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A non-starter this morning, (after some months unused) and after fresh fuel and a quick carb clean:
The engine starts smoothly, and seems to run great.
After a minute or two, it starts to cough, splutter and backfire, but mostly doesn't stall.
Then after a few seconds (10? 20?) it then bursts back into life and seems to run fine for a short while before repeating the process.
Fuel seems to be filling the carb, I tried removing the fuel filter as a test - no difference.
I also tried briefly without the air filter, maybe a little worse but not much different.
It does seem to run smoothly at low revs.
Any pointers to what might be wrong?
Thanks!

Dirty carb and or fuel system. Dry rotten fuel lines. Tank cap not venting. Trash in the tank and tank outlet.

Possibles that are secondary items. Dead cellulose air filter restricting air flow. Water in the gas. Valves never inspected for proper lash. Cooling fins probably never cleaned.

I think I have eliminated some of those with what I've done so far (see above) but cap not venting is something I hadn't thought of.
Not sure how valve lash would cause this run....won't run....run....won't run
Otherwise, probably a more thorough strip down and carb clean is next...
Thanks, your advice was appreciated.

SouthWestern said:

Otherwise, probably a more thorough strip down and carb clean is next...

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yes

slomo said:

Possibles that are secondary items.

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These are yearly maintenance items that should be done for a good running mower. Most are in your engine manual.

Boil clean the carb. Separate plastic and rubber parts from the carb body. Slow boil on a garage sale hot plate and pot. Little Pine Sol and water mix. Slow rolling boil for 45 minutes. Rod out and blow out with compressed air. Don't forget the pilot jet and those passages.

Fuel lines deteriorate internally. You won't see the trouble. Pull the hose off AT the carb inlet. Should have a solid flow of gas into a glass jar. Look for water in the bottom of the jar.

If I recall correctly, those carburetors have what is known as an air bleed jet. Remove the idle adjustment screw and under that in the aluminum housing is a little black plastic piece. Carefully pry that up and then clean it. They take a super fine wire that will fit through the tiny orifices. After that it will need little carb spray and blowing out with low pressure shop air.

Apply a little silicone grease to the tiny O-ring and slip it back into place. Then install the idle stop screw where it was and final adjust the low idle speed once running.

Edit:

Also, consider the fitting inside the fuel tank possibly having the screen choked up and debris or water in the fuel. Fuel tanks condense moisture in the air, especially during temperature swings at night. It is always best to keep the fuel tank full when not in use.

Run the tank empty, then remove the fitting to inspect it. While there, take some shop towels or rags attached to the end of a coat hanger wire and mop out the tank

That was an excellent and well timed post. My carb clean instructions simply say to "remove" and I have been looking at it wondering how it could unscrew... So pryed it out as you said: to find it blocked as far as I could tell, and with only one (and broken) o-ring. Not sure what size "super fine" is - that's an issue I have had a few times as instructions usually say "appropriate" or "suitable" or refrence a Honda tool that I don't have. Anyway - lowest cost way to get o-rings was to buy the complete assembly so we'll see when that arrives.
The fuel tank was emptied up front, and there always seems to be plenty of fuel running into the bowl but if the carb clean doesn't improve things, I probably ought to re-examine all that; actually clean the tank and pipes etc. Maybe do that while waiting for the parts... though I'd prefer to do one at a time so that I can find out what the problem was and post it for future use!

slomo said:

Tank cap not venting.

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Would have been great and a simple fix but no luck there.

SouthWestern said:

That was an excellent and well timed post. My carb clean instructions simply say to "remove" and I have been looking at it wondering how it could unscrew... So pryed it out as you said: to find it blocked as far as I could tell, and with only one (and broken) o-ring. Not sure what size "super fine" is - that's an issue I have had a few times as instructions usually say "appropriate" or "suitable" or refrence a Honda tool that I don't have. Anyway - lowest cost way to get o-rings was to buy the complete assembly so we'll see when that arrives.
The fuel tank was emptied up front, and there always seems to be plenty of fuel running into the bowl but if the carb clean doesn't improve things, I probably ought to re-examine all that; actually clean the tank and pipes etc. Maybe do that while waiting for the parts... though I'd prefer to do one at a time so that I can find out what the problem was and post it for future use!

Click to expand...

Those tiny O-rings are a gamble to remove and reinstall at times. Hope the new part fixes the problem. Maybe a shot of WD-40 will keep it from nicking the O-ring on the replacement part when you install it.