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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ShaunoS3 days ago
DallasY2 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)  DallasY2 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 B2 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)  DallasY2 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.

ThabisoD13 days 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)  ThabisoD13 days 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.
dxt73391 month ago
john n2 months ago
welder6533 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)  welder6533 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.

Jakeg633 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)  Jakeg633 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 B3 months ago
I've fully striped the carb and I don't lnow about ignition
Phil B (author)  Jakeg633 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.

welder6533 months ago

Hi Phil,I have a toro s 200 snowthrower i put in new coil,new points & condenser cleaned gas tank out put new mix gas in ,soak carb & rebuilt carb,new spark plug,have spark but intiment spark an will not start could you help please thanks

Phil B (author)  welder6533 months ago

Is the spark weak? Is the primary side wire between the coil and the points pinched anywhere? I once worked on a mower with a primary wire pinched by the cover over the points and condenser. I ould not see the pinch through the wire insulation until I picked up the wire from the engine case.

AndrewR83 months ago

I picked up a 2000 RM250 a week ago. The guy said it had not been started for a year, but ran great before he put it in storage. I took my mechanic savvy buddy and we looked it over. My buddy said it looked good and we probably just needed to clean the carb. So... I got it. Took it home and cleaned the hell out of the carb, the jets, drained the gas tank and put in new gas. Nothing! Wouldnt start. Put in new plug and checked for spark. Good spark and still wont start! Checked the reeds and they are like new. We thought it must be top end. bought a new top end and installed it last night. Great compression, but no start. The piston we pulled looked pretty good to. We tried to bump start it and it kind of starts, but no throttle response. I have no idea what is going on. I know the carb is getting fuel and the air filter is super clean... No idea where to go now?? Oh, I checked the kill switch. unplugged and still nothing...
Phil B (author)  AndrewR83 months ago

How old is the bike? As I mentioned, when the gaskets are more than ten years old, they no longer properly seal against air leaks entering the crankcase, and the fuel mixture becomes too lean to run. Of course, the spark timing must be set properly, too. Here are some Suzuki technical service bulletins from 1975 on problems they had with their bikes. Something in them might help. Otherwise, it sounds like you have checked most things. And, air needs to get back into the gas tank through the vent in the gas cap. Is there a fuel filter? Have you replaced it? I wish I knew exactly what the problem is and could give you better help.

smokedout104 months ago

Hello wondering if you can help i have 49cc goped it used to be weak when accelerated a little bit but would go up to top speed. When i was using it just stopped I heard a sort of popping noise and I stopped then the engine turned off I couldnt start it afterwards because i couldnt pull the cord and it was stuck at first i thought it was flooded so I removed the spark plug and it was still hard to pull so i remove the pull start rope to see if it was stuck and i tried to hand crank the engine it only moves two full turns either way before it sounds like it hits something and cant move it after that?

Phil B (author)  smokedout104 months ago
I do not know anything definite, but from what you describe, I would want to see what is inside the cylinder. I am wondering if the piston could be seized due to scored walls or a broken ring. You may or may not be able to see anything by looking into the spark plug hole. You may need to remove the cylinder from the engine case.

I have a Chinese clone 90cc 2 stroke engine,air box is coming apart and wonder if you can use a regular cone air filter or do you have to use the air box?,cover air intake and it starts and if keep parcial covered will run but when push throttle down speed up and down and dies when you let off.

Phil B (author)  robert.burrow.1215 months ago
I am not expert. My experience was that have modified an air cleaner and the engine ran, You may need to adjust the idle screws because the air flow through the new air cleaner may be a bit different. I think it should work.
coreystalvey5 months ago
There is no certain way if starting . Just hard to start and I don't know why! Maybe thinking the compression . But would it even crank?
Phil B (author)  coreystalvey5 months ago
I was thinking my son-in- law's Husqvarna chainsaw has six or so steps for starting, including a compression release valve. And, it is less horsepower than your engine. The steps must be followed in order for it to start. Even old John Deere tractors have a compression release to open before yanking the flywheel.
coreystalvey5 months ago

I have a 1972 mercury 20 hp 2 stroke boat motor. it is hard to crank but when it cranks it runs good. new sparkplugs , gas , carb cleaned , gas lines cleaned. any suggestions?

Phil B (author)  coreystalvey5 months ago
I am not familiar with your engine. I do know that pulling a starter cord on a 5 HP engine can really take all of the strength a person has, and you have 20 HP engine. I used my son-in-law's chain saw, and it was 5 HP or less. It came equipped ith a compression release button and very precise instructions on how to use it during starting. Are there any special starting instructions for your engine?
mashaca6 months ago
Hi! I have a Toro 2-cycle snowblower that was given to me in non-starting condition. The previous owner had mixed 10-30 oil with gas on the last winter he used it. Anyway, I replaced the spark plug and replaced the gas with the proper mixture. The snowblower started up no problem but would not run with the choke at full; engine was revving up and down but it ran for several minutes. Now I tried starting it and it will run but only if I use the primer bulb to feed gas, as if the carburetor is not feeding gas automatically. Please advise. Thanks!
Phil B (author)  mashaca6 months ago
Notice that in some of my responses to recent questions I referred to an on-line set of diagnostics charts and gave a link. Look for a symptom that most closely matches your problem. Then try the proposed solutions until you find one that solves your difficulty. You are likely correct that the flow of fuel is restricted. The cause could be due to a dirty fuel filter, but also to a poorly functioning air vent to the fuel tank. (Loosen or remove the cap on the tank and see if that makes a difference.) There could also be dirt in the carburetor. If there is a sediment bowl, dump it out and wipe it clean.

BrianJ46 months ago

Hi my friend baught a suzuki ts100 bike, it will start fine and idel fine, but when he puts it into first gear you have to have the revs basically full before it will pull away anything less it seems like its going to die yet in any other gear it is fine any ideas?

thanks in advance

Phil B (author)  BrianJ46 months ago

Take a look at the link in my response to bike22u7 immediately below. It is a link to general diagnostic charts for either 2-cycle or 4-cycle engines. I would guess the problem you describe would be considered "low power." This sub-link discusses multiple causes for that problem.

You will need to check out the probable causes one by one.

bike22u76 months ago
I have subaru 6hp motor on my power washer, it starts on the first or second pull but if i stop it when its warm and try to restart it wont start. Any suggestions. Thank you!
Phil B (author)  bike22u76 months ago

Is that a 2-cycle or a 4-cycle engine? I have not experienced the problem you describe and am not a lot of help. Here is a link to a diagnostic tool for getting a start on solving your problem:

When I was younger, I got my hands on a couple of auto repair manuals. They had diagnostic helps in a special section similar to the link above. It was always a matter of looking for the best description of the problem, then looking at possible causes, and trying them until I found and fixed the problem.

submarine247 months ago
Hello PHIL I have a scooter with 2 cycle engine that I bought used the guy said that it needed a new throttle and he also said in needed a primer bulb. He started it by spray the carb with starter fluid and it ran. So I only fixed the throttle cable but i didnt fixed the primer bulb and sprayed it with brake cleaner because I didn't have any starter fluid and it ran good for about a couple miles. But then I was about to run out of gas so I refilled it with gas but the scooter already had gas when I bought it. It still ran fine and it ran for like 5 miles after I started and sprayed. But then the next day I spray it and
pull it to start and it wont start it just sounds like its going to turn on but it doesn't take off it just turns off after a second. Is it because it doesn't have a primer bulb or could it be the gasoline I put in there I mixed it with oil according to ratio on the gas cap which says 25:1 ??
Phil B (author)  submarine247 months ago
I would begin by removing the spark plug and looking at it to see if it gives any clues. Often one can find photos showing color and types of deposits on a spark plug for various engine problems, and for a properly running engine. I do not know if such a chart exists for 2-cycle engines specifically. Also, when you have pulled the starter several times and then remove the plug, do you see evidence of too little fuel or too much fuel?
Hey Phil I have a echo ht225 hedge trimmer and when I accelerate I get sum leakage from fuel tank,what kind of sealant do you recommend?
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