QuestionEric, thank you for your prompt and thoughtful reply. Your single problem with ignition coils in 15 years is comforting, since I probably can’t locate a replacement. Decreasing its gap to the flywheel and decreasing the spark plug gap to the manufacturer’s minimum (.024”) had no effect, further suggesting the coil is OK.
I suspected the carb first. In this design there is no fuel pump. The tank is located on top of the unit and uses gravity feed. There are no diaphragms. The carb bowl is the world’s simplest. Needle float valve, float, and main jet. All were clean, but I cleaned them again in carb solvent and blew dry. I disassembled the main jet and cleaned it. I visually inspected it and know there is nothing blocking it. But the problem seems just like something is floating up and obstructing the main port, but I am working on a clean workbench with good light and proper tools. I’m baffled!
I don’t know what I would fix with a carb kit. The existing gaskets are in good shape and I don’t know what else I could replace.
I had a similar problem with a lawn mower several years ago. Turned out to be a blocked air breather hole in the gas cap. Caused the mower to stop after about 15 minutes. Fixed it by blowing out the dirt. I checked this leaf blower and the gas cap breathes easily.
Today I rechecked the fuel flow. I removed the plug from the bottom of the carb bowl; holding a clean container under the bowl, I opened the fuel valve. I collected a couple of ounces of fuel in about 10 seconds.
I have not measured the compression. Pulling the starter cord with and without the spark plug “feels” like it has normal compression. The engine starts perfectly and runs normally at full speed for the first 15 – 30 seconds. If you think it is important I will go borrow-back my compression tester and make a measurement. What should I get? 100 psi?
Today, I dismantled the fan assembly so I could confirm that the port that blows air on the cylinder was clear and not causing the engine to over-heat. It was clear.
Is there anything about the reeds that could cause a problem like this? Visually, they appear perfect.
Ken
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-----Question-----
I have a Sears Craftsman 37.7 cc Gas Powered Leaf Blower Model 636.796912. I bought it new about 15 years ago. I use it about 8 hours each fall removing leaves. Each year, I run it dry. The next fall it starts well on the new fuel mixture, usually on the first pull.
This year, after several hours of operation, it abruptly failed to run at high speed. It still starts perfectly and idles nicely. If cold, it will accelerate to full speed for 15 to 30 seconds and then slow up to an idle. The engine will quit unless I back off the throttle to the idle position.
At first, I suspected something was clogging the high speed jet. I have fully disassembled and cleaned the carburetor, filter screen, etc. I found no dirt or residue and the problem continues. The problem still happens when the air filter is temporarily removed.
I have replaced the spark plug. I replaced the fuel with a new mixture. I removed the muffler and inspected that the exhaust port is clear and confirmed that the spark arrester is clear. I inspected the reeds – they appeared like new; coated with a thin film of clean oil.
With no where else to turn, I suspect the ignition coil is breaking down. I decreased the air gap slightly between its laminations and the fly wheel – no improvement.
Sears no longer makes the ignition coil (11400331000) available as a replacement part.
Am I on the right track? What have I missed? What can I try? Is there a way I can check the ignition coil? How can I find out who made the engine?
Ken
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I have had one coil in 15 plus years that caused this problem.
I would suspect the carburetor. When you took it apart, were the metering and fuel pump diaphragm soft and flexible. Is the diaphragms are brittle then they need replacing.
Before blaming the ignition system you should purchase a carb kit and see what happens. Most any mower shop should have a kit if you take in the carburetor.
Let me know if a carb kit fixes it. There is one other test to do before replacing any igntion parts.
By the way, what is the compression reading?
Let me know.
Eric
Answer100 PSI with the throttle wide open should be engough compression. Check it cold and then with the engine warm.
There is a remote possiblity of a crankcase leak. Two cycles need a fairly sealed crankcase to push the fuel from the crankshaft area up through the ports to the compression area.
Most two cycles can handle 1 psi of pressure loss per minute. Several manufactures sell small hand pumps to check the crankcase pressure but you can use a MityVac Vacuum/Pressure pump. The difficult part is sealing the muffler and intake port but still have some method to pressurize the crankcase. Echo used to sell an adapter that screws into the spark plug hole. The adapter has a hole with a nipple on it to conntect the pump. Most manufactures sell special adapter plates to seal off the muffler and intake ports. Most of the time you can improvise using rubber from an inner-tube or rubber gasket material. Cut a piece large enough to cover each port then use a hole punch to punch the bolt holes; the holes should be very tight around the mounting are to prevent air leakage.
With the ports blocked, pressurize the crankcase to around 6 psi and check for leaks. You can even place the power head assembly in water to find leaks. Water will not enter as long as the crankcase is pressurized.
Again, leaky crankcases are rare but once in a while we do find them.
So, if the compression is good, the carb is clean and working properly, crankcase is not leaking too much, ports are clean then about the only item left is the ignition module. I don't like replacing parts unless I know 100% for sure they are defective.
The one faulty ignition module I had would allow the engine to start fine, idle as long as you wanted but would not allow the engine to rev up. I'm assuming there was a timing advance circuit in the module that was faulty.
Let me know if this helps.
Eric
