Stihl hs 87 R

[Trailoftears]

An unusual and possibly abtruse problem,but   I hope this may help someone somewhere.I started said machine up which has always been utterly reliable formerly,and could tell it wasn't a  happy German small-engined machine.It would grudgingly fire on choke,but due to the auto choke,no way of rev ing it up,without choke,it sounded 'dead' with no interest in firing.So,I did the obvious-brand new plug/checked air filter box and filter,also peered at the fuel filter hopefully.Knew the fuel was fresh.I had a gut feeling this was a breathing problem so that left me with the exhaust to check.Unusual config on this machine-3 extension tubes fitted on the exhaust outlet,all steered to the front end to keep fumes away from the operator.The furthest 2 are plastic sections with an internal diameter of say 8mm-both totally clear,didnt check the rubber hose that acts as a bridge between the exhaust outlet and the plastic extension pipes assuming that would be clear too.Whipped off the top plastic cowling to look for the dreaded spark arrestor on the exhaust box-not present,Coil and leads looked fine.So,turned the machine upside down and removed the rubber tube off the exhaust outlet and viola! A v. thin film of soft white ash no more than 1mm blocking the 8mm exhaust main outlet pipe-v.soft ash,not what I would call black gnarly carbon.Still,that tiny film was too much for the engine to expel/allow a successful firing process.As ever,you live and learn!

[Stubby]

10 hours ago, Trailoftears said:

An unusual and possibly abtruse problem,but   I hope this may help someone somewhere.I started said machine up which has always been utterly reliable formerly,and could tell it wasn't a  happy German small-engined machine.It would grudgingly fire on choke,but due to the auto choke,no way of rev ing it up,without choke,it sounded 'dead' with no interest in firing.So,I did the obvious-brand new plug/checked air filter box and filter,also peered at the fuel filter hopefully.Knew the fuel was fresh.I had a gut feeling this was a breathing problem so that left me with the exhaust to check.Unusual config on this machine-3 extension tubes fitted on the exhaust outlet,all steered to the front end to keep fumes away from the operator.The furthest 2 are plastic sections with an internal diameter of say 8mm-both totally clear,didnt check the rubber hose that acts as a bridge between the exhaust outlet and the plastic extension pipes assuming that would be clear too.Whipped off the top plastic cowling to look for the dreaded spark arrestor on the exhaust box-not present,Coil and leads looked fine.So,turned the machine upside down and removed the rubber tube off the exhaust outlet and viola! A v. thin film of soft white ash no more than 1mm blocking the 8mm exhaust main outlet pipe-v.soft ash,not what I would call black gnarly carbon.Still,that tiny film was too much for the engine to expel/allow a successful firing process.As ever,you live and learn!

Common problem on those . Can be caused by too much oil in the petrol , not revving  it flat out when cutting etc etc .

[GarethM]

What oil is being used?

[Trailoftears]

Stihl semi synth @ 45:1,the same as all my 2/ small fleet.I'm tempted to put the issue down to the fact that its a suprisingly low-revving machine for a 2/ engine?

[GarethM]

50:1 would be a smidge less oil.

 

Not sure about the low rev, usually it's balls out and steaming along for hours.

[pleasant]

1 hour ago, Trailoftears said:

Stihl semi synth @ 45:1,the same as all my 2/ small fleet.I'm tempted to put the issue down to the fact that its a suprisingly low-revving machine for a 2/ engine?

45:1 compared to stihls recommended ratio for their oil of 50:1 may not sound much more oil, but its over 10% more than recommended (50:1= 20ml per litre, 45:1=22.2ml per litre) See this a lot on modern stuff that is designed to run pretty much on fumes to pass euro specs. Everything is made narrower in the fuel and exhaust systems to pass emissions and doesnt take a lot of excess oil in the mix to build up and then you get a snowball effect and the engine cannot breath efficiently. 

Edited May 16 by pleasant

[GarethM]

@pleasant not sure if this was less of a problem when everyone ran the HP red ?.

 

I've vague memories of that being oil based, so burnt but left an black smokey residue Vs the white ash ?.

 

As the red I assume being a more basic oil is burnable or evaporates, leaving black soot. As the green is maybe less flammable at a lower temperature ?.

Edited May 16 by GarethM

[Trailoftears]

Hmm,i tend to drop a tad from 50:1 to 45:1,just I suppose as an insurance policy given my machines can run pretty much flat-out for 3 odd hours in increasingly hot summers,also I thought/hoped that the semi synth would/should run a bit cleaner and leave less deposits than the old standard red mineral stihl oil?But thats largely guesswork on my part.Either way,its the first time I've seen this happen on stihl b/cutters/blowers/chainsaws etc etc.Of course,given the guidance on birds and summer hedgecuts,this machine lies idle more than I'd like these days!But as far as I understand it this machine has low revving built in-v.low gearing,low flat out speed,large tooth gap,so I suspect it doesnt get the opportunity to clear out its lungs properly...!