Stihl MS260 Starts, Revs off Choke, then dies

[CanisiusHist]

A friend brought me his Stihl MS260 that he's had sitting for at least a couple years because it wouldn't start.

Before I started in, I verified that the saw wouldn't fire at all. Given that it sat, I replaced the plug, fuel line, fuel filter, air filter and pulling apart the carb and giving it a good cleaning. I didn't find any cracks or holes in the fuel line, but for the price of a new one, it's cheap insurance.

I don't have a compression tester or access to one, but my arm/shoulder tells me it has enough compression to run.

Put everything back together, started the single adjustable jet (presuming the Low jet on the WT-403B carb) at the 1 turn out from seated. Saw pops with full choke, flip up the master control, and it'll start and rev high for a couple seconds then die. Sometimes it won't just pop with the full choke on, but run with it on. When I try blipping the throttle, I don't get any increase in engine speed, it won't rev up like it does if I don't blip the throttle, and dies anyway.

I've checked the impulse line for leaks (rudimentary check of pulling it, plugging it with a finger and trying to blow air through it) and it seems alright, as does the intake boot (same rudimentary steps).

I didn't find any gunk in the carb, and quite frankly it looked very clean when I pulled it apart and went over everything with a bit of carb cleaner. Checked the IPL from Stihl to make sure I put it back together the right way.

Running the same 45:1 mix that I run through my other Stihl saws without trouble.

I just don't know what's going on with it at this point. Any advice is welcome. Just be patient with my responses - it's cold, my shop isn't heated, and I have a full time job that doesn't involve the chainsaws.

 

[old-cat]

Just for "gp" or cheap insurance, I'd put a new seal in under the flywheel.

 

[AKDoug]

Here is my process when reviewing a saw that comes to me with an unknown running issue:

• My first step when checking a saw like this is to drain the fuel, which you've done.
  
• Next I will visually inspect it, which you've done. Look for obvious loose wires, worn parts, worn AV, etc. On any saw with the choke in the filter like the 260 family, I take a really close look at that.
• I will then check spark, which you've done since it will run momentarily.
  
• Next I will pull the muffler (check the spark screen) and carb to visually inspect the piston and cylinder condition. I will take the tip of a screwdriver and make sure the rings have a bit of spring to them. Many saws are done at this point as an obvious scored piston cylinder will not warrant further inspection unless the customer wants a rebuild. If I'm going to rebuild it is important I find out why it died, so I continue.
  
• I get out my pressure/vacuum tester and test the fuel line from the filter end. This tests the fuel line as well as makes sure the carb will hold pressure and vacuum.
• At this point I will prepare the rest of the saw for a pressure/vac check. You have to block off the intake and exhaust and have a location to apply the vacuum and pressure to the case. There are numerous threads on this.
  
• I will vacuum check and pressure check the engine. If it fails on this test I locate the problem and decide how to proceed. A torn intake boot will often mask leaking crankcase seals, as will a leaking impulse line. Often you have to fix them as you go so you can complete the test. Yesterday I had a leaking flywheel side seal that I replaced. Then the clutch side leaked, so I replaced it too. Finally thinking that I had it under control I discovered that the crankcase was barely leaking into the oil tank. ARGHHH !@$#@#@. That's going to require a split of the crankcase and additional labor, which the customer now doesn't want.
  
• I will then pressurize the fuel tank through the fuel line to check the condition of the fuel line (again) and make sure the tank will hold pressure. I will then vacuum check the tank through the fuel line and it should not allow vacuum to build, showing that the vent works.
  
• Next I will disassemble the carb to inspect it for cleanliness and any obvious flaws.
  
• Usually by this point I have removed the recoil. If not, it is removed and I look over the flywheel and ignition. I will check the flywheel for tightness and make sure the clearance between it and the ignition is a business card thickness apart.
• Out of curiosity I might compression check the saw, but I find it unnecessary in 99% of the cases the cylinder passes visual inspection.
• Now, I'll put the whole mess back together.

I will fill the saw with fuel and oil and attempt to start it. If it won't start it's going to be a carb problem most of the time. I have amassed a collection of carbs for pretty much every modern Stihl saws, so I grab one of these known good carbs and try them out. If the saw fires I will either replace or rebuild the customer's carb depending on cost. Does this process help you? Probably not. But this is just about every dang thing you need to check on a saw and I've been wanting to write it down for a long time. We will cover every item I've written by the end of this thread most likely. Notice how important I place the use of a vacuum/pressure tester in this? There's a reason for that.

 

[AKDoug]

Btw- You saw sounds like a big air leak or bad carb.

 

[CanisiusHist]

Just for "gp" or cheap insurance, I'd put a new seal in under the flywheel.

That would be the oil seal, goes on without splitting the crankcase? If I got a vacuum tester and connected it where the impulse line connects, and it held a vacuum, would you still put a new seal under the flywheel?

 

[old-cat]

That would be the oil seal, goes on without splitting the crankcase? If I got a vacuum tester and connected it where the impulse line connects, and it held a vacuum, would you still put a new seal under the flywheel?

Follow AKDoug above. The Man with the plan!

 

[CanisiusHist]

Here is my process when reviewing a saw that comes to me with an unknown running issue:

• My first step when checking a saw like this is to drain the fuel, which you've done.
  
• Next I will visually inspect it, which you've done. Look for obvious loose wires, worn parts, worn AV, etc. On any saw with the choke in the filter like the 260 family, I take a really close look at that.
• I will then check spark, which you've done since it will run momentarily.
  
• Next I will pull the muffler (check the spark screen) and carb to visually inspect the piston and cylinder condition. I will take the tip of a screwdriver and make sure the rings have a bit of spring to them. Many saws are done at this point as an obvious scored piston cylinder will not warrant further inspection unless the customer wants a rebuild. If I'm going to rebuild it is important I find out why it died, so I continue.
  
• I get out my pressure/vacuum tester and test the fuel line from the filter end. This tests the fuel line as well as makes sure the carb will hold pressure and vacuum.
• At this point I will prepare the rest of the saw for a pressure/vac check. You have to block off the intake and exhaust and have a location to apply the vacuum and pressure to the case. There are numerous threads on this.
  
• I will vacuum check and pressure check the engine. If it fails on this test I locate the problem and decide how to proceed. A torn intake boot will often mask leaking crankcase seals, as will a leaking impulse line. Often you have to fix them as you go so you can complete the test. Yesterday I had a leaking flywheel side seal that I replaced. Then the clutch side leaked, so I replaced it too. Finally thinking that I had it under control I discovered that the crankcase was barely leaking into the oil tank. ARGHHH !@$#@#@. That's going to require a split of the crankcase and additional labor, which the customer now doesn't want.
  
• I will then pressurize the fuel tank through the fuel line to check the condition of the fuel line (again) and make sure the tank will hold pressure. I will then vacuum check the tank through the fuel line and it should not allow vacuum to build, showing that the vent works.
  
• Next I will disassemble the carb to inspect it for cleanliness and any obvious flaws.
  
• Usually by this point I have removed the recoil. If not, it is removed and I look over the flywheel and ignition. I will check the flywheel for tightness and make sure the clearance between it and the ignition is a business card thickness apart.
• Out of curiosity I might compression check the saw, but I find it unnecessary in 99% of the cases the cylinder passes visual inspection.
• Now, I'll put the whole mess back together.

I will fill the saw with fuel and oil and attempt to start it. If it won't start it's going to be a carb problem most of the time. I have amassed a collection of carbs for pretty much every modern Stihl saws, so I grab one of these known good carbs and try them out. If the saw fires I will either replace or rebuild the customer's carb depending on cost. Does this process help you? Probably not. But this is just about every dang thing you need to check on a saw and I've been wanting to write it down for a long time. We will cover every item I've written by the end of this thread most likely. Notice how important I place the use of a vacuum/pressure tester in this? There's a reason for that.

Fuel drained, visually inspected, everything good - replaced air filter (which includes choke assembly) as the previous filter was rather dirty.

Spark checked (nice dark blue with the plug pulled) and it'll fire with fuel in the spark plug hole and in the carb (nice having some small syringes for precise metering).

Muffler pulled, everything intact, no cracks, spark screen clean. Piston and cylinder both look good (no scoring, no scorching) from both the exhaust and carb side -- not sure what I should expect with the screwdriver tip and the ring/spring. Borrowed a compression tester from Autozone's Loan-A-Tool and it seems to be about 155.

Also borrowed a vacuum tester from Autozone's Loan-A-Tool and connected to the impulse hose after I used some scrap aluminum to make a set of rough looking plates to cover the carb and exhaust - stuck some gasket paper between the plates and the cylinders to get as good a seal as possible - turned out that the only place I found leaks were at those plates (put some pressure in the system and used some soapy water to find the leak). I pulled both the flywheel and the clutch to check both of those oil seals and they aren't leaking under vacuum or pressure (rotated the crankshaft to be sure there wasn't an intermittent leak).

Couldn't test the fuel tank precisely, as the tank vent is shot and I haven't got a spare to put in right now but the fuel line holds pressure.

Carb is clean and seals, but the metering diaphragm doesn't seem as pliable as maybe it ought to be -- ordered a carb kit for it, should arrive later this week.

Would a stiff metering diaphragm match up with what the saw's doing at this point? (Starts, runs for a few seconds, dies, will start on choke, run a few seconds, die, repeat ad nauseam).

 

[AKDoug]

The saw shouldn't run on choke. It should pop on choke and the lever then should be set the next notch up. If it runs on choke, even for a few seconds, it is getting air from somewhere and it possibly could be from a leak in the carb itself. If there are no air leaks in the engine, then it's the carb. Put the carb kit in and see what happens.

 

[lone wolf]

Fuel drained, visually inspected, everything good - replaced air filter (which includes choke assembly) as the previous filter was rather dirty.

Spark checked (nice dark blue with the plug pulled) and it'll fire with fuel in the spark plug hole and in the carb (nice having some small syringes for precise metering).

Muffler pulled, everything intact, no cracks, spark screen clean. Piston and cylinder both look good (no scoring, no scorching) from both the exhaust and carb side -- not sure what I should expect with the screwdriver tip and the ring/spring. Borrowed a compression tester from Autozone's Loan-A-Tool and it seems to be about 155.

Also borrowed a vacuum tester from Autozone's Loan-A-Tool and connected to the impulse hose after I used some scrap aluminum to make a set of rough looking plates to cover the carb and exhaust - stuck some gasket paper between the plates and the cylinders to get as good a seal as possible - turned out that the only place I found leaks were at those plates (put some pressure in the system and used some soapy water to find the leak). I pulled both the flywheel and the clutch to check both of those oil seals and they aren't leaking under vacuum or pressure (rotated the crankshaft to be sure there wasn't an intermittent leak).

Couldn't test the fuel tank precisely, as the tank vent is shot and I haven't got a spare to put in right now but the fuel line holds pressure.

Carb is clean and seals, but the metering diaphragm doesn't seem as pliable as maybe it ought to be -- ordered a carb kit for it, should arrive later this week.

Would a stiff metering diaphragm match up with what the saw's doing at this point? (Starts, runs for a few seconds, dies, will start on choke, run a few seconds, die, repeat ad nauseam).

You need to put rubber inner tube from a bike with grease on also to get the plates to seal before you can say its ok. After it holds then take it from there.

 

[HarleyT]

A friend brought me his Stihl MS260 that he's had sitting for at least a couple years because it wouldn't start.

Before I started in, I verified that the saw wouldn't fire at all. Given that it sat, I replaced the plug, fuel line, fuel filter, air filter and pulling apart the carb and giving it a good cleaning. I didn't find any cracks or holes in the fuel line, but for the price of a new one, it's cheap insurance.

I don't have a compression tester or access to one, but my arm/shoulder tells me it has enough compression to run.

Put everything back together, started the single adjustable jet (presuming the Low jet on the WT-403B carb) at the 1 turn out from seated. Saw pops with full choke, flip up the master control, and it'll start and rev high for a couple seconds then die. Sometimes it won't just pop with the full choke on, but run with it on. When I try blipping the throttle, I don't get any increase in engine speed, it won't rev up like it does if I don't blip the throttle, and dies anyway.

I've checked the impulse line for leaks (rudimentary check of pulling it, plugging it with a finger and trying to blow air through it) and it seems alright, as does the intake boot (same rudimentary steps).

I didn't find any gunk in the carb, and quite frankly it looked very clean when I pulled it apart and went over everything with a bit of carb cleaner. Checked the IPL from Stihl to make sure I put it back together the right way.

Running the same 45:1 mix that I run through my other Stihl saws without trouble.

I just don't know what's going on with it at this point. Any advice is welcome. Just be patient with my responses - it's cold, my shop isn't heated, and I have a full time job that doesn't involve the chainsaws.

First thing to try for testing purposes. Set the hi and lo speed screws at 2 turns out, and try to start. It may start and run, then after a few seconds of running, the fresh fuel may break up the varnish in the carb, and it ill start running slower/richer. Run like this for a bit, then readjust the screws properly while running.

 

[CanisiusHist]

You need to put rubber inner tube from a bike with grease on also to get the plates to seal before you can say its ok. After it holds then take it from there.

I should've been more clear - after finding the leaks at the plates, I added another layer of gasket paper with some grease between them and it sealed at the plates. It held pressure as well as the vacuum pump did with just the line on it closed by a set of vice grips. Just the pump with the crimped line on it leaked down 5inHg in 5 minutes - the same as the saw. Not an ideal pump, but the best I had available. Then pressurized it (after sealing the plates for sure) and found no other air bubbles.

 

[CanisiusHist]

First thing to try for testing purposes. Set the hi and lo speed screws at 2 turns out, and try to start. It may start and run, then after a few seconds of running, the fresh fuel may break up the varnish in the carb, and it ill start running slower/richer. Run like this for a bit, then readjust the screws properly while running.

Only has a low speed screw - Walbro WT-403B. Tried it one turn out, two turns out, three turns, four turns, and five. Any more and it's not in the carb any longer. Behaves the same regardless of where the lo speed screw is. When I say it ran on choke, it was for a few seconds, and was more of a stumbling around a dark room black out drunk kinda run - like when you're choking an old saw because the stop switch doesn't function and it's just barely hanging on running a few seconds.

 

[z71mike]

Yup. I think we're starting too deep. Go shallow. Richen her up.

 

[CanisiusHist]

The saw shouldn't run on choke. It should pop on choke and the lever then should be set the next notch up. If it runs on choke, even for a few seconds, it is getting air from somewhere and it possibly could be from a leak in the carb itself. If there are no air leaks in the engine, then it's the carb. Put the carb kit in and see what happens.

Carb kit on order, delivery date supposed to be Saturday via USPS.

 

[CanisiusHist]

Just how much flex should the metering diaphragm have to it? My 009L has an oil pump that runs off impulse, and it's diaphragm flops about like a rubber glove - this one seems almost as stiff as the gasket.

 

[lone wolf]

Carb kit on order, delivery date supposed to be Saturday via USPS.

Sounds like a bad carb alright. The intake manifold was left on at the time of vac testing right so that is not in question correct?

 

[z71mike]

With the choke engaged, it should be too rich to run, by design - introducing an artificial rich condition. So if the saw is actually running on choke, the rich condition, that you imparted by choking the saw, is actually making your lean saw run properly. Need to find the source of the leaning. Where is your L screw?

 

[CanisiusHist]

Sounds like a bad carb alright. The intake manifold was left on at the time of vac testing right so that is not in question correct?

Correct. Put a plate and gasket between the carb and intake manifold. Pulled the bottom off the carb so I could connect the vacuum pump via the impulse hose.

 

[CanisiusHist]

With the choke engaged, it should be too rich to run, by design - introducing an artificial rich condition. So if the saw is actually running on choke, the rich condition, that you imparted by choking the saw, is actually making your lean saw run properly. Need to find the source of the leaning. Where is your L screw?

Only has a low speed screw - Walbro WT-403B. Tried it one turn out, two turns out, three turns, four turns, and five. Any more and it's not in the carb any longer. Behaves the same regardless of where the lo speed screw is. When I say it ran on choke, it was for a few seconds, and was more of a stumbling around a dark room black out drunk kinda run - like when you're choking an old saw because the stop switch doesn't function and it's just barely hanging on running a few seconds.

 

[lone wolf]

The piston looked good right? Was a compression test done? The only thing left is carb I think, if the tests were done right.