Stihl br340 backpack blower spits back out the carburetor – page 2 – smokstak gastritis


2 reasons for spitting out the intake on a 2 stroke. 1) restricted exhaust, which you seem to have not found an issue. 20, poor cylinder seal, wether it be bad rings, poor piston fit or wear. The fact that the cylinder is polished, seems to point to wear. Only 30 PSI seems a bit low.Perhaps just a glaze break and a new set of rings may set you up.

Try this: With the saw assembled, grasp the starter cord and try to pick up the saw by the cord. if you pick it up off the ground, and it stays suspended, then there is another issue. If it mushes thru compression, and drops to the floor, then you have your answer. If you put some SAE 30 oil in the cylinder and turn the engine slowly thru a couple of revolutions to spread the oil, then repeat the test and it remains suspended, you will have confirmed poor cylinder seal. If you look in the exhaust port, and move the piston up and down, with the rings visible in the port, do they move instantly with the piston, or do they hesitate? If the latter, poor ring groove condition.

How fresh is your gas? if more than 30 days old, it will affect the engine performance. Correct fuel/oil ratio? Running too lean or rich on oil is a no-no on modern saws. Are you using the manufacturer’s oil or a generic? sometimes it makes a difference!

3. Gas is fresh. I have always been very liberal with oil in my mixes. I think this one calls for 20:1 and I usually shoot for something around 16:1. I use Wally World house brand 2 cycle oil that comes in a gallon jug. Never had a problem with anything that was traceable to oil. All of my machines have fully adjustable carburetors so I am not worried abut running lean because oil displaces gas in the fuel supply. And I always run my 2 cycles a bit rich anyway.

4. Motomowers commented that he has a two cycle machine that has spit back through the intake (as mine, I assume, NO flames or excitement) from new. This sounds inherently wrong but then I recalled a McCulloch 6 cubic inch saw I have that came with some sort of foam collar around the carburetor inlet. This thing has some sort of drain on the bottom. It is factory installed. Now why would they add something like this if not to catch the blown back gas? Still sounds wrong, but I have no other explanation.

5. The engine seems to have very reasonable compression on the top end. Certainly I have run engines, both two and four cycle, with worse. The compression is not enough to yank the cord from your hand but I do have to brace the machine securely to start it. And this problem had a relatively quick onset. Worn out cylinder/rings/piston would have caused problems of one sort or another for some time now. Crankcase compression, tough, seems uninspiring at best.

Next I plan to do two things. One, I will test the compression gauge in 5 psi increments to see if the thing is capable of working properly at all. I know, I have an unlimited air supply in my tank but at least I will be qualifying the gauge. I will then try the gauge on some larger displacement engines (like a Wisconsin AHH) to minimize the volumetric distortion caused by the gauge and its hose. I’m not sure why I will do this as I thought I was trying to fix a leaf blower but that’s life I guess.

Next I will set up a regulator at something like 5 PSI then bring the crank down to BDC and hold it in place while I apply the pressure to the pulse port, then see where (if anywhere) the pressure leaks out. I don’t have a feel for what pressure on the crankcase side should be so I will be gentle to keep from blowing out the shaft seals. I think this test ought to tell me something.

If I was forced to pick a likely part to replace right now, knowing no more than I do, as the most probable cause of the problem I would have to guess at a crank seal. I know, I said I did not see any evidence of leakage (wetness) at either end of the crank but I might not if the seal failure is recent.