Kikuyu grass keeps clogging the deck on an old Briggs rotary

[Mxolisi Jaxa]

Hi all, first post. I'm in Gqeberha in South Africa, so some of what I describe might be a bit different from what you get in the UK, but the machines are mostly the same.

I've got an old 18 inch rotary with a Briggs & Stratton 3.5 hp side-valve, no brand name left on the deck, bought second hand years ago. The lawn is kikuyu, which if you haven't met it is a thick runner grass that grows like mad in summer. If I leave it more than ten days the mower bogs down and the underside of the deck packs solid with wet clippings, then it starts throwing them out in clumps and leaving stripes.

Things I've tried: sharpened and balanced the blade, cleaned the air filter, cutting higher and doing two passes. It helps, but after about half the lawn I'm on my back scraping the deck again.

Is it worth fitting a mulching blade or a higher-lift blade on an engine this size, or will that just make it bog down more? And does anybody put anything on the underside of the deck to stop the build-up? I've read about silicone spray and also about people painting them, not sure what actually works.