Author Topic: 3400 - Hot start/vapour lock?  (Read 147 times)

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Offline Hoggwood

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3400 - Hot start/vapour lock?
« on: March 23, 2015, 01:11:28 am »
I've a stubborn 3400 that is giving me hot start issues.  It is a low hours saw, base gasket removed, muffler mod, new kit, lines, plug, filters etc. With a cold start it starts and runs really strong.  Metering lever is flush and L an H are 1 1/8 and 1.  I have had the carb apart at least half a dozen times now.

If I  stop and start within a minute or less it will fire immediately.  If I stop to refuel or let it sit for more than a minute it will not start.  Full, half or no choke and many pulls - nothing.  Pull the plug and it is usually dry or just moist.  Rarely flooded.  When pulling there is clearly fuel vapour venting at the venturi.  If I invert the saw and pull many times with plug removed I can get the saw to pop immediately with WOT.  This, though, usually takes 3-4 cycles of doing this.  Little flooding is evident throughout the process.

I am hoping someone may have insight into this and perhaps a systematic diagnosis/solution.  I am continuing to do the same things and hoping for different results. 

Thanks fellows,

Cory

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Offline aclarke

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Re: 3400 - Hot start/vapour lock?
« Reply #1 on: March 23, 2015, 09:58:29 am »
Sounds more like flooding Than vapor lock, vaporizing would tend to block fuel flow? Sounds like yours clears with full throttle eventually?  See if the tank vent is working right,may be pressure in the tank. 

Offline aclarke

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Re: 3400 - Hot start/vapour lock?
« Reply #2 on: March 23, 2015, 12:54:35 pm »
Fuel line would likely be hot to touch ( 140-150f )  if it's vapor locking in the line.  If the fuel line is clear you would be able to see the vapor "blockage"    Is the fuel line in contact  with the cylinder?

Offline 3000 FPS

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Re: 3400 - Hot start/vapour lock?
« Reply #3 on: March 23, 2015, 08:00:17 pm »
I would put another known good carb on it.   
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Offline Hoggwood

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Re: 3400 - Hot start/vapour lock?
« Reply #4 on: March 23, 2015, 11:07:12 pm »
Thanks guys,

Fuel line is run under the carb and in front of the intake block but I should check again.  I adjusted the linkage on the throttle lock to increase the throw, hoping that would help.  There is quite a difference in start RPM using throttle lock when saw is cold versus hot (when it hits).  Odd. 

I will pull a carb from one of my other runners after I run the problem saw again.  I put another new duckbill in and swapped another kit so I will run it through a tank first and go from there. 

 

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