chainsaw mystery, give it a try...

[Rubicon]

Hi there. I have a Husqvarna 51 chainsaw that will refuse to run properly. It seems like it is getting too much fuel, as raw gas spews out the air intake. I cannot get the revs up at all even at WOT and as soon as I let go of the throttle, the saw instantly dies. I have tested compression(fine), spark(fine), flywheel key(fine), air leaks(fine). The carburetor also does not appear to be the problem. I take pride in not being a "parts changer" as I love troubleshooting and finding the problem myself, but alas, I have been forced to become one in this instance. I have taken the following from a perfectly running Husky 51 to no avail. Carb, exhaust, flywheel, ignition coil. The piston and ring are in perfect shape as are the fuel line and filter. This is truly a mystery to me. I have worked on hundreds of saws with great results, but this one has me totally confused. The only thing I can think of at this point is a twisted crank. Is this possible? Any ideas guys and gals?

 

[tree monkey]

i think you need to look at the piston again. sounds like the intake side of the piston skirt is worn.

later scott

 

[Rubicon]

The piston is fine, as is the ring and cylinder. Just for fun, I swapped pistons with the good unit and got the same result. Man, this is a tough one. Thanks for the thought.

 

[Jake]

random thoughts

Have you tried fresh fuel mix? Different plug? My repair manual shows a governor in the carb. that dumps fuel to prevent over speed?

 

[oakman]

rubicon, you say you checked for air leaks. how? if it had reed valves, it would be a broken reed. but i know it's piston ported. i'm betting it has a leaking crank seal. the only way to check is to remove carb and muffler, make test plates to block off exhaust and intake ports. one of these needs a hole or some means of injecting compressed air into the crankcase. i simply drill a 1/4'' hole and use a rubber tip blowgun to put around 10psi max in. you can also remove spark plug and use the hole. do this with the piston at bdc and hold the blowgun in place while listening for air leakage. if you have the clutch and flywheel off you can spray the seal area with wd40 or soapy water or what have you. this will show tiny leaks. also a vacuum test should be done, but you need some way to apply vacuum, such as a mighty mite vacuum gauge kit for automotive use. usually though the pressure test will reveal the bad seal. or it could even be the crankcase halves leaking at the joint. 2 strokes are like air pumps. the crankcase/cylinder assembly must be airtight to perform properly. good luck.

 

[Gypo Logger]

My first thought is that the carb may be varnished up and the needle isnt seating and/or jets are obstructed etc.
In its rich condition, seat both needles till it runs out, then reset and try again, assuming metering diaphram is not stretched.
John

 

[G-MAN]

Take a look at your impulse and make sure it isn't blocked or clogged up.

 

[jbrody44]

My 55 was doing something similar and it turned out to be a leak in the boot that goes from the carb to the jug. It is the small black oval shaped rubber boot that attaches the bulkhead assembly or "carb mount" to the intake side of the jug. -JB

 

[Crofter]

Rubicon:
Was that saw at one time running properly and the went sour or did you inherit it in that condition without knowing its history. I am wondering if someone tried some port modification or there is a mismatch with the piston and jug.

Frank

 

[tree monkey]

seen you went as far as changing the piston, change the cylinder too. if it runs then its the cylinder. check where the magnets are at tdc on both saws to check timing.

and look at the intake port to see if its been opened up.

good luck

later scott