Mountfield HP185 (Briggs Stratton engine) surging/hunting?

[Ripp]

My Mountfield HP185's engine is surging when running, it's one of the ones with a plastic carb which has no adjustment screws on it at all anywhere.

Things I've done so far:

Stripped and cleaned carb
Put fresh fuel in
Cleaned Air filter
Gapped spark plug
Reset governor position (as per Briggs & Stratton instructions online)

When running I can see the governor arm constantly moving back and fourth which is causing the carb to constantly open and close. I disconnected the governor from the carb to see how the engine runs and for some reason when the linkage on the carb is manually pushed to the fixed idle stop the engine cuts out, so I'm wondering if because under normal operation the carb is pretty much supposed to idle that because mine won't idle the governor keeps kicking in and opening the carb and upping the revs momentarily then it just keeps getting stuck in a loop of opening and closing the carb, hence the surging.

So, am I right in thinking that with the carb linkage pushed up the the fixed stop that the engine should be idling quite happily and not cutting out?

Temporarily I've added my own adjustable stop so when it's idling it doesn't cut out but this isn't the best fix and isn't how it is supposed to be so any advice on anything else I can take a look at would be great.

FYI there is definitely no adjustment anywhere on the carb or the carb linkages.

This is the mower:

https://www.screwfix.com/p/mountfield-h ... ower/1323f

And the carb:

https://www.outdoorspares.com/Outdoor-s ... 979-595656

And my temp fix:

The screw prevents the carb from closing all the way when at idle

[dewaltdisney]

I found that the governor spring is usually responsible for surging. They tend to fatigue out over time and I tinkered with one by shortening it a bit by putting a fresh bend in the spring end. I view mowers as disposable these days and I put £5 a month away for replacement. As soon as it plays up usually four or five seasons, I replace it. I ponced around replacing a carburettor once and got another few months out of a mower before something else went. So the £60 I spent was a third of a new one. Same rule applies to washing machines.

DWD

[Ripp]

I found that the governor spring is usually responsible for surging. They tend to fatigue out over time and I tinkered with one by shortening it a bit by putting a fresh bend in the spring end. I view mowers as disposable these days and I put £5 a month away for replacement. As soon as it plays up usually four or five seasons, I replace it. I ponced around replacing a carburettor once and got another few months out of a mower before something else went. So the £60 I spent was a third of a new one. Same rule applies to washing machines.

DWD

I have actually ordered a new spring which hasn't arrived yet but after finding out that the carb won't run with its linkage in the idle position I suspect that maybe there is an issue with the carb itself.

[dewaltdisney]

Baring in mind what I said above, how old is it?

DWD

[Ripp]

Baring in mind what I said above, how old is it?

DWD

It's 3 years old but runs like a dream other than the surging. It gets yearly oil and filter changes and blade sharpening.

[Ripp]

I'll see what happens when my new spring arrives.

[dewaltdisney]

Yes, it should last longer than 3. The spring might do it. Worst is when the carb gets gummed up jets.

Lets us know how you get on. There is nothing a good sledge hammer cannot cure

DWD

[Ripp]

Yes, it should last longer than 3. The spring might do it. Worst is when the carb gets gummed up jets.

Lets us know how you get on. There is nothing a good sledge hammer cannot cure

DWD

lol. I've poked a small rod in all the jets and holes in the carb to clean them out.

[peter c]

Had the same problem with my Webb mower which has the same B&S engine. The spring stretches I ordered a new spring and being a tight wad bent the old spring to the new ones shape. The mower now runs well.

Just a small point do you drain the fuel tank at the end of each season? Now days I use fuel stabiliser which seems to help.

HTH

Peter C

[columbiers]

If it's not the spring- clean the carb again.Not just poking the holes through but a thorough soaking in carb cleaner and spraying through. Leaving any old fuel in over winter does this, classic issue.

As above, best to treat mowers as disposable items these days. Older B&S engines would run forever with a new diaphragm every few years, deck would rust out long before the engine died. Not convinced by the newer plastic carbs. Even if they last, 5 years from Mountfield deck is good going these days

[Argyll]

There's stuff you can buy from Halfords that you can put in the fuel when running that's supposedly meant to clean the carbs out.

I used it on an old strimmer last year and it worked a treat.

[Ripp]

If it's not the spring- clean the carb again.Not just poking the holes through but a thorough soaking in carb cleaner and spraying through. Leaving any old fuel in over winter does this, classic issue.

As above, best to treat mowers as disposable items these days. Older B&S engines would run forever with a new diaphragm every few years, deck would rust out long before the engine died. Not convinced by the newer plastic carbs. Even if they last, 5 years from Mountfield deck is good going these days

Not supposed to use carb cleaner on plastic carbs as they warp.

[Ripp]

Had the same problem with my Webb mower which has the same B&S engine. The spring stretches I ordered a new spring and being a tight wad bent the old spring to the new ones shape. The mower now runs well.

Just a small point do you drain the fuel tank at the end of each season? Now days I use fuel stabiliser which seems to help.

HTH

Peter C

I'm pretty sure its the spring, my new one should arrive soon.

[columbiers]

If it's not the spring- clean the carb again.Not just poking the holes through but a thorough soaking in carb cleaner and spraying through. Leaving any old fuel in over winter does this, classic issue.

As above, best to treat mowers as disposable items these days. Older B&S engines would run forever with a new diaphragm every few years, deck would rust out long before the engine died. Not convinced by the newer plastic carbs. Even if they last, 5 years from Mountfield deck is good going these days

Not supposed to use carb cleaner on plastic carbs as they warp.

Yep, heard people say that before- and indeed some plastics can be damaged by it. Can only speak from experience but i've done hundreds of these with no issues. I guess someone somewhere may have had an issue with a B&S carb but i've never heard or seen it.

[Ripp]

If it's not the spring- clean the carb again.Not just poking the holes through but a thorough soaking in carb cleaner and spraying through. Leaving any old fuel in over winter does this, classic issue.

As above, best to treat mowers as disposable items these days. Older B&S engines would run forever with a new diaphragm every few years, deck would rust out long before the engine died. Not convinced by the newer plastic carbs. Even if they last, 5 years from Mountfield deck is good going these days

Not supposed to use carb cleaner on plastic carbs as they warp.

Yep, heard people say that before- and indeed some plastics can be damaged by it. Can only speak from experience but i've done hundreds of these with no issues. I guess someone somewhere may have had an issue with a B&S carb but i've never heard or seen it.

Fair enough. I gave mine a good clean with an air line.