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

[JacobE]

I have a 660 that i bought used several years ago and i havn't had great luck keeping it running well. It definitely has a muffler mod of some sort because its crazy loud. I was told it has a meteor piston kit. It's been several times in to my local shop for tuneups. I got it out this year to saw some logs and do some chainsaw milling. It ran decently yesterday but not great after it was warm. I tried to start over with tuning the carb today after watching some videos. reset the Low and High screws to 1/4 and 3/4 out and then backed out the high just a little more. seemed to be tuned well. I was able to cut one slab (of pine, nothing super dense) but i did start to lose power as i went. On the second cut i made it halfway (2 feet) and it just wont go full throttle anymore. I can get it to start and idle but not go full power. this is after about 5-10 minutes of full power cutting.

Im using premix 50:1 trufuel.

Would appreciate any suggestions.

 

[lone wolf]

Well are you sure those are the correct carb adjustments,some of them call for 1 out on ea.?
Try one turn out on each H and L and see what happens.
Also pull the muffler off and inspect the piston for seize marks.

 

[Old2stroke]

It's critical that saws running at full power for a long period not be running a bit too lean, they will overheat and lead to piston/cyl damage. Are you sure you understand how to tune the high speed fuel system so that it "4-stokes" at full throttle without load and cleans out as soon as it is loaded? There is better tuning info available through a search on this forum than you can get on most videos. Might be time to pull the muffler and inspect for damage. Will the saw speed up to maximum rpm with no load on it or is it just when you load it down that it won't rev up and produce any power? When it looses power, does it run rough as though you are getting misfires from the ignition?

 

[Old2stroke]

If the saw is healthy and the tuning is correct, when it starts to lose power under full load, high speed fuel delivery is the suspect. Any restriction in the fuel supply will first show up when the carb needs to supply maximum fuel. Causes can be the fuel filter, fuel line, filter screen in the carb, stiff fuel pump diaphragm, etc.

 

[JacobE]

Appreciate the responses. Cylinder looks good to me. No I’m not sure I’m setting it up right. I had let it cool off and have it another shot and at least finished the cut I was on but it doesn’t run well. Doesn’t idle now and won’t run full throttle either. I keep fiddling with the screws up and down a 1/4 at a time without success.

I called a local small engine repair guy who suggested to order a new carb, since mine is of unknown age and I don’t know what the default settings are.

I also ordered a tach.

I’ll pull off the fuel hose and filter and see if they seem plugged. They don’t look like garbage from an external inspection.



This was what it was doing with about 75% throttle when I gave up fiddling.
View attachment IMG_5920.MOV

 

[lone wolf]

That could even be an air leak or the carb is f ucked

 

[Woodslasher]

1 and 1 are the default settings I use, that saw sounds like it's not getting a direct/consistent flow of fuel, I'd check the filter in the carb as well as the diaphragms, if they look good I'd do a pressure/vacuum test.

 

[Clyde85]

Don't forget the tank vent.

 

[JacobE]

1 and 1 are the default settings I use, that saw sounds like it's not getting a direct/consistent flow of fuel, I'd check the filter in the carb as well as the diaphragms, if they look good I'd do a pressure/vacuum test.

A replacement carb would cover these bits, yes?

Don't forget the tank vent.

I read somewhere about cracking the fuel cap to see if that improved anything, it did not seem to.

 

[Clyde85]

The replacement carb would cover the final filter in the carb and the diaphragms. It may not cover an air leak.

 

[Woodslasher]

A replacement carb would cover these bits, yes?

I read somewhere about cracking the fuel cap to see if that improved anything, it did not seem to.

Yes, but a Walbro kit for your carb is ~$20-25 while a new oem carb would probably be $80+, and chinese copies aren't worth the gamble in my book.

 

[cookies]

load the parts cannon. spark plug, carb, filters, impulse hose

 

[JacobE]

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.



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

 

[Clyde85]

You need to pressure/ vacuum test the saw for air leaks.

 

[Aknutter]

YouTube...donnyboy73 , he has many short videos of different stuff.

 

[cookies]

get a new 3.00 ngk, if the carb diaphragm is hard or the fuel pump flaps have any indentions or simply do not sit flat on their own they require replacement. A mighty vac kit will allow pressure/vac testing

 

[Woodslasher]

Pressure/vacuum testing a saw is easy. I bought a Mityvac 8500 "kit" and outside of that all you need is a roll of duck tape or some inner tube. Pull the muffler, place a piece of rubber or tape over the exhaust port, and bolt the muffler back on. Pull the carb, put some tape/rubber over the boot, pull the carb cover with the fitting for the impulse line off, and bolt the carb back on. Run the mityvac to the impulse line and pump it up to 15lbs (or whatever it says) for both the pressure and the vacuum and see if the # rapidly drops. If it does, dunk the saw in a bucket of water and see where the bubbles come from.

Or for a quicker cheaper (but less precise) method spray carb cleaner/brakleen behind the flywheel, around the clutch side crank seal, the cylinder base, the intake boot, etc and see if the saw stumbles/dies.

 

[JacobE]

Thanks for all the suggestions!

I think I’ve narrowed it down to a bad fuel line… to start with at least. I got some fresh fuel this morning and tuned the carb up. I ended up at about 1.5 out in the low and 1 on the high. Sharpened the chain and got a cut done. It started to big after a bit. Noticed that the fuel had dropped below half, the upper 1” of the hose was exposed. I put enough fuel in to cover the hose and it worked like a dream.

I hadn’t been using ethanol free fuel, had been mixing my own. Then it sat for a while. Maybe that degraded the hose? I’ll find a source of ethanol free fuel because 25$ a gallon is pretty steep for a lot of milling

 

[lone wolf]

Thanks for all the suggestions!

I think I’ve narrowed it down to a bad fuel line… to start with at least. I got some fresh fuel this morning and tuned the carb up. I ended up at about 1.5 out in the low and 1 on the high. Sharpened the chain and got a cut done. It started to big after a bit. Noticed that the fuel had dropped below half, the upper 1” of the hose was exposed. I put enough fuel in to cover the hose and it worked like a dream.

I hadn’t been using ethanol free fuel, had been mixing my own. Then it sat for a while. Maybe that degraded the hose? I’ll find a source of ethanol free fuel because 25$ a gallon is pretty steep for a lot of milling

View attachment 988159

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

 

[Clyde85]

It definitely sounds like you are on the right track. Good luck.