What's on your bench today?

[bmp01]

Thanks for the extra info openspaceman.  Seems like fuel line "rot" is the common theme. I wonder if it had some nasty fuel left in it, maybe followed up with alcylate fuel when it wouldn't run right...

[openspaceman]

5 minutes ago, bmp01 said:

Thanks for the extra info openspaceman.  Seems like fuel line "rot" is the common theme. I wonder if it had some nasty fuel left in it, maybe followed up with alcylate fuel when it wouldn't run right...

I don't know but I also replaced the pipes to the bulb as they were a bit stiff.

[openspaceman]

17 hours ago, openspaceman said:

I don't know but I also replaced the pipes to the bulb as they were a bit stiff.

Part arrived as per L&S usual prompt service, and fitted, much better  but air still getting into bulb and hi running erratic,  it's definitely fuel starvation as saw runs fine with finger over the stratification throttle body. It makes me think the fuel hose was were degraded and broke when I took the carb off.

 

Normally at this stage I would swap carburettors but don't know of another ms181c I can borrow off.

[spudulike]

What happens if you unwind the H screw more and more? Does the saw ever do what it should and the revs die back and go flubby as the engine gets saturated in fuel?

Did you check the gauze strainer? They often look good but when you take them out and look at them under a magnifying glass and a backlight, they are blocked.....had MattyFs MS200 in for repair around three times until I found that and learnt that lesson many years ago!

Did you lift the metering arm a bit? 

The primers usually have a bit of air in them from my experience. Are you sure you got the pipes on the correct way round? 

[openspaceman]

1 hour ago, spudulike said:

What happens if you unwind the H screw more and more? Does the saw ever do what it should and the revs die back and go flubby as the engine gets saturated in fuel?

No it never four strokes and  it misfires, almost like there was a bit of crap bouncing around in the jet but it's clean.

1 hour ago, spudulike said:

Did you check the gauze strainer? They often look good but when you take them out and look at them under a magnifying glass and a backlight, they are blocked.

I did but i can look again with a magnifying glass,

1 hour ago, spudulike said:

 

Did you lift the metering arm a bit? 

yes a bit to bring it up to level with the aluminium case and I do suspect this area is where the problem is because the purge bulb is sucking air back from the main jet, I think this can only be because there is a restriction getting fuel past the needle jet. How high above do you suggest I risk?  I only fitted a new diaphragm/gasket/ pump kit, no needle and fork  with the kit.

1 hour ago, spudulike said:

The primers usually have a bit of air in them from my experience. Are you sure you got the pipes on the correct way round? 

Yes I know and yes it is sucking fuel plus air  from the carb and pumping it to tank.

 

I wish I had ordered a new spark plug (as I don't have any small ones as all my other saws use 14mm) with the other bits just to be sure it's not that contributing to the misfire but the fact that it runs well with the stratification port blocked still suggests fuel starvation.

[bmp01]

Back ground  info - did you tell us the age of saw (sticker on brake handle will say)?  Also, what's your perception, is it a high mileage dog? Hardly used ? etc

 

Can you see the fuel line while running (to see if air is coming through during running), could be the air is only due to purge bulb use ?

 

[openspaceman]

2 hours ago, bmp01 said:

Back ground  info - did you tell us the age of saw (sticker on brake handle will say)?  Also, what's your perception, is it a high mileage dog? Hardly used ? etc

 

Can you see the fuel line while running (to see if air is coming through during running), could be the air is only due to purge bulb use ?

 

I'll check in the morning, it looks in good condition, it was part of a friend's estate and was in a collection of other non working machines. I replaced the bulb some time ago and the saw had not been used for 4 years but never tried it properly as I had no bar and chain. I got it back when a new bar and chain was fitted and it would not rev up.

 

So far the only one that has defeated me is the FR480c so I'll persevere.

[spudulike]

OK, the fact the thing works and revs out by reducing the air in to the engine which I guess is under no load suggests not enough fuel is getting through at high revs. I also guess if you put the saw in to wood with the strato blocked, there would be no power.

SO......you are lacking fuel.....if you turn the L screw out more than you normally would...does it rev out better?

This is sounding like your check valve is stuck shut and you are getting no fuel through.....you need to pull the H screw out and spray some carb cleaner down it and see if you get a good spray out of the check valve - easy enough to see. 

[openspaceman]

10 hours ago, spudulike said:

OK, the fact the thing works and revs out by reducing the air in to the engine which I guess is under no load suggests not enough fuel is getting through at high revs. I also guess if you put the saw in to wood with the strato blocked, there would be no power.

Not tried putting it into wood with strao blocked, maybe when it goes back together.

10 hours ago, spudulike said:

SO......you are lacking fuel.....if you turn the L screw out more than you normally would...does it rev out better?

This is sounding like your check valve is stuck shut and you are getting no fuel through.....you need to pull the H screw out and spray some carb cleaner down it and see if you get a good spray out of the check valve - easy enough to see. 

I've done the carb cleaner down the main jet and taken the brass main jet out and it is clean. I thought the check valve was only on the low screw circuit. I'll re check the strainer this afternoon.

 

@bmp01 it's a 2011 saw.

[bmp01]

3 hours ago, openspaceman said:

Not tried putting it into wood with strao blocked, maybe when it goes back together.

I've done the carb cleaner down the main jet and taken the brass main jet out and it is clean. I thought the check valve was only on the low screw circuit. I'll re check the strainer this afternoon.

 

@bmp01 it's a 2011 saw.

No,  that's wrong.  Check valve is typically only on high speed circuit, as part of the high speed (or main) jet. So while the jet is out make sure you can blow through it in one direction , as per the fuel flow , and not in the reverse direction.

Low speed circuit doesnt have a check valve but sometimes there is a one way valve on the whole fuel circuit ...I think....

 

Edit, the logic for check valve in high speed jet is ironically for idle operation.  It stops air going back down the high speed jet, passing through the fuel circuit and out the of the low speed jet (coz that would surely cock-up low speed fueling). If you think about idling and  the pressure drop across the butterly (or throttle) valve you can see:

- engine side of butterfly is at a depression

- airbox side is at ambient pressure

That's the driver for air going the wrong way through the high speed jet. 

Edited January 13, 2022 by bmp01