# Stihl TS 510 cut-off saw fuel problem



## FarmerBob (Nov 4, 2014)

Hello, I'm working on my Stihl TS 510 AV concrete saw. I've owned it a few years, but having trouble keeping it running now. Appears to be starved for fuel. I started by cleaning fuel filter, removing and cleaning carb, put a new carb kit in to make sure diaphragm pump was working well, clean air filters, no luck. With carb needles in factory position (H=1 turn out, L=1.25 turn out) it will start on choke, but die when I take choke off. I can make it run by turning needles out 4-5 turns, obviously not well but shows me I'm not getting enough fuel. 

I've worked on a lot of small engines, but am not familiar with the surge tank that this unit has. The service manual is confusing to me, as it describes the surge tank cap as a fuel fill cap, and the hose that runs into this tank has a pickup filter on it. I've never put fuel in this surge tank, it is supposed to have fuel in it?? I assumed it was just getting pressurized and pushing fuel through the main tank into the carb, can someone explain what exactly this tank does? 

If you look at the service manual (tried to attach .pdf but it's too large, if anyone wants to see it I can e-mail it), this model has a new style and old style fuel and air system, my saw is the old style. On the old style system they note that I must have the gray cap that is vented for the surge tank cap, my cap is gray but doesn't appear to have a vent in it. There is a bolt through a hole in the middle, but it has been like this as long as I've owned it and has run great in the past. 

Any help is greatly appreciated, I poured some concrete last Friday and need to get this thing running so I can cut my break lines in it ASAP! Thanks for the help.


----------



## paulr44 (Oct 14, 2008)

That saw is basically an 041 or such chainsaw that's been converted, and the surge tank is the oil tank, which isn't used on cut-off saws. You're barking up the wrong tree worrying about the surge tank...it's just the tank vent, and unless an engine runs a few minutes and stalls, the tank is venting.

NO, the fuel does not get pressurized, it has vapor pressure as a characteristic but the fuel is pumped by the one side of the carburetor with the diaphragm with flap valves.

On that unit, there's a cover on the carb. with a 90 degree fitting with a hose connected to it, that's the impulse line that makes the fuel pump function by means of being vented to the crankcase, and the impulses (pressure/vacuum) of the piston going up and down make the pump diaphragm flutter up and down. The fuel on the other side of that diaphragm is pulled in and pushed out because of the one-way flap valves.
IF THAT HOSE IS punctured/cracked or blocked, THAT'S THE PROBLEM.
Also check the fitting, ensure it's clear.

To check for impulse, remove the hose from the carb. fitting, put a dab of grease over the end of the hose to block it, and pull the engine over once. The grease should blow off a bit or get sucked in...get the idea?

For good theory on this, go to Walbro's website and download the repair manual for the HS series carb., which is the model of what's on your saw.

Knowing that model and the problem you describe, I suggest you consider a vacuum leak. Unfortunately without it running off choke it may be hard to determine. To test for a vac leak, get a can of carburetor cleaner and start the saw. Using a spray straw so you can zero in on specific areas, spray the manifold gasket ares below the carb. THE PIC attached shows you where I suspect you have a leak.
If the RPMs change, you've found a leak.
NOTE of being misled: if you spray the throttle shaft, IT WILL change the RPM as it will draw in the fluid.
NOTE of caution: You'll be near the muffler, be careful.

If you don't find a vac leak, you may have a restriction in the carburetor. If all you did was put a kit in it and not clean it, then I'd go back into it and clean the passageways with alcohol-base carb. cleaner. Xylene based cleaners can damage check valves built into many 2-stroke carbs.

It's unlikely, but vacuum leaks can occur elsewhere but for the symptom your saw has I kind of doubt it. A blown cylinder base gasket could give that symptom, but I've never seen a blown cyl. gsk. on a TS 510/760. A leaky crank seal would still allow the engine to run, just poorly.


----------



## FarmerBob (Nov 4, 2014)

paulr44, really appreciate the help on this one. Great info, I understand this engine better now. I dug into it again, looking hard at the line going from the carb to the crankcase, but it looked great, I did the grease test and it would blow the grease out of the line. I looked again at the fuel line from carb back to fuel tank, again couldn't find any problems. Looked at the line going from fuel tank to surge tank, and found a crack in that line when I stretched it. I replaced this line and that took care of my problem, saw runs again. The small crack must have been enough to suck air as the carb was trying to pump fuel from the tank. 

Thanks again!


----------



## paulr44 (Oct 14, 2008)

Excellent. Those saws are real workhorses, and their heft alone says something. Take care of the air filtration and it'll last a long time. Keep up on the belt tension, if the belt starts to slip due to slack or wear on cut-off saws (or any belt drive system) it can wear the pulleys out and that can be expensive. They don't make them like that anymore, surely due to cost as a model of that caliber would probably run about $2K U.S.


----------

