MS720 just rolled in to my shop

[junglebiker]

Yesterday a man brought me this very nice looking example of a MS720 to work on. Looks just like a 070/090 to me, are there any significant differences between the 070 and the MS720?

Guy says that it has not run in many years and he thought maybe the problem was the kill switch wire. That did not make sense to me so I told him I doubted that was the problem, but that I would look into it and let him know. Feels like it has great compression. I see quite a few of these, but usually they are labelled 070 or 090, and of course a number of the 070's have 090 top ends on them. I've also seen chinese clones available for sale in hardware stores in Lae, (usually in weird colors and with funny names on them like "Warrior" or some vaguely chinese sounding name) but so far I don't think any of those have been brought into the shop. Seen a few top ends brought in for spark plug hole repairs, but that's it.

 

[junglebiker]

Well! First things first, I checked for spark. It has a nice, hot spark, so that's good. Checked the fuel tank and it was nearly fuel of clean, fresh fuel, so I thought, "well, maybe this guy just doesn't know how to start a MS720". Tried it until I got tired, this105cc beast is not easy an easy one to pull. There was not even a hint of running, no popping, no backfiring, nothing. Pulled the spark plug and there was still a nice, fat blue spark. But the plug was really super dry, which seemed a little odd. Pulled the air filter looked down into the carb as far as I could, funny, looks really really dry in there, too. Better check the carb.

First I sprayed a little carb cleaner into the intake and tried starting it. Yes! It started easily, and I always forget what noisy, vibrating monsters these guys are! It quickly ran out of carb cleaner to run on and didn't seem to be picking up any fuel. I pulled the carb, not a drop of fuel in it. Drained the gas out of the tank and I could see traces of varnish on the floor of the tank, so that tracked with what he had told me about this saw sitting for several years--the old gas had evaporated away over time and just left goo. When this guy decided to try it again, he put fresh fuel in it on top of the goo. Not a huge deal, fuel here doesn't have as many additives in it as it does in the US, so when it evaporates it doesn't leave much behind. Nevertheless, I decided to pop the bottom off of the carb, just to make sure it wasn't all gummed up. Nope! Super clean and really, really dry inside.

Carb cleared of guilt, I turned my attention to the fuel lines. The external looked okay--not pristine, but not gummy or cracked or hardened, and more importantly, I could see right through it from end to end and it was clean. Pulled the fuel tank internal line and, ah, ha! This line was gummy to the touch, and if I pinched it shut, it would stay pinched shut for a moment, and it had cracks in it. It was not blocked, however. Checked the in-tank filter and found the problem. It looked clean, but I could hardly get any air to pass through it. So! I will order a new filter and new fuel lines just as soon as I get the okay from the customer to do so.

 

[Stihl 99]

Still watching this, how is the saw now?

 

[junglebiker]

Still waiting on parts!! I literally live in the middle of nowhere. Usually takes 2-4 months to get parts, even simple little parts. I think the next flight of the plane that usually collects cargo for us from Australia is in August. Yeah, I could have ordered the parts through the mail, but then it tends to get stuck in customs in the capital for 2 months, so it's not really any faster. Just part of living in Papua New Guinea!

 

[blades]

get er all done and back to running shape?

 

[junglebiker]

Finally got the parts, installed them, and I can get it to run for a second by dripping fuel into the carb, but it doesn't seem to want to run by itself. It's an absolute beast to start, a yank on the rope will pretty much just pull the saw out of my other hand, so I have to put it on the ground and put a foot through the handle and then pull on the rope--it has a compression release, but it's not the type you push and then it automatically closes again when the engine starts--you have to hold it down, but then it won't start because the compression is too low. I guess it's only for clearing the saw out if it's flooded? Anyway, still fiddling with it. Almost seems like it's not drawing fuel for some reason. The old "hold the rope and drop the saw" method is no good, it will just hang there against compression and won't drop at all.

 

[Old Kiwi]

Is the pulse tube ok ????
seen these beasts in Sarawak very abuse tolerant

 

[junglebiker]

Pulse tube? Do you mean inside the carb? Everything looks super clean in the carb. Gas here is not like gas in other countries, it usually evaporates pretty clean doesn't leave a lot of goo behind. Usually. There was a short period a while back where the gas was like gas in the US, and left a huge mess behind when it evaporated. But this saw looks like it was last used before that time. There was some gum on the bottom of the fuel tank where the gas had evaporated and left the oil behind, but I cleaned all that out. I've had to put it aside to work on other stuff, but now I can get back to it.

 

[Old Kiwi]

The pulse tube conects the pump side of the carb to the crankcase it provides the alternating vacuum pressure pulses to pump the fuel to the carb
The pump side of the carb is the side with the diagram that has the cutouts
Is the carb a tilerton or aftermarket zama carb kits are avaliable from farmertec
Do you have any way of determining that the cases are airtight
Make sure fuel is getting pumped to the metering side of the carb
the diagrams in the carb may be stuffed due to age
Is the carb air tight ie will it hold presure against the valve seat apply 10 psi no more into the fuel line
Farmertec in china make copies and spares Might be easier in your part of the world. they call there copy a G070 ,

 

[junglebiker]

I replaced everything in the fuel system up to the carb--in other words, the in-tank filter, the line in the tank, the big plastic dooflicky that fits in that line at the top of the tank, and the line between that and the carb, all new, all genuine Stihl parts. I had the carb apart when the saw first came in and everything inside of it was clean and the diaphragm and other rubber parts appears to still be supple. I will open it up again. I'm glad it's a MS720, at least they are super easy to get the carb off of.

 

[Old Kiwi]

Passage 9 is the pulse tube it must connect to the crank case some how to provide alternating vacuum and pressure to pump the fuel
By the look of the diagram it may connect through the manifold this must be airtight . A common mistake is to have the gasket round the wrong way blocking the connection, There is a chance that this may be a rubber pipe /hose to connect the carb and the crankcase
now you have the fuel hoses replaced remove the filter of the inlet end use something like a mighty vac or copy to hold 10 psi to the fuel line at the pickup end where the tank filter came off it must hold pressure "all day long "
While you are at it make sure the crankcases are air tight both vacuum and pressure it should now run if you have spark