Briggs Misfire

[Ropi Jo]

Hello.

First post here. Won't be my last.

I'm in the UK. Lincolshire, near Skegness.

I have a very old and rusty MTD Yard Bug. Looks like a VW Beetle coloured like a ladybird. It's badged as a Lawn Flite 503. To be honest it's a wreck, left outside and neglected for ever. Picked it up for free a few years ago.

Anyway, it's been stripped and 'repaired' (mainly bodged) and now runs and drives. The variable CVT works and the previously siezed transaxle is running nice and smooth.

The mower deck has had the rot cut out and plated and treated and is currently being re-fitted. I wish I had taken pics of the deck before stripping 2 yrs ago as I'm really struggling to work out how the various bracketry goes back together. I've found various manuals online and none of them match my particular version of this machine.
But it's getting there and today it will run and cut grass... hopefully.

The main issue I have is with the Briggs engine. There's no identification on it but I've read in some manuals it's a 8HP single cylinder OHV.



The issue I've got, and need some advise on, is... it will run sweet for about 10 seconds, then there is a small audible tick, at which point the revs drop and the governer kicks in. This repeats in approx 5-10 second intervals. Always with the tick. I'm very hard of hearing but even I can hear the tick.

Innitially thought it may be an electrical arc from the plug lead, so replaced the plug and magneto with parts from the next project, but no difference.

I've set the valve clearances according to multiple instructions. Innitially the engine would not turn on the starter as the clearance on the inlet was too loose giving too much compression. Common apparently.

Also followed instruction for setting the governer which appears to be doing it's job.

The engine does smooth out a little when under load going forward and reverse, as is probably expected.

Will hopefully smooth out a littlemore when the mower deck is engaged. We'll see today.... maybe.

Appreciate your thoughts, even if it is "what do you expect from an ancient machine that's rusting away with 1000's of hours on it."


Thank you reading, even if you cannot give me any suggestions. Have a great day all.

[Para1]

HI ROPI JO, I've never had much success with Briggs and scrap 'em. good luck, and welcome.

[Ropi Jo]

Well, I think the tick I hear is actually the governer kicking in. So that's the result of the drop in revs, not the cause. My thinking is coming back to the cheap after market carb. I read stories of drilling out the jet(s).

Found some reference to the engine being a 9.5HP, and these aftermarket carbs not being jetted large enough straight out of the factory due to some regs so need drilling.

But... I refitted the refurbished deck yesterday and found the belt was too long. The replacement belt is listed as 52" OD. Measured this one and it is 52". Well worn, but the correct length. This strongly suggests I've got something wrong with the multitide of brackets when re-fitting the deck, but it looks to be correct. Goes up and down parallel to the ground, and is central on the machine.

Unfortunately cannot find any pics showing the same assortment of brackets that I have.

Working away for a few days so I'll have another look at it when I have the chance.

[Ropi Jo]

This is the carb I've fitted....

https://www.amazon.co.uk/EVTSCAN-Engine-Carburetor-Briggs-Stratton/dp/B0B7RV8RC5

[Mick]

Hi, and welcome to TGMForum.

To be honest it's unusual to hear the governor make a clicking sound,  The thing you hear commonly is the decompressor kicking in especially if the revs drop below a certain point at idle speed, The decomp will kick in and the engine will run lumpy. I'm wondering if this is what you're hearing. 

[Ropi Jo]

Hi Mick.

I used to live in Essex.

Can you elaborate on the decompressor please?

Do you mean the decompress for starting to momentarily open a valve?

If so, that is a result of the revs drop, not the cause of the revs drop, I believe.

The revs dropping happens at higher speeds as well.

So I'm still looking the cause of the revs dipping. And there's not much other than the carb.

[Mick]

Hi Mick.

I used to live in Essex.

Can you elaborate on the decompressor please?

Do you mean the decompress for starting to momentatiliy open a valve?

You used to live in Essex, sounds like you had a luck escape. lol..

Re: The decomp. yes the compression release that lift's a valve slightly on start up. If the revs drop too low while idling  it can suddenly kick in.

[Ropi Jo]

The engine is sorted. I was overthinking it. Just needed a smear of sealant on the gasket as there was air being sucked in.

As for the deck fittings, my guess at the orientation of the hanging brackets was as correct as it could be. Any re-position by even 1/2 inch in any direction would mean the grass collector would not fit.

Although the belt measured the correct OD, it had 'thinned' out sufficiently over the years such that it was seating deeper into the V pulleys than it should have, thereby giving too much slack. The new belt was noticably wider and sat less deeply in the pulleys and worked perfectly.

My efforts to shoe horn the wrong size tubes (left over from other projects) into the front (very poor condition) 12-5.00-4 tyres was a failure. The tubes were way oversize and scrunched up inside the tyre and this caused pinch points that developed pin holes. New correct tubes are on the way. Still way cheaper than new tyres.

Only problem left to overcome is that I mulch most of the time, rather than collecting the cuttings. I don't have a mulch setup for this mower. Need to bodge something together.

All told I'm into this project for about £90 - £100. now, plus loads of work. Probably not worth it for most people, and if I was paying somebody to do this, or looking for a sell-on and some profit, it would have been a non starter.

So, I've ended up with a very rusty, very old, but still fairly solid and very capable sit-on for £100 and some elbow grease, and I've leaned a lot.

Compare this with the previous sit-on I fixed up for my son.. a Mountfield 827H, again for a total cost of about £100, he definately go the better deal!