Chainsaw engine experts?

[trader4]

I have a 40 year old Stihl chainsaw that is giving me fits. It had not been used in probably 6 months or more. Even back then, it was having problems, with not wanting to start, stalling when going back to idle, etc.

I got it running this time and after fooling around with it was able to actually use it to cut up one large tree. It usually will eventually start from cold, after a lot of fiddling between choking, not choking, etc, but then has trouble transitioning to full throttle. It won't idle, it stalls if you try to let it idle. But at full throttle, it ran OK and cut like a champ. You just can't let it go back to less than maybe 1/4 power or it stalls out. I also observed gas dripping from the carb at times. And at times after it's run, it's very difficult to start again.

I took the Stilotson carb apart and it all looked good, except for a small fuel screen in the carb. It was packed with gunk. Otherwise it looked good, no rust, no other signs of fouling. I cleaned it out with carb cleaner, compressed air, put it back together. Same problem.

I ordered a carb kit and took it apart again today. Cleaned it again with compressed air, including this time I could take out the welch plug. All the passages look like brand new. The kit included two new diaphrams that form the fuel pump, new needle valve, gaskets. etc. The old diaphrams were still intact, but they\had more stretch/play then the new ones. Was hoping this was the problem. Put it all back together again, following the service manual, so I'm pretty sure it's right. Still same problems.

Tried squirting a bit of fuel into the carb directly and it fires right up. Did that several times. Without the squirt, it's much harder to start.

I've taken it apart to inspect the fuel line to the tank and filter which is inside the tank. I can suck or blow on the line, pinching off the filter end and no evidence of any air leaks. Fuel filter is clean.

I put a new plug in and also have tried pull starting it with the plug out, but connected. Good spark. Also when taking the plug out after having tried to start it for a long time, the plug is dry. Acts like it's not getting fuel....

So, from a bit of searching, it looks like one of the other potential problems is the impulse line. This carb uses a line from the crankcase to supply alternating pressure and vacuum that works the fuel pump diaphrams in the carb. Apparently if the crankcase seals are kaput, then it may not get enough pressure/vacuum. I've inspected the hose, it looks OK. With the sparkplug out and pulling the starter, I can feel some suction, some puffing on the end of that impulse hose with my finger. But not a lot. Anyone know how much you should be able to feel? Or have any other ideas?

[clare]

Make SURE the crankshaft seals are good. If they leak the engine runs lean - more noticeable at idle - and is hard or impossible to start. 2 stroke engines are VERY fussy that way.

[trader4]

From where I've been so far, that seem like one of the posibilities left. Only problem is, how can you tell if they are leaking or not? The service manual has the test procedure, which boils down to removing the carb, removing the muffler, bolting a testing adapter where the carb was, bolting a blocking plate over the exhaust, getting the piston to TDC, then applying pressure and vacuum to the test adapter, therby pressurizing the crancase, to see if it holds about 7 psi.

Only problem is, I don't have the test adapter or blocking plate. Not sure much can be determined by finding the seals and looking at them. And by the time I'm there, might as well just replace them. Any ideas on another way to test?

[SRN]

[Stormin Mormon]

A mechanic I knew, used to check for vacuum leaks by spraying ether on the vac line, while the engine was running. If the line was leaking, the motor speed would change. Not sure that technique applies, here.

Christopher A. Young Learn more about Jesus

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From where I've been so far, that seem like one of the posibilities left. Only problem is, how can you tell if they are leaking or not? The service manual has the test procedure, which boils down to removing the carb, removing the muffler, bolting a testing adapter where the carb was, bolting a blocking plate over the exhaust, getting the piston to TDC, then applying pressure and vacuum to the test adapter, therby pressurizing the crancase, to see if it holds about 7 psi.

Only problem is, I don't have the test adapter or blocking plate. Not sure much can be determined by finding the seals and looking at them. And by the time I'm there, might as well just replace them. Any ideas on another way to test?

[The Daring Dufas]

You can also unscrew the tip from a propane torch, slip a length of fuel hose on it for convenience and crack the valve open and run the tip of the hose over suspected leaks and the engine will speed up. I think the propane would be much safer to use than ether. My personal choice for finding vacuum leaks is my ultrasonic leak detector. I use it mostly for all types of pressurized gas leak detection. ^_^

TDD

[harry]

Did you clean out the fuel tank? Points in magneto need replacing/cleaning? (Weak spark) Did you replace spark plug? Have you tried fresh fuel? Did you check the fuel tank cap vent is clear?

[trader4]

Thanks for pointing me in the right direction. I found another video on youtube that shows the actual crankshaft seal removal/installation, as well as testing procedure:

That video gave me some excellent ideas. I was already thinking about how to plug up the intake and exhaust to be able to test it. They used a piece of rubber inner tube, placed it between the carb and intake and tightened it. Used a metal plate on the exhaust side with similar inner tube to block it. Then they used vacuum tester on the impulse line. I even have the vacuum tester, if I can find it.....

From what Clare said here, what I've read other places, and what the video you had about crankshaft bearing/seal symptoms, it sure sounds to me like it's very likely the crankshaft seals are shot. Damn, they only lasted

40 years..... I'm sure the bearings are OK, the saw has only had light, irregualr use.

But, the next problems are to see if new seals are still available for a 40 year old Stihl saw. And then, according to the manual, to get the seal out on one side, you need a fancy puller tool that has fingers that grab the seal from the inside. The second video shows it being used. Not sure if I can get it out without the tool, but maybe it's possible.....

[Steve Barker]

put a new carb on it. They are not worth the time to rebuild. as a matter of fact the last one i did, the kit was more $$ than the whole carb.

[gfretwell]

Two things to consider. There is usually a filter inside the tank on the end of the intake hose. The muffler can get plugged up with carbon.

[trader4]

I've checked the filter and entire length of fuel hose. Filter is perfectly clean, no signs of leakage in hose and it looks in good shape. I doubt it's a muffler problem, because when it decides to run, it will run at full power, take down a tree, pretty much OK, yet won't idle. Would think if there was blockage, full power would be a problem, no?

I really think it has all the symptoms of leaking crankshaft seals, which as I understand it are:

won't idle difficult to start balks at going from idle to full throttle adjusting the needle valves doesn't help

just about everything else has been ruled out. I disassembled, cleaned, rebuilt carb. has good spark

Even though it's 40 years old, I found the seals on Ebay. Only problem is, don't know if I can get the old one out on one side without the special puller. Debating whether to try to do some pressure testing to verify it's the seals or just order/replace them.

[trader4]

The rebuild kit was $10. A new carb is $100 and there is no sign that anything is wrong with this one. I've had good results with rebuild kits on other carbs, like my Sears snowblower, which I've rebuilt twice now.