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Needles General Concencous 790cc Bonnie 2003

1403 Views 9 Replies 8 Participants Last post by  RedBird
OK my Bonnie's a 790cc T100 2003 centennial.

I have finally collected all the parts to tune it how I want, which is not overly radical.

Since I started collecting the parts, which has taken me a year now ( things weren’t so readily available when I started this process ) there have obviously been other developments, leading me to possibly revise my options.

I'm using the Jenks Bolts tuning notes as my template and I have a spare set of standard carburettors.

My question basically is do I go Thruxton needles or Stock needles as per the Jenks tuning notes for "off road exhausts and free flow air filter


.Primarily the most important thing to me is to retain or improve low end power and get rid rid of the flat spot in the mid range without loosing or even possibly gaining top end power.

Which in everyone's opinion is the combination most likely to give me the best starting point.

Stock Needles, 130 mains, 42 pilots, 1 shim, 2.5 turns, 2.5mm air hole.


Thruxton Needles, 120 mains, 40 pilots, 1 shim, 3 turns, 3.00mm air hole.

I've got a local dyno booked for tweaking but which would you say is the best way to go.

Once again thanks for your time.

Jon (UK)
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what parts have you collected, and why a year?

I'd try the thruxton needles straightaway.
Its taken a year due to finances and other commitments. Plus I've collected stuff from all over the place.
England, France, Canada, USA.
The dealers over here in the UK aside for a few don't seem to be all that keen on going into after market parts, possibly because of the European Type Approval Laws.
Which for some reason seem to apply to all bikes except Harley's. I have no idea how they can possibly comply with our noise regulations.

Anyway thats enough excuses.

Thruxston's weren't invented when I started and I only found out about anyone tuning these bikes after stumbling across Bella Corse and this site by accident. Which opened my eyes to a lot of avenues, so thanks to all concerned.

OK the parts: AI removal kit job done.

Norman Hyde Classic Toga Silencers.
Single Unifilter
Norman Hyde Belmouth
Jenks main jets, 120, 125, 130, 135
Jenks 42 pilot jets
Jenks shims .5mm
Thruxton Needles

Plus a spare set of complete standard condition Carburettors and the carbs currently on my bike which are jetted for TOR's with standard air box and filters.

Thanks for your interest

Jon (UK)
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I have a 03/04 bonnie with Staintune reverse megaphones, Thruxton needles, 1 shim, 115 mains, 42 pilots, 2.5mm air hole and 3 turns out. I've also removed the snorkle and installed a uni filter.

I haven't put her on a dyno as yet, don't know if I ever will, but I'm very happy with my set up. IMO I would go with the Thrux needle set up.

Thruxton needles will address your mid-range concern.

I'm running virtually the same set up you've got, and I found the sweet spot to be with 122.5 main jets and 42 pilots. My cans are pretty free flowing though..... your 120's might be fine, possibly asking for a single shim on your Thruxton needles. 42 pilots will make cold weather start up a breeze.

As good as the bike is running (having tried 120's and 130's) I'll be having the bike sniffed for air/fuel when my Predator pipes arrive (WooHOO!!).

By the way, I left the divider plate in place this time. I think the power delivery is more linear and provides better mid-range than without the plate.

In addition, my ProCom ignition module has since been removed when I discovered that it was the cause of my cold starting problem. I guess they're good for 20,000 miles, and my 24,000 miles on it was simply too much...... I oughta try it now that the weather is no longer sub 30's. I haven't yet decided if I'm gonna sping another $100.00 for one via ebay....
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This is all good stuff guys.
Thanks a lot.

Jon (UK)
The thruxton needles will be better than the stock but they (thruxton) tend to be too rich at the bottom. A better needle is the Thunderbird Sport needles (TBS).

The TBS has the same profile as the thruxton but is longer overall. This means you would need 3 x 0.5 mm shims to get the TBS to be the same as a Thruxton needle. This lets you effectively with a thruxton profile that can be leaned off.

However, if you already have the Thruxton needles it is probably not worth worrying about.
I had my 790 checked for air/fuel ratio last week end, see dial-in that I posted last week end. My midrange with the Thruxton needles worked best with no shims on the needles.
Dinqua's jetting calculator will put you right on the sweet spot for a 790.
All carbs are NOT identical...if you have a flat spot in the mid-range now, I would suggest the TBS needles like Darcy said. They can be easily shimmed when on the dyno to get the mid-range spot on. My 790 '03 is a little rich in the mid range with STOCK needles...the other needles were way too rich. The needles should have NO affect on the size main jet will see this on the dyno with EGA if you change needles and main jets.

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