Changing Mikuni’s to Keihin’s and installing pod filters

I had got a bee in my bonnet about swapping the carbs ! A few folk on the site had gone that way and it inspired me to have a go, some of them had posted up their settings so with all their hard work done I figured I would have at least a bench mark or close to the finished set up.

I could have just stuck pods on the Mikuni’s, rejetted and seen how it went but fancied the better mpg of the Keihin CVK’s. There is also a bit of talk that the Mikuni’s slides & guides wear a bit more over time, this is because they are plastic as opposed to the metal Keihin’s, this would only really come into play on higher mileage bikes (of which mine was slowly getting there..!)

What I wanted to achieve was..

Removal of the original air box chambers & tubes
Better Mpg
If poss better power…but would be happy if it was just as good
Maybe better looks

I bagged a set of low mileage CVK’s off ebay (they go for between £80-£120+ depending on how lucky you are), they are fitted to 98> bikes Ie: The Legend, Thunderbird, Thunderbird Sport, Sprint, Sprint Sport & Trophy. The classic bikes have a single fuel hose feed and the Trophy, Sprint etc have the same dual hose fuel feed.

Thanks to others I was awear that a few bits would be necessary to get the carbs on the bike, Choke and throttle cables remain the same carb to engine rubbers can remain but I opted to get new ones as they would be the only thing holding the carbs on part No. T1240086 plus some pod air filters to replace the original airbox, you can buy cheaper dry (no oil) pods to fit for about £28-£35 for three or K&N do some for about £100 (ouch !!..muggins here went for the K&N’s..what can I say they looked trick..I couldn’t help myself..ok..) they needed to be 54mm diameter to fit the Keihin’s.

Now part of this project was to make it all look good so I also set about painting the float bowls black & replacing all the nuts bolts with stainless plus stainless jubilee clips (clips need to be 8mm wide). I also had a new throttle cable lying about so set about fitting that too

Fitting the throttle cable required me to grind down a 10mm spanner to squeeze between the carbs and get to the lock nuts..

And to seat the end of the cable I used two screw drivers, one to hold the end of the cable in place and another to push the cable around to access the slot to get the cable home…

So all he tarting about done down to the serious part of rejetting the carbs..Matt off had a setting of 152 mains, 42 pilots, 3 turns out of the pilot air screws, and two 20 thou (0.5mm) shims under the needle..I used small electrical washers they seem to mic up ok for the job.

So… shimmed the needle, remove the carb top and diaphragm to get to the needle…

Popped it all back together then turned the carbs upside down & removed the float bowls and replaced the jets in each carb…mains first

Then pilots, you’ll need a thin screw driver to get in and at them, it has to be a very good fit if the jets are stuck you will only get a few few good chances to get them out before you damage the brass heads

Adjust the pilot screws, wind them all the way in then count the number of turns out..

So time to pop the carbs back on with their new K&N filters on

The Keihins I got only had one fuel feed where as the fuel tap on the Speed Triple has two This meant a T piece had to be installed to bring the two pipes together, whilst at it I routed one fuel line around the back of the frame under the tank mounting bracket and the other the normal way under the frame. I also fitted quick release fuel couplings to aid tank removal in the future…

Only having one fuel in to the cabs doesn’t seem to affect fuel supply at all. Note that it is only the Thunderbird Keihin carbs that have one fuel feed the ’97 on Sprint and Trophies have the twin feed as per the Mikunis.

Got it on the dyno recently and here are the before and after….the bike is slightly weaker lower down by a few HP but because of the larger rear sprocket on the road I’m carrying more revs more of the time anyway & from about 6K on it’s stronger…still some tweaking to do

Now due to a fuel starvation problem caused by an experimental fuel filter I’d fitted !? I ended up down sizing the main jets as initially I thought it was just running too rich…but that had nothing to do with it in the end once the fuel filter was junked the bike ran fine….

So now I’m running
142 mains
42 poilots
3 turns out on the pilot air screws
Thruxton needles with 1 shim
Heavier springs

All good so far..I might pop those 152 mains in when I’ve next got the carbs off just to see if I’m missing anything.

Update: 152s are in and nothing detrimental to performance so they are staying in for the time being ! I have chopped out the K&N pods and fitted some RamAir foam pods mainly because I understand that CV carbs like to breath from the back and the blanked off rear of the K&N pods may work against them a tad. These foam pods are lighter and breath all round.

I re routed the breather as well as with my Hi comp pistons in I was misting a little oil out of the shortened crankcase breather so I extended it with a bit of black braided hose (see the crank case breather page 😉


The Ramair filters being made of a soft rubbery compound I found didn’t clamp up well onto the Keihin carbs. A friend made me some aluminum mounting bosses that served another purpose by unblocking the idle air intake in the venturi. A common problem with pod filters and part of their issues with poor set up.

I bonded them on with some Silkaflex I had lying around and cut some clamping slots in them.

Since all of this I have fitted an Ignitech ignitor box and had it on the Dyno for set up

Final settings running through 3-2 open reverse cone mega exhausts are

148 mains

42 pilots

2.5 turns pilot screw

Thruxton needles

3mm off the float height

Needles shimmed with 2 washers (20thou)

And ignition retarded slightly at the top end (probably because of higher compression).