Makita DCS540 and 520i -- sudden hard starting with kickback

[Molecule]

I have two Makitas, a 520i and a 540. From the beginning they were very reliable at starting, often on the 1st pull, even on old gas after a few months non-use. I'm pretty sure I had rebuilt one or both them, with new pistons and rings, and possibly new seals for one of them. (Sorry, but I can't remember if I replaced seals on one of the 520/540s, or on an older 6800.)

Now when I try to start either of them, I don't get that old smooth easy pull of the chord. As the piston turns over through the first compression, it hauls off and gives me a kickback that literally either (a) violently yanks the starter handle from my hand, or if I muscle through the "strike" using quite a LOT of FORCE (which I don't like to do), it still doesn't want to start.

They eventually start, and when they do, they run just fine. Like before. But, wow, what a change on that first pull. Could it be the fuel? Also, I've also been using up some of my older 2-cycle oil mix, with the old 32:1 ratio. (Makita specs call for 40 or 50 for synthetic.) Could it be I need new seals? Or maybe the fuel pump in the carb needs rebuilding (damage from ethanol fuel?). Maybe a thicker gasket at the base of the cylinder, between cylinder and the case body to reduce compression? I had removed the cylinders several years ago (holy yikes! decades ago?) to replace pistons and the rings. I have no clue where to start.

When either saw kicks back on that first compression, the kickback is so forceful, I'm actually afraid of bending or breaking the starter dogs that engage the flywheel.

Once these saws are started and running they are the nicest re-starting saws I've ever used. After a short break, a tiny little 8-inch pop on the chord is all that's needed. It's actually pretty impressive. But now, let her sleep over night, and wow -- this new kickback thing is mean as hell.

What could be causing this?

 

[Roll Tide]

My first thought would be flywheel key missing or sheared but with it running fine I'm not sure. I'm sure someone with much more knowledge will be along soon.

 

[alexcagle]

Weather is heating up. Maybe your saws are seeping fuel past the needle that enters the crankcase and is causing some hydraulic lock?
It's odd that both are doing it at the same time.
Does the place you store them get really hot during the day, causing a buildup of pressure in the fuel tank that is higher than what the carb needle can handle.
At night when it gets cooler, the air and fuel shrink inside the tank creating a vacuum that causes the tank vent to let air in.

The higher the fluctuation in temperatures, the more likely the fuel vent, if holding higher pressure than the needle can handle, pop off to relieve the pressure.
If the piston is up and the intake port is open, will go into the crankcase.
I would try some new premium fuel first and see if that helps. If it doesn't help you might try dumping the fuel and run the saw until it runs out of fuel. Next time you use it, put fresh fuel in and see if that makes a difference.
Could your carbs need kits too?

 

[Molecule]

belated thanks to Roll Tide and alexcagle!!

Nice to learn about the consequences of fuel pressure fluctuation. I'd been using regular gas 89 octane 10% ethanol, and that with temperature changes may have been the problem. Once the saws warmed up and pressure in the fuel tank equalized, they ran normally. I also pulled the flywheels on both and cleaned up all the exhaust oil+dust that gets behind them. No damage to the keys or keyways. I did notice that the keyways are slightly wider than the keys, so when I replaced the flywheels, I used the slop to rotate the flywheel a tiny bit to retard the spark, maybe about 1-deg or so. Also when replacing the ignition coil, there was some slop as well, so I also "rotated" that too so as to slightly retard the spark. Probably combined effect is 2-deg or less.

I switched to the ethanol-free puregas from Home Depot, premix and higher octane (92?) and -- $20 per gallon! Ouch! (Otherwise I'd have to drive 120 miles to find a rural gas station that carries ethanol free. It's like Homeland Security won't allow sale of real gas or something in urban to suburban areas.) They both are starting normally now.

@alexcagle -- where can I learn more about "hydraulic lock?" Interesting that the tank vent both lets air in on cooling or as fuel is consumed preventing formation of a vaccuum, but, it also lets off internal pressure if it becomes excessive during storage. I take it that if the tank pressure rises (yuk to the ethanol fuels) and the pressure seeps past the control needle in the base of the carbs (the 520i has a Tilloton HU with injection choke, the 540 has a Walbro WT with valve choke), and if the piston is up high enough to open the intake port, then you can get a liquid fuel flow (not aspirated or carburated) into the lower unit of the saw. The raw fuel in the lower case would then raise the gas pressure below, such that when trying to first start the unit, and the piston moved down, allowing intake port to connect the lower unit to the piston cylinder area, the cylinder would get in effect a super charge, litterally causing something like a hammer lock -- just WAY too much compression -- the increase in compression during startup only actually had me thinking of adding an extra gasket between cylinder and lower unit.

Did I get the "hydraulic lock" correct? Also, if the piston is up, then as the piston moves down it will start to hyper compress the lower unit, which is already under excessive pressure from the evaporation of a few drops of liquid fuel.

My air filters had gotten old, and I once made the mistake of cleaning them in fuel. Bad mistake. I also finally got around to replacing them, which took a while.

I haven't rebuilt the carbs in about a decade now, so that's my next step. Plus, both fuel lines had burst, dried out, cracked, leaked, whatever, and I had fashioned some replacement lines from available parts. The Dolmar fuel lines are soft rubber, easily expansive. I replaced them with standard hard plastic fuel lines, from a local Stihl saw dealer (which was a trick because I had to keep the old seal between the tank and fuel line). The harder fuel lines may have exascerbated the pressure problem.

Thanks MUCH for your guideance!!

Bert

 

[alexcagle]

You're Welcome.
US EPA regulation (for the good of the planet), doesn't want fuel venting into the atmosphere, so they hold more pressure than earlier designs. I've seen fuel boil in saws when the temperature was 95 degrees outside.

 

[PhilKawasakiuser]

Well what would the fuel do when the temperature is 115 outside or when at parker when its 125 and the low is 110? Here it can hit 115 and drop to 50 with hail in about 3 minutes then go back up to 105 a half hr later with humidity. Winter can get up to 90 and down to 25 in the valley. 55 high and -15 plus 75 mph sustained wind in the Mts. More normal is 40 high and 5 low there. A few yrs ago I heard it hit 131 and 133 even according to some sources luckily I aint there that time of yr ever anymore. I try to run stuff dry if I am leaving it

 

[alexcagle]

Hydraulic lock just means that when the piston goes down, it can't physically compress the liquid like it would an air/fuel charge.