Cleaning carb on smoking Stihl

[spudulike]

Metering arm - get a rule and lay it over the flat surfaces the diaphragm/gasket sits on, the metering arm should be 0.5mm - level with the rule and no higher - get it too high and the needle valve will leak, too low and no fuel will flow through.

[bmp01]

9 hours ago, coppicer said:

This one feels like plastic, not rubber (or metal). Will have a poke around.

.... plastic not metal.... fair enough. Its just to stop the rubber manifold from collapsing in under closed throttIe vacuum.

[coppicer]

No improvement. Might be running a tad longer.

[coppicer]

On 02/08/2020 at 11:23, bmp01 said:

-main jet one way valve not doing is job,  happens if an air line has been used to clean main jet. Check by bIowing each way through the jet with a piece of soft tube.

 

This is probably the next stop. If the main jet isn't doing its job - and yes, I did blast the external surface of the carb with air, but not the valves directly - is replacement the only way to fix it?

[bmp01]

Blowing outside of carb clean is fine, to be expected on a well maintained saw.

What some folk do when blocked internal passageways are suspected is to whip the adusting screws out, pop the airline  nozzle tight against the vacated hole and give it "what for" ..... that's not the best thing do.

 

So, this is what I've done to test main jet one way way valve: Carb off, metering cover off, high speed screw out. .... soft piece of hose (silicon is ideal but small bore fuel hose is ok) over the adjuster screw hole, now health and safety aside, blow through the hose. You will find the fuel feed hole (below the diaphragm) let's air escape so figure out which one it is and block that with a finger (from your pictures there are 3 holes I think you want the one without a brass jet, but could be a second one too).  Now,  blowing and sucking through hose should allow air in but the one way valve should seal and stop air returning. ...

 

Aternatively attach a hose through the middle of the carb and onto the exit end of the main jet, same blow suck routine but expect results to be t'other way round. I suspect this approach is neigh on impossible due to size of the carb, unless you want to remove the butterfly valve...

 

Alternatively 2, to be 100% sure, remove the jet and test out of the carb. Its just pressed in, pretty easy to drift it through into the middle of the carb with an appropriate parallel punch thats in good condition. Hold the carb in soft jaws in a vice and take care.... I've heard of someone pressing the jet out on a pillar drill using it as a press. NB I've done this on a 211 carb, pretty sure there is just enough space for the jet to come out into the centre on the carb....

 

Main jet has its own Stihl part number and is completely replaceable, go by your pictures for how far to press it back in.

 

Now, I know Spud is going to hate me for going on about accelerator piston getting shagged but it happens way too often IMHO. IF the accerator piston is shagged its a source for air to get sucked into the meterring side of the carb. The accerator circuit feeds the mainjet too, so if its shagged that's going to throw your result in above first way of testing one way valve. Anyway, lets see how you get on.

Edited August 7, 2020 by bmp01

[spudulike]

18 minutes ago, bmp01 said:

Now, I know Spud is going to hate me for going on about accelerator piston getting shagged but it happens way too often IMHO. IF the accerator piston is shagged its a source for air to get sucked into the meterring side of the carb. The accerator circuit feeds the mainjet too, so if its shagged that's going to throw your result in above first way of testing one way valve. Anyway, lets see how you get on.

The only thing I would say is that drifting the check valve out is sort of simple but it is fairly unlikely to be the issue. It gives a saw a real lumpy pickup and massively over rich running - I do around 2-4 a year and isn't a common issue.

On the accelerator pump....I actually agree with what you say 100%,  if the C1Q carb is over 7 years or very heavily used then yup, it may give issues and I replace them as any decent tech will rather than replacing the carb and charging the customer £130 for the techs lack of skill!

Each saw reacts differently to a bad pump, MS200Ts have a completely unstable idle, rising high and then dying, MS201s do run with dodgy pumps but generally I have found that they are stuck rather than worn giving a flat unresponsive lag to the throttle.

Replacing the pump and check valve is relatively easy after you have done 2-3 carbs but they aren't what I would call easy for the uninitiated and can go completely wrong!!!

What is the current state of the saws running, this banter has been going on that long, I have lost track of where we are now

[coppicer]

32 minutes ago, bmp01 said:

Anyway, lets see how you get on.

Christ. The pressure! ?

[coppicer]

2 minutes ago, spudulike said:

What is the current state of the saws running, this banter has been going on that long, I have lost track of where we are now

Diaphragms, needle and sundry other bits replaced. I adjusted the metering arm, which was OK but possibly a fraction low, and reseated the fuel filter so it was no longer skewed. Then I did the thing with the idle screw that you mentioned to just fractionally open the brass plate. Reassembled. The saw will fire and idle better and longer than it used to, but dies if I rev it. Further messing around with screws has achieved nothing useful, so I reset them. Back to square one-ish.

 

I was wondering if it could be something other than the carb, but the saw seems to rev fine on initial startup - there's nothing obviously wrong with it to my ears.

 

 

[spudulike]

OK, call me a loon, a freak, a dipstick, master of the universe but.........just try winding the H screw out a couple of turns and try again. The symptoms don't strike me as being accelerator pump or check valve unless the valve is stuck shut!!

[Khriss]

On 31/07/2020 at 20:26, frammo said:

just a shame she was holding an assault rifle (with grenade launcher) and not a 357xp?

You need to calm down ?  K