What's on your bench today?

[Rich2484]

Yeah sorry I forgot you need the small adaptor kit for the tester. A car tester will still work. If you could get it tested by a dealer just so you know the psi that will give you a good clue.

[GardenKit]

Can't do, my compression tester is too large to seal on the tiny plug hole, it's really for car/motorbike application.

You will need a screw in comp tester with a 10mm adapter, which are not as common as the 14mm ones. A push fit will be no good at all.

 

Did your dealer not test it when it was in? What was his diagnosis on its problem?

 

Edit

Sorry Rich, was writing while you posted.

Edited November 5, 2012 by GardenKit
Crossed in the post

[JamesNI]

No, the dealers ultra sonic'd the carb and cleaned the exhaust of carbon. They could get them to run fine under no load, they seem to behave a lot differently once under load and or being moved about through various angles as they do when cutting.

[JamesNI]

Morning gentlemen. My dad butchered an old plug and mod'd it to fit the comp gauge! Did a cold compression test this morning. On a single pull, with the carb removed I must add the machine is making 110-120-125 psi or there abouts each time. If I get a chance i'll stick them back together for a hot comparison. Just really need them fixed and reliable!

[GardenKit]

Oh well, compressions not the problem then, thinking cap back on!

[Rich2484]

Hmm this one has me stumped a bit. Usually the checks that have been made will bring up any problems.

 

Are all the pipes in good nick, none split or holed? Does it run when turned a certain way? You said you have done a pressure test on it, did it come out ok as I'm lost in you saying no joy? Des this mean no leaks?

 

I will have a look at one and see if anything jumps out that could be causing your problems.

 

It's ound alike there is too much fuel getting through the carb. If the exhaust is getting wet.

 

But the only thing causing that could be the metering valve, you said it was checked? Is the air filter getting soaked aswell when the trimmer is left?

[JamesNI]

Fuel pipes were replaced at the dealers when the carb was cleaned.

Sometimes, but not always they bog down when tilted with exhaust side of engine facing ground and carb facing up.

The airfilter has been wet, though I haven't noticed it recently.

Metering valve is set level with the top of the body of the carb.

I'm glad it's not just me that can't work it out! Many thanks for the help so far.

[spudulike]

Just been thinking of what I would do to a saw if it had these symptoms.

 

The poor idle - I would wind the idle in until it would either fast idle or not even run with the screw turned fully in.

 

If it won't idle with the idle screw turned fully in, I would see what effect adjusting the "L" screw has, typically on half a turn out, the engine will race as the mixture will be a bit lean!

 

Sometimes the L screw needs to come out more than one turn, have you tried 1 1/2 turn, 2 turns, 2 1/2 turns?

 

The poor starting and weak idle could be an over weak low speed mix!

 

Have you checked the spark when the engine wont start, can you try this with a mild speed start to see if the spark is a bit iffy at low crank speed?

 

Other than that, it sounds much like lack of compression but know you have tried this with a gauge - if all else fails, drop out the base gasket and seal with a little liquid gasket and see if that gets the motor working, if it does, it looks like the piston is just worn out. Have you checked the ends of the top ring and compared their depth to the depth half way round - they can be thin on high hour engines. I had an old hedge trimmer with the same fault so did this quick fix as it was just for my use and gets 1-2 hours use a year!

 

The last thing to check is for carbon in the exhaust port and also for the exhaust being blocked with carbon and crap - this can cause lots of oily deposits around the exhaust - has it got a spark arrestor that has got blocked?

 

Thats about it - hope you rebuilt the carb with the pump membrane gasket closest to the carb body and the spacer between the carb and the diaphragm.

 

Oh - also check the fuel line and filter are in good nick

 

Carbs can be un-fixable, I had an 020 carb recently, just wouldn't idle consistantly so must have had a low speed issue but nothing fixed it - can you try a known good carb?

 

If it happens again, try running it with the fuel cap off - I know the vent has been changed but worth a punt!

[spudulike]

Right back to chainsaws:thumbup: got a Stihl 036 in, dirty but in good condition, I can still see honing marks on the bore. Symptoms are intermittent severe over revving.

 

I have seen similar on a MS440 I had in recently, to be due to the owners, in both cases they recognised the danger signs and sent their saws in suspecting possible air leaks.

 

With this one I pulled the carb apart and noticed the welch plug sealent had broken up and was floating round the diaphragm cavity blocking the main jet now and then causing lean mix at high revs!

 

I will still do the pressure/vac check and tach when finished - thorough to the end:thumbup:

[GardenKit]

Back to the HS81

Its almost impossible to offer any diagnosis on this problem from afar

1) the symptoms have changed from post to post.

2) every possible cause has been investigated and fixed or discounted, but has the work been done properly? if so the problem should be sorted.

 

One possible ,but very remote chance left. I once had a machine in which would not run when on its side. Eventually traced it to a pea sized lump of carbon which floated around inside the exhaust and occasionally blocked the outlet from the exhaust when it was facing down.

 

Otherwise take it to a decent techie, but be warned, if I was that techie I would do everything again, not trusting the work done by others, so the bill could be quite large.