Stihl 660 starts and runs then loses power and quits when hot

[JacobE]

If the hose is soft it collapes,see if is real spongy. Is the fuel filter missing by chance?

Fuel filter is still on. Seems nice and pliable, I wouldn’t call it spongy.

 

[lone wolf]

Fuel filter is still on. Seems nice and pliable, I wouldn’t call it spongy.

Are you getting a new carb?

 

[JacobE]

Are you getting a new carb?

I was but I’m going to try the fuel line bits first.

 

[lone wolf]

I was but I’m going to try the fuel line bits first.

Good idea. Are you able to do a vac test? Can you tell if the breather is working? Does it run bad right away or act up after a minute or two I wonder about the coil?

 

[JacobE]

Good idea. Are you able to do a vac test? Can you tell if the breather is working? Does it run bad right away or act up after a minute or two I wonder about the coil?

Neighbor says they have a pressure test kit I could borrow if I keep having trouble. Haven’t tried yet.

Got back to it this afternoon and it was bogging down under full power again, despite a full tank. Idled well and at full throttle would start fast then slowly throttle back. I gave it another 1/4 or 1/2 turn out (I was frustrated and not taking good mental notes) of high speed jet and it stayed stable and I was able to finish out this log.

Tach showed up in the mail, what should I target for rpm for a 660?

 

[Clyde85]

Ok took the whole carb out and disassembled and cleaned with carb cleaner in every orifice. Even backed out the high and low needles and cleaned down there. Diaphragm seems intact, the float slides freely. Cleaned the filter on the bottom side but it didn’t seem funky. Not sure how to score this plug.

View attachment 988028View attachment 988029View attachment 988030View attachment 988031

Fuel filter and hose were clean and I removed and tested the fuel tank vent.

Will get some fresh fuel tomorrow and see how it goes. If nothing else I have one of those “Chinese copies” coming with a whole set of filters and tubes and bits.

Appreciate all the suggestions. I’ll search on how to find an air leak, unless someone wants to drop me a link here

The diaphragm in the second picture looks as if it is starting to curl. I think this is the one that is recommended to be replaced with the black rubber one that comes with the rebuild kit.

 

[jon@camano]

One additional thought- if you have a spark arrestor in your muffler, make sure it is not plugged up.

 

[stihltech]

You have a few choices. Keep fiddling with the carb and take out the piston and cylinder.
Order parts and hope for the best.
Or do a complete diagnosis and find the problem. Do the pressure and vacuum test on the motor. NO leakage. Pressure vacuum test fuel fuel tank. No leaks he under pressure, vent should not hold under vacuum. Pressure test carburetor to 10 psi. Must hold. Inspect all rubber parts for cracking and/or too soft. Inlet screen clean an no corrosion on body. NEW fuel filter, preferably orange Stihl unit. Check fuel in a glass jar for debris and water. (Over 50 percent of my repairs are caused by this. It does not matter what fuel you are using.)
Truthfully this is just a start to find the issue. But without it you may as well start burning currency for heat.

 

[JacobE]

You have a few choices. Keep fiddling with the carb and take out the piston and cylinder.
Order parts and hope for the best.
Or do a complete diagnosis and find the problem. Do the pressure and vacuum test on the motor. NO leakage. Pressure vacuum test fuel fuel tank. No leaks he under pressure, vent should not hold under vacuum. Pressure test carburetor to 10 psi. Must hold. Inspect all rubber parts for cracking and/or too soft. Inlet screen clean an no corrosion on body. NEW fuel filter, preferably orange Stihl unit. Check fuel in a glass jar for debris and water. (Over 50 percent of my repairs are caused by this. It does not matter what fuel you are using.)
Truthfully this is just a start to find the issue. But without it you may as well start burning currency for heat.

Alright y’all have me convinced to do the pressure test. Have a pressure/vacuum combo tool on the way with a full set of replacement rubber bits. Tried to locate some genuine orange stihl fuel filters but even my stihl dealer didn’t have those.

 

[cookies]

Orange filters are designed for mtronic saws and are preferred but the white filters are just fine. Take the old filter out and see if you can blow through it some what easily, toss a carb kit and plug in it and test cut. Filter, plug and walbro carb kit is under 35 bucks

 

[JacobE]

impulse line isn’t even attached to the nipple on the back of the crankcase.

Makes for quite a leak.

Now how to get in there?

 

[JacobE]

OK after some time on the struggle bus to get the new impulse line installed, holds pressure like a champ.

Followed some Directions I found to unscrew the rear anti-vibration bushings and loosen the front ones. Had to do a lot more removal than I thought I would have to to get there. But was able to drop the tank down enough to fish things around with some long hemoststs.

 

[Vintage Engine Repairs]

Pressure doesn’t test the seals, pull a vacuum of 15 inHg and then rotate the crank a number of times and see if the needle returns or drops

 

[JacobE]

Pressure doesn’t test the seals, pull a vacuum of 15 inHg and then rotate the crank a number of times and see if the needle returns or drops

I did a vacuum test as well and it held, didn’t realize I was supposed to rotate the crankshaft as part of it though. As stihltech suggested also did pressure/vacuum test on the fuel tank and it was all good.

If I continue to have issues I’ll redo the vacuum test.

 

[rmh3481]

I would take the bar off and box that bad boy up and send it straight to Stihl Tech for new bearings and seals. That power head has to be 20 years old. Stihl Tech can replace the carb guts, do the tests and set it up for milling. Use a different model to learn on.

 

[JacobE]

I would take the bar off and box that bad boy up and send it straight to Stihl Tech for new bearings and seals. That power head has to be 20 years old. Stihl Tech can replace the carb guts, do the tests and set it up for milling. Use a different model to learn on.

What’s the fun in that?

 

[lone wolf]

Pressure doesn’t test the seals, pull a vacuum of 15 inHg and then rotate the crank a number of times and see if the needle returns or drops

It tests the outer seal area I have seen many leak there.

 

[Vintage Engine Repairs]

It tests the outer seal area I have seen many leak there.

That’s very true especially on clam shells, It’s the sealing lip that I was really referring to, but I certainly see your point

 

[lone wolf]

That’s very true especially on clam shells, It’s the sealing lip that I was really referring to, but I certainly see your point

agreed.

 

[JacobE]

All reassembled, runs much nicer on the bench than before. Used my tach and it’s at about 11000. I’m using a 42” bar. From other threads this sounds right, please correct me if I’m wrong.

View attachment IMG_5970.MOV

Running 50:1 premix fuel right now but will try some 45:1 ethanol free mix.

Sharpened up the chain and will try to mill tomorrow depending on how early the storm rolls into Denver…