small engine: carburator confusion

I have a Stihl FS36 trimmer, which has a Walbro diaphram carb.
It starts OK (after priming), but only runs for 10-15 secs before rapidly losing power and dying. After it dies, I can restart it (after pumping the primer bulb), and it does the same again.
IMO, it acts like it's not getting gas. There's no smell or other evidence of it flooding. But, (a) pumping the primer bulb while it's dying doesn't seem to help; and (b) when I take the carb apart post mortem, there's gas in the chamber under the diaphram. Also, when I operate the needle valve, gas flows up into that chamber. So, the gas seems to be there.
Any thoughts would be appreciated.
Thanks, George
(NB: I have removed the spark suppresser screen.)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

First make sure the gas cap vent hole is open (not clogged). If that doesn't do it, blow air back thru the fuel line to the tank. There is a small filter in the tank, you may be able to use needle nose pliers and pull the tube out of the tank and check the filter that is at the end of the tube.
Hank
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Did you ever leave gas in it like overwinter, My fs 36 got like that so I had the carb cleaned after leaving gas in it one winter.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Jet plugged.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 05 May 2009 22:09:49 -0400, snipped-for-privacy@snyder.on.ca wrote:

I wondered a little about that. But, during its brief burst of life, it seems to have full power, and good throttle response. Would you still think the jet?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

the crab is fouled......usually from leaving gas in the unit
BTDT with a Husqvrana chain saw...exhibited the the same behavior.
not getting gas, runs that short spurt on gas "primed" in
clean the carb, it'll work
I learned this the hard way, paid to have the card "rebuilt"
cheers Bob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Sure sounds like it. Carbs usually have several jets. One for idle, one for midrange and sometimes more. Your mid-range jet is very likely clogged. Enough gas dribbles past it to make the engine rev for 10-15 seconds, and then it starves. It's the same performance you would get by sqirting some ether in the throat and then starting it. As soon as the ether burns of, the motor dies.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 05 May 2009 22:09:49 -0400, snipped-for-privacy@snyder.on.ca wrote:

I just flushed it with Gumout, and that seemed to do it. Thanks to all.
G
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Your carb has 2 jets. One is the pilot jet which controls rpm's from idle to 1/3 throttle, the other is the main jet which controls rpm's from 1/3 to full throttle.
If it runs for 10-15 seconds, we know you have compression and spark, and spark timing. Therefore, the only thing left to check is Air/fuel mixture. Most motors won't run 10 -15 seconds from fuel delivered by the prime bulb, but could I guess. Most only run 3-5 seconds at best.
Have you tried adjusting the needle jet screws (one is for the pilot, one is for the mani)?
If you have checked the gas cap and lines going to the carb and are sure they are clear of any debris, then try adjusting the needle jets. To start, screw them in all the way, then back out 2 turns, start it up and then adjust the pilot jet at idle speed and the main jet at full throttle.
Hank
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Site Timeline

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.