# Help with Stihl leaf blower



## Weesa20 (Jun 4, 2014)

Got a Stihl BR 340 I got off ebay for repair. Put in a new set of rings as it had no compression, piston and cylinder are perfect. Problem, is I haven't been able to get it started and it fire everywhere except in the combustion chamber...out the carb and out the muffler, and it won't really even "pop"...so I thought, bad timing- changed out the ignitionmodule, no change. Looked at the flywheel- looks like the timing marks are hitting at TDC. Changed out 3 carbs- no change. Changed out muffler, no change. Has spark. Plug is always fouled with raw gas, but I think that is from pulling on it without firing.

The real kicker, though, is that I just rebuilt a BR420Mag from the crank up and it runs great but has a knock like the con rod is coming out the bottom end....ran 3 tanks at full throttle though it with no change and I can't find the knock...this other one spins as smooth as can be and I can't get it run....go figure.

I've built a lot of these in much worse condition, many from the crank up and have always gotten them to run, but this one has me stumped...the only things I can think is that the timing is still off or the second ignition is bad.... (when I have time, i'll pull the carb and iginition off the runner)...any other thoughts?


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## AVB (Jun 4, 2014)

Try a different spark plug. Plugs can misfire under compression. Also check the compression as you will need at least 100+ PSI for it to hit.

As for the one with the knock are sure it not hitting the top of the cylinder. You just need to add a second jug gasket to get clearance right.


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## Weesa20 (Jun 5, 2014)

Used a known good plug so I don't thinks that the problem- I think compression is okay with new rings and good piston/cylinder.

I'll check the other one out, too, but the knock is very noticeable with compression and not audible without the plug in- if it was hitting, I would think it would be the same with or without the plug in place- and I don't see any damage on the top of the piston so I don't think it is piston/plug contact. Used a new AM crank, bearings, seals, new crank bearing but reused the wrist pin as I didn't see any wear and it fit tight. 

I think it may be piston slap as the skirts are a little worn- don't know what else it could be.


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## AVB (Jun 6, 2014)

I am still fairly new at this 2 cycle stuff only been fixing for 5 yrs now. I still see strange problems now and then. The 340 stills sounds like compression problem with the raw fuel in the cylinder.

On 420 it could be the piston slap under compression. I just don't know here. Is the knock there only at low speeds or is it at all speeds. I just trying figure this one in my head as I like to know what causing it too as with my luck I will run into one like it soon.


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## Weesa20 (Jun 6, 2014)

Yeah, I appreciate being able to bounce ideas around...I agree with the compression problem on the 340...gonna go with a new top end and see if it fixes it since everything else has been changed out (I changed the carb off the runner with the non-runner and the runner still ran and the non-runner still didn't run, so not a carb issue.) Have great spark- the only other thing I'm gonna do is pull of the flywheel and make sure the timing is correct- could have good spark but if the timing is off, it would look like this and would explain backfiring through the carb and muffler- and since the fan is bolted to the flywheel, it could have shifted under load, I suppose (or the previous owner messed with it and didn't get it right)

Gonna go with a new topend on the 420 as well, I just don't know what else it could be- it is definitely audible at low speed and if I pull it over and it doesn't start or when I shut it down right when it is on its last few revolutions--clunk clunk clunk. At speed it is barely audible and it runs great. I would think if it is something rotational, it would have let go after 3 tanks at full speed. Could be the used wrist pin, but it felt pretty tight with the new bearing and didn't have any vertical movement when I checked it before putting it back together.


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## Weesa20 (Jun 6, 2014)

well, def a topend problem with the 420....not so sure with the 340 now...I took the P&C off the 420 and put it on the 340 bottomend...same knock and it wouldn't start. Thinking it's timing or spark on the 340 now.


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## Weesa20 (Jun 7, 2014)

Well, I was right on both accounts. New piston in the 420 and knock is gone. Flywheel key way was sheared off on the 340, reset the timing and it started in one pull. Darn I hate wasting time...anyway, other thing I learned is that a MS280 piston is interchangeable with BR380/400/420 pistons.


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