Lawnmower Engine Repair

[DocTom]

Help wanted pease!

My lawn mower has developed a problem, and I want to try to repair it, but need some advice. The mower starts OK, runs for about a minute, then dies. Cleaning the plug, or fitting a new one, facilitates a new start, but then one minute later it dies again. This happens repeatedly.

The plug after one minute is black and dirty. I thought that this indicated a rich mixture - too little air - so I removed the air filter, but there was no change. At this point my diagnostic ideas ran out, hence my cry for help in detirmining the cause and the cure.

The machine is a Briggs & Stratton 3.5 hp side discharge engine, purchased in Chiang Mai about 7 years ago. (I now live 3 hours' drive for C.M.) I tried the B&S website for help, but it is very user-unfriendly - one cannot email a query unless one has a US address and phone number. Nor does their site recognise my model number (98002).

So I hope someone can help, please.

DocTom.

[Crossy]

Is the plug wet with fuel when it stops?

Check the float in the carburettor, over rich running and choke out could be that it's sunk (could be full of fuel).

This may be useful http://www.ehow.com/briggs-and-stratton-engines/

[MESmith]

Can't help you fix your problem, but there is repair shop that works on Briggs & Stratton engines in Chiang Mai if you have to make the journey.

Discussed here

[VocalNeal]

My money is on the choke stuck shut. If it was the float then all the petrol would "leak" out of the tank. Although not of course if a new fangled B&S has a vacuum operated gas tap.

[Crossy]

True, but if the float is only half sunk it would result in the fuel level in the bowl being too high and causing rich running, still worth checking

[DocTom]

Is the plug wet with fuel when it stops?

Check the float in the carburettor, over rich running and choke out could be that it's sunk (could be full of fuel).

This may be useful http://www.ehow.com/...ratton-engines/

Thanks for your help so far. No the plug is not wet when removed, just black and sooty.

I found a video on You-Tube on removing and cleaning the carburetter, so I shall have a go at this forthwith. (Unfortunately I cannot get my laptop to play the whole video, I think because of interruptions in the Wifi signal, so I will have to go to an internet shop to view it.) Then to the tools!

Many thanks.

DocTom.

[Artisi]

Black and sooty = overfueling or faulty spark-plug, or faulty coil or faulty high tension lead.

[SteeleJoe]

Call Karl.

Sorry. Just watched "Sling Blade" (again) last night so a mention of lawnmower repair...

[DocTom]

Further to my Briggs and Stratton lawnmower engine problems, following your advice.........

First, the carb. does not seem to have a float at all. It seems to be the Pulsa Prime carb. which sucks fuel up from the tank by means of a flexible plastic membrane (diaphragm) which is activated by low pressure.

I must confess that I do not fully understand its mode of operation.

There is a video re. cleaning this carb. on YouTube by a guy called Donyboy73.

First I tried running the engine with the air filter removed - no change - ran about a minute then died with a sooty plug.

Then I stripped and cleaned the carb., following the guide by Donyboy73. All cleaned now, but curiously my carb. does not have a diaphragm spring, unlike the example shown by Donyboy73.

Reassembled, tried - same result!

I am now wondering if the absense of the spring is the cause of the inability to run ( I guess it may have been removed and not refitted by local engineers who did a crankshaft replacement.).

Any further advice will be appreciated, but I am getting close to seeking out the repair shop in Chiang Mai, or buying an electric mower.

DocTom.

[krading]

Have you tried a new plug of the correct heat range?

Does the engine blow smoke when it runs?

Have you laid the mower on its side or backward to change the blades or clean underneath lately?

Is it easy to pull over when starting? ie Can you feel the piston compressing the mixture in the cylinder when you pull the starter slowly?

Does it start within a couple of pulls?

You haven't put 2stroke fuel in the tank have you?

[DocTom]

Have you tried a new plug of the correct heat range?

Does the engine blow smoke when it runs?

Have you laid the mower on its side or backward to change the blades or clean underneath lately?

Is it easy to pull over when starting? ie Can you feel the piston compressing the mixture in the cylinder when you pull the starter slowly?

Does it start within a couple of pulls?

You haven't put 2stroke fuel in the tank have you?

Both the older and a new Bosch plug (correct for the engine) behave similarly - they run the engine for about a minute then it dies with a sooty black plug.

There was some black smoke when the problem first began, but now little or none.

I have not turned the machine over or on to its side.

There is compression, and it does start within a couple or so of pulls, but then dies after a minute or so.

I did not put an incorrect fuel into the tank.

Thanks for your ideas,

DocTom.

[krading]

How much work has it done?

Been a while since I worked on one but barring any fuel problems I'd lean toward a valve clearance problem.

It starts and runs ok for a minute until the engine heats up and the exhaust valve expands and won't seal properly thereby losing compression and fouling the plug.

I've seen it on near new ones that come out of the factory without the correct clearance.

Intake valve about 6 thou. and exhaust about 10 thou just off the top of my head.

good luck

[howto]

Ruling out the choke not unloading...

and not "really" blowing black smoke...

and is fouling plugs...

and is dying after a few minutes (heating up)...

Fits the pattern of a coil going bad, or perhaps the transistor.

I attempted to locate that manual, to no avail.

Cheers.

BTW, Briggs has a bad reputation among Thai's

as having terrible parts distro and low quality.

I agree from experience.

I do prefer the Honda GXV series engines.