Hi,
I have a Briggs and Stratton 475 sprint engine in my lawnmower and it's running very poorly. It starts fine but soon after starting it starts to "hunt" as if it is running on choke when it shouldn't be. Sounds like, "Brr..Brr..Brr..Brr" and sometimes comes close to stalling. There is also a lot of soot around the exhaust exit which leads me to believe it is running too rich.

I've changed the spark plug, cleaned the air filter, changed the oil and checked the armature air gap but nothing seems to help. I'm pretty sure it is fuel related but the carburettor doesn't seem to have a mixture adjustment.

Anyone know what might be wrong or how to adjust the mixture on these engines?

Cheers,
Craig.

last time mine sounded like that, it was a build up of grass/crap under the air cowling, the fan moves a flap to act as a govenor on mine
cleaned it out and ran sweet after

Yep, a stiff or partialy seized governor linkage will make then hunt very badly.

Hi guys,
I had the top part of the engine in bits when I set the armature air gap and I cleaned it out then. The governor seems to be free and unhindered but I'll double check.

Thanks,
Craig.

quote:

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Originally posted by dave r
last time mine sounded like that, it was a build up of grass/crap under the air cowling, the fan moves a flap to act as a govenor on mine
cleaned it out and ran sweet after

---

Hi Paul, thanks for your advice but I'm pretty sure the flappy bit is working okay. Everything is clean and moving freely at least.

What I have found elsewhere is some evidence that it might be the diaphragm fuel pump which might be damaged. I have been googling and found a couple of people describing the exact same symptoms and it turned out to be a split diaphragm in one case and a stiff diaphragm in another case. I have ordered a carb rebuild kit from www.briggsbits.co.uk for £13.90 and also a new pull-starter assembly as mine slips quite a bit and you end up falling on your ar$e...

Apparently this engine uses the manifold suction to act against a spring which then pumps fuel into the carb. If the diaphragm is damaged then it will either pump too much or too little I guess. Not quite sure how these pumps work exactly but it at least sounds plausible...

Will let you know what happens.
Cheers,
Craig.

I'm no expert Craig but faulty fuel pump would supply less fuel, and you say that you
think it's running rich, i.e. black sooty bit. (another techy term)

The flap will appear free but unless sufficient air can pass, it will not move enough.

Just to please me poke out the fins or blow out with an air line.
Oil collects at the base of the engine and grass sticks to it in this area.

Paul G

what about build up off carbon on the head wont take alot to cause problem

quote:

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Originally posted by 907
I'm no expert Craig but faulty fuel pump would supply less fuel, and you say that you
think it's running rich, i.e. black sooty bit. (another techy term)

The flap will appear free but unless sufficient air can pass, it will not move enough.

Just to please me poke out the fins or blow out with an air line.
Oil collects at the base of the engine and grass sticks to it in this area.

Paul G

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If it's the model with the carb built ontop of the tank, check the strainer onthe bottom of the fuel feed pipe.

The symptoms sound similar to my B&S last year.

I had a cycle of normal revs - nearly stall - normal revs - nearly stall - etc. It nearly stalled at around 1 second intervals. Black smoke as well, I think the smoke was happening on the way up to normal revs each time.

If you take the air filter off can you see the sprung throttle link thing working on-of-on etc. I could hold the link open and it would hold a steady speed but it was not happy.

A simple carb clean sorted mine. It was easy to do.

quote:

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Originally posted by gazza285

quote:

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Originally posted by 907
I'm no expert Craig but faulty fuel pump would supply less fuel, and you say that you
think it's running rich, i.e. black sooty bit. (another techy term)

The flap will appear free but unless sufficient air can pass, it will not move enough.

Just to please me poke out the fins or blow out with an air line.
Oil collects at the base of the engine and grass sticks to it in this area.

Paul G

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As they work off manifold vacuum, if they are split, instead of the diapragm pumping fuel, it sucks it through the split hence rich running.

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Yes they hunt and run rough if short of fuel -

Wow - loads of replies while I've been out all afternoon working on my Locost. Thanks guys!

Yes the thing about the diaphragm is as gazza285 said, if it is split then the fuel just keeps on leaking through instead of pulsing through in time with the engine cycle. Just to confirm it is the type with the carb on top of the tank although I misled you earlier - it's not a Spring 475 it's a Sprint 375 with engine model 98902.

I think the cycle of accelerate and stall is happening because it gets fuel, speeds up to normal speed, the governor then backs off the throttle which raises the manifold vacuum which draws in too much fuel through the split diaphragm which causes vacuum to drop which draws in less fuel which causes it to run better and speed up and so the cycle repeats....

I'm kinda reluctant to take the carb apart because I'm pretty sure you need new gaskets and seals to put it back together. So if I get the carb kit which I've ordered then I can completely rebuild it and hopefully solve the problem at the same time. I can also fix the slipping pull-start clutch which will prevent a few injuries which have occurred in the past when you give it a big pull and meet with no resistance...

Paul, I will do as you suggest while I've got it in bits and clean out the governor but I'm pretty sure I cleaned it to a sparkle when I changed the oil.

I should get the spare parts during next week so I'll post back with the results.

Thanks again for your suggestions,
Craig.

i had exactly the same problem a couple of days ago. take off the fuel tank which has the governor mounted on it and strip down governor and give it a good clean/ blow with airline. mine only had a slight amount of dirt in it but it made all the difference. Ive seen this twice how on that type of engine, once on mine and once on my mums. they both had the same symptoms as yours.

cheers craig

Hmm, might be worth a try after all. I might have a look tomorrow.

Thanks,
Craig.

I did not use new gaskets or seals and mine was fine. There is hardly anything to the carb on my model - cleaning it made all the difference though and is only a 30 min job. The problem seemed to start when I ran out of fuel and gradually got worse.

What sort of air filter does it have? Is it foam?

The Haynes lawnmower book has this on fault finding:
Rescued attachment faultfinding.jpg

Craig,

I'll have a word with a friend and work colleague on Monday if your prob's not resolved.

He used to service mowers so knows them inside out and upside down, though just giving him your post might not be enough for him to go on

How's the car coming on?

ATB

Simon

Thanks guys, I've checked the filter - it is the foam type but the engine runs just as badly with it removed completely so I'm pretty sure it's not that.

Simon, I was working on the car today and hope to spend a bit more time tomorrow. Working on the rear hood. Still haven't had much time on the car recently but I am getting rid of some of the ToDo list...

Catch you later,
Craig.

Ah - that's why I asked. I had something similar with mine and the (foam) filter is supposed to be soaked in oil, strange I thought but I followed the instructions and it ran like a dream. Here's the paragraph from Haynes again.
Rescued attachment img003.jpg

Hi Geoff,
Yes the service manual tells you to soak it in oil by squeezing through and then using a paper towel to squeeze out the excess.

The thing with my problem is that just after firing up the engine runs perfectly for maybe 20 seconds and then starts this cycle of rev-die-rev-die. Every time it revs I get a puff of black smoke indicating that excess fuel is being purged.

Cheers,
Craig.

Craig, mine was EXACTLY the same. The 'lulling you into thinking it's fixed' first reasonable running bit and then up-down-up-down. I was looking for a deeper reason, I trawled the net for solutions, but it was a carb clean that sorted it.

I will admit it was a last straw (after changing the plug and the petrol) move for me as the symptoms did not point to a blocked jet etc.

The only other thing I could have fixed in dismantling and reassembly was the armature gap - and you have already tried that.

Strip the carb and clean it out - you know you want to.

David

Hi,
Yes your symptoms do sound familiar. Maybe the carb is gummed up with old fuel or something. I know you can get a special additive to prevent this. Once I receive the bits from briggsbits I'll try a carb clean first and if this works then I can either return the carb kit for a refund or sell it on ebay unopened. I don't really want to start the job unless I know I have all the bits that I might need to finish the job.

I can't wait to get the engine purring nicely as it has never run properly in all the time I've had it!

Cheers,
Craig.

With an engine in such a low state of tune as this there is obviously something up, and is certainly pretty basic. Mine is less than ten ears old but the carburetter has less components than my old FSIE (70's moped).

If you can make a V8 Locost then cleaning out the carb, even without a repair kit, is a doddle.

You have nothing to lose by stripping and cleaning as it's not working at the moment.

What really p***ed me off with mine was the initial OK start bit wen you think you have fixed it. That and the 'have you fixed i yet?' fromw SWMBO while the grass (and weeds) grows ever taller.

The thing is I do have something to lose as it is working well enough to cut the grass at the moment as this is the way it has always run since we got it second hand. If it wasn't working at all then I would have had it in bits long ago!

Don't get me wrong, I'm not worried at all about the technicalities of stripping the carb - I have the exploded diagram of the entire engine and as you say there's not much to it. I've previously rebuilt gearboxes and have reconditioned a couple of 16 valve twin cam cylinder heads before so this will be a doddle. I just don't like going to the effort of stripping it all down without at least using new gaskets and seals when I build it back up. The diaphragm is £2.23 and the inlet seal is £2 to it's just not worth doing the job without replacing them in my opinion. I've got little enough time to get on with my Locost without having to repair the lawnmower so my aim here is to rebuild the carb, replace the pull-start clutch, renew the throttle cable and renew the air filter. That should keep it going for a couple of years at least.

Good news is that I got to spend a good 4 or 5 hours on my Locost today. Got the rear luggage bay cover 90% completed with just the two corners to pleat nicely round the curve now.

Cheers,
Craig.

Craig,

Had a word with Russ today and he says it's probably the diaphragm. You'll need to separate the carb and tank and hold dia up to the light (while stretching it slightly). Even a pin prick hole is enough to make them run rough, so replace.

Yet another suggestion for you to look into, I'm afraid.

ATB

Simon

Thanks Simon, that's pretty much what I'm expecting to find and I received confirmation today that my parts have been dispatched so I might tackle it tomorrow evening.

Cheers,
Craig.

Another vote for the diaphragm, my parents B&S mower had the same symptoms. New diaphragm sorted it out.

Not too dificult to change either.

Purrrrrrrrrr!

That's the sound it's making now!

Replaced the carb, diaphragm and gasket along with a new throttle control, air filter and starter clutch. What a massive difference!

Thanks for your advice all, much appreciated!
Craig.