MS181 spluttering and bogging down at high revs

[Ratman]

Hey guys, anyone ever had a baby MS181 that starts and idles perfect yet when you go to rev and cut the saw bogs down? The saw doesnt have to be in the cut either for it to bog down. Can press trigger and she’ll rev out fine, no messing at all, then all of a sudden it’ll miss and start to bog?! Other times if wont rev out clean at all?! Really weird and really random [emoji35]

Up to now i have had exhaust off to check everything, all looks good on piston side, compression is 125psi, no spark arrestor fitted to exhaust and carbon build up is minimal if i’m honest. I’ve fitted new diaphragm kit to carb, new fuel line, air filter, plug etc and cleaned around the drum and chain brake as this was rank and proper made up!!! (Idle farmer) its a 2007 model, so reluctant to throw big money at it. Anyone else had any similar issues?

 

Cheers

Andy

[spudulike]

125psi sounds low but will depend on what a well running  machine registers on your gauge. Other then a worn or nipped up piston, it may be a split fuel line or carb issue but check out that compression again.

[bmp01]

Low compression doesn't usually cause random behaviour. If this random behaviour happens even when the engine is warm then my guess would be the carb. You say you've put new gaskets in but did you check the accelerator piston o ring. It's boringly frequent that this little carb feature causes random behaviour. If you can swap in a known good carb that will set you off on the correct path of fault diagnosis.

Presume you've tried a new plug, unlikely to fix it but as its easy ....

 

  

 

[Ratman]

Hey spud, 127psi on my stihl compression tester, but there isnt an ounce of marking or blemish on the pistion looking through exhaust side, like i say i’m shocked theres so little carbon build up or anything. Will do the check again tho [emoji106]
bmp01 this is the first newer type carb i’ve played with, its usually older type 180 i mess with the 181 is new to me but i suspect the theories are very similar apart from the added on top section / butterfly which will be for emissions etc. Will go through it all again tho for sure, my gut feeling is fuel related (carb) cos like i say its had new fuel line & filter (all genuine stihl too)
Cheers guys, will come back to you on further findings and developments [emoji106]

[Ratman]

May be next week tho guys, cos i’m on my silly shift at work and currently mid way through re-roofing my house [emoji30] [emoji27] not enough time in a day for everything at min. Thanks for the help tho guys [emoji106][emoji108]

[peatff]

A friend of mine has MS181 and his does the same until he gets it really warmed up then it works fine.

[Ratman]

Hey guys, re-roof on house done now, back on my late shift so have stripped the carb down again.... all looks mint ?‍♂️ what do you call the “accelerator piston o-ring?” ?

[bmp01]

Yeah, it's not obvious is it. ..

The throttle shaft (the one with the butterfly valve) .... where it goes through the carb housing isnt round it has a flat machined into it. That operates on the accelerator piston, which is buried in the carb body.

Wish I had a picture. ... but anyway,  if you look at the face of the carb,  engine side,  there is a hole parallel to the inlet port, 5 or 6 mm diameter. Sometimes they have a blanking plug.

You should be able to see the throttle shaft running across the hole, you might be able to see its not round.

 

So boringly you've got to disconnect levers,  springs,  butterfly plate etc and slide out the throttle shaft and the accel piston will pop out.  It has a small spring behind it.

Usual precaution,  take some pictures, lay out the bits in the correct order etc so you can get it back together. You also need to be able to align the butterfly on its shaft and re-peen the end of the butterfly securing screw when you reassemble... If the accel piston is tired the plating is usually worn through see piccy. Worn o rings are often visibly small. Trouble is once the piston is worn there is a good chance the housing is too. The next alternative is to disable the mechanism, seal up the hole and retune the carb a bit richer.

All in all - not one of the easiest jobs and time consuming. That's why they would just lob a new carb on at a dealers.

 

HTH, if i can clarify anything just ask. 

 

 

 

[Ratman]

Top man! [emoji106] will strip in bits and have a butchers, you had any experience with the aftermarket carbs?
Reason i ask is i dont think this is a genuine carb as no markings or branding on the bottom plate as you would normally find? I would assume it should be a zama carb on the 181?

[bmp01]

Nope,  my normal approach is to chuck the accel piston, make an ally plug to stuff in the hole and retune the carb. ... so far so good - I've not needed replacement carb. 

 

Yes it's a Zama carb as std eqpt.

 

If you have an after market carb that probably doesn't have an accelerator circuit at all. The whole concept was necessary so the manufacturers could run a leaner mixture (for emissions purposes) with good throttle response.  Aftermarket carb manufacturers are not subject to the same emissions legislation so they don't bother.

Quality and life span of aftermarket is questionable by all accounts.

 

 

 

 

Edited July 10, 2018 by bmp01
Clarifications.