankcase gaskets at 10 years because they were no longer resilient enough to seal well. I wrote about gasket leaks because those were an unexpected problem I had, but none of the diagnostic helps I found seemed to mention that.

I think the manufacturers of these machines do not expect owners to use them a quarter century. As an example of that sort of thing, I read the bicycles you might buy for your kids at a discount store are expected to be ridden only 250 miles before they are junked, and are designed accordingly.

Vapor lock certainly could be the problem on your machine from the symptoms you describe. Someone I know just built a very expensive custom automobile for a customer, and the factory new engine in the car suffers from vapor lock after a longer period of operation. In this case, the vapor lock is in the fuel line. He will resolve it with insulation on the fuel line. I know another solution is to add a thicker gasket between the carburetor and the engine block (small engine) or manifold (automotive engine).

toddk63 Phil B2 months ago

Thanks for your thoughts , Phil. Its in my nature to keep a machine running "till the wheels fall off". Kind of a pride and bragging thing. My wife seldom gets new things as a result. I tore the Ryobi open last night completely...almost. I could not get the crankshaft out of the crankcase. It was supposed to come off the bearings with some hammer taps. I tapped really hard and only got the shaft and bearings to move together inside the crankcase (bearings stuck on the shaft) That didn't seem right so I quit and tapped it back into place. That said, the bearings and shaft seemed good. No axial or radial play. So I figured the seal was OK also. It will have to be what it is. I did find that a piece of a rubber spacer was pinched in the sealing surface of the crankcase during original assembly. I don't know if this contributes to my problem , but it will be corrected on re-assembly along with new gaskets. Then cross my fingers. I will also try the fuel line/carburetor insulation thing if the above doesn't work. Anything low $. My time is cheap, especially since I enjoy tinkering on this stuff.

Todd K.

Phil B (author)  toddk632 months ago


Thank you for the report. I suppose it is always possible the crankshaft seal hardened and split. But, if an air leak at crankshaft seal is your problem, it seems strange that the engine runs until you try to restart it hot. Also, if vapor lock is the problem, it seems strange that the problem did not present itself much earlier. "What changed?" is always a good question to ask.

A couple of years ago I had to junk a good alternator just because I could not get the rotor out of the case so I could clean and grease the rear bearing. In that case, it was not stuck bearings, but wire connections I could not separate to pull the halves of the alternator apart.

Is the crankshaft bearing pressed on?

toddk63 Phil B2 months ago

Now that you mention it. The "cylinder gasket" (to the crankcase) has an extra long flap on it that extends beyond the cylinder and hangs over the muffler. It did not occur what it is for until just today. I presume it acts as a heat shield to protect the carburetor that sits just above the muffler. That flap became brittle and fell of in pieces a while ago. We could be on to something!

Phil B (author)  toddk632 months ago

That is a good observation. The few books I read on troubleshooting computers always said to ask what was the last thing that happened before the problem developed. Is there a way to engineer a metal replacement heat shield without disturbing the old head gasket, just in case that gasket is no longer available, if it is not too late for that already?

toddk63 Phil B2 months ago

I'm afraid its "game over" for the old Ryobi 31cc trimmer. I was really sure this afternoon that I would put it all back together with new gaskets and at least get it running like it was before. I ran into trouble putting the flywheel back on (tapered fit) no matter how hard I pounded it would not seat firmly. I even heated it up with propane torch, no go (prolly should of gone with MAPP). At that point I figured I had nothing to lose pounding the crankshaft (and the bearings) out of the crank case. It came out easy enough. Even the the outboard bearing finally gave up its grip of the shaft. The inboard bearing however was on tight. I tried driving a screwdriver (like a wedge) to work it off. It was very stubborn and I only got it about a 1 cm further before I damaged the shield. So at this point I am giving up...but I would like to know from you, Phil...How does one put a flywheel back on one of these? And how do you get the bearings off in one piece. The only thing I can figure is that during assembly they may have frozen the shaft to slip the bearings and flywheel on.

I hate losing!

Todd K.

Phil B (author)  toddk632 months ago

A friend acknowledged his occasional (expensive and frustrating) defeats, but consoled himself by recalling he had experienced more successes over the years, on balance.

I once did an Instructable on a cord snapping backfire problem we had with our Sears (Tecumseh) mower engine. The flywheel on a Briggs & Stratton engine always seated on its tapered shaft with no special effort, but something about the Tecumseh flywheel was similar to the problems you had. I had to find the factory torque figure and even the proper torque was barely enough to keep the flywheel in place and avoid overloading the soft key so that it sheared and failed. In my limited experience, very small engines like weed trimmers and chainsaws have a small steel key that keeps the flywheel in place after the nut is tightened.

I can only guess the crankshaft bearing was pressed onto the shaft at the factory. It may be you are supposed to replace the crankshaft and bearings as a unit. If you would choose to look for parts, a lot depends on Ryobi's policies. Some companies have continued to make parts available for all of their products back to day one. Others keep parts available for maybe ten years. Sears (in our experience) might still have parts available, but the price increased steeply after a few years as an inducement for the customer to buy a new machine.

toddk63 Phil B2 months ago

If only that flywheel had a nut holding it on. It was just pressed on. Next time (in another 25 years) I will know not to mess with the flywheel if not needed on a small trimmer

Thanks for all your support, Phil

Phil B (author)  toddk632 months ago


I am sorry it has come to buying a new machine. Maybe the new machine will have a nut on the flywheel. It has been nice chatting with you.

toddk63 Phil B2 months ago

hoo! I get a do-over. I called Karl today. We bought the exact same trimmer at the
around the same time in 1995 (so that makes it 20 yrs old, not 24 yrs
like I first thought). He gave up on his a couple of years ago. I
asked if he still had it...YES!...Now I have couple of choices. I can
strip his down and get the donor crankcase / flywheel (still assembled)
and slap it in mine (with the new gaskets). Or I can do some quick
diagnostics on his (I've gotten real good at that in last 2 weeks). Make sure compression and spark is there. Slap on my just rebuilt carburetor and bust it off.
He said he took apart and cleaned the carb, but did not put a rebuild
kit in it. Silly Karl. And even better, I get all his attachments that
will work on it and my new Husky! I owe you a beer, Karl

Todd K.

toddk632 months ago

Great article. I have read ALL the comments.

I have a 24 yr old Ryobi 31cc trimmer. Starts cold and runs great, but wont restart when hot if I let it sit for more than a minute. A shot of ether in the spark plug hole will get it going again usually. Spark plug is dry after attempts to restart. When cold, primer bulb is firm and full of liquid. When hot, bulb is not as firm and doesn't seem to be moving much liquid. Compression test on top end 105 psig cold and hot. Spark seems OK cold and hot.

Everything seems to indicate fuel problem and maybe vapor lock when hot. I see you are all about crankcase gaskets on old machines, but seem a little stumped on hot start problems. Could the two be related?

Here is what i have done chasing this problem in no particular order:
- drain gas tank refilled with TruFuel 40:1 (seemed to help a little)
- rebuilt carburetor with kit (twice)
- new spark plug
- new fuel filter
- tried loosing gas cap

Anticipating your comments

Todd K.

ShaunoS3 months ago
DallasY5 months ago

I have a tiller, a Bolens BL425 that I have recently replaced the spark plug and carburetor. I can only get this to run with the plug wire disconnected. It sparks just fine, but with the wire fully connected it won't run longer than a second. It seems like there might be an exhaust leak near the plug, but I can't tell for sure, it could be coming from inside the plastic case. Any help on this would be appreciated.

Phil B (author)  DallasY5 months ago

I am not sure how an engine runs with the plug wire disconnected. I found a photo of your model tiller and can see the location of the carburetor. Is the muffler on the backside facing the operator? Can the exhaust leakage be coming from a leaking seam in the crankcase, or perhaps where the cylinder head bolts onto the crankcase?

Disconnecting the plug wire to make an engine with a fouled spark plug run is an old mechanic's trick. Let me explain before you get too skeptical. A fouled plug doesn't fire because the fouling creates a leakage path from the center electrode to GND that is of lower resistance than jumping the gap. Current will always follow the path of lowest resistance, so instead of jumping the gap it finds its way to GND through the fouling, hence no spark. A fouled plug also requires less kV from the ignition coil’s secondary so the KV builds up only until it’s able to bleed down the fouling.

Disconnecting the plug wire but leaving it close enough to the spark plug terminal so that it'll still arc over (creating a visible spark in the air between the wire end and the plug terminal) will often make that cylinder fire.

Why does this work? The introduction of the additional air gap causes the coil secondary voltage to build up higher – it must be higher in order to jump that first artificial gap you made by pulling the plug wire off the plug. Once the ignition KV builds high enough to jump that added gap, it is high enough to jump the fouled plug electrode gap before it has a chance to bleed off down the plug’s fouling.

Have you seen those "Spark enhancers" sold at county fairs and swap meets for $20? They were common back in the 70's, 80's and 90's, don't know if they're still being sold or not. Those guys usually had a car or truck with no hood demonstrating how much more spark and mileage you get by installing this magical little black gizmo between your coil and distributor. All you got for your $20 was a little box with an air gap in it.

Even today, some spark plugs use an internal air gap in their design. That allows the breakdown voltage of the capacitive discharge spark burst to be controlled by the auxiliary air gap and not by the spark plug electrode gap itself. This has been quite common in plugs for two stroke outboard engines to prevent cold fouling & long duration idle fouling. For some small engines & easy plug fouling 2-cycle applications Champion offers the "easy start" and U series booster gap spark plugs.

DallasY Phil B5 months ago
The muffler is on the right side facing down. I will likely be tearing it down. Just going to need a good portion of a day, and havent had that opportunity yet. I was hoping this may have been a somewhat common problem. I will have a go at it in a few days and report back if figure it out or have any additional informatipn. Thank you for the attention and reply.
Phil B (author)  DallasY5 months ago

I am suspicious the crankcase gasket is not sealing tightly. If the machine is over ten years old, the gaskets may have lost their resiliency and need replacement. If that is the case, replace all gaskets. Non-sealing gaskets is a common problem.

ThabisoD3 months ago

Hi Phil, I've got a Senwei Chinese 950 generator set. It is fairly new but has just gone dead. It looks like the fuel does is not pulled up into the carburator after igniting it with quick start carburator sprays. Could there be something that I'm not doing right, I cleaned the fuel filter between the tank and the cargurator. Please help me I'm stuck.

Thank you. Dube Zim.

Phil B (author)  ThabisoD3 months ago
You have done some good work analyzing the problem so far. Can you remove the fuel line from the inlet tube on the carburetor to see if gas flows freely as far as the carburetor? I expect the carburetor is like those used on a chainsaw with no float chamber. A small piece of dirt can sometimes get into the passageways. A crack in a gasket, a gasket installed backwards, or an air leak can keep these carburetors from bringing fuel into the engine. (Check for any screws that are even a quarter of a turn loose from vibration.) If the engine was stored for several months without running, varnishes from the gasoline may have formed inside the carburetor, and it may need to soak with some cleaner in it. And, air must get back into the gas tank as fuel is drawn out of it. There is a small vent in the gas tank cap, and if it does not let air back in, the engine stops after starting, although that is not the problem you described.
A friend have me a 2 stroke leaf blower that quit running. The carburetor looked fine but the piston and ring was ugly. The compression ring was compressed into the groove and held the by carbon build up. I replaced the ring and it now runs great.
A friend have me a 2 stroke leaf blower that quit running. The carburetor looked fine but the piston and ring was ugly. The compression ring was compressed into the groove and held the by carbon build up. I replaced the ring and it now runs great.
I have a TB425cs trimmer that had lost compression. After replacing the compression ring still no compression. After further inspection I found that the exhaust valve was not seating and would not seat even with pressure. After tearing it down further I found that the exhaust valve guide had dropped and spun giving it a slightly different angle thusly not letting the valve seal. I realigned the guide and staked it in place and it now runs better than it has in a long time. Just thought I'd share my experience of my little 4 stroke. The answer is there, just keep looking.
dxt73395 months ago
john n5 months ago
welder6536 months ago

Hi Phil,Bob again,i took the thrower back apart nothing was pinched,i tightened all the screw's on everything,took muffler off cleaned out port,kept it off,try to start but still won't start,could i have hooked something up wrong or could it need new gaskets on everything,i am just lost and banging my head against the wall,don' know what else to do anymore please help Thanks so much again Bob.

Phil B (author)  welder6536 months ago

Somehow I missed your post. Are you getting any wetness on the spark plug after repeated cranking of the engine? If not, there may be an air leak in the crankcase that keeps the gas/air mixture from reaching the cylinder.

Jakeg636 months ago
Hi I have a jawa 50 got it for 20 quid it hasn't been run for two years has loads of compression and it's getting fuel and air but the spark isn't the best and it only uses to start with petrol pored in the cyclinder but don't know help me please
Phil B (author)  Jakeg636 months ago
Was it stored during the last two years with old gasoline in the carburetor? If so, there are vanishes and gums that will need to be cleaned out of the carburetor, and maybe the fuel lines, too.

As regards the spark, some conditions, like an old spark plug, will appear to produce a reasonably good spark when no compression is present, but will not produce enough spark under the amount of compression found in a running engine. Is the ignition the points and condenser type, or an electronic ignition?
Jakeg63 Phil B6 months ago
I've fully striped the carb and I don't lnow about ignition
Phil B (author)  Jakeg636 months ago

As regards how ignition works, powerful magnets in the flywheel move rapidly past the coil to create a rising current. At a precise moment timed to correspond to the maximum compression of the gas and air mixture, ignition points open and break the primary side of the circuit. That causes a rapidly falling current in the primary windings, which generates a powerful current in the secondary windings of the coil. It's exit point to ground or earth is through the gap in the spark plug where it ignites the gas and air mixture with explosive force. The condenser or capacitor absorbs most of the sparking that occurs at the ignition points in order to preserve their life.

Newer engines replace the points with an electronic trigger that delivers a spark at the right moment. These pointless ignition systems work well without attention for the life of the engine. In recent decades after-market systems have been available to replace the points with an electronic triggered. I once bought and used one of these that was very inexpensive, was easy to install, and worked very well.

In order for an engine to run it needs clean air, fuel properly mixed with the air, a good spark, and good compression. Getting an engine to run is a process of checking for each of these, and then proceeding in logical steps to correct any of these not working as needed.

Since you engine runs when you inject fuel directly into the cylinder, it is logical to think something is wrong with the fuel delivery system. Does the spark plug appear wet after cranking the engine multiple times (with the spark plug in the engine, then removed)? If so, that indicates fuel is reaching the spark plug.

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