Instructables

Curing 2-cycle engine problems

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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tezwarby771 month ago

Hi I would appreciate some advice on how to get the generator belt pulley off the crankshaft end, on a G1 Yamaha Golf buggie, and the magneto plate which is fixt with a woodruff key .

Phil B (author) 3 months ago
Tezwarby77, adding extra fuel to the mixture and choking the carb. air flow means you have extra air leaking into the engine someplace. Look for loose screws and hardened gaskets that no longer seal.
tezwarby77 Phil B2 months ago

Thanks Phil it looks like I have a crankcase seal gone on the clutch side of the engine so I will replace both sides.

Thanks Terry.

Phil B (author)  tezwarby772 months ago
Terry,

There was a time when people would bring an engine to me and ask me to get it running. It is a new experience to troubleshoot engines I have not even seen. Thank you for reporting back and letting me know what the problem was. It makes sense. I think you will be pleased with the engine's response when you get the new seals in place and the air leaks are gone.
who8dapple3 months ago

I followed your advice and tried some starting fluid (2-3 second blast) about a half dozen pulls to no avail...took out the Spark Plug for a look see. It was pretty wet, so certainly getting fuel. At this point, I decided to dry it out with a little butane torch and gave the cylinder case the same treatment for approximately 30 seconds. Reassembled and managed to start it with about 5 pulls...(no priming, just full throttle) I think heating it up was just a fluke...i think it was getting flooded!

I ran it for about 10-12 minutes at full throttle and noticed how much fuel it was consuming...quite a bit more than normal. I did a few cool down restarts 15, 30 and eventually 60 minutes apart. It started up within 3-4 pulls with no priming and just full throttle. My guess, 3-4 push of the bulb is enough to flood it. Ryobi manual does suggest 10x for a cold start, and i've never done more than 4-5x in the past...

Now to figure out how to adjust the carb without the special tool.

Big thanks for all your helpful suggestions Phil!

who8dapple3 months ago

Need Help...Ryobi CS26 String Trimmer, about 2-3 yrs old. Briefly started 2 weeks ago, before i did the following; Put in new Carburetor and Carb Gasket, Fuel Filter, Fuel and Air Line, New Spark Plug (Sparking) fresh mixed gas. I've attempted to start it more than a half dozen times, and can't get it to start...any suggestions would be appreciated!

Phil B (author)  who8dapple3 months ago
With problems of all sorts it is good to ask, "What was the last thing that happened before it quit working?" Although every engine needs spark, compression, and clean air in proper mixture with gasoline; chances are your problems are related to something you did to the fuel system. Are the mixture screws set properly on the carburetor? The "L" (low) screw should be about a turn and a half backed out from fully seated. The "H" (high) screw should be about one full turn out from fully seated. Make certain the air filter is clean, itself, and not restricted in any way that impedes the flow of air. Remove the fuel line from the carburetor. Gasoline should flow fairly freely from the fuel line. Remove the spark plug. Is it clean, or caked with carbon deposits? Wrap a bare copper wire tightly around the body of the spark plug and connect it firmly to the metal frame of the engine. Pull the cord rapidly a couple of times with the engine kill switch open so it does not short the spark voltage to ground. There should be.a nice blue spark. If the plug is more than a year old, replace it. An old plug can look very good, but it fails to spark when under normal engine compression. Was the plug wet with gasoline when you removed it? If so, that is evidence the engine is flooding. If the plug was dry when it was removed, try putting the plug back into the engine and torque it properly for a good seal. Then spray ether starting fluid into the air cleaner opening and pull the rope. Did the engine fire or run? A very dry plug likely indicates fuel starvation, which could be caused by an air leak or by improper carburetor adjustment, or by a blockage in the carburetor. The compression on a small engine is not always easy to measure with a compression testing gauge. Often you try to spin the engine by hand, and the compression causes the crankshaft to stop and bounce in reverse direction, if it meets minimum standards.

I am curious why you decided to change so many parts of the fuel system on a machine that is only about three years old.

I hope something her helps.
who8dapple Phil B3 months ago

Hi Phil, thank you for your response! The unit was running rough at the end of last season, and I did manage to start it 2 weeks ago while cleaning it with carb cleaner. This particular model does not have a carb rebuild kit available, hence purchasing a new one for $22 didn't seem too outrageous. I cleaned the (basic) Air Filter, Checked the Exhaust for Clogs/Carbon build up, replaced the Priming Bulb, Fuel and Air Line, and also changed the Spark Plug.

I did check the plug for spark, and it produced a nice blue spark. I mixed fresh fuel with a 50:1 mix, but the only thing that was new to the equation was the addition of Ethanol Stabilizer as per recomendation from the guys at HD (possible culprit?).

I will get a can of Starting Fluid to see if that may help!

Thanks Kindly

RY28020.MOV(402x714) 23 KB
Phil B (author)  who8dapple3 months ago

My son-in-law has a Husqvarna chain saw. He always gets gasoline made without ethanol to use in it. The starting instructions say to apply full choke and squeeze the primer bulb five times. Then pull the cord five times. Open the choke and continue to pull the rope as needed. But, in my experience, the engine fires and dies after about three pulls of the cord. Then it starts and continues to run on the next pull. That is from a full cold start.

who8dapple3 months ago

Oh, i forgot to add that I installed a new primer bulb as well!

Phil B (author)  who8dapple3 months ago

You should not need 50 pulls of the starter cord to make the engine run. I would try some ether starter fluid into the air intake before the first pull and see if the engine runs or at least fires. I would also want to remove the plug after 5 to 10 pulls and see if it shows evidence of flooding or fuel starvation.

who8dapple3 months ago

update...it finally started after approximately 50 consecutive pulls. It ran great and idled well. Did a few hot starts in 2-3 pulls, did a cool down for 45 mins and started right up. But, after about 1 1/2 hrs cooling, it will not start up again. Can I upgrade to a hotter spark plug, and would that help?

tezwarby773 months ago

Hi I have a Yamaha G1 golf buggy with a 215cc 2-cycle engine, it will not run without the choke on, I have richen it up on the carb screw as much as I can, it has a new plug in , the filters are good . Can you give me any idears please

sean72397 months ago
Thank you for your reply. Yes when I removed the bottom of the carb I filled up the bowl with gas to see if the floats were working correctly and not being stuck nor cracked and they both floated to the top. The only other thing I can think of is that the tab on the float valve has been weakened and moved thus not allowing the floats to fully go to the top and controlling the amount of gas.
Phil B (author)  sean72397 months ago
I am assuming your floats are either cork or a solid block of lightweight semi-porous plastic. Even when they have become too heavy they are light enough to float, but not with the required buoyancy. I had this problem with a car I once owned. I got the car new. After seven years gasoline would mysteriously run out of the carburetor and onto the exhaust manifold when the engine warmed up. The float was a black block of semi-porous plastic. It seemed nice and light. But, when I pressed a fingernail into it, I saw a little moisture where my nail made an impression. The float was light enough to float, but had absorbed enough gasoline to be too heavy when the engine was warmed and the gasoline in the carburetor became less dense. The float worked well when the engine was cold and the gasoline was more dense, but not when the engine was warm. When i got a new float, I could suddenly tell the old float was a bit heavier. (We are talking about only a few grams.) And, the engine suddenly performed perfectly when I installed a new float. You would need to weigh your old floats with a scale capable of measuring a small number of grams accurately and you would need to know the factory specifications for the weight of a new float to determine if your float is not heavy. If your carburetor rebuild kit did not include new floats for your carburetor, replace the floats. I think you will be surprised.
sean72397 months ago
Hello I was wondering if you can help me? I have a 1990 Polaris Trail Boss 250. I am having the problem where it will start and run great when its cold but will die when its hot. It will not start again once hot until it cools down. The bike is oil injected and I have checked the system. I changed the head gasket as I had a compression leak. The compression is at 150psi cold and about 144psi warm. I have cleaned and replaced all the carb jets and needles in the carb. I have a shop manual that I got and set the carb specs back to factory settings. It seems that the carb is getting way to much gas. I have tried to adjust the amount of fuel going to the engine but with no success. I have checked the stator and spark. I have great spark cold or hot. I am all out of ideas on what to do next. Bolts are all tight around the engine. Could you please help me? Thank you!!!!!!!!
Phil B (author)  sean72397 months ago
If the engine is getting too much fuel and you cannot adjust by any usual means, have you checked the carb. float. Even the plastic block floats can absorb gas and become heavy.
TexasBuilder9 months ago
I'd like to add my two cents for troubleshooting the dreaded "two cycle engine starts when cold, but dies when hot"...

I used to have a '76 Yamaha Dirt Bike that would start like a champ and run peppy; However, once you took it out, just as you started to break a sweat, the peppy 125 would die, and not restart...

So, upon troubleshooting everything else in the book, it turns out that the problem was that as the engine heated up, the scalloped out cylinder wall would begin to lose compression. This was not very evident, as the visual inspection of the engine cylinder wall looked all nice and smooth. Calipers revealed that the cylinder wall was scalloped. Once the engine was given a machine job and new piston / rings, it ran like a new engine.
Also, another thing to check are the reed valves, which I think may also be prone to temperature related changes.

So, now I also have a chainsaw that exhibits this same behavior. I will troubleshoot it and report back what I find...

I was thinking that I will run the engine until it heats up and fails, then quickly perform a compression test, and see if that differs from a cold compression test... One theory of mine is that running a two stroke with insufficient lubrication in the fuel could cause this phenomena of scalloping out or "honing the bore" with the piston rings such that as the engine heats up to operating temperature, the engine loses compression, but starts fine when cold.
Phil B (author)  TexasBuilder9 months ago
Thank you. That should be helpful to many. I had never considered what you described.
botrys9 months ago
As I said I rebuilt the carb. with new gaskets. Could it be something to do with the exhaust since it starts to die after about 30 seconds as the motor starts to warm up?
Phil B (author)  botrys9 months ago
When I mentioned gaskets I did not mean only the gaskets in the carburetor, but also those between the engine castings that are held together with screws as well as the gasket between the carburetor and the engine. It still sounds like the engine is suddenly running too lean after warming up. When an engine warms up, tolerances change. Perhaps you heard the SR-71 Blackbird leaked fuel like a sieve when rolled out of the hangar, but when it reached altitude and the friction of the air at those speeds caused enough heat for seals to snug up. In your case it may be loosening up. Gaskets lose their resiliency with age and do not seal. Most small 2-cycle engines also have one mixture circuit for low r.p.m.'s, but another for high r.p.m.'s. When the gaskets on my weed trimmer were getting old, it would stumble and die when I revved it up. The problem was solved after I replaced gaskets between the cylinder and the crankcase and between the carburetor and the crankcase.
botrys9 months ago
I have a Tanaka strimmer, the motor starts, no problem, and will will rev at the beginning, however after about 30 seconds the motor tries to die, if I release the throttle it ticks over no problem. After that will rev a little but if I try to rev full it dies again. Have rebuilt the carb (new gaskets, etc), replaced the fuel line and filter, but no improvement - ANY IDEAS????
Phil B (author)  botrys9 months ago
I am guessing it is fuel starvation. That could be caused by an air leak in a gasket. If the machine is ten or so years old, get or make new gaskets. The carburetor adjustment screws for rich/lean could also be set too lean, especially on the "high" side.
Redneck036510 months ago
I have a 1975 Suzuki GT 185 that a purchased as a project and it will only start and run at high rpms (around 4 thousand) with the exhaust pipes taken off. I rebuild the carbs and used fresh gas, new air cleaner new spark plugs, good compression, good spark. I can not figure out why it will only run at high rpms. I have had the help of a former mechanic and we are both stuck does anybody have any ideas about why it is not working.
Phil B (author)  Redneck036510 months ago
When you rebuilt the carburetors is there any possibility the same gasket in each could have been installed wrong side up? (Usually the gaskets go in only one way, and it is more probable such a mistake might be made in one carburetor, but not two.) Is there a low idle adjustment and a high or run adjustment screw, or only one idle mixture screw? If there are two, it may be that the low idle adjustment screw are turned in too far. Are the parts of the throttle and choke linkages installed correctly? Is there a governor that might be stuck? Is the spark advance stuck? This has a couple of centrifugal weights that can be stuck. (These are under a round removable plate on the side of the crankcase.) Is it possible for the ignition timing to be set incorrectly, or is it impossible to adjust? I do not know what the problem is, but am suggesting some things to check.
charlestonump711 months ago
I have a compression problem, well my neighbor does but I am the fix-it guy. So his Ryobi string trimmer is working fine one day but will not start the next. When the choke is set to full or when the throttle is not held open (with the choke in any position) it feels as though there is no compression when pulling the starter string. However except for on full choke, if you hold the throttle open there appears to be the same amount of compression as it normally would have. Although I have not visibly checked the plug but it appears/sounds like it is getting fire when pulling the starter string when the throttle is held open. I have checked the carb mounting bolts, etc for obvious spots where air may be leaking out of the cylinder but did not see anything alarming. However I do not think that there is any type of continuous air leak air as the compression appears to change depending on the amount of air being allowed in from the carb. Does anyone have any ideas on what could be causing this? I am stumped!
Phil B (author)  charlestonump711 months ago
While it seems logical that the spark plug is firing, you really cannot know for certain without removing it and checking. As you know, assumptions are often very misleading. Something you would not expect often hides under an assumption. It is odd that the compression comes and goes with changes in the throttle setting. When you find the reason why, it will all make sense.

Some engines, like Stihl chainsaw engines have a compression release valve that makes the engine easier to turn over for starting. I have been unable to find any mention of such a system on a Ryobi string trimmer. But, if there were such a system, it could be linked to the choke.

Here is a link to a brief article on compression problems related to Ryobi string trimmer engines. I would take most of the engine apart, one system at a time, and make notes on what I find because it is easy to become confused later about what you saw or did not see. As the article mentions, check every inlet and outlet for air. Also, it might be good to do an actual compression check on the engine with a gauge for that purpose. And, it is always possible that a piston ring suddenly broke, which would explain the sudden onset of the problems.

I wish you well. Please let us know what the problem was when you figure it out.
southcoastdj11 months ago
Hi, I was hoping you could shed some light on my 2stroke/cycle scooter, basically I am having trouble starting the bike if left overnight or for 5hrs, in order to get it started I have to cover the air filter intake with my finger in order to get it to start, once started all is fine, ticks over as it should and drives as it should with no loss of power or performance, I have changed the autochoke for a new one, installed a new spark plug but still it will not start if left for a period of time, I am at a loss trying to figure out why its doing this as it was fine up until last sunday, any ideas?
Thanks
southcoastdj
Phil B (author)  southcoastdj11 months ago
If you must block the air intake to get the engine to start, it means air is entering the engine elsewhere, too, and the engine is getting a lean mixture. But, it is not so lean that the engine will not run when it is warmed up. Since it happened suddenly, I would suspect a screw loosened. It may be that the low circuit idle screw has been bumped and is not adjusted properly. These are usually 1 1/2 turns loosened from bottoming out. It is more likely a carburetor mounting screw or two have/has loosened. If a carburetor mounting screw loosens a quarter of a turn, that lets in too much air and you will have starting problems. If the engine is ten or more years old, consider that the gaskets may have hardened and do not seal properly. That lets in too much air, too. While you are at it, make sure the fuel flows as it should from the gas tank into the carburetor. If the engine has an old fuel filter, you could replace it with a new one.
keegan10111 months ago
Hello hello, can anyone figure this out? I have a 1985 suzuki 185 dirt bike, starts first go, idles ok, but when the throttle is applied it conks out. and after 3-4 attempts the engine revs uncontrollably to the point where it sounds like it wants to detonate and the kill switch wont turn it off nor will the key. carby looks clean on the inside, the float is ok and the throttle valve doesnt seem to stick at all. please help if you can, thanks, Kf : )
Phil B (author)  keegan10111 months ago
Find and check the spark advance weights. It sounds like they are sticking. They are under a round plate and work by centrifugal force.
trance72811 months ago
Hi Phil,
Maybe you can help me out. I have a 2 stroke powered skateboard, probably around a 32cc. It has worked fine in the past but it sat for a while. I recently began trying to tune it and have replaced fuel lines, cleaned the muffler and carb and added a new air filter. Everything runs fine, except when trying to go to its top end it becomes jerky, almost a sticky feeling. At mid range it runs fine, and if I full throttle when I am not riding on it seems to be fine with no jerkiness. I'm not sure where to concentrate. I have played around with the air/fuel screw with no effect on fixing it. This is the carb that has the idle screw on top and the air/fuel on the side. Could this be related to the gasket from the carb to the engine not being in the best condition? Any help would be greatly appreciated.

-Dustin
Phil B (author)  trance72811 months ago
If you have a gasket that is not in the best condition, I would replace that quickly and then see what the engine does. It may not be sealing as it should and be leaking air, which would lean out the fuel mixture. When the idle screw no longer makes any changes in the way the engine runs, it is likely too much air is getting in someplace.
MurphyDIY11 months ago
Rest of the Story... After disassembling and cleaning the carb, it started right up. I'm guessing it was a slight bit of debris (almost a smudge) on the flapper valve that is paper thin and resisted the vacuum action. Thanks!
Phil B (author)  MurphyDIY11 months ago
Thanks for the report. It makes sense. I am glad it was that easy.
Paulygriff11 months ago
Hello,
I was wondering if anyone could help me with my 2 stroke engine problem I have with my bike?
Recently bought a motor and fitted it on my bike, broke the engine in properly for the first 50 miles. Running it's first gallon on a mix of two wheel two stroke mineral (Comma) oil at 25 to 1 ratio. This went really well for 220 miles. Then I put a 2 stroke motorcycle oil XL (Carlube) in at 20 to 1 ratio for the next gallon and that's when I started having problems.
The problem Is now that it starts fine and goes for a little way and that conks out. After pedalling for a while it will start again, go for a while and then conk out again.
I think maybe that I have put the wrong type of oil in on the second gallon mix?
Can anyone help me please,

Many thanks Paul
Phil B (author)  Paulygriff11 months ago
I would take the spark plug out and examine it for any deposits on it. What is their texture, thickness, color, etc.? Compare the appearance of the spark plug and any deposits on it to a spark plug chart to get clues about what may be happening with your engine. For certain, I would go back to the oil mixture that worked, even if I threw out what did not work.
Phil B (author)  Phil B11 months ago
There is the possibility the oil in the second gallon of mix does not provide enough lubrication and the engine runs extra hot, which causes it to bind up with extra resistance and quit. Pedaling for a while allows the engine to cool so clearances can go back to normal. This is a very brief article, but it has a couple of possible clues.
MurphyDIY12 months ago
Ok, thanks. I'll see if I can pull it apart to inspect and clean. It's labled "ZAMA", without bulb on top and quite small. I think it has diaphram which I saw after taking off two screws on side (I believe) - was intact. Definitely flooding. Thanks again for your help and contributions to instructables! I'll tinker a bit more, but may have to make a sacrafice to the landfill gods (hate doing that!). Any suggestions as to brand I should buy instead for lead blower?
Phil B (author)  MurphyDIY12 months ago
I remember small carburetors like that with a name like "wilco" or something similar. I think they are all very similar. Carburetors have a needle valve that regulates the flow of gas into the carburetor. Anything that causes that valve not to seal causes flooding. That could be a little piece of dirt, like a shred of grass. A broken or missing spring can also be a problem leading to flooding. I think you can buy those carburetors new or used quite reasonably. I have now doubt your leaf blower will work well again.
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