Triumph Rat Motorcycle Forums banner

1 - 10 of 10 Posts

·
Registered
Joined
·
41 Posts
Discussion Starter #1
I bought some Thruxton needles to throw in my Scrambler during the re-jet, but last night I read that the stock needles are already different than the stock Bonnie needles. Does anyone know how they are different?

Today, I put on the Thunderbike exhaust. What a cool sounding set of pipes! Definite power increase as well.

Thanks for any feedback.

mike
 

·
Registered
Joined
·
3,588 Posts
I believe the Thrux needles are used in both the T-100 (865cc) and Thruxton. The 865cc 270 degree motors may have a slightly different needle (as evidenced by a different part #); however, it will probably be very hard to tell the difference. It only takes a few minutes to pull one and compare. If it is the same or nearly the same as the Thrux needle, sell the new Thrux needles to somone who has a 790 cc bonnie. :-D

Larry
 

·
Registered
Joined
·
2,472 Posts
Believe it or not, if you put a K&N air filter (or Uni) on your bike at the same time that you put your Thunderbike cans,

then a simple exchange of your factory needles with some Thruxton needles may be all you need to get close to the correct air/fuel ratio..... without having to change out the Mains.
 

·
Registered
Joined
·
41 Posts
Discussion Starter #4
Yeah, that's what I was thinking. It runs really well now, but until I screw up and getting running worse, I gotta try to make it better. My thinking is Thruxton needles with one shim, 120 mains, 40 pilots, 2-3 turns out, and possibly drilling the air hole in the slide to 3mm.
 

·
Registered
Joined
·
679 Posts
What do you hope to improve by drilling the slides?
Be careful you may not get what you wish for. It is pretty hard to undrill a hole.
 

·
Registered
Joined
·
2,054 Posts
Also, Thrux needles do not normally need a shim.
They are pretty rich already
G
 

·
Registered
Joined
·
2,472 Posts
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 ]
 

·
Registered
Joined
·
1,576 Posts
Mike,

I sent my Scrambler needles to Peter Jenks at Jenksbolts when I bought Thruxton needles from him. He measured them and found the Scrambler needles to be somewhere between the older Bonnie needles and the Thruxton needles, ie. richer than standard Bonnie but not as rich as Thruxton. If you email Peter he'll give you the info' you need :)

[ This message was edited by: johnyC on 2006-12-15 11:01 ]
 

·
Registered
Joined
·
41 Posts
Discussion Starter #10
Thanks for all the info. I'm just waiting on the carb stainless screw kit to come in the mail before I continue my jetting experiments.

As far as drilling out the air hole in the slide, I got that info from the Jenks guide. I'll probably never do it since I don't want to have to buy new slides if I screw it up. But it's like driving by a car wreck and not looking; once you read about a possible performance increase, it's hard not to try it, especially when you're as dumb as me.

Mike
 
1 - 10 of 10 Posts
Top