Triumphrat

funhunter63 · 04-09-2007

I just got a 99 Thunderbird. I like it a lot and it runs great. I have been reading all these threads about mods on the airbox, carbs, inlet rubbers. I think I want to try some of those mods. How do I know if the inlet rubbers are restricted? I don't see the dimples on them. I has aftermarket silencers on it and the guy I bought it from said the previous owner had those put on and the carbs tweaked during that process. Maybe the mods have been done already. How could I tell be looking? I am trying to make a plan and order the parts I'll need like jets. Maybe some of you have some jets that would work. How do you know where to set the air/fuel adjustment without a CO reader?

idahotb · 04-09-2007

Fun-
Congrats on a great purchase. Others may pipe up with a lot more knowledge than I have, but I will try to answer some of your questions.
Restricted carb rubbers were used in some of the early T-birds with Mikuni carbs. I think that by 1999 all of the T-birds were using Kheihin carbs. "Tweaking" the carbs can mean a couple of things, Usually folks who put on less restrictive exhausts richen the mix by changing the mixture screws (to 2 1/2 - 3 1/2 turns out). You could probably tell is these had been adjusted because the mixture screws are covered at the factory by little aluminum caps (aka EPA or "Dork" plugs). If you use a mirror to look at the bottom of the carb bank and see the screws, they were tweaked. Typically that just helps the bike run a little better and pop less on deceleration. If you want to improved performance you have to rejet the carbs. You can't really tell this without tearing down the carbs, but for a re-jet to be really effective you have to modify the airbox too (by adding holes), and that should be visually apparent.

For more on all of this see Jimmy's article entitled Tuning the Hinckley Classics in the Downloads section.

funhunter63 · 04-09-2007

I can tell that the EPA or "Dork" plugs have been removed when they put on the new exhaust "silencers. I can tell that all three of these screws are not set the same. The center screw is a lot further unscrewed toward the spark plugs. I can see the brass from the outside. I can't see it on the other two carbs. I guess they might be that way because whoever did it had a EPA CO tester? Any idea on the size of jets/shims needed and number of holes in the air box to start with. It would be nice to just use a good know setup. Maybe it is not that easy.

idahotb · 04-10-2007

The middle carb mixture is often richer, because it "steals" it's fuel feed from the outside carbs. There seems to be a consensus that the "best" tune for the Hicnkley classics is a Factory-Pro jet kit with their Titanium needles 4 from the top, 38-40 Pilot jets, and 120-130 main jets, with 4-6 1 1/2" holes in the the air box. I haven't done this personally, mostly because I don't have a dealer near by, and don't want to screw anything up. But if you do a seach on carb re-jet in the Hinckley Classics forum, it seems pretty formulamatic. As I said before, check the Classics Tuning doc in the downloads section for more and better information than I can give.

Oh, and let us know how it turns out!

jimmyj900 · 04-10-2007

Quote:

How do I know if the inlet rubbers are restricted? I don't see the dimples on them. I has aftermarket silencers on it and the guy I bought it from said the previous owner had those put on and the carbs tweaked during that process. Maybe the mods have been done already. How could I tell be looking?

Verify which carbs you have -- Mikuni or Keihin CVK. There have been a bunch of carb swaps and tuning for one brand will be a disaster for the other. On a '99 you should have the Keihin CVKs.

If there are no dimples in the sides of the carb rubbers (engine side) then you've got the unrestricted version. These were installed on all Keihin models and the restricted rubbers were used on the Mikuni models.

The only way to determine what has been done with the carbs is to remove and inspect them. The stock Keihins use #28 pilot jets and #98 main jets with the stock mixture screw setting at about 1 turn. The stock needle is not adjustable and stamped N3RF with no shims under the needle head.

With the stock (unmodified) air filter box the biggest main jet you should install is a #100 and the biggest pilot jet is a #40. Larger jets will make the carbs too rich for performance. Power drops faster with an overly-rich mixture than a lean one, so take it easy on the jet sizes.

If you don't want to tear the carbs down, get a dyno run with a printout of the air:fuel mixture versus torque/horsepower and post it here. I'll take a look and see what's going on.

Take a look in my photo album for jet locations on the Keihin carbs.

Jim

Rhinoe · 04-10-2007

I think Jimmy means #38 pilot jets are stock (not #28).

Jawsh · 04-10-2007

By the way, put some Galfer stainless brake and clutch lines on when you get a chance. They really improve the feel and progression of the brakes.