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14666Re: [Small4-strokeEngines] Re: motorcycle engines

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  • factoryfit
    Sep 1, 2014

      You can't just bracket behavior on two and four strokes, stop a two stroke by switching off then full choke to flood the crank with gas and oil and they do last better, run them regularly to stop the crank bearings rust-pitting and they do even more. Rotax agent in the UK used to advise huge amounts of servicing, Air Creation in France just run them to 750 hours without stripping, then throw the entire thing away. Servicing at high hours risks things like mag coils and ignition coils failing, seals leaking and so on.

      I've had MORE failures with the Briggs than past two strokes, as the 2-cycles were factory mounted and pretty well sorted out so ran OK if looked after. The aim of using the Briggs has succeeded beyond my hopes though, as it burns 6.99 litres per hour (1.85 US gal) pushing a draggy naked weight shift around the UK. Had her to 8,490 feet trying to get into calm air above the thermals, and that meant a long time, perhaps an hour, at full throttle and still got 7.5 litres per hour.

      So failures? first one was instead of using the stepped flywheel key to advance the ignition as it didn't seem accurate enough, mounted the coils on alloy plates to shift them exactly where required, used CAD to work it all out. They lasted maybe 50 hours before dropping a mag coil into the plastic fan and busting the nose off the starter motor; the little 4mm screws didn't like having a large lever moment to cope with. Bigger screws fitted, 5mm metric countersunk and lots of Loctite, messed up one thread and managed to get 6mm into the mounting tower off the crankcase. Staying put now.

      Next stoppage was the terminal vibrated off the little KTM carb heat element; for some reason Briggs ice the carb rapidly and mine needs constant 20 Watts, it fades inside five minutes without. As I'd flown the Channel to France earlier that year it now has two carb heaters, and screw clamped terminals...;-) 

      Just managed to get to 100 hours and checked the valve clearances, am using Al Hodge's roller-tipper billet rockers, which are 1.5:1 ratio instead of 1.1 for more valve lift, in conjunction with valve spring packing washers to stop float. Clearances had opened maybe 2 thou. on the inlets, exhausts staying set. For some reason the clearances are 12 thou" inlet and 16 thou." exhaust set hot, wish he had them at stock 6-7 thou. as they clatter a lot on tickover, but as his camshaft is the Performance V-Twins Chevy grind and 280° duration I worry that closing up the clearances will increase the duration too much.

      So the point is halved fuel burn, my range is superb with a 35 litre tank, recently flew down the Duxford to see the American aviation museum into a headwind, set off at 08:18 and got there at 12:00 exactly, burned 26.5 litres thrashing it. Ground speed 45, air speed 60mph.

      This weekend am hoping to get to Isle of Wight curry and beer fly in, 252 miles with a tailwind, expecting to burn 29 litres so will have a reserve of 6, or nearly an hour's flying time.

      447 Rotax was a tad more powerful but twice as thirsty for the same cruise.


      Kev Armstrong (no relation to astronauts or cyclists)

      PS multi Vee belts are brilliant, mines a 14 row and the factor you have to add for an I.C. engine takes care of the issues. Redrive is $450 plus carriage made to my design from Ace Aviation in India, by a Brit who lives there. I've no commercial connection but the redrive is tasty.

      Briggs Vanguard Ultralight Engine ground run

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