I'd never been entirely happy with the fuelling on my bike. It had been fitted with a pair of Highway Hawk straight-throughs, which the previous owner had obviously just bolted on & done nothing with.
Fuelling was abysmal & it backfired on over-run constantly. Removing the PAIR valve improved it a great deal & my mechanic "fettled" with it & made it better. I then chopped them down & had to add end-cans
, which made no difference to performance to it all (not that I was expecting, or hoping, them to). It would occasionally "stutter" on acceleration & pop a bit.
I'm not confident enough on carbs to piss around with them myself, so I found a local company, SPR Racing
, who had a rolling road dyno tester. They'd do a run on the rolling-road & decided what they could do about it. If it needed any parts, they'd order them in & have the bike back to fit & re-test.
Took it over & they strapped it down to the bed, ran it up & the results came up on the screen - now it don't mean a lot to me, but I thought it may be of interest.
The dotted red line (on the bottom graph) is the optimum fuelling, the solid red line is where we started off at. Running lean, not a lot, but enough to be causing the problems I was having. So I though "this is where the expensive remedy is discussed". Initially, they were speaking about a jetting kit with a K & N filter, then one of the guys decides he wants to try something. He ran it again with hand over one of the inlets at the back of the air filter cover. Immediately the dyno showed it was over-fuelling. He then has an idea that if we could sleeve-down one of the intakes, it would increase the fuel/air mixture, effectively having the same effect as changing the jets. They found a rubber grommet with a 10mm hole in it which fitted the air intake hole exactly. Fitted it & re-ran it - still rich, so they trimmed a bit off the hole. After trimming it a couple more times, they then came up with this test result.
They reckon that's the closest you're ever going to get to fuelling perfection. If you have a look at the first graph, you'll see that it's also added another 2.5 hp to peak performance (red line before, blue line after).
This is what they fitted instead of a (very pricey) jetting kit
In situ in the air filter cover (super-glued in place so it doesn't disappear)
The cost - £60 - they only charged me for the dyno run - all things considered, I didn't think that was bad. I know some peolpe like tinkereing with carbs, but unless you have access to a dyno tester rig, youll never going to know if you've got it right. In my experience, you end up stripping your carbs down more than once 'cause the first jet is not the correct one.
All in all, it was an interesting experience, seeing your bike hammering along at about 100 mph & going nowhere. It was also great to deal with a company who didn't immediately see pound signs flashing. On the way home the difference was amazing - pulled cleanly all the way through the rev range with no hesitation - money well spent