TUNING FOR SPEED
RC readers with long memories will recall Lasse Horskjær’s rebuild of his own Nimbus (RC123-126). Not long after it was complete he set off on a trip from Copenhagen to Czechoslovakia, but in Germany things went wildly awry…
‘Without warning my Nimbus suddenly started to sound like a machine gun. With the throttle part closed the sound almost stopped, but it came back in full force with any extra power. I wasn’t in any doubt. This sound could only be because the big ends had lost their white metal bearings surfaces. So very, very slowly I rode uphill in first gear back to our hotel: end of adventure!
‘Looking at the dipstick, I found lots of oil, so the problem wasn’t caused by lack of oil. I believe long distances on autobahns had been too much for the old-fashioned white metal bearings. However, when the Nimbus had been running it was very, very good; comfortable and responsive. So I decided to bite the bullet and treat the motor to a major overhaul (again-again), but differently this time.
‘Luck was with me. A veritable Nimbus guru, Niels Nielsen took interest in my project and allowed me to work in his workshop, doing the rough stuff while he did the brainy jobs. By a miracle, he found one of the very last original Nimbus crankshafts – brand new. He declared he had to “ruin it” right away, and grind all four journals! Why’s that? Niels knows a very smart trick, used in the days where Nimbus machines were raced. He substituted the vulnerable white metal bearings with Fiat 1200 Sport shell bearings which he reckoned would suit my“press-on driving style”.
‘So in went these modern (1976!) halfshell bearings. Niels also talked me into new piston rings, total valve seat renovation, new valve springs, as well as eight new H-D stainless valves, a new oil pump together with a lot of other smaller improvements.
‘The end result? A very, very lively Nimbus, definitely with more power and a 100% clean pick-up from any speed. It now has an absolutely safe cruising speed in the region of 70mph. Top speed is over 85mph but that’s not to be used except in an “emergency”. The funny thing about my new motor is that I don’t need to give it so much throttle as before to hold a certain velocity. At half throttle the bike holds a constant speed of 60mph. Great!’
Lasse Horskjær’s Nimbus; somewhat modified
Left: The con rods have been converted to take shell bearings to replace the original white metalling
Right: Neat and simple; the engine from underneath shows off the four con rods and two big main bearings
Right: H-D valves have replaced the originals