Strimmer engine conundrum

[Tim+]

A volunteer organisation that I work with has three 4-stroke Honda strimmer/brush cutters like this one.

Ours are about 12 years old but have always been easy to start and pretty reliable.

Recently one of them started hesitating and dying when run continuously at full throttle. Not regularly but intermittently. You could always “catch” the engine by briefly coming off the throttle and then reapplying it. It would then run fine for a period until it’s next “fainting attack”.

It went of for repair, and supposedly lots of dirt was found in the carb. This didn’t fix the problem. It went back to the repair shop and came back still trying to die intermittently. To be fair to the repair shop, the symptoms only generally occurred after the machine had been used for a period.

Anyhow, I thought it was worth trying a new carb and fuel filter. Only £14.99 on a popular online retailer with next day delivery so it seemed worth a shot.

I rinsed out the tank with clean fuel, changed the filter and the carb and it started and ran beautifully. Hooray!

That was until today when after about 15 minutes of use the engine hesitated and just died completely despite coming off the throttle. Unlike before the engine didn’t restart easily and after a few minutes of pumping the priming bulb, it started and ran fine again.

So, what’s going on? I was sure it was fuel starvation but now I’m not so sure. The spark plug looks clean and I’m pretty sure it was recently replaced. I cleaned and re-oiled the air filter just in case that might be a factor.

Generally the engine still starts and runs well so I don’t think it’s an ignition problem but I’m running out of ideas. Is it remotely possible that the issue could be E10 fuel related? One of the other strimmers is also starting to display similar symptoms which makes me wonder about the fuel.

Tim

[Thomas Prufer]

New fuel filter, so: Tank cap vent hole clogged? So once fuel gets used, the pressure in the tank can't equalize and the engine starves?

(Doesn't readily explain why it works if you go off the throttle for a moment...)

Thomas Prufer

[The Natural Philosopher]

That symptom is consistent with rstricted fuel flow. My chainsaw does that until its warm as its set up very lean and once off choke, it has to warm before it works properly.

Things that could cause that are

- the afore mentioned pinhole in the filler cap is blocked.

- if it has a clunk fuel pickup, that may be clogged as well - a feedpipe may be kinked - the main jet may have dirt in it.

All of these require a couple of hours of patient disassembly, cleaning and reassembly. l

[Brian Gaff]

Could air be getting in where it should not? Obviously something is happening when it gets hot and it makes me wonder if a seal is leaking as the metal expands and causes enough of an imbalance in the mixture to make it misfire. Brian

[charles]

or, as happened to me with a lawnmower, the spark creating system failed.

[Tim+]

It’s not really misfiring, it’s just behaving like it’s suddenly starved of fuel. I can’t imagine that an ignition fault would instantly cure itself when you come off the throttle. If it is an ignition system fault, it’s a lot more subtle than outright failure.

Tim

[Michael Chare]

Maybe try one of the spark plugs from a different strimmer. Especially if they are the type that have an internal resistor to reduce interference.

[Michael Chare]

Google knew what the fault might be when the Honda enging on my grasscutter refused to start. Have you asked it?

[The Natural Philosopher]

So what, he is right, ignition issues tend to start as irregular misfires but fuel starvation is highly RPM dependent as the engine simply doesnt get enough juice to run beyond part throttle

[David Wade]

You have not ridden enough small Japanese motor cycles. Did you check the plug gap? I find strimmer plugs have a short life. Mine had similar symptoms and a new plug fixed.

Dave

[The Natural Philosopher]

I have never changed a plug in any of my two stroke garden equipment. Ever. Modern oils, lower oil to fuel ratios and better fuel control have made them almost immune from sooting up.

[Rod Speed]

Or why it runs flat out when it chooses to misbehave.

That has to be due to a throttle cable problem.

[Tim+]

If it was a two stroke I’d agree that plugs can be an issue but it’s a four stroke and I don’t find those anything like as temperamental. I guess trying a new plug wouldn’t hurt.

Tim