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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 rotted 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.

Still struggling with this one; after carb clean and new pilot jet, fresh fuel, just to recap:
The engine starts smoothly and easily, and seems to run great at low revs.
After a minute or so at high revs, it starts to cough, splutter and backfire.
Then it may burst back into life and seems to run fine for a short while before repeating the process.
Can anyone think of anything specific that might cause this behaviour?

Possibly a wrong spark plug. Verify you have the correct plug it calls for. If it has an Amazon plug, those are almost guaranteed to be bad.

The next suggestion is to pull the fuel tank and remove the tank fuel nipple to clean it.

Most generally about the only thing that is reasonably priced about Honda engines is a replacement carb. (and I've had good luck with the Honda carb replacements)

This is just one of the examples of where a Ultra sonic cleaner really shines.
I've tried just a general cleaning of (especially) the Honda carbs and if they still surge I send them to the USonic and usually get a good run and not even have to install a kit. If they still surge and hunt I try a cheap replacement. If the bowl is rusty it's a hint you need to replace, it's probably had ethanol fuel inside and the fuel lines are also going bad internally.

One some of the Honda powdered equipment they hide a fuel filter on the end of the fuel line poked into the plastic fuel tank. If it's restricted it can cause erratic surging and hunting.
Lots of info on-line about this in tank filter that is on the end of the fuel line on some Honda powdered Equipment.

I am going to take the tank off and clean it (again) but I am a bit uncertain this is going to help: if I take the fuel line off the carb then fuel literally jets out.
The engine is not really hunting and surging: it virtually dies with just a few pops and coughs... then usually recovers. If I turn the fuel off at this point, (to test if the carb is now empty) it may still recover and run or run/cough/run for a while before finally stopping for lack of fuel - as you would expect with the tap off. A puzzle.

The engine would not run at all when I started - I just emptied the bowl, cleaned the bowl and main jet and it the started fine with fresh fuel - but with the erratic problem. Part of my second carb clean was an ultrasonic clean of the whole thing (as well as detailed cleaning/replacement of jets) there was no discernable difference after all that! So that maybe points away from the carb... let's hope the answer lies in the tank.

This is starting to sound like you have some bad fuel in your gas can from a station recently.

I consider any fuel without a stabilizer "bad fuel" after 15 days here in South Texas, due to the heat and humidity.

I have also had these "self-inflicted" episodes just from leaving a fuel can sit open in a shed when there is high humidity.

Ethanol fuel is "hygroscopic". That is a fancy word which means it absorbs moisture from the surrounding air.

Most people never check their fuel cans/jugs and just keep putting new fuel in when they get at or near empty.

To verify: Empty most of your fuel can into one of your vehicles then remove the nozzle from the can and look at what is left behind in the fuel container bottom. Usually trash from dust and cut grass along with a little water stays behind.

Flush out your fuel can really well with water, then let it sit to dry out upside down (without the nozzle installed) for a couple of days. After that refill the can with new mid-grade fuel from a reputable station. Don't use the low budget Ethanol fuel. If you have a none-Ethanol fuel available, that would be even better.

A product called Ethanol Shield works really well and lots of professionals use it. Sta-bil has lost popularity in recent years because it is not living up to its claims (from my own personal experience).

Teryl Fixes All (You Tube) has conducted several tests on fuel stabilizers, so check out his videos. Chickanic is another great small engine repair tech who has also verified Ethanol Shield is some great stuff.

Install a new spark plug, but it has symptoms of bad water contaminated fuel.
After you clean the tank use fresh fuel from a different container. The symptoms you describe indicates bad/water contaminated fuel, but
If it's a push mower with the presence switch lever make sure the brake, kill is not at the critical point of interrupting the engine kill. Sometimes it gets at the vib critical point of ignition run, kill on the brake cable.

Also monitor the spark with a Briggs or Oregon 19051, 19368 spark tester. $15
Connect it in-line and watch the spark through the view window while the engine is running.

Do not rely on a light bulb type ignition tester.

I'm in UK, so standard grade petrol/gasoline has 10% and "super" 5% Ethanol. So we have to live with that... having said that it normally works fine, I think as long as the engine is in regular use. I use an alkylate like Aspen or an additive over winter on anything which is not in regular use.
But bottom line, I need to look at both tank/fuel and thge spark plug next.
PS: Chickanic has a great channel and helped me with a few chainsaw problkems!