Anyone had experience of using thundercat carbs on a 3TJ hybrid?

I tested reducing main jet size today from 155 to 125 and it was horrible; jolty and bugger all power from 6-7K and wouldn't rev out. Going back to the standard 155 main for the moment but where from here?

It was OK with the standard thundercat carbs but did not want to pull properly from 6K in top (although it would in lower gears). I though it was rich due to th 4JH carbs having 118 mains, so leaned it out.

It's probably something to do with the needle/slide opening rate, which is my next adjustment but has anyone any other thoughts?

try putting the airbox snorkel/lid back on buddy you sound like your going thru exactly what i did trying to jet it right,it will only take 10 minutes and at least you will be starting from a standard place to gauge your results from my 4jh wouldnt entertain running nicely without the lid or snorkel and i really did try every jet and needle combo i had and it drove me mad.just for good measure whip the filter out and run the airbox empty and see how it feels and adjust fuelling from there.the carbs need a steady flow of air and with the lid off the air is flowing and tumbling about everywhere over the hole and then pressure over top of the airbox where the snorkel should be will create all sorts of different pressures and the airbox will be getting an unpredictable flow of air which the carbs dont like.the japs are pretty god at making airbox,s(especially yamaha).or finances allowing get it set up on a dyno so you can see where its running rich or weak and where it needs more or less air/fuel thru the rev range.try starting with the standard 4jh carbs on their standard settings and go from there and try to keep a record of your results so you can refer to them later when you start to feel suicidal like i did!!!!-good luck

a complete aside which I'm sure someone will know the answer to if they've used Tcat carbs on one of these, but -

on the cat because it has ram-air, there is a bleed pipe in the nose of the ram air system which I believe goes to pressurise the float bowls so that the forced air through the venturi from the ram air is matched by a similar pressure in the bowls pushing the fuel through a little - on this conversion does that get blanked off or attached, and given that there is no ram air on the fzr does it even matter?

I also agree that airboxes are essential - I have a YZF750 track bike that had just got the induction rubbers on it when i bought it, ran like shit. airbox added, sweet as a nut.

I also had my FZR600 dyno'd back in 93 with no airbox, and when we put pods on it, it ran much better and also weaker - the mixture being the absolute reverse of what was expected, but proving that carbs won't grab enough air from loose space above the venturi.

good luck with it.

toosmooth

plod

try filling in the left air jet when looking down at the carbs (where the air box rubbers sit) this will allow you to run without air bok lid and no air filter and should smooth out the problem.

Shall take a photo for you later to show how.. also you can drill the lift holes on the sliders to 3mm and this will make throttle response quicker. set needle slide to two from the bottom.

Good luck

LAJ: I tested blocking the pilot air jet on the FZR600 carbs. Yes it did help midrange but was so rich the UHCs stung your eyes.

cagiva996: I know where you're coming from but that's not the route I'm going to go down. Yamaha didn't design the 3TJ airbox to work on the 4JH engine, so it's already compromised. I have FZR600R carbs working (almost*) very well with no snorkel and a #38 pilot jet out of the thundercat carbs (*still a bit lean). Hopefully #42 jets en-route from NZ will be right and not too rich and then those carbs will be finished.

toosmooth: have you got more details of what you're saying here? On the thundercat carbs, there is a series of interconnecting hoses below the bowls. I don't know what they're for and have just left them open, assuming they are a level overflow. The bowl vents connect together and then through an inline filter canister. I've just left this open too. Do you know what these things connect to on the cat?

toosmooth: have you got more details of what you're saying here? On the thundercat carbs, there is a series of interconnecting hoses below the bowls. I don't know what they're for and have just left them open, assuming they are a level overflow. The bowl vents connect together and then through an inline filter canister. I've just left this open too. Do you know what these things connect to on the cat?

my top to bottom knowledge of this is as follows:

When I first bought my racing Thundercat back in 99, one of the first sessions I ran it in at Snett, it ran like a bit of a dog.

I phoned the mechanic who looked after it for Shane Byrne (oops, did I drop a name there) and he said to make sure that the ram air bleed pipe was connected to the ram air in the nose. All I knew was that it was supposed to go to the bowls, and add the same pressure increase in there as was going through the venturi from the ram air.

Mine was disconnected, and when reconnected, hey presto, running problems gone.

I'd guess, using nothing more than common sense, that it was only causing a problem on mine because I had half the ram air system working. If I'd got rid of the ram air to the airbox I suspect it would also have cured the problem. This might translate as on yours meaning it makes not a blind jot of difference if its connected or not, but I've seen lesser things cause bigger problems, and thus it's worth ruling out.

Best way to resolve this is if anyone else has used cat carbs successfully on one of these conversions and then mimic whatever they did!

sorry I can't help more.

toosmooth.

Cheers for that. I've had a look at the parts shematics since I posted this and the ram air has a lot of bits to it by the look of it. However, I've not got a factory manual

Do the thundercat carbs have a water connection? If so then that explains the bottom connections and confirmes that the top connection through the filter goes to the snout.

I believe the pipes on the bottom of the carbs are for heating.. they are connected to the coolant run on the thundercat. I've got mine disconnected on my 1wg/yzf.

Thanks for that, it confirms what I thought.