Triumphrat

Flash451 · 11-17-2009, 07:01 PM

Hi all

I currently have a 1997 Thunderbird with Mikuni Carburetors. I have decided to swap them out for a set of Keihin carburetors off of a similar age used bike to try to get much better gas mileage.

I am not certain about which parts I will need to purchase to properly fit the different carburetors. Does anyone know what engine number on the 1997 TBird that Triumph started to install the Keihin carburetors?

Also, I did a bit of research on the subject here in the forum and seemed to get conflicting information.

I think the airbox is the same (correct me if I am wrong) but:

I assume that I will need new rubber and clamps between the airbox and carb?

Will I need a new expansion ring or will the one from the mikuni rubber fit?

Will I need new rubber and clamps between the head and the carburetor or will the ones for the Mikuni fit the Keihin as well?

The carb I want to fit is from a 1998 Thunderbird sport. Should I buy the parts listed for the 1998 TBird sport or from the 1997 Thunderbird with the engine number that would be stock for the Keihin carb? It is confusing trying to figure out which parts to order when trying to create a hybrid like this, I have been looking at the Bike bandit site.

Thanks

Slinky · 11-18-2009, 02:36 AM

David,

I switched carbs on my 98 bike. Everything will fit- throttle & choke cable, inlet stubs, main jets etc if you want a quick and easy option.

I went with a later airbox, only for the reason that I got it free with the carbs. I would expect the original airbox will work just as well.

You might want to switch the inlet stubs to later ones, your originals will be restricted. if you keep the stock 98 main jets in the Keihins then it would be best to switch the inlet rubbers too.

IrlMike · 11-18-2009, 05:24 AM

I agree with Slinky, I did the swap on my previous 98 Tbird. The TBS Keihins will go straight in with just new rubbers needed on each side. (No need to touch jets etc.) I've read here that the airbox rubbers from the Mikunis will fit ok, but they are slightly different. You will want the intake side ones. Ordering items for a TBS will get you what you want - Miks were never fitted to the TBS. If the TBS carbs are from a US bike, you may want to drill out the pilot (fuel) screw plugs so you can use EU adjustment setting to improve starting.

Flash451 · 11-18-2009, 01:07 PM

Quote:

Originally Posted by IrlMike

I agree with Slinky, I did the swap on my previous 98 Tbird. The TBS Keihins will go straight in with just new rubbers needed on each side. (No need to touch jets etc.) I've read here that the airbox rubbers from the Mikunis will fit ok, but they are slightly different. You will want the intake side ones. Ordering items for a TBS will get you what you want - Miks were never fitted to the TBS. If the TBS carbs are from a US bike, you may want to drill out the pilot (fuel) screw plugs so you can use EU adjustment setting to improve starting.

Yeah, the pilot screw caps are still in place. Any Hints on how to remove them without damaging the internals? I can't remember, but my Haynes manual may have a procedure for removing them. Sorry for the dumb question, but what is the EU adjustment setting for the pilots?

I also need to go through the carbs and give them a good cleaning.

Do you know if the internal expansion ring (between the airbox and carb) are the same? Am I safe in assuming that the external clamps are the same?

IrlMike · 11-18-2009, 01:24 PM

Yes, the expansion rings are the same. The stock pilot screw setting, widely suggested here is 2.5 turns out. I've read here that careful drilling with a small bit, just to the point where you're thru' the thin alloy plug, then yank it out with a small spike is the easiest way to go. (I did this with the carb butterfly shaft plug when I fitted a throttle position sensor.)

GrvThang · 11-18-2009, 04:22 PM

You're doing it to increase gas mileage? How much better gas mileage can you expect to get? If you were to get 33% better mileage, which I would consider to be unrealistically optimistic, and went from 30mpg to 40mpg, with gas at 2.50 a gallon, by my calculation it would take you roughly 5,000 miles to save $100. I don't know if that's worth the hassle and expense - is performance increased as well?

Flash451 · 11-18-2009, 06:22 PM

Quote:

Originally Posted by GrvThang

You're doing it to increase gas mileage? How much better gas mileage can you expect to get? If you were to get 33% better mileage, which I would consider to be unrealistically optimistic, and went from 30mpg to 40mpg, with gas at 2.50 a gallon, by my calculation it would take you roughly 5,000 miles to save $100. I don't know if that's worth the hassle and expense - is performance increased as well?

Hey, thanks for the input and the math, but you do not know what I have been through, it is definitely worth the hassle and much more expense than I will actually end up spending.

I have spent this last entire summer riding season trying to tweak these POS Mikuni carburetors into running correctly. I have had them on and off more times than I care to remember, each time changing one thing or another on the carburetor and testing it out. I bet I have pulled them 15 or 20 times (On the plus side I am getting good at it).

My mileage has never been better than about 28mpg and as low as 23 mpg. It would probably average out at about 25mpg. I have replaced needles, needle jets, emulsion tubes, slides, air filter, changed pilot screw positions, needle clip positions, adjusted float heights, etc. you name it I adjusted it, cleaned it or replaced it. The bike still carbon fouls plugs after all of the settings and changes and gets what I feel is excessively low gas mileage. I have come to believe that my Mikuni carb set is essentially a lemon and want to try the Keihin carb. There are pretty good things said about that carb on this forum.

It is a huge pain in the back end to have to flip to reserve at about 70 miles, especially when riding with others. I feel like I am riding with a peanut tank.

Well worth it if I could get 120 miles to reserve and run without the carbon fouling of the plugs. I want to be set up to actually bond with the bike and ride for fun vs. constantly messing with the carb and worrying about the excessive carbon build up. My goal is to not have to pull the carbs again anytime soon, it is a PITA.

Thanks

GrvThang · 11-18-2009, 11:18 PM

Sounds like you're having major problems and need to change carbs anyway. I thought your Mikunis were running fine and you were just doing it for better gas mileage alone. Good luck with the switch.

Flash451 · 11-19-2009, 06:21 AM

Quote:

Originally Posted by GrvThang

Good luck with the switch.

Thank You, I am thinking that it can't be worse and hopfully much better.

cafetbird · 11-19-2009, 12:40 PM

Wish you the best... old Keihins aren't exactly trouble free either. They are both CV carbs, and behave much the same in many ways.

The Keihins do get a bit better mileage, and they don't have plastic slide assemblies. Other than that - not much diff in the way they function...