Stihl 026 - Pulsating Ignition?

[SteveSr]

Hello,

I have an 026 recently stripped to the crankcase and rebuilt (mostly rubber parts). Went to run the saw and it starts and idles fine. It will accelerate fine at WOT but when it gets to a certain RPM it dies just like the kill switch was engaged. Once the RPMs drop to near idle it will pick up again and the process repeats.

Anybody seen these symptoms before? I can't believe that a carb defect would cause this behavior so am thinking a bad ignition system (coil). Your thoughts?

Thanks,

Steve

 

[Mad Professor]

If you replaced AV mounts check your ignition/coil wires. Bad mounts can pinch those and wear off the insulation.

 

[SteveSr]

If you replaced AV mounts check your ignition/coil wires. Bad mounts can pinch those and wear off the insulation.

Yea, I know about the wiring pinch between the recoil/top cover and the crankcase. I replaced the sleeving and ignition wire. No shorts here.

 

[SteveSr]

UPDATE: I removed the kill wire from the coil and also tried increasing the coil/FW gap which made no apparent difference. I then went out and actually put the saw in some wood. It ran good as long as I kept the bar in the wood and the RPMs below about 10K. Out of the cut same issue.

 

[Ozhoo]

Can you make a video? ... of the saw? Running?

 

[Ozhoo]

And give it a try without the air filter installed

 

[SteveSr]

And give it a try without the air filter installed

I don't see the significance of this. Air filter is brand new. When the saw catches itself it doesn't appear to be rich like the the choke was closing when it shouldn't. Just sounds like the ignition has been cut.

 

[SteveSr]

Can you make a video? ... of the saw? Running?

Not easily... don't have a way to hold the phone (camera) while running the saw. It sounds just like 5 O'clock Charlie on M*A*S*H!

 

[Mad Professor]

I don't see the significance of this. Air filter is brand new. When the saw catches itself it doesn't appear to be rich like the the choke was closing when it shouldn't. Just sounds like the ignition has been cut.

Is air filter OEM or aftermarket? I've had AM filters do bad things.

Are you sure it's not fuel related? Sometimes disturbing things like an old fuel line will open up a crack you don't notice, or some crap gets into carb and plugs up the screen inside.

 

[backhoelover]

the coils have stages in them. your third stage might be going out. only way to test is which a new coil. can test the coil with they way then are set up with resisters etc

 

[backhoelover]

cant test the coil sorry. how about the carb has it had a new kit in it. when the diaphragm start to get hard it will not pump the fuel you need at higher rpms

 

[backhoelover]

what about impulse hose did your replace it also ?

 

[SteveSr]

Is air filter OEM or aftermarket? I've had AM filters do bad things.

Air filter is OEM

Are you sure it's not fuel related? Sometimes disturbing things like an old fuel line will open up a crack you don't notice, or some crap gets into carb and plugs up the screen inside.

Fuel wouldn't explain why it runs and cut up to a certain RPM before it starts misbehaving.

 

[SteveSr]

what about impulse hose did your replace it also ?

Yes, New OEM

 

[SteveSr]

cant test the coil sorry. how about the carb has it had a new kit in it. when the diaphragm start to get hard it will not pump the fuel you need at higher rpms

I know the coil is not testable except by substitution. Found a cheap used one on FleaBay. I'll find out in about a week if that was it or not.

Carb had a kit installed by a local dealer just before the original owner sold the saw on CL. He originally complained of hard starting which I confirmed. This is the main reason he was selling it. When I finally got it started it was running pig rich which I put down to an almost completely clogged AF. This is the first time I attempted to run it at full WOT RPMs.

 

[drf255]

Do one run with no air filter.

What have you got to lose?

The choke flap can be closing shut, even on oem filters.

A new oem flocked filter is still quite restrictive on this model.

I've been fooled before.

 

[Millerhill520]

I read through this post, and I wanted to suggest one other thing on top of the impulse line, and fuel line and filter.

Pop the carb off and make sure the intake flanges were put back in. If a dealer rebuilt the carb, I have seen once or twice that someone had said they put a kit in the carb, and the saw wanted to run good, but was acting funny, and they were at a loss. It was dismissed as a bad carb. So, a new carb was bought, and when the one that was rebuilt was removed, it was found that the inner and outer metal flanges were missing off the boot. It happens, and I have done it once or twice while kind of rushing to get to the finish line. That will create a non rigid and secure mount of the carb to intake boot area, making enough of an air pocket to really mess up the air fuel fix. Also, check that boot. Fire her up and spray the boot with some starting fluid, or brake cleaner is better, or carb cleaner. Just enough to see if the saw stalls, or races slightly. Of course another thing, although I don't believe it is the problem, is give a quick spray to the base gasket area. Maybe not sealed up there.

With the wt carbs, when I rebuild them I am about 50% successful with them. Half of em rebuild really well, the other half, I have to adjust em quite a few times to finally dial it home. And sometimes I find my final settings to be a half a turn almost past factory specs on the low. High is often fixed, non adjustable, or if it is, never seems to need too much change. Those little diaphragms are so sensitive as well. The smallest little non visible pin hole or tear from handling it too hard can wreak havoc in the end.

There's a seller on eBay selling new genuine Walbro wt carbs. I picked one up for 40.00, and it came even with the stihl plate on it! They are 44.99 now, but they aren't aftermarket, they are genuine Walbro. It works awesome. It came not pre set, so if you buy one, dial it to one turn on both screws. But it's a high low adjustable.

I hope you didn't pay too much for the coil, and that it was an Oem one. I have seen aftermarket coils not work right out of the package. I have 3 or 4 coils for the 026 here that work, they are used, but Oem. If there's an issue with the one you bought, let me know. It may be a good idea to see if your flywheel is ok. I have seen a flywheel with a bad scratch or mark/ding on the magnet before, which upsets the spark. Very rare though. That took a lot of testing different parts before I found that problem.

 

[SteveSr]

Pop the carb off and make sure the intake flanges were put back in. If a dealer rebuilt the carb, I have seen once or twice that someone had said they put a kit in the carb, and the saw wanted to run good, but was acting funny, and they were at a loss. It was dismissed as a bad carb. So, a new carb was bought, and when the one that was rebuilt was removed, it was found that the inner and outer metal flanges were missing off the boot. It happens, and I have done it once or twice while kind of rushing to get to the finish line. That will create a non rigid and secure mount of the carb to intake boot area, making enough of an air pocket to really mess up the air fuel fix. Also, check that boot. Fire her up and spray the boot with some starting fluid, or brake cleaner is better, or carb cleaner. Just enough to see if the saw stalls, or races slightly. Of course another thing, although I don't believe it is the problem, is give a quick spray to the base gasket area. Maybe not sealed up there.

I am the last one who had the carb off and remember replacing these parts, plus I had NO parts left over! The boot was thoroughly inspected and in good shape before it went back in. Air leaks will usually affect the saw at idle and the saw idles fine. The crank seals were changed and the saw pressure/vacuum tested so there should be no leaks.

With the wt carbs, when I rebuild them I am about 50% successful with them. Half of em rebuild really well, the other half, I have to adjust em quite a few times to finally dial it home. And sometimes I find my final settings to be a half a turn almost past factory specs on the low. High is often fixed, non adjustable, or if it is, never seems to need too much change. Those little diaphragms are so sensitive as well. The smallest little non visible pin hole or tear from handling it too hard can wreak havoc in the end.

There's a seller on eBay selling new genuine Walbro wt carbs. I picked one up for 40.00, and it came even with the stihl plate on it! They are 44.99 now, but they aren't aftermarket, they are genuine Walbro. It works awesome. It came not pre set, so if you buy one, dial it to one turn on both screws. But it's a high low adjustable.

I have similar luck rebuilding carbs almost to the point it ain't worth it anymore. The last one was a WT-22 that would load up at idle and couldn't be adjusted on the low end. I think there was a leak into the idle chamber around the welch plug. A new WT-194 fixed it.

I hope you didn't pay too much for the coil, and that it was an Oem one. I have seen aftermarket coils not work right out of the package. I have 3 or 4 coils for the 026 here that work, they are used, but Oem. If there's an issue with the one you bought, let me know. It may be a good idea to see if your flywheel is ok. I have seen a flywheel with a bad scratch or mark/ding on the magnet before, which upsets the spark. Very rare though. That took a lot of testing different parts before I found that problem.

The coil was a real Stihl (MS290) and at less than $10 shipped! For that price it is a good spare. Based on perusing FleaBay it appears that Stihl (and probably the rest of the industry) had issues and kept changing suppliers over the years. Some that I noted were - SEM, Ducati, Walbro, and Stihl. Who knows who made the ones marked "Stihl"?

Thanks for the offer for a used coil. Hopefully, I won't need it.

I have only seen one other coil failure and that was on an FS-85 weedeater that ran until it heated up and then just died and wouldn't restart.

 

[lone wolf]

Try it with the filter off like someone said .

 

[SteveSr]

Try it with the filter off like someone said .

I will, but it will be the end of next week after the snow melts and the rain stops! Winter has arrived just in time for spring!