Anyone know anything about lawn mower carbs?

I have a Mountfield SP470, with a Briggs & Stratton LS45 engine.

It has a carb very similar to this...



there is no manual throttle, you just start the mower and it maintains steady revs. If something is starting to stall the engine, it adds revs until it has overcome the resistance.

To do that the lever attached to the top end of the spring mechanism moves back and forward to open and close the throttle automatically (a governor of some description?).

The sprung mechanism has started oscillation open and closed, so instead of a steady brrrrrrrrrrrr at steady revs, I'm getting brum, die off, brum, die off, brum, die off. If I hold it still with a screwdriver I get steady revs, so the engine is fine, it's definitely a carb problem, or whatever it is that controls this throttle mechanism.

Any ideas?

(other than rip it off and put a manual throttle on which I am tempted by)

Looks like it's a 'pulsa-prime' tank mounted carb

Think I may have found the answer at 6 mins 5 secs

[maviczap]

A thread answered by yourself Ralph

I have one similar. Only problems I had were air filter dirty (cleaned, oiled fixed that) and spark plug failed. Fitted new spark plug and work once, next time gave similar symptoms to yourself. Bought yet another spark plug now without any problems for months. Spent ages stripping thing down finding out how it all work. carb is held open by fan on top of the engine. Hand control is just a brake.

You can manually bend/alter the part (above part B on the diagram?)under the air filter holding the spring tensioned to find the right revs at full throttle. You should always cut at full chat. (6min 5 sec on the vid)

We do this using some device on the spark plug lead which measures rpm and alter the part (moving back or forth) until the revs match what they should be.

If it doesn't have a throttle, its the same principle as its always at full chat rather than being able to idle.

[mister c]

If it's hunting then it's sucking air from somewhere. If the video doesn't help, then clean the foam filter with petrol, let it dry then soak in engine oil & squeeze out the excess. You will probably find that is the problem.
I have no sound at work, so don't know what the bloke is babbling on about, but also check the filter on the bottom of the carb for dirt.