Hobbyist Forums banner

1 - 4 of 4 Posts

·
Registered
Joined
·
2 Posts
Discussion Starter · #1 ·
I've got a Stihl 440 that we recently rebuilt. We're a conservation corps in Arizona (relevant because of a wide range in elevations our saws work at as well as lots of users) that does nearly all of our chainsaw repairs in house. We rebuilt it because of substantial scoring on the piston.

The saw has a new carburetor, air filter, fuel filter, and gaskets as well as the new piston, cylinder, and rings. The spark arrestor and all components in the drivetrain were cleaned thoroughly and inspected carefully. We adjusted the carburetor using a tachometer to appropriate settings at the same elevation as the project (which is 15 minutes from our shop). We ran about a tank of gas through it on idle to break in the new parts and then had an experienced user handle it for its first project back in service -- falling recently burnt Ponderosa pine for a fence line.

The saw started up easily and cut well with plenty of torque; however, it was noticeably reluctant to slow back down to idle after being revved. I ensured that the chain tension was correct, once again inspected the filters and spark arrestor, and started it back up. Its idle fluctuated wildly (from around 1900 up to about 3400 -- I typically see our saws fluctuate by 2 or 300 rpms, not 1500), even when set flat on the ground. I picked the saw back up and checked its rpms with a tach in different orientations -- at first it appeared it may be speeding up while being tilted forward and back, but I couldn't persuasively demonstrate any correlation.

The same tachometer was used successfully to adjust other chainsaws and its readings matched the saw's sounds well. Furthermore, the gasoline we use is high-octane gas purchased from a service station which guarantees under 10% ethanol and was mixed recently by our experienced shop technician. We use Stihl brand two-stroke oil.

I brought the saw back to the shop for further inspection. It did have a bent and shortened oiler arm, which I replaced -- I have had broken oiler arms produce similar symptoms before, by misaligning the clutch drum and causing resistance with the chain brake. The clutch and the clutch drum appear to be in good shape, though, and after warming it up and measuring its rpms at idle in our parking lot for a while, it's still experiencing the same fluctuation. There's no evidence of fresh scoring, its carburetor holds pressure when tested, the carb screws have good threads and are straight, and upon inspection, the saw's gaskets are fine, too.

I'm stumped. Have any of you seen this before? Does anyone have any ideas about what's going on?

Many thanks. I appreciate your help.

Too long, didn't read: A saw that's been recently rebuilt by a competent mechanic and appears in good shape otherwise has a wildly fluctuating idle. Any ideas are much appreciated!
 

·
Registered
Joined
·
1,767 Posts
Since it has had experience people working on it, I can't add much other than the fluctuations is usually related to a lean condition and on a 2-cycle would indicate an air leak, so with that said, while it is running spray some carb/brake parts cleaner along all the mating surfaces, if the engine tempo changes you have found a leak and can take the appropriate action. Have a good one. Geo
 

·
Registered
Joined
·
2 Posts
Discussion Starter · #3 ·
I'll try that out today. I'm waiting on a new carburetor I can use as a test carb, and I think I'll try vacuum testing it next week with our shop tech.

Thanks for the ideas!
 

·
Registered
Joined
·
128 Posts
I agree with geogrubb,espesially since you said it's slow to return to idle.
On this saw I would carefully inspect the rubber boot and impulse hose between the cylinder and carb to be sure the are connected and not damaged or cracked.The carb cleaner trick will work for this,just be sure it is'nt being pulled into the air filter.The crankshaft seals are another place to check,but it's the last place I check for leaks as it requires some disassembly.
I'm sure you already know this but operating a saw with this type of problem will ruin the piston and cylinder quickly so I can't help but wonder if this problem did'nt exist before the saw was rebuilt.
Personally I think"the more info the better"so I do'nt mind long the post.It explained the situation a lot better than the shorter version.
Don
 
1 - 4 of 4 Posts
Top