MS170 fuel delivery problem

[Woodchopper]

I have an older Stihl MS 170 chainsaw that is giving me fits. At first, with the original carburetor, it would not start at all on its own. Even with priming the cylinder it would like run for a short time. Rebuilt the carb. and no improvement. Bought a highly rated carb from Amazon, with new fuel filter, air filter and spark plug; installed it and it started on first pull and at that time after 3or4 shutdowns it would start on first pull.
After sitting for a week or so, I tried to start the saw and no soap. Checked the plug and it was dry (even after pulling on it with the choke on). I primed the cylinder with a little gas thru the plug hole and it started and continued to start after subsequent shutoffs in about an hour of use. The saw ran great! Next day same problem: the saw would only start after pulling the spark plug and priming the cylinder with a small amount of gas. Subsequent startups, after initial start up were achieved with just pulling the recoil starter as previously described.
I’m stumped! I have good gas/oil mixture, great spark, good compression and have inter-changed fuel filters all to no avail.
Any suggestions at this time would be very much appreciated. (bought the saw used; thinking of buying a new factory carburetor-Zama).

 

[CutTheNoise]

When you say you have to prime the cylinder and then it runs fine, that suggests the carb is working but it's not getting fuel initially.
The check valves and diaphragms might not be quite right, especially on the metering side, so it's not pulling fuel from the tank unless the engine is already running.
Since you've tried new filters and confirmed spark and compression are good, a genuine Stihl or a quality Zama carb is probably your best bet.
A new OEM carb is a good investment that will likely solve the problem once and for all.
I know it's frustrating but a factory part will have the right settings and tolerances to work correctly with the saw's fuel system!

 

[Woodchopper]

Thank you, CutTheNoise. Your recommendation is exactly what I was thinking to do next. But before I replace the repo with a factory Zama carb, do you know if I can get to the cylinder-to-block bolts - just to check that all are tight and right? Or, would this be a waste of time because the saw runs well after it’s primed?

 

[Paul]

The cylinder bolts are underneath the saw. The handle assembly has to be removed in order to get at all the bolts. Since saw runs ok after starting they are ok. I think your problem is with the choke not closing all the way.

 

[STIHLTECH]

I don't see a fuel hose on here. I am seeing cracks in those with age.
The linkage and arm wear on these.
If the case bolts are loose, you need to tear down and reseal. But unless it will not idle, you are probably OK.

 

[Jean Christophe Chuine]

Pour un problème similaire mon spécialiste stihl m'a fait changer le joint de cylindre et c'est reparti impeccable

 

[SawdustKing]

Hope you’ve got it all worked out now. That carb swap will likely tell you what you need to know

 

[Leon Couper]

I would also (my pet saying) pull the fuel filter out of tank and pressure test.
If you have a mityvac 8500 or similar pull the filter off and plug straight in.
It should hold 8 to 10 pd and if not it will have a leak between there and carby.
I have had this trouble with older saws and same thing would have to prime
down plug hole to get them to start. It is worth a try before spending to much
money or pulling it down.

 

[SawdustKing]

Absolutely. Pressure testing the fuel filter while it’s connected is a smart move.
Holding 8-10 psi confirms there are no leaks, and avoids any unnecessary disassembly and confusion. Having a mityvac 8500 is definitely a bonus.

 

[Alan]

I would also (my pet saying) pull the fuel filter out of tank and pressure test.
If you have a mityvac 8500 or similar pull the filter off and plug straight in.
It should hold 8 to 10 pd and if not it will have a leak between there and carby.
I have had this trouble with older saws and same thing would have to prime
down plug hole to get them to start. It is worth a try before spending to much
money or pulling it down.

Appreciate the tip. Pressure testing really saves both time and stress. That mityvac 8500 sounds like a super useful tool as well.

 

[Leon Couper]

Well the way I look at it by doing it that way you are testing your fuel lines for cracks or holes
and also if line has been semi pinched and also the needle valve and seat. I can tell
you now it has saved me a lot of time with that same problem woodchopper is facing.
At the end of the day if it does not hold pressure you have to go through the pull down thing
but that is a quick way of testing for leaks in your lines and carb.

 

[Alan]

Yep, love a method that saves time and headache.