MS211C Primer circuit

[bmp01]

Hello all, I'm after some opinions and experience please.

 

I've got this MS211C that I'm sorting out - its been seized so has had a replacement engine (genuine parts).  I'm VERY keen to avoid a repeat engine seizure, so have been trolling through the fuel system just incase, although i suspect crap fuel was the root cause of the problem.  That said i did find the main jet one way valve was fubarred so thats been replaced. 

 

Here's the thing,  when i use the fuel purge bulb to suck fuel through the carb i get a surprising amount of air coming through with the fuel, was worse before the main jet one way valve was replaced. 

 

So the question, what's the usual fraction of fuel and air drawn through with the purge bulb on an MS211C? I recon I get 50:50.

 

FYI, I've looked in the carb and can't see any sign of one way valves on the low speed fuel circuit - i think it's impossible to get 100% fuel and 0% air. In the past I thought strimmer engines etc pulled through pretty much neat fuel. 

 

BMP01

[Dan Forsh]

If you are saying that you constantly get the bulb only half fuel half air, then something isn't right. Are you pumping the bulb for the required number of times and still getting 50/50?

 

If that's the case, then I'd think it's more likely a badly fitting hose, but then I'd also expect fuel leaks.

[billpierce]

Does the bulb inflate by itself easily of a bit slowly?

[spudulike]

The fuel is being pulled up through the fuel filter, if the fuel filter is fully submersed in fuel, no air will get pulled up UNLESS the fuel line is holed!!

[bmp01]

On 16/09/2017 at 01:28, Dan Forsh said:

If you are saying that you constantly get the bulb only half fuel half air, then something isn't right. Are you pumping the bulb for the required number of times and still getting 50/50?

 

If that's the case, then I'd think it's more likely a badly fitting hose, but then I'd also expect fuel leaks.

Yep,  can pump the primer bulb as much you like. ...still the same. ...just didn't feel right. 

On 16/09/2017 at 07:26, billpierce said:

Does the bulb inflate by itself easily of a bit slowly?

Inflates by itself and not slowly,  if anything a bit quicker than you'd expect. 

On 16/09/2017 at 09:28, spudulike said:

The fuel is being pulled up through the fuel filter, if the fuel filter is fully submersed in fuel, no air will get pulled up UNLESS the fuel line is holed!!

I started with a part full tank but quickly brimmed it - to rule out air through the filter. Lines all look good. 

 

Thanks for replies - it isn't right is it. ...  

Here's an update:

After much head scratching I've established that the air (that's all the air) is being pulled into the fuel circuit via the jets in the carb. Specifically,  through the low speed circuit. And I confirmed this by temporarily blocking the low speed circuit with bluetack (a 8ugger to clean afterwards !). It's worth noting, the primer bulb sucks from the metering chamber - so you can see how it will suck air through the jets if there is nothing to stop thIs back flow.  Still surprised you can suck so much air through the tiny dia 0.4mm jet though.

Further,  I have a spare (high mileage) carb from a MS171 and in this one there is a one way valve in the low speed circuit (as well as the more obvious main jet /one way valve for the high speed circuit). Now, the MS171 doesn't have a primer circuit which makes me wonder why they bother with the one way valve in the low speed circuit - I can't see it serves any purpose....and pretty sure most carbs for chainsaws don't have it. Parts consistency I suppose. 

 

So,  given the main jet/one way valve was fubarred (see post one) and now it seems the low speed circuit one way valve has gone the same way, I'd be tempted to say the carb has been 'cleaned' with an air compressor or maybe the engine has been 'popping' back through the inlet (due to the  engine seizure)....  who knows?

 

Looks like new carb time or just live with it. 

 

bmp01

 

 

 

[billpierce]

Does it run ok? My bg55 sucks v slowly and often has a bit air in bulb. Runs fine and fuel lines are fine and starts fine. Been like that a year or so.

When I did carb kit on it one of the diaphrams was lacking a hole on the edge to allow the bulb to fill. I punctured a small hole and it fills better now. If a little slower that it should.