What's on your bench today?

[billpierce]

Thay reminds me. Another vital clue is the hi revs dont go higher than 12k even with the h screw wound in, which blows my mind. Both carbs the same both original one with oem rebuild kit (have changed Welch plugs but did seal them with super glue but they looked fine) and cheap chinese test carb. I dont get that!

Yes, throttle is opening fully@sawchip

[bmp01]

Left your cleaning rag in the inlet port again did we ?

 

It is 4 stroking at 12 k rpm is it ? 

Spark plug gone bad ?

Genuine top end ? Aftermarket piston with thicker rings will not seal at high revs.

Decent compression?

 

 

[spudulike]

I have had this on high use 020Ts before. If the saw makes enough compression to fire but compression is pretty low...135psi or so, sometimes the saw will just not rev out as it should.

The compression on a 346 is ideally 170psi on a nice top end or 150+ on a well used one. If the base gasket is dropped, expect 180-190 psi and if you are only making 150 with the base gasket gone....that will be the cause.

Have you tested the compression? 

Other than that, as BMP said, is it four-stroking? What colour is the plug and has the saw got good power!

[billpierce]

. OEM pot. Not had it off but piston isnt scored and comp is ok, will run and idle fine, just doesnt feel as zippy as it ought to ( this was bought by a friend and he wants me to fix it so no little of its history).

Sounds like its four stroking and just wants tuning up, but nowhere to go with the H screw!

Have changed sparkplug, coil, checked gap, new OEM carb innards and also tried new carb, and filter

[spudulike]

Yes but.....what is the compression reading? Even OEM cylinders and pistons wear out. I have seen this before and bores can wear out. 

Is it the older 45cc machine as this was the older version.

I once had a 024 and it had compression of 180psi as I had dropped the base gasket, it would barely spin the chain and there was nothing on the handle - the inlet side of the bore had no plating left!

I would measure the compression or check the bore, the ring end gap and then the piston in the bore.

[billpierce]

Yes but.....what is the compression reading? Even OEM cylinders and pistons wear out. I have seen this before and bores can wear out. 
Is it the older 45cc machine as this was the older version.
I once had a 024 and it had compression of 180psi as I had dropped the base gasket, it would barely spin the chain and there was nothing on the handle - the inlet side of the bore had no plating left!
I would measure the compression or check the bore, the ring end gap and then the piston in the bore.

Kinda lost faith in my gunson, it constantly reads low - even on saws that run perfect. Had the top off no real sign of wear, decent Mahle piston and pot, ring doesnt look worn. There was a bit brown residue on the piston which I have cleaned off, deleted base gasket and put back together,m - sadly no different.

I have ordered meteor piston to try

[openspaceman]

3 hours ago, billpierce said:

ring doesnt look worn

Did you check the ring gap in the bore?

[Khriss]

3 hours ago, billpierce said:

Kinda lost faith in my gunson, it constantly reads low - even on saws that run perfect. Had the top off no real sign of wear, decent Mahle piston and pot, ring doesnt look worn. There was a bit brown residue on the piston which I have cleaned off, deleted base gasket and put back together,m - sadly no different.

I have ordered meteor piston to try

Professional Digital LCD Tyre Tire Air Pump Pressure Gauge Tester 200PSI Car Van

WWW.EBAY.CO.UK

---

Suitable for all kinds of vehicle like car and truck. - Simple push power button to turn it on, select the desired...

Might help, will need adaptor making.   K

 

 

[spudulike]

The Gunson gauge is one of the more reliable ones, I did keep blowing the schrader valves on modified saws so fitted a Presta one and it has lasted well. 

A decent 346 should have 170psi.

What was the end gap like? 7 thou is good, below 20 is OK, anything over and it is looking a bit iffy. 

Sometimes you have to tear it all down again and then build it up again to get to where you need to be.

Are there signs of it being over-fueled  such as a wet oily residue, black plug etc. 

The only time I have come across this sort of issue is with saws with excessive top end wear, the last one was an 020T that was doing exactly the same thing, couldn't get the revs above 12k even with the H screw turned in!

It is possible the HT cap is leaking down making the high revs miss or the carb having a leaking check valve or the coil being like your 361 one but that was a limited one.....weird one but my diagnosis head still comes back to engine wear.

Stick a new piston in it😉

[bmp01]

On 05/12/2020 at 08:03, billpierce said:

Kinda lost faith in my gunson, it constantly reads low - even on saws that run perfect. Had the top off no real sign of wear, decent Mahle piston and pot, ring doesnt look worn. There was a bit brown residue on the piston which I have cleaned off, deleted base gasket and put back together,m - sadly no different.

I have ordered meteor piston to try

Think you've done enough engines by to know what to look for, residue on piston ie combustion gases passing the rings never a good sign.  If the piston is unworn ie. still showing turning marks on the skirt and rings are unworn  maybe the ring grooves are choked up with crud ? Piston ring does some sealing on its own but it also needs the combustion gas to get on top AND behind the ring to force it onto the cylinder wall. Obviously if the ring groove is too tight (piston made wrong or clagged up) - then that doesn't happen.... result is blowby past the ring. Swaping in new piston is obvious solution.

 

Is the cylinder bore shiny ? Might be worth a gentle deglaze ?

 

As an aside, ever heard of ring groove porting? Several small drillings to get combustion gas to the back of ring. Drillings in top edge of ring groove only.