GOLD STAR SET-UP
Sometimes you can’t solve a problem without modern technology. This month Rick took his spluttering Goldie to Hitchcocks Motorcycles – Amal parts stockists with an in-house dyno to reveal where he’s been going wrong
I’m rich up top and have a lean bottom – or was it the other way round? Rick sorts his fuelling
That won’t save you!” grinned Dan Hitchcock, noticing my grip tighten on the front brake lever as the BSA Gold Star’s revs climbed toward 7000rpm. He was right, of course; if the dyno roller stopped rolling both bike and I would be catapulted through the wall...
But it was worth the risk to get my Gold Star carburation right, since finally curing the bike’s 18-year lack of performance it’s been clear that the carburation is all over the place. Trouble is, Gold Stars are fast and noisy, making it difficult to find anywhere you can set them up on the road. When Allan Hitchcock offered the use of their dyno, the chance to rev the bike to illegal speeds within an arm’s reach of tools while the gas analyser figured out what was needed was too good to miss.
Now first, I need to stress that Hitchcock’s don’t offer dyno testing as a customer service. As Dan Hitchcock, who does most of the dyno work, explained: “We use it mostly for programming fuel injection; carbs have so many variables, it can take forever to get right – plus if it’s a customer bike the problem might be ignition related.” I nodded, my eye falling guiltily upon the Lodge FE70 plug in the Goldie – older than I am, I kicked myself for not replacing it.
Hitchcocks mainly use the dyno to evaluate their tuning modifications for both old and new Enfields, but I soon realised that it was not some kind of internal-combustion crystal ball – you need to know how to interpret the readings, and Dan’s expertise was essential.
The plan was to set up both the original Amal GP and also the MKI Concentric I’ve been using lately and see how they differ. Starting with the Concentric, we warmed the bike up before Dan poked the sensor into the exhaust and I started the nerve-wracking business of revving up to maximum revs in top gear. The engine went flat near full throttle, with the fuel/air mix reading in the 12:1 region. Dan indicated to shut down. “Too rich?” I queried. Dan nodded, “You want about 13½ for optimum power.”
This was a surprise having been taught it should be 15:1. Dan shook his head: “14.7 is a theoretical perfect for the cleanest burn; it’s possible on modern fuel-injected engines, but an old design like this will start melting!”
Dropping to a 320 main jet sorted the Concentric out and I eagerly swapped over to my beloved GP. Straight away it was obvious that the bike revved much harder, but near the 7000rpm redline there was an occasional misfire, completely confusing the readings. Dan took over the throttle here because (frankly, nervous about my engine) I kept backing off slightly, which is another thing that’s guaranteed to mess up the readings.
Lean at the top end, there was some richness lower down. We upped the main jet from 350 to 370, but the misfire was still there. “That could be ignition...” Dan pondered, sending my thoughts back to that ancient spark plug; but there was that richness lower down, and before altering the needle I recalled that the GP pilot screw affects a wider range than usual – a half-turn inward leaned everything off, chased away that misfire and allowed us to find the optimum main jet – which turned out to be 370.
Ha! I knew that plug was alright...
‘LEAN AT THE TOP END, THERE WAS SOME RICHNESS LOWER DOWN’