Just a note to readers of the comments…not only does the crank seal leak in air to the combustion chamber , it also can draw in engine oil from the clutch side and cause the engine to bog. Also noted in my case (gas gas ec300) that the engine breather was spoofing out black oil , indicating the extra pressure into the crank cases. Thanks for explaining the basics of what’s going on. Greetings from down under 🇦🇺
Hi mate can a crank bearing/ seal cause a bike not to start have got a kmx and it has good compression have taken head off and barrel and piston and rings look good I even taken it to a shop and they said my top end looks fine so anyway I have checked my fly wheel and it has movement when I pull it up and down not loads but its there will this be the problem do u think
Epic video. There is also a condition where crankcase oil lives on the other side of the crank seal, which causes an overly rich condition (as that oil or tranny fluid doesn’t burn the same) as such you foul plugs and/or saturate your muffler packing.
I have a yz 80 i haven't ever been able to get to run properly. Kicks over first time almost everytime. Wont rev up. Tons of oil coming from the pipe. Any ideas?
I really liked your videos. You are very thorough in explaining. Your video on metering diaphragm was really helpful for me understanding small engine carburetor
LoL! I have two mopeds with crank seal leaks on opposite sides from one another. One was too rich and the other too lean. Replacing the seals fixed them both! Thanks!
When you mentioned pressure in the crankcase is the key word... What nobody seems to understand these days is the fact that two-cycle or two-stroke if you want to call it engines are self supercharging engines. At a certain RPM the pressure in the crankcase becomes higher than atmospheric pressure. That's what we call supercharging.. for a two-stroke engine this is when the power band starts and continues all the way through to the RPM range ... Something I've never seen done but I know that manufacturers are aware of.. Let's put a tapped hole in the bottom of the crankcase using something small like 1/16 NPT tap and then attach a pressure gauge and while the engine is running under load let's see how much crankcase pressure (boost) two-stroke engines can produce..
I am proud to be the very first to comment this video,as the others,very clear, usefull,and with drawings very precise,I am the one from Brasil.Congratulations,and keep up with your nice job.
André Welser, thank you very much indeed for taking the time to write your comment. It really does motivate me to keep going with these videos. Thanks again, Craig
I assume if there's oil mess around the crank seals that,d be a good indication of blowby? I know that term is generally used for the rings wear but in this case I guess it'll be alright. I'll tell you my friend, your videos make us think, as well as learn. thanks so.
A two stroke engine has a total loss lubrication system and not a wet sump like a four stroke. There are three things in a two stroke sump, mostly air, and then fuel and 2-stroke oil. Leakage around here is not very prominent unless the engine has been flooded.
Thank you for a very informative video! I have a Kawasaki ultra 150 jet ski with 3 cyl and 3 carbs. I rebuid the entire engine with new crankshaft etc. and after that the engine dies after about 10 seconds of idling. The only way to keep the engine from not bogging down and die is to keep it above 3500rpm. And to get it running again i have to choke it between every bog down. I comp tested the cylinders and they reach 120 but if i let it sit for a couple of minutes the pressure drops to 100 and stays there. Would you say my symptoms are due to leaking crank seals? I tried the light oil trick but I couldn't get it to work.
I wonder if you can have an air leak at the crank seals, but not have any fuel and oil mix leakage? On my engine i see no seepage, but suspect a possible leak? I know that i need to use some shims on the sides of the crank shaft because there is excessive play with the crank and bottom end🤦 Thanks for the video 😊
I have to check further on my 3.5 horsepower Briggs & Stratton mower engine as after I lapped the valves in,the engine still had a bit of a misfire at idle speed. I want to check it by plugging off the PCV valve tube then by inserting an airline into the oil filler hole,then to pressurise the crankcase with air. With the magneto removed,spray some soapy water around the top crankshaft oil seal then watch for air bubbles. The top of the engine is caked with dirt,grass & oil so I assume that the top crankshaft oil seal is leaking & upsetting the fuel mixture, the bottom one doesn't leak. If it misfires at idle speed,then it could be a lean fuel mixture,I have trouble getting it to run rich when I tune it so that could be the case.
Russell Booth thats a four stroke engine, did you check your valve clearance after lapping? To adjust the clearance you have to grind the valve stems a little bit at a time, with piston at tdc on compression stroke, check the clearance with springs removed and feeler gauge inserted in the gap between the cam tappet and valve stem. Clearance should be .006 for intake and .009 for exhaust. Grind a little at a time until proper clearance is met.
Thank for the vid. Very informative. I have a problem with my 100hp 2 stroke mercury, starts and slowly increases revs in idle and then puffs out smoke, reduces revs and then increased again and keeps doing the while puffing experience over and over again, could this be the seal also? Thanks again
Hello Can a leaking crankshaft seal cause the bike to rev out uncontrollably. I had a bad crash on Saturday during a motocross race and I hit the face of a jump in over rev the bike completely reved out , I’m trying to figure out what happened
My engine belt is loaded with oil and I know it is not my power steering oil. It leaks a court of oil every 40 miles. I am guessing it is coming out of the crankshaft seal but the engine performs fine with no bogging at all. Is my guess unfounded?
got a polaris indy 500 2 stroke twin once fully warmed up she bogs at about 2000rpm and wont gain rpm unless i feather the throttle but blipping the choke lever works best and once above bog spot clutch engages at correct rpm and runs really well also no problems starting motor like ive seen once in a bad crank seal before any ideas ? cleaned carbs new fuel pump
I bought a 2022 brand new motorcycle that has a leak at the front sprocket shaft. The dealership refuses to recognize that there’s a leak, but I say the PCV valve is bad and stuck or even partially inoperative or another venting port is blocked. My question is if a motorcycle is brand new what could possibly cause an oil leak at the front sprocket shaft? ?
I know this post is old. Would the crank seals cause a no start/no run condition? I have a Husqvarna 240 chainsaw that suddenly died while in use. I’ve examined every other possible cause. Including installing a new OEM carb, new spark plug. I have good spark, good fuel flow, fresh fuel (verified in another machine), flywheel key-way is in check (the key way is cast into the flywheel on this model) I have checked and double checked everyyyything. I also tried starter fluid and not even a pop. The compression feels just as strong when I pull start as it always has, it has good resistance as I pull. Consensus on a Facebook post is it’s the crankshaft seals. They are visually intact, I can’t imagine a seal on this, when the compression feels strong, could possibly cause a no start condition.
Actually measure the compression with a tester...I had same problem w/ mine died while running, wouldn't start back up even w/ starter fluid, EVERYTHING else checked out... I then measured the compression and it could only muster 35 psi! Took the top-end off and piston had a huge crack & the rings were stuck solid, pretty new engine so I think it sucked in some dirt or something 🤷♂️
It can cause it to over revv and starting problems mainly. It can be very temperamental with trying to set the carb as well. Almost impossible to get the settings just right depending on how advanced this is
@@TheRepairSpecialist Thank you sir,my crm 250 won't stay idle but when choked, it idles,I think this is the problem or it should be a air leak,do you have any idea?
You are technically correct but to put it simply if your seals are shot your crank case will not build vacuum or pressure, without vacuum it cannot draw in the fuel air mix, without pressure it cannot transfer that mix through the port to the combustion chamber.
@@sunflxwergamer303 bogging can come from bad seals, bad reeds, dirty or damaged carburetor, incorrect carburetor adjustment, bad fuel. 2 strokes are finicky, I’ve seen bad spark plugs cause it as well as sheared key on flywheel and even a weak magnetic field on flywheel. I tend to look at case leaks last as the leak would have to let pressure out but not leak on vacuum to cause a bog, it it leaks on vacuum it generally will cause the engine to rev uncontrollably. It is nearly impossible to diagnose without being able to test the engine all we can do is tell people how to diagnose by themselves but it will not replace experience in knowing the sound of the bog, what point it starts and so on. I turn down a lot of work on small 2 strokes simply because the diagnosis cost and repair could exceed replacement cost and quite often I will not get paid for the hour or two it took to figure it out
One thing about this video is not quite complete. The best way to test crank seals isn’t by pressurizing, it’s by creating a vacuum. By the nature of how these seals are constructed, using pressure instead of drawing a vacuum can give a false negative result.
i had gearbox oil leaking into the crankcase and i changed that seal but not the other, no oil leaking now but is bogging and wont idle consistantly and is smoking alot
Hi any one with knowledge when I try start bike even with fuel directly down cylinder it just makes a loud pop after like ten kicks have got good spark and have taken cylinder off and bore rings piston look good have check my fly wheel and it looks to have play when I move it up and down can crank bearing and crank oil seal cause a none start ?
I have an interesting issue with my 2 stroke 800cc snowmobile motor. This is one of those motors that used grease for the main bearing from Ski-doo. The grease pushed out of the seal on both sides. I checked if the seals where leaking air by doing the light oil trick and that didn't do anything. The motor runs great, and I don't see a lean condition on the spark plugs. So I repacked the bearings with grease and no issue as far as running. From what I could tell is that I have a high crank pressure when riding hard, and that pressure is more than the seals can handle. I guess something has got to give.
The problem im having is the exhaust temperature being different on each cylinder , i have a 3 cylinder engine in my snowmobile and the PTO side has higher exhaust temperature then the other 2, trying to figure out whats going on there.
Why does my bike sounds like it's flooding and does not want to start and feels like it have an little less compression than before but i have put in a new piston and rings and gaskets
Most light oil spray cans have a straw that fixes onto the spray end of the can, which direct the spray to the main seal area much better and without contact with the clutch. Thanks. Craig
@@chrisheffernan7540 Primary compression is not the compression on the compression stroke but on the down stroke to pressurize the crankcase, with leaky seal the primary is affected.
@@UpInSmoke54 you asked how is primary compression affected, it is not affected. Crankcase pressure is less due to the leaky crank seals, which will lean your mixture.
Wow... important stuff about 2 stroke... ❤❤❤.... bro my bike isnt consuming gear oil... both crank seal are perfect... no air leaks... but it still does not acclerates like real 2 stroke .... feels like a four stroke... very less power at lower as well as higher rpm range.... i am stuck... please help....
A very common thing with two stroke engines, that I have seen more than once, is that most people put the clutch side oil seal in backwards, which would be right on a four stroke engine, but on a two stroke engine you are try to keep oil out of the crankcase, because it gets it's oil from the gas mix. If the seal is blowing out of the holder it's because it is trying to keep pressure in the crankcase, and not the oil out, I know it looks backwards, just remember that the part of the seal with the little spring on it goes toward the fluid that you are trying to keep in, or out, whichever the case may be.this is part of the reason for the vent tube on a two stroke, and also the reason why the tranny oil should be changed more often, on a two stroke.
Would this explain why I’m only getting 60 ish percent throttle and then after that is a 4 stroke sound until I keep on reviving up and then I get full throttle?
Just a note to readers of the comments…not only does the crank seal leak in air to the combustion chamber , it also can draw in engine oil from the clutch side and cause the engine to bog. Also noted in my case (gas gas ec300) that the engine breather was spoofing out black oil , indicating the extra pressure into the crank cases. Thanks for explaining the basics of what’s going on. Greetings from down under 🇦🇺
Hi mate
can a crank bearing/ seal cause a bike not to start have got a kmx and it has good compression have taken head off and barrel and piston and rings look good I even taken it to a shop and they said my top end looks fine so anyway I have checked my fly wheel and it has movement when I pull it up and down not loads but its there will this be the problem do u think
What a fantastic well explained video. A delight to listen, learn and watch.
It should be mentioned also that the lean condition caused by a leaking crank seal on a 2 stroke can lead to premature piston failure or ring failure.
Epic video. There is also a condition where crankcase oil lives on the other side of the crank seal, which causes an overly rich condition (as that oil or tranny fluid doesn’t burn the same) as such you foul plugs and/or saturate your muffler packing.
I have a yz 80 i haven't ever been able to get to run properly. Kicks over first time almost everytime. Wont rev up. Tons of oil coming from the pipe. Any ideas?
I really liked your videos. You are very thorough in explaining. Your video on metering diaphragm was really helpful for me understanding small engine carburetor
LoL! I have two mopeds with crank seal leaks on opposite sides from one another. One was too rich and the other too lean. Replacing the seals fixed them both! Thanks!
When you mentioned pressure in the crankcase is the key word... What nobody seems to understand these days is the fact that two-cycle or two-stroke if you want to call it engines are self supercharging engines.
At a certain RPM the pressure in the crankcase becomes higher than atmospheric pressure.
That's what we call supercharging.. for a two-stroke engine this is when the power band starts and continues all the way through to the RPM range ... Something I've never seen done but I know that manufacturers are aware of..
Let's put a tapped hole in the bottom of the crankcase using something small like 1/16 NPT tap and then attach a pressure gauge and while the engine is running under load let's see how much crankcase pressure (boost) two-stroke engines can produce..
That is a brilliant idea actually.
Great easy to understand video. I've also used carb clean to see if the revs recover.
I am proud to be the very first to comment this video,as the others,very clear, usefull,and with drawings very precise,I am the one from Brasil.Congratulations,and keep up with your nice job.
André Welser, thank you very much indeed for taking the time to write your comment. It really does motivate me to keep going with these videos. Thanks again, Craig
I assume if there's oil mess around the crank seals that,d be a good indication of blowby? I know that term is generally used for the rings wear but in this case I guess it'll be alright. I'll tell you my friend, your videos make us think, as well as learn. thanks so.
A two stroke engine has a total loss lubrication system and not a wet sump like a four stroke.
There are three things in a two stroke sump, mostly air, and then fuel and 2-stroke oil.
Leakage around here is not very prominent unless the engine has been flooded.
That is absolutely correct. A telltale sign of failed crank seals is black, oily residue around the seal area.
@@TonyLing
Baloney! Read my comment.
Very helpfull trick for quick diagnose! Greetings from Argentina
Thank you for a very informative video! I have a Kawasaki ultra 150 jet ski with 3 cyl and 3 carbs. I rebuid the entire engine with new crankshaft etc. and after that the engine dies after about 10 seconds of idling. The only way to keep the engine from not bogging down and die is to keep it above 3500rpm. And to get it running again i have to choke it between every bog down. I comp tested the cylinders and they reach 120 but if i let it sit for a couple of minutes the pressure drops to 100 and stays there. Would you say my symptoms are due to leaking crank seals? I tried the light oil trick but I couldn't get it to work.
You're a very smart man. You go above. My little brain u always lose me!!!
Amazing video on explaining why engines with bad or worse crankcase seals can run funny and bog ...
I wonder if you can have an air leak at the crank seals, but not have any fuel and oil mix leakage?
On my engine i see no seepage, but suspect a possible leak?
I know that i need to use some shims on the sides of the crank shaft because there is excessive play with the crank and bottom end🤦
Thanks for the video 😊
I have to check further on my 3.5 horsepower Briggs & Stratton mower engine as after I lapped the valves in,the engine still had a bit of a misfire at idle speed.
I want to check it by plugging off the PCV valve tube then by inserting an airline into the oil filler hole,then to pressurise the crankcase with air.
With the magneto removed,spray some soapy water around the top crankshaft oil seal then watch for air bubbles.
The top of the engine is caked with dirt,grass & oil so I assume that the top crankshaft oil seal is leaking & upsetting the fuel mixture, the bottom one doesn't leak.
If it misfires at idle speed,then it could be a lean fuel mixture,I have trouble getting it to run rich when I tune it so that could be the case.
Russell Booth thats a four stroke engine, did you check your valve clearance after lapping? To adjust the clearance you have to grind the valve stems a little bit at a time, with piston at tdc on compression stroke, check the clearance with springs removed and feeler gauge inserted in the gap between the cam tappet and valve stem. Clearance should be .006 for intake and .009 for exhaust. Grind a little at a time until proper clearance is met.
Briggs Stratton ? 4 stroke then you plumb
Thank for the vid. Very informative.
I have a problem with my 100hp 2 stroke mercury, starts and slowly increases revs in idle and then puffs out smoke, reduces revs and then increased again and keeps doing the while puffing experience over and over again, could this be the seal also? Thanks again
Left hand crank seal or your pilot jet is blocked dirty sort that out before crank seals
Thanks for making this video very helpful and educational 👍🏽👍🏽
very informative video. thank you and i look forward to your next videos.
So beautifully explained!😎
Thank you so much 👍👍
Hello
Can a leaking crankshaft seal cause the bike to rev out uncontrollably.
I had a bad crash on Saturday during a motocross race and I hit the face of a jump in over rev the bike completely reved out , I’m trying to figure out what happened
My engine belt is loaded with oil and I know it is not my power steering oil. It leaks a court of oil every 40 miles. I am guessing it is coming out of the crankshaft seal but the engine performs fine with no bogging at all. Is my guess unfounded?
Brilliant video...explained so well!!
Wonderful job with this video! Thank you
Excellent, informative video, thank you!
got a polaris indy 500 2 stroke twin once fully warmed up she bogs at about 2000rpm and wont gain rpm unless i feather the throttle but blipping the choke lever works best and once above bog spot clutch engages at correct rpm and runs really well also no problems starting motor like ive seen once in a bad crank seal before any ideas ? cleaned carbs new fuel pump
Amazing video
I bought a 2022 brand new motorcycle that has a leak at the front sprocket shaft.
The dealership refuses to recognize that there’s a leak, but I say the PCV valve is bad and stuck or even partially inoperative or another venting port is blocked.
My question is if a motorcycle is brand new what could possibly cause an oil leak at the front sprocket shaft? ?
I know this post is old. Would the crank seals cause a no start/no run condition?
I have a Husqvarna 240 chainsaw that suddenly died while in use. I’ve examined every other possible cause. Including installing a new OEM carb, new spark plug. I have good spark, good fuel flow, fresh fuel (verified in another machine), flywheel key-way is in check (the key way is cast into the flywheel on this model) I have checked and double checked everyyyything. I also tried starter fluid and not even a pop. The compression feels just as strong when I pull start as it always has, it has good resistance as I pull. Consensus on a Facebook post is it’s the crankshaft seals. They are visually intact, I can’t imagine a seal on this, when the compression feels strong, could possibly cause a no start condition.
Actually measure the compression with a tester...I had same problem w/ mine died while running, wouldn't start back up even w/ starter fluid, EVERYTHING else checked out... I then measured the compression and it could only muster 35 psi! Took the top-end off and piston had a huge crack & the rings were stuck solid, pretty new engine so I think it sucked in some dirt or something 🤷♂️
PW80 redlines when I kick it. It only runs on carb spray which gets sprayed through the carburetor. Could a bad crank seal be the issue?
Thank you and Well done
Sir does this case also cause to engine runaway
It can cause it to over revv and starting problems mainly. It can be very temperamental with trying to set the carb as well. Almost impossible to get the settings just right depending on how advanced this is
@@TheRepairSpecialist Thank you sir,my crm 250 won't stay idle but when choked, it idles,I think this is the problem or it should be a air leak,do you have any idea?
Also I wondered, Would the carb adjustments compensate for the air fuel mix in the crankcase?
Depends how bad the leak is at the seal. If it's a large leak, the amount of air from the carb will be small in comparison
Thanks so many chanies saw is that problem
Would wd 40 do? thanks for a great vid!
i think so but dont spray too long or you will seise ur engine
i have 2 stroke engine that seams to be sucking oil into the exhaust an flooding the exhaust with oil
You are technically correct but to put it simply if your seals are shot your crank case will not build vacuum or pressure, without vacuum it cannot draw in the fuel air mix, without pressure it cannot transfer that mix through the port to the combustion chamber.
You have a point Sir, so any other suspect to look around ? Assuming oil seals are both ok
@@sunflxwergamer303 bogging can come from bad seals, bad reeds, dirty or damaged carburetor, incorrect carburetor adjustment, bad fuel. 2 strokes are finicky, I’ve seen bad spark plugs cause it as well as sheared key on flywheel and even a weak magnetic field on flywheel. I tend to look at case leaks last as the leak would have to let pressure out but not leak on vacuum to cause a bog, it it leaks on vacuum it generally will cause the engine to rev uncontrollably. It is nearly impossible to diagnose without being able to test the engine all we can do is tell people how to diagnose by themselves but it will not replace experience in knowing the sound of the bog, what point it starts and so on. I turn down a lot of work on small 2 strokes simply because the diagnosis cost and repair could exceed replacement cost and quite often I will not get paid for the hour or two it took to figure it out
Will the seals affect compression?
One thing about this video is not quite complete. The best way to test crank seals isn’t by pressurizing, it’s by creating a vacuum. By the nature of how these seals are constructed, using pressure instead of drawing a vacuum can give a false negative result.
Mine leaks so much that you can hear it and even blow through the carb with your mouth, is that bad or can it still work?
what kind of light oil do you spray around the crank seals?
Steve R, i personally have used WD40 on my machines.
TheRepairSpecialist thanks!
Steve R m- thought most crank seals are located inside the case, not easily accessed
Propane works also. Non chlorinated brake clean also. Non flammable brake klean is worthless as tits on a boar hog.
@@Community-Action mostly the early engines
What specific tool would you use to pump the air in? Through the spark plug thread in?
nope
@@integrationofmanandmachine4714
Wrong, totally wrong! Spark plug hole adapters are made for this exact purpose!
Many tools are sold for this. The most well known is MityVac.
i had gearbox oil leaking into the crankcase and i changed that seal but not the other, no oil leaking now but is bogging and wont idle consistantly and is smoking alot
James Blake main bearing, and both crank seals.
Hi any one with knowledge when I try start bike even with fuel directly down cylinder it just makes a loud pop after like ten kicks have got good spark and have taken cylinder off and bore rings piston look good have check my fly wheel and it looks to have play when I move it up and down can crank bearing and crank oil seal cause a none start ?
very good ...
I have an interesting issue with my 2 stroke 800cc snowmobile motor. This is one of those motors that used grease for the main bearing from Ski-doo. The grease pushed out of the seal on both sides. I checked if the seals where leaking air by doing the light oil trick and that didn't do anything. The motor runs great, and I don't see a lean condition on the spark plugs. So I repacked the bearings with grease and no issue as far as running. From what I could tell is that I have a high crank pressure when riding hard, and that pressure is more than the seals can handle. I guess something has got to give.
The problem im having is the exhaust temperature being different on each cylinder , i have a 3 cylinder engine in my snowmobile and the PTO side has higher exhaust temperature then the other 2, trying to figure out whats going on there.
So 3 years later what did you end up doing?
Why does my bike sounds like it's flooding and does not want to start and feels like it have an little less compression than before but i have put in a new piston and rings and gaskets
Bennie Coetzer They might have been installed incorrectly. You can also try adjusting the carb.
How did u learn all that u know about small engine ????
William Isen a small book
How do u test with squiring oil with a clutch and flywheel in the way ?
Most light oil spray cans have a straw that fixes onto the spray end of the can, which direct the spray to the main seal area much better and without contact with the clutch. Thanks. Craig
@@TheRepairSpecialist nice
Good video, but you didn't explain how it effects the primary compression.
It has Zero effect on compression. It causes a lean fuel mixture with a leaky crank seal
@@chrisheffernan7540 Primary compression is not the compression on the compression stroke but on the down stroke to pressurize the crankcase, with leaky seal the primary is affected.
@@UpInSmoke54 that's not what he asked
@@chrisheffernan7540 who?
@@UpInSmoke54 you asked how is primary compression affected, it is not affected. Crankcase pressure is less due to the leaky crank seals, which will lean your mixture.
Wow... important stuff about 2 stroke... ❤❤❤.... bro my bike isnt consuming gear oil... both crank seal are perfect... no air leaks... but it still does not acclerates like real 2 stroke .... feels like a four stroke... very less power at lower as well as higher rpm range.... i am stuck... please help....
Dod you fixed that issue? I am having that problem with my 2 stroke now 😔
A very common thing with two stroke engines, that I have seen more than once, is that most people put the clutch side oil seal in backwards, which would be right on a four stroke engine, but on a two stroke engine you are try to keep oil out of the crankcase, because it gets it's oil from the gas mix. If the seal is blowing out of the holder it's because it is trying to keep pressure in the crankcase, and not the oil out, I know it looks backwards, just remember that the part of the seal with the little spring on it goes toward the fluid that you are trying to keep in, or out, whichever the case may be.this is part of the reason for the vent tube on a two stroke, and also the reason why the tranny oil should be changed more often, on a two stroke.
Would this explain why I’m only getting 60 ish percent throttle and then after that is a 4 stroke sound until I keep on reviving up and then I get full throttle?
your main jet is too lean
yup sounds like bad crank seals symptons
Is there a way to keep your seals conditiond or get them to seal again? 😒 🤔