Stihl BR550 backpack blower issues. Carb?

[canucklehead]

I have a Stihl BR550 backpack blower that is causing me grief. It ran fine last year. I winterize ALL my 2-strokes by dumping out fuel and running them dry. Some will say that's hard on carbs, but it's actually how the mfrers tell you to winterize. In addition, I've had more problems with old fuel gumming up carbs, especially ethanol fuel.

anyways, everything has started up and run well so far, except this BR550. I try to prime and then choke it..several pulls , then half choke..more pulls. Then WOT with no choke and it just starts but not revving. Then I pump the primer bulb like mad and the RPMs pick up. If I stay on it for several primer pumps, it holds the revs and then I can use it. I used it for 30 minutes and it revved well, but only after I did that painful start procedure.

I removed carb and cleaned it in ultrasonic cleaner. The diaphragms seemed okay. One odd thing when I pump the primer bulb, it doesn't seem to fill with gas, yet it increases my RPM. Not sure what that means. My small engine buddy said to replace carb; the fact I have to pump primer bulb to make it run tells him carb is no good. Considering my symptoms, I'm wondering if fuel pump/diaphragms are maybe worn out and a carb kit would fix it. Carbs are about $60, carb kit is $15.

It is a Zama C1Q-S101 carb.

Any ideas? What would you do? Does my primer bulb not filling with fuel make sense?
thanks :thumbsup:

 

[J_J]

The primer bulb has to pump out the air and then it will pump fuel if there are no holes in the bulb.

There are also little tiny check valve in the fuel path.

Check/replace the filter in the gas tank.

Does your exhaust still have the wire screen in the exhaust muffler port. Mine was blocked with carbon. Removed it and it ran great.

 

[Bengineer]

The primer bulb ... if there are no holes in the bulb.

There are also little tiny check valve in the fuel path.

Check/replace the filter in the gas tank.

...

I'm thinking this too - look the bulb over carefully for cracks. ... If you get into the carb to replace the diaphram, etc and squirt carb cleaner into the primer base where the check valve is, the valve will seize up. The plastic parts in the valve swell up when exposed to carb cleaner. Dry it out in the sun & after a while it will work again.

Another thing to check is the welsh plug under the diaphram. I had one blow out.

Zama has exploded parts diagrams, but they're for multiple carbs - it's key to get the metering & pump diaphrams & gaskets back on in the right order. pix when disassembling work pretty good too.

 

[canucklehead]

I re-assembled the way it was, not 100% certain it was correct before, but i think so since it previously ran better.

When I have had dried/cracked/leaky primer bulbs in the past I could feel they wouldn't pressurize or fuel would leak out. I can feel this bulb pressurize as if there is no leak, and no fuel is leaking out. And it also seems to be pumping something to get this to run at higher RPMs.

I believe I replaced the tank filter last year. They typically last quite a long time, and I rarely use this blower.

So would a carb kit fix this, or do I need to dig deep into wallet for a carb?

 

[canucklehead]

JJ, where is the check valve? maybe the fuel is being blocked to primer? It doesn't start well at all when primed and choked. Eventually seems to fire when I WOT & no choke (after trying the choke & prime method first).

 

[J_J]

When you remove the covers to the carb, you should see the check valves.

The primer draws fuel through the check valve, and the check keeps it from returning to the in port.

Excess fuel goes back to tank via another line.

On this carb, the screen/check valve is item # 15,16, 52

Walbro Parts & Service

Link does not work, but look for a BR-420. Could not find the 550

I have rebuilt some of those Stihl carbs, some worked and some did not.

As a last resort, purchased a new carb, and fixed most carb problems.

 

[canucklehead]

looks like it's inside the bulb area. I'm not sure if I played with that when I was cleaning carb.

 

[JimRB]

My limited experience for poor running 2 cycles have always been fixed with a carb kit or new fuel lines. Diaphragms just get stiff and do not pump. Kind of cheap test putting in a kit assuming your cleaner did a good job. Yes I have put together a carb that I thought the old diaphragm looked good but wasn't. Fresh diaphragm/fuel pump gasket fixed it. I do have eight 2 cycle engines in the garage. I think I have done carb kits in three of them in the past 3 years.

 

[Bengineer]

My experience is similar to JimRB's. I don't know if it's our special CA gas or just the prevalence of ethanol, but I've been into the carbs of 3 4-cycle and now 3 2-cycle small engines. I'll likely get into the rest of them before it's over. It seems ethanol is not kind to the rubber & plastic bits in carburetors. Disassembly, cleaning the jets & crannies with carb cleaner and install of new gaskets & diaphrams has made each one run perfectly.

IME, assuming you can get a spark, a really dirty air filter or bad spark plug are about the only other issues and they're rare if you do any kind of maintenance. IME, it's usually the carb diaphrams.

But back to the OP's issue. I think at this point, I'd pull the lines off the primer hook a short one from the intake to a cup of gas and see if it fills the bulb & pumps gas out the exhaust. If not, either the bulb is cracked or the check valve has seized up (see note on carb cleaner above). If it does, then either the filter is clogged or the carb is stopping the flow...