090 irratic tickover

[Big Beech]

Long story short after getting the old girl runnin and using for nearly two years a small problem has cropped up.all gaskets where changed oil seals etc. Apart from crank gasket.The engine would not drop back to tick over even tho tge throttle was back and no amount of twiddlin the tick over or low jet would help.

Due to this the chain kept running on So new clutch springs where put in but still no help Thinking they where at fault.It would still get full revs under load but wouldnt idle.

 

It went down hill quite quickly. Used it fine. Got it out chain ran on used it for a couple of cuts. Put away, got out again and no amount of twiddlin sorted it. Hence where we r now.

Spark plug however was almost white even though it had been running fine and the right choclate colour Previously. the L screw also made little difference to it either so perhaps worn jets? We then pressure checked it all thinking an air leak due to the plug colour.

So exhaust and carb off and blanked. Red oil added under pressure and no leaks found. Except from the small finger nail size oil gauze under the carb on the oil filler side. Does this gauze have somekind of seal inside or a gasket on the oil pump itself and is likely to be the cause of my issue? If oil gets out, air can get in?If not should we go back and look at the carb itself?

I may buy a cheap new carb and thro that on and see if it works or not.

 

Any input most welcomed as i need the saw!!

Edited July 9, 2014 by Big Beech

[Pommie]

Sounds like you still have an air leak somewhere. Have you done a pressure/vacuum test ?

[Big Beech]

Sounds like you still have an air leak somewhere. Have you done a pressure/vacuum test ?

 

As described, 3rd paragraph

[Horatio]

I think you should just sell it to me !!!!!!

[spudulike]

As described, 3rd paragraph

 

Yes but it sounds like you are using fluid under pressure rather than air under pressure and then vacuum. Different leaks behave differently under vacuum and pressure.

 

Holding on to revs is a classic sign of a seal leaking under vacuum FYI.

 

You should check that the throttle valve is fully closing and the closing spring is still working well, also make sure the throttle linkage is free moving and loose when the throttle is closed.

 

Also try manually shutting the throttle valve with your fingers to see if the engine revs drop if you push it closed.

 

I am not familiar with the 090 but doesn't it have a aerovane rev limiter on it - the Contra I worked on recently did! the plug going white will be down to a lean mix and over-revving!

[Big Beech]

Thx spud.

So how do diagnose under vac what gasket is the issue?.

Throttle was pushed back manually with no effect on revs.

Yes has a vane, this has not been touched. The saw was tuned with 4ft bar on in 4ft wood and rev limiter still kicks in and is or was tached to 6k if i recall.

The plug has only recently run white. Was nice colour b4.

[spudulike]

Thx spud.

So how do diagnose under vac what gasket is the issue?.

Throttle was pushed back manually with no effect on revs.

Yes has a vane, this has not been touched. The saw was tuned with 4ft bar on in 4ft wood and rev limiter still kicks in and is or was tached to 6k if i recall.

The plug has only recently run white. Was nice colour b4.

 

You are checking the crank seals, inlet manifold if rubber, impulse line if rubber and all gasket joints - usually cylinder to crank and the crankcase vertical joint....and decomp valve if fitted.

 

You do this by sealing the inlet and exhaust port, sealing the impulse if separate and then attaching a holed spark plug hole adaptor so a pressure/vacuum gauge and pump can be connected - this can be a purpose manufactured bit of kit or home grown like mine - the pressure you need is 10psi and the vacuum is -15 inches of Mercury.this

 

There is no real alternative to doing this correctly - it is best to get zero leakage on the engine even when rotating the crank slowly. A water and detergent mix can be painted over suspect leaking parts to see if it bubbles.

 

I would have thought that the airvane would have stopped the saw over revving but may need checking again.

 

Seals tend to last for 5-10 years + but you may have an issue somewhere.

[Eddy_t]

Don't forget to check the oil pump! If the diaphragm has split on that (or the pipe) that will cause an air leak!

Other things to check are the crank-lock plug (may be loose), decomp valve and spring arm, the bottom of the carb where the gauze is for the fuel, check the plastic inlet thing for cracks. If all those are ok then it's either the seals, crank gasket, cylinder gasket, one of 3 (or 4 if you have the heat shield for the carb) carb to intake gaskets, muffler gasket (if you have the older, non-metal type), the bolts on the muffler, or at very worst, the crankcase has slid out of alignment! If it's the crank seals then you will need to check the crank bearing cages, washers, c clips, big and little end bearings. Nice checklist for you Simon

 

 

Sent from my iPhone using Arbtalk

[Big Beech]

Thanks Eddy & spud!!

One ditty of info i had forgotten about.

 

When i was on a 5ft oak when it hadnt even really started, when i pulled the saw from the cut the saw would rev ever so slightly on each tug to remove it. That was b4 the carb issuse started. So given a tug on the bar, may that suggest crank seals as it was tweaking the crank on yanking?

[Pommie]

Thanks Eddy & spud!!

One ditty of info i had forgotten about.

 

When i was on a 5ft oak when it hadnt even really started, when i pulled the saw from the cut the saw would rev ever so slightly on each tug to remove it. That was b4 the carb issuse started. So given a tug on the bar, may that suggest crank seals as it was tweaking the crank on yanking?

 

You could be tugging the impulse hose too as the AV mounts move