M18QS Problems

I have a M18QS built in 1990, and installed on a 1991 Craftsman GT6000. I bought the mower used when it was a year old. Last year I started experiencing the "runs fine for 20 minutes, then dies" problem. I'm an average backyard mechanic, but this has me baffled - here are the details.

I've known for a while that the ignition modules on these were prone to issues. After inspecting the wires, and seeing that they were pretty well cracked and worn I decided to go ahead and replace the ignition module. While I was at it, I put a fresh oil filter and oil in it (easier to do with the shields off). It also has a new ignition switch less then a month old. It started right up, was idling smooth, took it out to mow, was running fine with power, and 20 minutes later it sputtered and quit. Again, acting like it was fuel starved. Started checking the fuel lines - no fuel between the filter and the fuel pump. Went and got new fuel line and a new fuel filter. When I removed the old fuel filter entirely, there was also no fuel between the tank and the filter. Drained and removed the fuel tank, removed the fuel pickup and found it full of crap. Fired up the air compressor and blew out the fuel pickup. Reassembled everything, jumped on and cranked the motor - watched the fuel filling up the new filter as I cranked (which implies the fuel pump is working - right?)...but...the engine wouldn't start. Sprayed a bit of carb cleaner in it which has served to help start it in the past and still nothing, not even a burp from the plugs igniting the carb cleaner. 30 amp fuse is good, but replaced it just to see if it made a difference - nope. Disconnected the kill wire to see if it made a difference - nope.

Anybody have any ideas for what I should try next?

Check for spark, in-line tester , or pull the plug and ground it to the eng.

So...I left the key in the on position and ran the battery down (duh!). After recharging it, I grounded a plug because I don't have an in-line tester, then shut the shed doors so it was dark. When I cranked the engine, the spark was very strong and obvious. I'm assuming I still have a fuel issue...somewhere. The fuel pump pulled the gas from the tank, is there a possibility that it wouldn't push it on to the carb, though? When I cleaned the fuel pickup, what came out looked like a dissolved chip of wood. Thinking out loud...maybe some of that got past the old fuel filter and fuel pump and is gumming up the carb jets?

Update: Today I disconnected the fuel pump from the carb, put a longer piece of fuel line on it, and cranked it. After several short quick spurts of fuel, I determined it was working. Then I removed the carb, blew out cleaned all of the needle ports. I didn't see anything obvious. Hooked the carb back, and checked the spark plug again. Put the plug back in, and low and behold it fired up. I mowed for about 15 minutes, then the same old, same old. It sputtered and died. I am at a loss.

Four things that happen with time
1) Vacuum lock in fuel tank. check by removing fuel cap
2) debris blocking the fuel outlet in tank check for fuel pumping STRONGLY out of the fuel pump just after it stops
3) cracked wire in ignition module gets hot then either shorts or loses contact , check using in line plug tester while problem is happening
4) overheating causing problems in the head

bertsmobile1 said:

Four things that happen with time
1) Vacuum lock in fuel tank. check by removing fuel cap
2) debris blocking the fuel outlet in tank check for fuel pumping STRONGLY out of the fuel pump just after it stops
3) cracked wire in ignition module gets hot then either shorts or loses contact , check using in line plug tester while problem is happening
4) overheating causing problems in the head

Click to expand...

1) I checked the cap previously. It's not plugged but the hole is small. Still could be causing a vacuum in the tank possibly.
2) The fuel pickup in the tank did need cleaned, and I did that already, and replaced the entire fuel line and filter, but it didn't fix the problem.
3) The ignition module is brand new with about 35 minutes total of run time on it, and while I don't have an in-line tester, a grounded plug showed spark after it quit.
4) Maybe, but what's causing the overheating? Oil change is fresh, and cooling fins are clear. Carbon build up?

Next steps: I'm going to pull the fuel pump and inspect it closely, and I'm going to check the kill wire for shorts. It's possible that I have one of the older engines with the diode in the kill wire, but if that was bad I don't think it would start at all. If everything checks out, I'm going to use some sea foam on it, and then I'll test the gas cap theory. Just waiting for the rain to stop right now.

It stopped raining, and I pulled the old fuel filter off. When I opened it up, the small spring, washer and nub were just rolling around in there. I didn't find any holes in the diaphragm, but it doesn't look in very good shape elasticity-wise, either. I've seen some videos of people converting to a pulse pump, and I think this is the route I want to go. I can gut the old fuel pump, plug one of the ports, and use it to drive the pulse of a vacuum pump. I have a concern about oil blow-by, though. Every conversion like this I've seen shows a valve cover mechanical pump to pulse pump conversion. Mine runs off a camshaft eccentrical. Does anyone know where else I might be able to access a pulse vacuum on this motor?

Swapping a mechanical for impulse pump can cause lean running problems with some engines.
If the carb has a different set up for the float valve designed to work at higher fuel delivery pressures.
In these cases I pull off the hood, fit an in line plug tester ( red neon tube type ) the go mow.

When the mower starts to play up I watch the tester to see if it is flashing when the engine is not firing thus it can be isolated to one system or the other.
Otherwise you just end up going round & round in circles.

Problem Solved! It was the broken check valve in the old fuel pump. I went ahead and converted it over to the pulse. It started right up and has ran all day while I mowed and moved a bunch of stuff with the garden trailer. It isn't running lean - in fact it might be running just a tad rich. I'm pretty sure I adjusted the needle valves back when it first started acting up, after I removed and cleaned out the carb. I might of inadvertently opened them up if the old fuel pump wasn't pushing enough fuel out. One more carb adjustment and I think I'm good. Thanks for all your advice.