Main Motorcycle: Can't decide now!!
Join Date: May 2008
Location: Merseyside, UK
Other Motorcycle: Scram + Thrux
Extra Motorcycle: '71 BSA Thunderbolt
Air intake mods...... Definitive results
As some of you here may know, a while back I fitted an Innovate A/F gauge on my Scrambler.
This now gives me real-time readout of the air:fuel mixture, as I make adjustments and as I ride.
After much tinkering I had the fueling just about spot-on for my then set-up, giving as close as possible to 13:1 to 13.7:1 A/F across the board.
This definately being the ratio where optimum performance is obtained. Anything below 12:1 and above 14.5:1 can certainly be "felt" by the rear-end dyno!
This set-up consisted of the usual initial mods we all carry out i.e. free'd up exhaust (in my case Zard 2:1), snorkle removed and Unifilter fitted.
Jetting for this set-up was 115's and 40's, no shims, and 1.5 turns.
There have been so many questions in various threads over the years as to exactly what effect the various mods we carry out actually have. With the A/F gauge I can now quantify these effects.
Firstly I refitted the snorkle. The imediate result was to run WAY RICH (11.5:1) which shows how much the snorkle actually does restrict air flow. A rejet would certainly be required in this case if I was to leave it that way.
Next I refitted the stock air filter. This made NO difference to the readings, either with or without the snorkle. This shows that the Unifilter may be a more efficient type of filter, but it does not improve air flow to the engine.
I then returned to my normal set-up (Unifilter and no snorkle) and then pulled out the air-box restrictor plate.
This certainly caused the biggest effect, this time running WAY LEAN (15.5:1), and I lost a great deal of performance. This plainly shows how much restricting the restrictor is actually doing!
I've now left the restrictor out and had to step-up one main jet size and fit a shim to compensate.
This set-up, with no restrictor, no snorkle, Unifilter, 120's, 40's, 1 shim and 1.0 turn has now resulted in much improved performance and sweet throttle response across the entire rev/throttle range.
It's also worthy of mention that I found very little difference in readings between my original NH Off Road mufflers and the Zard 2:1's, which would indicate that there isn't a great deal between types of exhausts. If they have no baffle then they are very similar.
The above info may help in deciding which air intake mods to carry out, and what effect they will have on fueling, and the adjusments required.
Others opinions and experiences may differ for all this but, as I see it anyway, being able to actually see as well as feel the differences certainly makes for a more accurate means of tuning.
2006 Scrambler, VARK'd, FCR39's & DNA's, 790 In-cam, Pieman'd igniter, Zard 2:1.
2005 Thruxton Black, with ARK, FCR 39's & Uni's, Retimed 790 cams, Barnett clutch and suchlike!!