OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Chainsaws and Concrete Cutters Petrol Chainsaws and Concrete Cutters Chainsaw bogs on full throttle

Hi guys. Picked up a little MTD saw from the tip for free. Replaced the primer bulb and fuel lines, which fell apart in my hands, and it is now a runner but bogs down on full throttle. It will take full throttle with half choke on but adjusting the H fuel mix doesn't appear to make any difference. Would an air leak at the carby gasket be the most likely problem?, or should I look for something else?

It's not likely to be the carburetor to insulator gasket aussietrev, because that has its greatest effect when the intake vacuum is highest, and that is at idle. Because the H needle has no effect, the problem is lack of fuel supply, so the problem will be a clogged fuel pickup filter, cracked fuel suction line, defective fuel pump diaphragm, or a blockage in the mesh strainer in the pump chamber. You should attend to all of those anyway, before you give it a clean bill of health.

Thanks grumpy. I fitted a new pick up as part of the replacing of the fuel lines. Guess its time to pull the carby down and check the diaphragm and mesh.

I would lay odds on it being a plugged strainer or clogged H speed circuit. Metering diaphragms usually gives more trouble at low speeds doe to stiffing. If not then crankcase vacuum test for bad seals or could be a bad ignition coil. Usually when I get in a 2C that needs fuel lines or broken built in primer bulb I pull the carburetor, disassemble it to the cube (removing mixture screws) and toss into my ultrasonic cleaner while I am replacing the fuel lines. If gaskets and diaphragms are still in good condition I reuse them if not I install a carb kit. To me it just waste of time not go ahead and clean the carburetor while I at it for if I don't I usually end doing it anyway. Don't forget to set metering lever to the correct height.

Pulled the carby off and removed top and bottom plates and mixture screws. It was spotless inside and no holes in the diaphragms. I think it is a WT/WA Walbro from the reference pdf. So I checked all pasages and made sure every thing was clear, put it all back together and set the screws at out 1 turn low and 1.25 high. Now it will rev its little head off but I cannot get it to idle. The screws did not appear damaged or worn in anyway. Any advice on tuning or possible lead on what I might have got wrong?

Why did you set the idle mixture screw to 1 turn? For the WA, the manual says start with both idle and high speed needles set at 1.25 turns. At that adjustment the H needle should be somewhat rich, and when you tune it properly you'll probably end up at about 1.125 turns if the metering lever height has been set accurately.

Getting those little engines to idle is an art, especially if you have the bad luck to have an engine with a slight crankcase leak, though in that case it will probably be quite difficult to start as well. Just set the idle speed high at first, and follow the instructions. These are for the WA:

Walbro are normally preset at 1-1/2 both L and H and fine tuned from there. First make the metering lever is set at the correct height. The W tool makes it easier but it normal even with the small valley it retracts into for the WA/WTA/WT carbs.

Set the L mixture to best idle speed plus a tad richer then adjust the H speed for best top speed but don't exceed max rated rpm. If it wants to do this enrich the fuel to limit the top speed.

Also Walbro has both IPL and service manual for their carburetors at http://www.walbro.com/partsservice