Fixed no run issue - Mcc 3616

[ToniG]

I picked up a 14 year old Mcculloch 3616 (Husqvarna) for $10, I needed a smaller saw for fiddly bits.
Replaced perished urethane fuel lines, air filter & a full cleanup & inspect. Looks to be hardly used.
But it would not run well & was starving for fuel. I noticed the fuel tank was very pressurized, after releasing pressure it would run ok.
As the the finger pump drains back to the tank its a closed loop, excess pressure likely affecting the pump diaphram in the carby.
The fix was to make a vent, finding a high point on the tank left side I drilled a small hole & press fitted some PTFE capillary tube 1.8mm OD, It now runs well.
This might help someone with the same issue.

 

[bertsmobile1]

The actual fix is to replace the fuel cap which is probably 2 way vented
The tank is supposed to be pressureized so the engine will get fuel when the tank is lower than the carb. much the same as old outboard boat engine tanks were .
They are supposed to run from 5 psi to 10 psi and the metering diaphragm usually is set to pop off at around 15 psi because most tanks will either split or blow out bungs when they get to 20 psi ( yes I have tested this )
However what you have done should work ok and if you never cut blade high should not be a problem

 

[ToniG]

>>"The actual fix is to replace the fuel cap"
Do you mean my cap has a faulty pressure relief valve & is too much pressure ?
.
I am happy with how it runs now with the tank at atmosphere & no fuel leak.
The carby pump lifts fuel ok from the tank so should not need additional tank pressure to push fuel up the line (the return line via the manual prime pump is to the tank so there is no pressure differential here.)

 

[bertsmobile1]

Yes
The caps are vented to allow air in and hold pressure
The pump is there to pressurise the tank & PUSH fuel into the carb
Volatile liquids do not respond well to pulling
Most pumps actually push
But if it is working then leave it alone
When you are done cutting tip the fuel out start the saw then run it dry and you will not have many problems with it .
When stored long term it is a good idea to slip it in a bag or box to deter mud wasps from making nests in the exhaust .