Stihl KM94 RC: petrol dripping out of carburettor into air filter

[Barcombeboy]

Hi.
Suddenly the petrol in my Stihl KM94 RC (2-stroke) has started dripping out of the carburettor into the air filter. The machine still works (although I have had to remove the sodden air filter) but it is getting through petrol mix at a right old rate (and is probably dangerous too). The petrol leaks out even when the machine is turned off, lain on it's back and even when the tank is virtually empty. It's almost like some siphon action is at work. I've looked around a bit and all I can find is that either the meter lever spring in the carburettor has become dislodged (or too relaxed?) or an air vent (on top of the petrol tank?) has become blocked. Neither seem that likely though. The controls seem to work OK and as I say this continues to happen even when the machine is lain on its back.

Has anyone experienced this and know what the problem might be? I've attached a photo in case it helps. It drips every 5 or 6 seconds or so.

Thanks for any help.

 

 

Edited July 6, 2022 by Barcombeboy

[spudulike]

Normally this type of leaking out of the back of the carb is down to the needle valve leaking which may be a faulty needle, weak spring, badly adjusted metering arm or the metering diaphragm has gone hard or baggy.

A new carb kit should do it.

The hot weather can bring this issue on. If you fill your tank in a cool garage with cool fuel and then leave the equipment in a hot van or in the sun, the pressure will build in the tank and show up any weakness in the parts I have mentioned.

On hot days, filling up on site and leaving your fuel and unused kit in the shade will help just watch out for those types that want a strimmer but have no money and don't want to buy one!!!

[Dan Maynard]

Surely quick check on tank pressure is open the filler lid? Does this stop the dripping?

 

Definitely had saws that don't want to restart when too hot, let tank pressure out and off they go.

Edited July 7, 2022 by Dan Maynard

[spudulike]

8 hours ago, Dan Maynard said:

Surely quick check on tank pressure is open the filler lid? Does this stop the dripping?

 

Definitely had saws that don't want to restart when too hot, let tank pressure out and off they go.

Most tanks for build pressure and it isn't always that obvious that the needle is leaking as the fuel can enter the engine rather than flooding the air filter.

Anyway...just service the carb as I said. 

You can check the valve with a pop off tester or Mityvac but won't be 100% conclusive as the fuel tank can build up a lot of pressure.

[Barcombeboy]

Thanks. The engine also fills up with petrol. In fact the carb just seems to be a straight through conduit for petrol to go from petrol tank to air filter, exhaust and combustion chamber.

Have ordered a carb kit. Not the official Stihl one as it's almost as much as a new carb but the one from Amazon for less than a fiver.

[spudulike]

That should sort it. Take a lot of care on how the diaphragms and gaskets are fitted. There are two types of carb but guess you know that...not sure if the same carb kit fits both.

Just make sure the spacer gasket is fitted between the diaphragm with the metal disk on it and the carb body and make sure the thin polyester pump diaphragm is fitted touching the carb body. Make sure the metering arm is adjusted correctly for height and the spring is located correctly and square.

Beware that the gaskets may have been fitted the wrong way round in the past....seen that before!