Stihl 024 air leak

[ihatesaws]

Hi

I'm looking for help from anyone with experience with stihl 024/026.I have a 024, a 024 super and a ms290. The latter is heavy and I prefer to use the 024 for most jobs. The original piston and cylinder got scored. Given what I know now I think it was probably an air leak in the impulse hose that caused to saw to run lean as the old hose was in terrible condition when I replaced it. I installed a non oem cylinder kit. It has 120psi compression. Since then I bought the 024 super on ebay which has a compression of 170psi so with hindsight I should have just bought an old saw in good condition rather than repairing the first one. Both run fine at WOT and am happy with how they cut. I can get them to idle at the recommended 2500rpm or thereabouts. In practice with a saw that is tuned properly how much should the rpm fluctuate about this value? Is +/-100 okay, +/-200, +/-300? I'll focus on the 024 as I've not looked at the 024S recently although it demonstrated the same behaviour as the 024, though to a lesser degree.

There is still an air leak. If I lift the saw off the ground or turn it on one side the rpm increases by several hundred rpm. If I remove the airfilter and spray carburetor cleaner through a tube over or under the carburetor so it comes out at the front where the carburetor meets the rubber boot from the cylinder the saw dies immediately. I am pulling my hair out because no matter what I do I can't seal between the boot and the carburetor. I don't think there are any other air leaks as I've pressure and vacuum tested both saws to +/- 0.5bar while turning the crankshaft without finding any leaks. Problem exists with new impulse hose, fuel hose and a different carburetor. The rubber boot from the cylinder has a metal insert and a steel washer on the outside. Here is an image with the parts:

http://i.ebayimg.com/00/s/MTIwMFgxNjAw/$(KGrHqFHJEoE88d-iMCKBP,PMdhHzg~~60_35.JPG

All parts looked a bit worn so I got a new boot which had a new washer and insert. I thought maybe air was leaking in around the collar screws as they are officially called (the two studs that stick out of the tank housing through which the carburetor is fixed). I had previously tried putting hylomar gasket sealant in this area between the washer and the carburetor with no effect. Anyway the new boot and parts didn't have any effect either. I've even replaced the two collar screws. I have the feeling I am not able to squeeze the carburetor tight enough up against the boot to seal it. When tightening the M5 nuts on the collar screws I wasn't getting the impression that it was getting harder to tighten themt.I might have over tightened at some stage and the threads on the screws or the nuts got stripped. I would turn and turn but the nut would not progress anymore. The nuts do on occasion seem to eat into the back of the carburetor metal causing small shavings to come off . Unlike the ms290 there is no washer plate between the back of the carburetor and the nuts to protect the carburetor. So I got new collar screws and got new M5 nuts from a hardware store. I bought a 1/4" drive torque wrench so I could tighten to 3.5Nm as recommended in the manual but I didn't know anything about torque wrenches so I bought one with a range from 2 to 20Nm. I've been reading since that they are not reliable at the bottom or top of the range . I tried using it anyway but didn't hear it click tightening the nuts.I got the nuts reasonably tight and tested the saw again with no change in its behaviour. I tightened the nuts some more, got one very tight and then tried the other but instead of getting tighter it got looser. It is now stuck on the collar screw and am unable to get it off. I think the threads in the nut are damaged. 3.5Nm or 31 inch lb isn't very much, is it? I never used to tighten the nuts very tightly on the ms290 and didn't have any problem but there there was always the sensation with that saw that the more you tightened the harder it became to tighten but this is not always so with the 024 . I will eventually get the nut off or cut it off but don't know what to try next. I'd appreciate someone to put me out of my misery.

[Stubby]

Hi

I'm looking for help from anyone with experience with stihl 024/026.I have a 024, a 024 super and a ms290. The latter is heavy and I prefer to use the 024 for most jobs. The original piston and cylinder got scored. Given what I know now I think it was probably an air leak in the impulse hose that caused to saw to run lean as the old hose was in terrible condition when I replaced it. I installed a non oem cylinder kit. It has 120psi compression. Since then I bought the 024 super on ebay which has a compression of 170psi so with hindsight I should have just bought an old saw in good condition rather than repairing the first one. Both run fine at WOT and am happy with how they cut. I can get them to idle at the recommended 2500rpm or thereabouts. In practice with a saw that is tuned properly how much should the rpm fluctuate about this value? Is +/-100 okay, +/-200, +/-300? I'll focus on the 024 as I've not looked at the 024S recently although it demonstrated the same behaviour as the 024, though to a lesser degree.

There is still an air leak. If I lift the saw off the ground or turn it on one side the rpm increases by several hundred rpm. If I remove the airfilter and spray carburetor cleaner through a tube over or under the carburetor so it comes out at the front where the carburetor meets the rubber boot from the cylinder the saw dies immediately. I am pulling my hair out because no matter what I do I can't seal between the boot and the carburetor. I don't think there are any other air leaks as I've pressure and vacuum tested both saws to +/- 0.5bar while turning the crankshaft without finding any leaks. Problem exists with new impulse hose, fuel hose and a different carburetor. The rubber boot from the cylinder has a metal insert and a steel washer on the outside. Here is an image with the parts:

http://i.ebayimg.com/00/s/MTIwMFgxNjAw/$(KGrHqFHJEoE88d-iMCKBP,PMdhHzg~~60_35.JPG

All parts looked a bit worn so I got a new boot which had a new washer and insert. I thought maybe air was leaking in around the collar screws as they are officially called (the two studs that stick out of the tank housing through which the carburetor is fixed). I had previously tried putting hylomar gasket sealant in this area between the washer and the carburetor with no effect. Anyway the new boot and parts didn't have any effect either. I've even replaced the two collar screws. I have the feeling I am not able to squeeze the carburetor tight enough up against the boot to seal it. When tightening the M5 nuts on the collar screws I wasn't getting the impression that it was getting harder to tighten themt.I might have over tightened at some stage and the threads on the screws or the nuts got stripped. I would turn and turn but the nut would not progress anymore. The nuts do on occasion seem to eat into the back of the carburetor metal causing small shavings to come off . Unlike the ms290 there is no washer plate between the back of the carburetor and the nuts to protect the carburetor. So I got new collar screws and got new M5 nuts from a hardware store. I bought a 1/4" drive torque wrench so I could tighten to 3.5Nm as recommended in the manual but I didn't know anything about torque wrenches so I bought one with a range from 2 to 20Nm. I've been reading since that they are not reliable at the bottom or top of the range . I tried using it anyway but didn't hear it click tightening the nuts.I got the nuts reasonably tight and tested the saw again with no change in its behaviour. I tightened the nuts some more, got one very tight and then tried the other but instead of getting tighter it got looser. It is now stuck on the collar screw and am unable to get it off. I think the threads in the nut are damaged. 3.5Nm or 31 inch lb isn't very much, is it? I never used to tighten the nuts very tightly on the ms290 and didn't have any problem but there there was always the sensation with that saw that the more you tightened the harder it became to tighten but this is not always so with the 024 . I will eventually get the nut off or cut it off but don't know what to try next. I'd appreciate someone to put me out of my misery.

 

Your best bet is to ask spud. He would advise savaging the o.e.m. cylinder if at all poss . Some of the after market kits are not so good . any way he will read this soon and be able to advise/sort it .

[spudulike]

Blimey, thats a long first post:lol:

 

Gotta say I have had problems with the 024/026 series as well, the bit in your post where you are spraying carb cleaner in to the carb and around the carb has lost me??? Is the saw running when you are spraying it or are you spraying it in a non running saw and it is coming out from between the carb and the rubber boot?

 

One thing I have noticed is that on non OEM impulse lines, the part that sticks out of the airbox back plate is sometimes not quite long enough and the impulse connector on the carb is a loose fit in to it! That one has caused me problems in the past - just fit the OEM part or add a small length of tubing to the impulse connector.

 

On the carb nuts - they should be tight but shouldn't need excessive tightening and the union between the rubber boot and carb should seal well!

 

One thing you can do is to pressure check but do it with the carb in place and seal the back end of the carb. You generally get a very small amount of leakage around the throttle linkage but will give you a good indication if you have problems within this area - leaks are best found by hearing and soapy water!

 

The revs shouldn't vary by much more than 250 - 300rpm, older saws will vary more than the new ones.

 

120psi sounds low, I would imagine when warm, this saw will be a mare to start - worth checking! It is what I have been saying about the build quality of these parts.

 

When you pressure check, try pushing the end float of the crank shaft in and out - worth a check and also have you vacuum tested the saw - sometimes faults are hidden on one test and obvious on the other!

[ihatesaws]

Thanks for the reply, spud.

 

The saw is idling. I'm not spraying carburetor cleaner into the carburetor

through the venturi opening. I'm spraying it around the outside where the boot

meets the carburetor. As there is an air leak it sucks in the carburetor cleaner

instead of air which changes the fuel/air mixture going into the cylinder.The

mixture is now too rich and the saw dies.

 

I did the vacuum and pressure tests with a piece of rubber wedged between the carburetor and the boot. Once I get the nut off the collar screw I will try to put the piece of rubber behind the carburetor. I don't know anything about end floats of the crankshaft. The flywheel is still on the saw and I've never taken it off. After the saw passed the vacuum and pressure tests I had no desire to take it apart any further as it's unfamiliar territory. It took me ages to even figure out how to get off the old cylinder before I realised that the holes going down through the cooling fins weren't there for decoration but for a T27 screwdriver which I didn't have. I was wondering how I could come in diagonally from the side with a socket wrench.

 

I've done a fair bit of cutting with the saw and the new cylinder. I've not

noticed any problem restarting when warm. It is a shiny cylinder and piston.

Doesn't that mean it's nikasil coated and therefore okay quality?

 

I'm using this impulse hose

Impulsschlauch passend für Stihl 024 024AV AV MS240 MS 240, 4,95 € - SÄGENspezi

I don't think there is any length problem. It is not very easy to install

though. I stick it through the hole in the tank housing, loosen the three screws in the antivibration mounts so I can get a bit of extra space to stick in some pliers and coax the hose onto the nipple sticking out of the crankcase. In fact the impulse connector on the carburetor fits so well into the impulse hose that when I pull the carburetor off the collar screws the hose comes with it and I have to hold it in position when I pull the carburetor back.

 

Do you use a torque wrench for tightening nuts and screws on saws? Is there one you'd recommend?

 

I'll post back as soon as I get the damn nut off.

 

>>>>Life is tough but it's tougher when you're stupid

Don't I know it!

[spudulike]

OK, understand the carb cleaner bit now, the inner metal insert holds the inner shape of the boot and the outer metal surround stops you flattening the rubber flange on the carb side of the boot.

 

One thing to check is that with both metal parts mentioned above in place, the rubber flange the carb is supposed to sit on is actually proud of the meta surround - this must happen so the carb compresses against it!

 

You could try making a paper washer and greasing both the rubber flange and the carb and then bush the carb home against it and then dissassemble to see if there is good connection between both parts.

 

The pressure test with the choke side of the carb blanked off and a bit of fairy liquid solution will show you where the leak is and would appear to be that the metal surround around the rubber carb boot is keeping the carb from sealing on the boot manifold.

 

The impulse line should push through the back plate and be held pretty firmly in place but sounds like this is sealing OK.

 

Many of the Chinese parts state Nikasil but there is no easy way of telling what you have. I aim for 150psi on a good running saw with 200-220PSI on my fully tuned ones:thumbup: The type of compression gauge does give a varied measurement.

 

End float on the crankshaft is moved by grabing hold of the flywheel or clutch and pushing and pulling the thing back and forth - you may have no play but sometimes this end float can cause air leaks in one position and not others.

 

Your description of taking the cylinder off is classic:lol:

 

Torque wrench - only on car bearings and cylinders etc. I use a small 6" tommy bar or T wrench as it allows me to crank the bolts down but not strip the thread. The nuts you are doing up are simple to do up, not technical - do them up with a deep socket using your fingers and then circa 1/2 turn until reasonably tight and the jobs a good un!

 

It sounds like you know where the air leak is, if the boot is in good condition, you must have some sort of mechanical interference stopping the carb pushing on the rubber flange.

[ihatesaws]

Sorry I've not reported back sooner. It is so demoralizing not making any progress that I only manage to muster up the motivation to look at the saw at protracted intervals.

 

I managed to cut the nut off with a hacksaw.The collar screws seem undamaged. Two new nuts screw on and off easily anyway. It seems that the overtightening caused small pieces of the metal washer and the rubber boot around the collar screws to wear away. I guess this is also what happend to the previous parts that I replaced with this new set.

 

The rubber boot does protrude slightly higher than the outer metal washer. The inner insert ring however has a lip on it which keeps it on the surface of the boot that meets the carburetor. So it would be first this ring, then the boot and finally the washer that make contact with the carburetor. I can't see anything that would be forming a barrier when I put on the carburetor. I made the paper washer and put grease on the boot and the carb, put everything together and took it apart. The grease seemed to have formed a circle on both sides of the paper without any gaps.

 

One thing I have noticed is that the hole in the plastic tank housing is more ovular than circular with the axis perpendicular to a line between the two collar screws being slightly longer than the distance between the screws.The rubber boot and the metal ring tend to deviate from their initial circular shape and take on this ovular shape. There could therefore be a mismatch between this and the perfect circle at the back of the carburetor. I do squeeze the ring back into a circlular shape with a visegrips and I think it deforms more when I overtighten. I've started tightening like you suggested and the boot seems to be remaining more circular.

 

As I had replaced the boot on the cylinder I did the pressure and vaccuum tests as I had before with the piece of rubber between boot and carburetor to make sure I hadn't introduced any new air leaks. It seemed fine on both tests. I then put the rubber after the carburetor and used the old metal washer from around the carb boot to seal the rubber evenly on the carburetor with the two nuts. I couldn't get the gauge to register any increase in pressure or draw any vacuum. I poured in the soapy water the best I could and there seemed to be bubbles coming from the junction between the boot and the carburetor above and later below the throttle linkage and also on the opposite side but it was difficult to see exactly where. (Am I right in thinking that the pressurized air I'm pumping in (through the impulse hose on the cylinder btw) may make it through the three low fuel holes into the metering side of the carburetor but shouldn't make it past the inlet needle into the fuel pump side of the carburetor? Just concerned that the air could leak out the fuel line of the carb.The high fuel hole has a check valve, doesn't it? I fear I may have damaged a different carburetor blasting compressed air into that hole.)

 

So no progress, back to being demoralized I guess .

[spudulike]

Sorry I've not reported back sooner. It is so demoralizing not making any progress that I only manage to muster up the motivation to look at the saw at protracted intervals.

 

I managed to cut the nut off with a hacksaw.The collar screws seem undamaged. Two new nuts screw on and off easily anyway. It seems that the overtightening caused small pieces of the metal washer and the rubber boot around the collar screws to wear away. I guess this is also what happend to the previous parts that I replaced with this new set.

 

The rubber boot does protrude slightly higher than the outer metal washer. The inner insert ring however has a lip on it which keeps it on the surface of the boot that meets the carburetor. So it would be first this ring, then the boot and finally the washer that make contact with the carburetor. I can't see anything that would be forming a barrier when I put on the carburetor. I made the paper washer and put grease on the boot and the carb, put everything together and took it apart. The grease seemed to have formed a circle on both sides of the paper without any gaps.

 

One thing I have noticed is that the hole in the plastic tank housing is more ovular than circular with the axis perpendicular to a line between the two collar screws being slightly longer than the distance between the screws.The rubber boot and the metal ring tend to deviate from their initial circular shape and take on this ovular shape. There could therefore be a mismatch between this and the perfect circle at the back of the carburetor. I do squeeze the ring back into a circlular shape with a visegrips and I think it deforms more when I overtighten. I've started tightening like you suggested and the boot seems to be remaining more circular.

 

As I had replaced the boot on the cylinder I did the pressure and vaccuum tests as I had before with the piece of rubber between boot and carburetor to make sure I hadn't introduced any new air leaks. It seemed fine on both tests. I then put the rubber after the carburetor and used the old metal washer from around the carb boot to seal the rubber evenly on the carburetor with the two nuts. I couldn't get the gauge to register any increase in pressure or draw any vacuum. I poured in the soapy water the best I could and there seemed to be bubbles coming from the junction between the boot and the carburetor above and later below the throttle linkage and also on the opposite side but it was difficult to see exactly where. (Am I right in thinking that the pressurized air I'm pumping in (through the impulse hose on the cylinder btw) may make it through the three low fuel holes into the metering side of the carburetor but shouldn't make it past the inlet needle into the fuel pump side of the carburetor? Just concerned that the air could leak out the fuel line of the carb.The high fuel hole has a check valve, doesn't it? I fear I may have damaged a different carburetor blasting compressed air into that hole.)

 

So no progress, back to being demoralized I guess .

 

Think this may be the longest post in history:lol:

If the saw is getting you down that much - bung it on ebay:lol:

 

Having read your post, it sounds like you have a good seal between the carb and the inlet manifold, the hole the manifoldgoes through being slightly oval is probably not an issue.

 

I would say that you have a bad leak around the throttle valve shaft that is letting in air and letting out air on your pressure and vac tests - this would cause issues with idle speed fluctuating.

 

It isn't an easy one to test but on your pressure test with the carb in place, I would expect leakage from 10psi down to zero over 20-30 seconds and not well under.

 

There are a few carbs for this model on ebay that I have seen, for around £15 it may be a simple way of getting the saw working right - the latter carbs have a different diaphragm cover that plugs in to the air filter and the 026 carbs are a mm or so larger in the bore but it will work OK once the diaphragm cover is swapped over.

 

No need to get depressed over it, just dig in and not get beaten by the saw. The throttle shaft is where my money is - not an easy one to check either.

 

To check the carb needle valve is good, stick your pressure gauge on the fuel inlet connection and pump the carb up to 7psi - you should get no leakage at all. It is unlikely you will damage the carb in this way as there is a "pop off" pressure on the valve.

 

Good luck - let us know how you get on, you are not alone:thumbup:

[ihatesaws]

Thanks spud. I'm afraid I can't give up and put the saw on ebay. I have spent too much time and money already. A positive outcome is required.

 

The carburetor wt-194A is new and cost more like £45 than £15. I have seen cheaper ones on ebay from china but they looked a bit different and had a "compensator" connection????? Is this what you're talking about? The old carburetor is wt-22b. It looks almost identical to the wt-194A. I was doing the "pop off" test you describe a while back with the 22B and it was leaking.I stuck it in a bucket of water and there were air bubbles slowly coming out of the impulse connector. This was with a new carb kit so I thought maybe the carb body was deformed and that was the reason for my idle problem.I therefore decided to eleminate the carb as the source of the problem by buying a brand new one. I since discovered however that the gaskets are only air tight when they are wet. Just now I sprayed carb cleaner in the fuel pump side of the carb, screwed it back up and pumped it up to 7 psi.It seems to hold. I did the same with the new 194A and no leak. I stuck the 22B on the saw, put back on the rubber patch but again no increase in pressure. In with the soapy water and pumping gives lots of bubbles allover the place down at the carb boot. Anyway it is not like I can pump up the crankcase to 10psi and see if it takes 30 seconds to leak out through maybe the throttle valve shaft. I can't even pressurize it to 1psi. Two bad shafts seem extremely unlikely.

 

It just doesn't make any sense. I think I will put the 024 super back together and do the pressure test on that with the carb in place. One thing that might be stopping the carb from sitting properly (during the test anyway) on the carb boot is the impulse connector.I have the pressure pump connected to the impulse hose so the impulse connector on the carb is digging into the side of the hose instead which obviously is not ideal. I suppose I could not have the hose come through the tank housing and out the side instead to the pump. I would love to get a new ms240 out of a shop and do the same test on it but I guess I would probably just end up wrecking a brand new saw

 

I will soldier on.

[spudulike]

Thanks spud. I'm afraid I can't give up and put the saw on ebay. I have spent too much time and money already. A positive outcome is required.

 

The carburetor wt-194A is new and cost more like £45 than £15. I have seen cheaper ones on ebay from china but they looked a bit different and had a "compensator" connection????? Is this what you're talking about? The old carburetor is wt-22b. It looks almost identical to the wt-194A. I was doing the "pop off" test you describe a while back with the 22B and it was leaking.I stuck it in a bucket of water and there were air bubbles slowly coming out of the impulse connector. This was with a new carb kit so I thought maybe the carb body was deformed and that was the reason for my idle problem.I therefore decided to eleminate the carb as the source of the problem by buying a brand new one. I since discovered however that the gaskets are only air tight when they are wet. Just now I sprayed carb cleaner in the fuel pump side of the carb, screwed it back up and pumped it up to 7 psi.It seems to hold. I did the same with the new 194A and no leak. I stuck the 22B on the saw, put back on the rubber patch but again no increase in pressure. In with the soapy water and pumping gives lots of bubbles allover the place down at the carb boot. Anyway it is not like I can pump up the crankcase to 10psi and see if it takes 30 seconds to leak out through maybe the throttle valve shaft. I can't even pressurize it to 1psi. Two bad shafts seem extremely unlikely.

 

It just doesn't make any sense. I think I will put the 024 super back together and do the pressure test on that with the carb in place. One thing that might be stopping the carb from sitting properly (during the test anyway) on the carb boot is the impulse connector.I have the pressure pump connected to the impulse hose so the impulse connector on the carb is digging into the side of the hose instead which obviously is not ideal. I suppose I could not have the hose come through the tank housing and out the side instead to the pump. I would love to get a new ms240 out of a shop and do the same test on it but I guess I would probably just end up wrecking a brand new saw

 

I will soldier on.

 

To get zero pressure when pressure testing you would need a serious air leak, you have taken the crankshaft seals, inlet boot and impulse line out of the eqation as well as the crankcase gasket and cylinder base.

 

Personally I don't see how you can have an air leak between the carb and the boot from what you have said - I have part WT110 listed for a Stihl 024 without the later diaphragm casing compensator! the WT195 for an 026 and not sure if your carb is correct or not:confused1:

The £15 I mentioned was for a second user part and not new - £45 is a good price for a new part!

 

Have you tried removing the steel ring around the inlet boot and the steel inner ring and try the pressure test again with the carb in place, there has to be something stopping the carb mating with the inlet boot from what you have said.

 

The only thing left is to make up a spacer gasket to fit between the carb and the inlet boot but really don't understand why the seal isn't being made without a non standard part???

 

Are you sure the carb is free to move up to the inlet boot and compress against it??

 

Weired?

[ihatesaws]

Well, Spud, I see they don't pay you the big bucks for nothing. The throttle valve shaft does seem to be the culprit (or at least one of them).

 

I tried the pressure test without the metal insert ring but that didn't make any difference. I then put pressure with my thumb on the piece of rubber at the back of the carburetor and I was able to get the gauge needle to move a bit. The metal washer wasn't any good at keeping the rubber pressed against the carb.It is weak enough that you can bend it in with your hands. I made a solid steel plate to replace it. I can now get up to maybe 6 or 7 psi if I pump constantly. If I stop pumping it drops down to zero, certainly within 10 seconds. I poured in the water around the carb and there were bubbles coming from the throttle linkage. On the opposite side of the shaft there were bubbles as well although it is possible that they were coming from the carb-boot seal instead. There is less space on the left side of the carb to see anything. That was with wt-194a. I then put on wt-22b. This had bubbles coming out of both sides of the shaft as well with quite alot on the left side.

 

Would the following work to test the carb for air tightness off the saw? I would make a second rubber and steel plate and seal it at both ends using two long 5mm bolts. I would remove the inlet needle from the metering side of the carb which would allow air from the pump to pass from the fuel pump side into the metering side and from there through the L and H holes into the venturi. I would then stick the carb in a bucket of water and pump to see exactly where the bubbles are coming from.

 

How can a new carb have this problem? Maybe they figure the air leak isn't large enough to be of significance and all carbs are like this? There is a certain constant amount of air coming in through the choke butterfly anyway so a small air leak isn't a problem as long as it's constant? But I seemed to get an rpm increase by lifting the saw or turning it on one side so the air leak isn't constant.

>>I would expect leakage from 10psi down to zero over 20-30 seconds and not well under.

You mean you'd expect this on a normal saw that works fine?

 

I'm pretty sure the 22b is a compatible carb as both the 024 and the 024S came with one of them. I read somewhere else on the internet too about some guy with a 024 and a 22b on it. I see in the parts manual for the saw that the walbro carbs shown are 110 and 426. In the manual for the 026 there are 427, 590, 195, 155, 403B and 493A shown. What is this compensator thing? A carb with one of them wouldn't fit older saws? Where did you see a carb for £15? I see at the moment you can get a 426, a 403 or a 493 on ebay from the US for $30 including postage. But what's the point if they are all leaky anyway?