070 poor running

[Robert Raven]

Hi there,

 

My Stihl 070 (new to me) will start well and rev up, but under load it hesitates and runs intermittently, I have to back off and 'hunt' the throttle to keep it running. Felt like fuel starvation so I cleaned the fuel filter, new petrol, checked fuel line for leaks and replaced it, and reset carb to manual settings. After that it still didn't run great, the governor seems to be fidgeting about so the engine won't settle enough for me to tune the carb. I took off the fan housing and altered the governor spring, but it didn't seem to make much difference.

 

Does this sound like a governor problem, or perhaps the fuel pump diaphragm has got stiff and can't keep up with fuel demand? What's the next line of enquiry?

 

Really frustrating because for the few seconds it runs well, it cuts really well, just wish it would settle at full power.

 

Cheers,

Rob

[Megatron]

It doesn't sound like a governor problem. What settings is your carb at?

To "tune by ear" you need to disable the governor.

 

Check the fuel tank vent before anything else.

Have you installed a carb kit? or cleaned out the carb. That would be the most likely explanation. Diaphragms go hard with age and can warp, meaning they no longer pump fuel efficiently. Gunk can build up in the carb orfices and also partially block the fuel strainer/filter on the carb inlet.

[Robert Raven]

Hi Megatron,

 

I've had another look today and have made progress, I think it is a combination of small problems which is fair enough given it's grand old age and unknown provenance. I replaced the fuel strainer in the tank, all the fuel tubing and tank vent, and dismantled the carb and gave it a good clean out. The strainer in the carb was pretty scaly. Also I noticed an obvious wear ring on the needle. The pumping diaphragm was pretty wrinkly across it's moving surface, not sure if this affects performance? Where would you order a carb kit from?

 

I reset the carb to factory setting, 3/4 L, 1H. After all this it runs acceptably but still dies away unexpectedly from flat out, after much fiddling I think its th HT lead which has been taped up by a previous owner, so I guess is damaged. It seemed that if I squeezed and wiggled the HT lead during running problems, it would improve. Can I replace the HT lead, or is it permanently attached to the coil?

[Robert Raven]

Also, it's not oiling properly, I had to keep pumping manually to keep the chain lubed. What could be the problem if the manual pumping is fine, but the auto pumping is very very slow? I saw under the carb is a very short rubber hose, from the crankcase to the oil tank, does this transmit the pressure variations from the crankcase to the oil pump? I guess if it were gunged up it could reduce the rate of auto oiling?

 

Pretty impressed with the saw when it was running well, it made pretty short work of milling some ash, 17ft long and 20" wide.

 

 

Thanks, Rob

[Megatron]

First have you got the muffler cover installed yet?

 

The HT lead can be replaced, it is screwed into the coil. You have to remove the flywheel and stator plate to do so though. If you do, mark the stator plate and crankcase with a sharp chisel before removing the stator plate or you will have to reset/readjust the timing when you reinstall. However, usually in my experience with a HT lead problem the saw will cut right out instead of die back. Hard to tell for sure but it still does sound like a fuel starvation problem, especially if babying the throttle brings the rpm back up.

 

If the pump or metering diaphragms are warped/wrinkled/stiff they need replacing and could well be the cause of the problems. If the inlet needle has a groove on the sealing rubber tip then that should be replaced too.

A tillotson DG-5HL kit is for the gaskets and diaphragms only, RK-113HL for gaskets, diaphragms, needle, seat etc

e.g.

Carburetor Diaphragm Kit Fits Stihl 08 08 SEQ 70 090 AVR TF350 AVE TS08 TS350 | eBay

 

Tillotson HL Carburetor Repair Kit Replaces RK113HL | eBay

 

(just to check i'm assuming your carb is an HL-244 or HL-324 ?)

[Megatron]

With regards to the oil pump. The short black hose behind the carb does transmitt impulses from the crankcase to the oil pump. If it has a hole in not only will the auto oiler not work properly the crankcase will suck in excess air causing it to run lean!

 

It is also possible the pumping diaphragms in the oil pump have gone bad or like you say the oil strainer is partially blocked. Although the manual and auto pumps use the same pick up, the manual pump will create greater suction. Under the oil pump cover;

[Robert Raven]

Hi, thanks for the reply. Muffler cover is fitted. Sounds like I need to give the carb a total makeover, I'll check the model and order the kit. I'll forget the ignition system until I've totally ruled out fuel supply issues.

 

I'll have a look at that oiler as soon as I get a chance, sounds like a good clean, new pipe and maybe a new diaphragm should sort it. Sorry to be a pain but do you have the part number for the oil pump diaphragm? Or know where I can get a parts list? I downloaded the workshop manual but the link to the parts list was broken.

 

I've also arranged to borrow a compression tester from a friend, what compression is ideal/acceptable? Right now it's not too hard to turn over.

 

Thanks so much for the help,

 

Rob

[Robert Raven]

Oh, if I pull the oil tank cover, do I need a new gasket to put it back on?

[Megatron]

Pump diaphragm; 1106 640 3100 (I wouldn't order this until you know it is at fault, as it is about £14 exc VAT!)

 

Compression should be 140+psi. If you can, check the compression tester against a known good saw before testing the 070 to ensure the tester isn't reading low etc

 

If you pull the oil pump cover you will almost certainly have to replace the gasket as usually half sticks to either side

 

1106 359 0700 for the gasket. However, you can also get this gasket in the complete gasket and seal set (worth doing if you haven't got new seals yet and includes all other gaskets), part number 1106 007 1050.

The gasket and seal set looks like;