I have learned that every carburated bike is it's own entity, unique and with it's own set of characteristics. What is applicable to one, may not work for another...... except for perhaps mechanical Truths.
I put 22,000 miles on a Bonnie 790 with it's divider/restrictor plate removed, complete with open pipes, free'er flowing air filter, and a bellmouth intake. It ran great, and consistently read at 59 to 61 horsepower to the rear wheel on different Dyno's under different conditions. Unfortunately I can't recall what size main jets I settled on.....
So to start anew on a brand new 790:
This time I left the restrictor plate in the airbox. I'm using the same free'er flowing K&N, similar pipes (modified D&D's), and the same NH bellmouth (now powdercoated). And it's already been read at 59 hp even when VERY lean (for those interested in the restrictor plate 'out' argument -vs- restrictor plate 'in').
My 118 mains resulted in a very lean condition for the first half of the trip to 8000 rpm at wide open throttle. And my 40 pilots, with three turns open, made for a VERY difficult start when the motor was cold.
I already have some 122.5's sitting around, but given how lean I was running I decided to stick my old 130's in. The pipes seem quieter, there is less popping, it starts up quite easily when cold, and has a great deal of 'pep' while the motor is still cold.... then the performance falls off as it warms up.
I have a slight hesitation between on-throttle / off-throttle and concluded that I ought to add a shim...... NOT!
First things FIRST!
Settle on the correct size main jet before coming to any other conclusions. The symptoms as stated above is an indication of too rich a main jet (great 'pep' when cold, turning anemic when warmed).
I plan to install my 122.5's simply because I already have them..... I'm a big trial & error kinda guy. AFTER I settle on my choice of main jet (122.5 or 125), if I find that I've got a stumble (lean condition) between on-throttle / off-throttle, I will NOT hesitate to shim my Thruxton needles. It is said that the design of the Thruxton needle can be likened to a standard Bonnie needle with shims, but I KNOW from experience that a single shim on the Thruxton needle can in fact resolve a problem (some people insist that a Thruxton needle should NEVER need to be shimmed).
I also plan to replace my pilot jets with the 42's still sitting in the kitchen. I'm sure they will address any cold starting problems I might have. The bike fired up effortlessly when it was totally stock, but once you open things up..... the WHOLE package needs to be synched to work together. By installing larger pilot jets, the idle screws don't have to be advanced as far out as when you were seeking the right main jet size (maybe 2 1/2 turns -vs- 3 turns, as opposed to the factory 2 turns).
Drilling out that air hole? I've concluded that if you like to mess 'round with stuff needlessly..... drilling that little hole out to 3mm will do nothing more than give you more efficient throttle response..... no more horsepower, no more chicks, no bling bling factor..... and some people even suggest that simply replacing the spring in you carb's can have pretty much the same effect. I'm not gonna bother, with either.
I've also discovered that my butt-Dyno is getting an edumacation from all of this trial & error, and though I am learning to recognize jetting symptoms, an air/fuel ratio sniffer on a Dyno is far more sensitive than my limited experience (one year of messing around).
And when I finally get it all down, only the old fashion Triumph lumbering piece of archaic Bonneville iron, with it's anemic horsepower, poor brakes, cheap suspension, and obsolete carburators, will be the only bike around for me to have all this fun I've been having on...... And I'm love'n it!
[ This message was edited by: FattRat on 2006-12-15 09:45 ]
