Stihl MS 180. Bogging down at 3/4 Rev Range

[Dave Tansley]

I have recently completely overhauled my Stihl MS 180 following a problem with bogging down at 3/4 Rev range.  My machine had been working brilliantly and has carried a fair bit of work.  But suddenly it started to play up.  I use Stihl synthetic oil at 50:1 mix as required. I had refreshed the fuel with a new mix, but suddenly encountered the problem.

I have twice replaced the carburettor with a patterned model. The fuel line, air filter, spark plug and ignition assembly have all been replaced without solving the problem. On reading articles about this issue, I again stripped the unit down to re-check to see if I had missed something.  I decided to replace the piston rings, main bearings and oil seals.  The cylinder was I perfect condition with no scoring marks and the piston rings only had approximately 0.003" 0.004" end-gap wear.  The only thing I can think of is that the 2 new carburettors are mid priced units and not OEM replacements.

Has anyone had a similar issue?  Has anyone found problems with patterned carburettors?  Having spent so much time and expense trying to sort out this problem, it seems that the only thing left is to try an OEM Stihl Zama carburettor.  Your comments would be very welcome?

Dave in the UK

[sawguy21]

Have you looked at the spark arrestor screen for signs of clogging? I don't have much confidence in aftermarket carburetors, oem would have been my first choice.

[Tom King]

Take the spark arrester screen out. It's just held in with one screw.   Take that screw out, and slide the little screen out.

 Holding it with an old pair of pliers, just on one corner, heat it to read hot with a propane torch.  Once it's cooled, rub it between two fingers of an old glove.  You should be able to see through it like a new one after that, and the saw should run fine again.

[Canuck123]

No where in your description , have you mentioned tuning the saw after replacing the carb . Oem on Stihl is normally the way to go. However all carbs need to be tuned to the saw . Prior to tuning ensure air filter is clean or new , fresh tank of properly mixed fuel and yes spark arrestor is clean . Then tune as normal , low speed 1st to ensure no hesitation from idle . Then high speed to have a slight 4 cycle out of the cut then smooths out within the cut . A tach is valuable within high speed adjustments . Final adjustment is idle speed just high enough to prevent chain from rotating is optimal . Hope this helps !

[hms11]

We're the replacement lines OEM? I've heard of aftermarket fuel lines kinking and pinching off flow. 

[Dave Tansley]

Thank you for your comments.  In response, I have removed the spark arrestor from the exhaust, so there is no longer any exhaust restriction.

Secondly, the only adjustment or tuning on the carburettor is the idealing speed.  Regrettably there are no other means of carburettor adjustment on this model.  It would be helpful if another type of multi adjustable carburettor were available.  

Looking at one other comment, another carburettor, but an original OEM might be a good idea.