Author Topic: Lawnmower Woes  (Read 869 times)

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Lawnmower Woes
« on: May 03, 2020, 09:33:57 am »
Petrol lawnmower, about 20 years old, Briggs and Stratton 375 engine, fixed self regulating throttle, manual drive.

Started running really rough for some reason.
On start, a bit temperamental
On warm running, rough, sounds like a bit of valve overlap/backfire
Hunting throttle

Carb assembly replaced last year, so I know it's not that. Spark plug gap is OK, I checked it last week. Oil level is good.

Thoughts
1) replace spark plug - it could be breaking down, several years old
2) clean air filter
3) Throttle regulator springs

Any other ideas?
“Procrastination is the thief of time, collar him.” –Charles Dickens

Re: Lawnmower Woes
« Reply #1 on: May 03, 2020, 09:55:55 am »
Air filter would be my first thought. They get blocked. They are usually foam and can be washed out. Try running it with no air filter to see if it sounds better, if it does you know its that.
I think you'll find it's a bit more complicated than that.

Tim Hall

  • I want to eat the fleeting shade of your lashes
Re: Lawnmower Woes
« Reply #2 on: May 03, 2020, 10:58:51 am »
How old is the petrol?

</mode=Bonnie & Clyde>
Dirt in the fuel line. Just blowed it away
</mode>
Crap in the carb?
There are two ways you can get exercise out of a bicycle: you can
"overhaul" it, or you can ride it.  (Jerome K Jerome)

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: Lawnmower Woes
« Reply #3 on: May 03, 2020, 11:17:57 am »
Petrol is from last year in the can, sealed over winter.  Petrol in the tank has been replaced this year.  Need to refill my fuel can anyway.

Removing air filter has no effect, but it will be cleaned anyway.

Nothing visible in the carb, see above replaced last year, along with in-tank fuel filter mesh as part of the overall assembly.

One of the governor springs looks a little floppy on inspection, they are 20 y.o.
“Procrastination is the thief of time, collar him.” –Charles Dickens

Re: Lawnmower Woes
« Reply #4 on: May 03, 2020, 11:39:11 am »
'carb assembly replaced' begs the question 'with what, exactly?'.  Between the ever-declining quality of original B&S parts and the potential for Chinese knockoffs, I certainly wouldn't dismiss the possibility that the carb assembly has gone tits up again.

IIRC the carb in a 375 engine  has a horizontal diaphragm at the split line; this (amongst other things) functions as a fuel pump, driven by crankcase pressure.   Common faults with this arrangement include that

a) the diaphragm splits or swells or goes bad because modern petrol is different from what it used to be or the diaphragm is just crap.
b) that the engine is producing the wrong amount of crankcase pressure to drive the pumping action.

Re a) if you can't find any other problem just replace this bit if you are in any doubt about it
Re b) the crankcase pressure can be too little (eg if the pipe splits or falls off) or (inevitably, sooner or later) too much.  B&E engines don't even use a cylinder liner; they seem to think it is OK to run a piston straight in the cylinder casting. This means a B&E engine will usually turn its oil black instantly, and no matter how well you try and look after it, it will clap out  (I estimate) after a couple of  hundred hours of use.

Towards the end the engine won't start (not enough compression) and/or it won't run well (too much crankcase pressure/carb wear overfuels the engine).

IME when a B&E mower engine is about half-worn out, the fuel consumption starts to become excessive; about double that of a similar Honda-powered mower. By contrast with a good Honda engine, you can do a year's mowing on half the fuel and the oil will often stay clean, even though there is no oil filter.

cheers

Re: Lawnmower Woes
« Reply #5 on: May 03, 2020, 12:07:08 pm »
The air governors are awful.  I rebuilt a Briggs engine years ago, with new rings and stuff, but could never get the governor right once it had been off.

Standard advice: buy a Honda Izy  ;D
And Darkness and Decay and the Coronavirus held illimitable dominion over all.

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: Lawnmower Woes
« Reply #6 on: May 03, 2020, 12:55:24 pm »
'carb assembly replaced' begs the question 'with what, exactly?'.  Between the ever-declining quality of original B&S parts and the potential for Chinese knockoffs, I certainly wouldn't dismiss the possibility that the carb assembly has gone tits up again.

IIRC the carb in a 375 engine  has a horizontal diaphragm at the split line; this (amongst other things) functions as a fuel pump, driven by crankcase pressure.   Common faults with this arrangement include that

a) the diaphragm splits or swells or goes bad because modern petrol is different from what it used to be or the diaphragm is just crap.
b) that the engine is producing the wrong amount of crankcase pressure to drive the pumping action.

Re a) if you can't find any other problem just replace this bit if you are in any doubt about it
Re b) the crankcase pressure can be too little (eg if the pipe splits or falls off) or (inevitably, sooner or later) too much.  B&E engines don't even use a cylinder liner; they seem to think it is OK to run a piston straight in the cylinder casting. This means a B&E engine will usually turn its oil black instantly, and no matter how well you try and look after it, it will clap out  (I estimate) after a couple of  hundred hours of use.

Towards the end the engine won't start (not enough compression) and/or it won't run well (too much crankcase pressure/carb wear overfuels the engine).

IME when a B&E mower engine is about half-worn out, the fuel consumption starts to become excessive; about double that of a similar Honda-powered mower. By contrast with a good Honda engine, you can do a year's mowing on half the fuel and the oil will often stay clean, even though there is no oil filter.

cheers

It's a 20 year old mower, as above, so likely well over a "couple of hundred hours of use", I was wondering what the average lifespan of one of these might be.  It really owes me nothing, but it's going to be a bugger to replace right now.

Started running a bit rough last year, repalced the carb assembly and those gaskets you mention with B&S parts.  With it running slow and rough, I do think it's going over rich rather than lean, which i associate with fast idle on a carbe'd petrol engine.  This one is a fixed jet of course, so no scope for adjustment.

Sounds like it might be worth a few tens of quid on part to see if I can get it through another season, and do a bit of research on a new mower.  Hmm, rotary or cylinder, push or driven?
“Procrastination is the thief of time, collar him.” –Charles Dickens

Re: Lawnmower Woes
« Reply #7 on: May 03, 2020, 01:03:00 pm »
Stuart at Fenland Spirit, just along Ten Mile Bank road, Littleport, sorts my mowers out if I can’t, and does secondhand mowers for little more than a service. I am not sure if he is trading at the moment. Try him on Facebook. If the mower has a float chamber, try taking it off and cleaning. Most B&S ones have the carb sitting on the tank, though.
If it ain't broke, fix it 'til it is...

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: Lawnmower Woes
« Reply #8 on: May 03, 2020, 01:26:49 pm »
This is the carb on the tank job, dip pipe into the tank with filter on it.

Will check those guys out, cheers.
“Procrastination is the thief of time, collar him.” –Charles Dickens

Re: Lawnmower Woes
« Reply #9 on: May 03, 2020, 01:50:05 pm »
mad thought; IIRC the main jet dips into a chamber which is full of petrol. The level in this chamber is maintained (when the mower is roughly level) not by a float, but by excess fuel simply overspilling the sides and running back into the main part of the tank. I have often wondered if altering the overspill level could alter the mixture strength.

FWIW I used to maintain my neighbour's mower and it used to need a new diaphragm every other year (or sooner).

cheers

Re: Lawnmower Woes
« Reply #10 on: May 03, 2020, 01:56:54 pm »
For garden machinery which gets little use and is laid up over winter, either a fuel additive or Aspen 4 or 2 is worth the extra cost over normal fuel

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: Lawnmower Woes
« Reply #11 on: May 03, 2020, 05:48:39 pm »
As it's been a mild winter, it's not really had that much of a layup vs colder winters, and it's been used maybe once a month just to stop things going mad.  So I'm apt to discount fuel issues just in favour of engine age.    Will be buying a new can-full this week in any case.
“Procrastination is the thief of time, collar him.” –Charles Dickens

Re: Lawnmower Woes
« Reply #12 on: May 04, 2020, 10:27:06 am »
For the small cost I'd try a new spark plug.  I had an issue a couple of years back where I just couldn't find the problem and had cleaned the spark plug/carb/air filter etc.  In the end, it was a "hidden" spark plug issue and a replacement resolved it. Probably not that, but worth a try. 
Good luck.

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: Lawnmower Woes
« Reply #13 on: May 07, 2020, 02:22:58 pm »
New spark plug, new governer springs, cleaned airfilter.  Running slightly better but still rough and hunting. 

Looks like it might be new mower time come the end of lockdown
“Procrastination is the thief of time, collar him.” –Charles Dickens

Re: Lawnmower Woes
« Reply #14 on: May 07, 2020, 04:28:41 pm »
New spark plug, new governer springs, cleaned airfilter.  Running slightly better but still rough and hunting. 

Looks like it might be new mower time come the end of lockdown

Get a Honda, you know it makes sense.
I think you'll find it's a bit more complicated than that.

Re: Lawnmower Woes
« Reply #15 on: May 07, 2020, 04:57:21 pm »
New spark plug, new governer springs, cleaned airfilter.  Running slightly better but still rough and hunting. 

Looks like it might be new mower time come the end of lockdown

Get a Honda, you know it makes sense.
I only buy Honda now for the business. Briggs & Stratton are hard to start in the morning and take the fun out of the job.


Sent from my iPhone using Tapatalk

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: Lawnmower Woes
« Reply #16 on: May 07, 2020, 05:00:24 pm »
A new one will either be Honda or electric, when we bought the current one I was earning half what I am now and we couldn't afford a Honda
“Procrastination is the thief of time, collar him.” –Charles Dickens

Re: Lawnmower Woes
« Reply #17 on: May 07, 2020, 09:35:44 pm »
I have used Honda mowers for quite a long time and the only real engine fault I have had (between two different mowers and about twenty years with each) has been an intermittent ignition switch (sometimes gives trouble after winter hibernation) in one machine.

However I have noticed that some cheap mowers come with engines that are pretty blatant Honda knock-offs; Are these any good, I wonder?

cheers

Re: Lawnmower Woes
« Reply #18 on: May 09, 2020, 12:04:21 pm »
If it's old, rattling, down on power and difficult to start it's probably clapped! If changing the external bits doesn't make a lot of difference, suspect valve clearances and look at the state of the bore. I have a Quantum 35 that I think we managed to seize in some very heavy mowing. When it refused to start and the compression seemed a bit low I took the head off. No inlet clearance at all and when I looked at the bore there was a good score in it. Sorted the valve clearances and polished out the score and it did at least run but lacked the power to do the sort of job that killed it in the first place. The same mower now has a Quantum 60 motor (again a gift from someone) which hunts because it is turning a smaller blade than it is regulated for. I am hoping I don't kill it before I retire!
I also have a B&S 148cc Magnetron which must be getting on for 40 years old. Wonderful motor! B&S still make it, called the Classic now.

I have known Honda motors have regulator problems, easily fixed if you're into regulators (I'm not!)

One client has a cheap mower with a chinese Honda clone (might even be an ohv B&S clone but more likely Honda). It goes very well and so far very reliable (about 5-6 years of fairly hard work). I wouldn't like to look for parts for it but otherwise pretty good. Chinese superstore gardening stuff is sold on the basis that it won't break in the warranty period and if it does it's cheaper for the store to replace it than fix it! Outside warranty it is best considered cheap enough to be throw-away. (The same is true of B&S motors fitted to superstore mowers; the pros can tell from the factory code which is a good one and which is throw-away)

ian

  • fatuously disingenuous
    • The Suburban Survival Guide
Re: Lawnmower Woes
« Reply #19 on: May 11, 2020, 09:32:35 am »
I bought an electric mower about 10 years back. As the gardeners have run off to their plague bunker, I've had to dig it out. Works fine and far less palaver then wired electric or petrol (not to mention quieter). Did I mention quieter. There's no better sound to wake up to on a Sunday morning than an ancient petrol mower being dragged back into life, like a man attempting to strangle a small motorbike. Oh wait, there is. I hunt and kill these people with their own gardening implements. It's the only way they will learn.
!nataS pihsroW

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: Lawnmower Woes
« Reply #20 on: May 11, 2020, 03:41:23 pm »
you would have rich pickings in rural Fenlandia
“Procrastination is the thief of time, collar him.” –Charles Dickens

diapsaon0

  • Advena ego sum in terra
Re: Lawnmower Woes
« Reply #21 on: May 11, 2020, 03:51:00 pm »
Old petrol seems a likely culprit.  The chap who serviced my outboard motor told me that it starts to go off in a few weeks.
Advena ego sum in Terra

ElyDave

  • Royal and Ancient Polar Bear Society member 263583
Re: Lawnmower Woes
« Reply #22 on: May 11, 2020, 05:49:39 pm »
new fuel, new spark plug, cleaned filter, new regulator springs and it's running better but still hunting.  I found the original purchase document last week, it was Feb 2002, so 18 years old, which I think is not bad by the sounds of things
“Procrastination is the thief of time, collar him.” –Charles Dickens