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bgsengineclinic
bgsengineclinic, Small Engine Technician
Category: Small Engine
Satisfied Customers: 220
Experience:  Over 20 years of experience in Small engine repair, Kohler Certified Expert Technician, Self-Employed in my own dealership.
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have 026 stihl chainsaw fires up but when itake it off choke

Resolved Question:

have 026 stihl chainsaw fires up but when itake it off choke pull throttle it revs up then dies
Submitted: 4 years ago.
Category: Small Engine
Expert:  bgsengineclinic replied 4 years ago.

bgsengineclinic :

Carburetor needs to be adjusted - if it has limiter caps, you need to have it done at a professional shop that has the proper tools to adjust the carburetor to EPA specifications. However it is possible you also have an air leak - these have a rubber intake boot that sometimes can be torn, or the steel boot support insert is missing. Can also have a leaking crankcase gasket or leaking crankshaft seal.

bgsengineclinic :

basically it is running lean and needs more fuel - usually if it is a new carburetor, you'll more likely have an air leak problem - if fuel lines and filter are new, and carburetor is new, it's likely got a crankcase air leak somewhere - pulse hose, crankcase or cylinder gasket, crankshaft seals, or intake boot isolator, among other locations that can leak.

bgsengineclinic :

Would have to block off intake and exhaust and test with a crankcase pressure tester to check for air leaks, requiring special tools, and if it is an emissions compliant machine, carburetor adjustment requires specialized tools as well.

bgsengineclinic :

based on your symptoms, however that sounds almost classic case of leaking intake boot - what happens is it leaks air and goes lean - choke keeps it rich enough to run, but as choke comes off, it goes lean and revs up until it runs out of gas.

Customer:

ye idid adjust carb idid compression test it tested great .

bgsengineclinic :

compression test is not same as a leakdown test

Customer:

ok so it still may have air leak

bgsengineclinic :

leakdown test checks crankcase seals, gaskets and intake boot for air leaks - compression test won't check that. it requires steady crankcase pressure pulses to pump fuel in the carburetor

bgsengineclinic :

yes

bgsengineclinic :

leaking seals or gaskets it will usually start and run fine and then run out of gas when revved up

bgsengineclinic :

leaking intake boot is more of a match to your symptoms

bgsengineclinic :

other possibility, but remote, is plugged exhaust , but that acts a little different - more like it will idle but when revving up it stalls out

Customer:

i checked exhuust is good

Customer:

so i should start with replacing intake boot?

bgsengineclinic :

also if you can start and run at idle just fine, but it dies when you pull the trigger, may just be simple hesitation and idle side of carb needs to be adjusted richer - but, if it is like on throttle lock position, it starts on choke, and you take off choke, it then speeds up by itself and dies out without touching throttle, that's classic air leak in intake

bgsengineclinic :

you can check the boot by removing carburetor, check for the steel boot insert support, then use a finger to probe the boot and feel/ look for any tears or cracks in the rubber boot

bgsengineclinic :

may have to pull handle away from crankcase a bit to stretch the boot a little to find the crack or damage

bgsengineclinic :

or loose boot clamp where it clamps to the cylinder

Customer:

i did look at the boot it seemd fine

bgsengineclinic :

in a pro shop like ours, we will block off the intake and exhaust, plug the pulse hose, and pressurize the cylinder to 7 PSI through the spark plug hole, and then squirt soapy water over everything to locate the air leak - it should hold 7 PSI for 2 minutes

bgsengineclinic :

if it does not, it has an air leak.

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