Mower engine probs

[robgraham]

I have a mower with a Briggs & Stratton SP470 engine. Mostly it runs fine but occasionally it decides to run rich and hunts, sending out puffs of black smoke. I had thought that this was due to an oil-clogged air filter and cleaned this up. This cured it, but the problem has returned, and the air filter is clean. I speculate that the cure was pure coincidence.

The problem is that there are next to no adjustments. Throttle position maybe, although that's regulated by a spring. No choke, no air control, no fine running adjustment. What can the matter be?

Rob Graham

[cj]

Hi whip off the air filter and see if the governor linkage spring has snapped .This is a very fine spring slid over the likage wire which takes the slack out of the linkage they are favourite for snapping off at the butterfly linkage end.

HTH CJ

[robgraham]

No, it's not that. I've checked.

Rob

[cj]

Hi Seems you've checked the obvious ,is the auto choke set correctly? sometimes this linkage works loose or even falls off. If not is the plug oiling up? If so suspect worn valve seals or rings. Strip the carb' and check the float needle some times the snap. Again just a matter of ruling out what it's not. good luck CJ

[robgraham]

Now, autochoke. Where's that? There seems to be a fairly large steel pressing that is in the form of levers, where the springs are attached. It will swing 10 - 20 degrees or so one way or the other by hand, but doesn't seem to have any effect. Is this part of the autochoke?

Rob

[cj]

Hi the auto choke operates a second butterfly valve across the inlet of the carb' and is operated by fully opening the throttle if you watch the linkage at the carb' you should see that after it hits the stop there is a further travel as the actuator plate pushes closed the auto choke butterfly this is returned by a small spring as you close the throttle. These used to seize with grass cuttings or the spring snap causing the choke to bounce on & off. Bear in mind this is old school B&S so they may have altered the choke arangement since 1990. Again B&S engines are very simple so start by removing the air filter completely so you can see the throttle/choke system ,move the remote throttle on the handle and see what moves,if the cable does not cover the whole range of movement from stop to choke use the ajuster (this used to be simply unclip the cable outer pull it into position and reclip).

HTH CJ

[robgraham]

Thanks CJ. I honestly don't think my engine has an autochoke. I've taken the carb apart and there is no butterfly additional to the throttle so I've ordered another diaphragm hoping this will be the cure. I think the lack of an autochoke is explained by the presence of a priming button instead. And this doesn't work now! So hopefully the diaphragm will sort it.

Thanks for your help.

Rob