Hi all,

Just a little update and pic of my thundercat engine 3tj race bike:

So without any testing I decided to pit my newly built bike against the Thunderbike Sport and Extreme classes at Bemsee's opening round at Brands Hatch. With no real idea about correct gearing I opted for 14-46 and set the suspension up as best I could and headed out for the first qualifying session with the sport class. I knew the bike would be close to the power to weight limit of the class but couldn't do a check on the dyno before going out so I put 15l of fuel in just to be on the safe side (The bike weighs 158kg and makes anywhere between 88-100hp depending on the dyno). My intention in the first qualifying session was to get used to the handling and engine of the bike and took it easy. The bike was fast in a straight line but the front suspension was too soft However we still managed to qualify in 4th position which I was happy with.

I decided to not alter any settings but just try alternate lines out of clearways to avoid the worst of the handling and headed out for the second qualifying session with the Extreme class. I felt a bit more confident with the bike and started to push a little managing to qualify in 8th position with a best time of 52.4 which I was impressed with considering the gearing wasn't quite right and I was still getting used to the bike. Unfortunately that was the end of the road for the little Yam. The extra weight of the engine was compressing the front under braking more than normal and the tyre wore through the radiator and holed it. Despite efforts to fix it before race one, the rad still leaked as I headed to the grid and so I decided to run my ZXR400 in both classes for the day. Given that I had changed bikes from the qualifying sessions I was forced to the back of the grid for both the opening races which was unfortunate but fun. Sunday produced wet weather and I opted to stick with the Kawasaki for fear of crashing the oh so beautiful Yam!!.

All up a very promising start for the Yamaha as it produced faster laps in qualifying than the Kawasaki did in race one. The conditions were excellent and the Kawasaki got down the a 51.6s lap and so I feel that the Yamaha has the potential to go under 51s at brands. Cadwell is up next if we can solve the radiator issue. I suspect I will need to stiffen the front up somewhat and move the rad up and back if at all possible

It sounds positive apart from the wheel travel holing the rad.

I guess this is down to the header pipes pushing the rad forward as well as the softish suspension. You can use heavier fork oil to dampen the action, that's fairly common on a race bike. You should be able to get stiffer springs from one of the suspension companies.

What carbs, jetting & airbox specs are you finding works ? Got a 3tj/4dx thundercat with (from memory) cat carbs, opened up 400 airbox, but it's not been used in anger yet. Last owner said it ran up Street OK, but I'd like a known Base setting if you'd share. Many thanks

Unfortunately , no money for any new suspension, I've now managed to move the rad up and back and have got about 3cm of clearance with the forks fully compressed so hopefully that is sorted now.

The carbs are from a yzf600 thundercat, I don't have an airbox and I am running a standard main jet, pilot jet etc (155 from memory). I have blocked off the air correctors which made all the difference. Oh, also shortened the bellmouths by about an inch, but that was only so they would fit under the tank. Hope that helps and no totally don't mind sharing the info, that's what this thread was all about. I added pics of the carb set up in workshop section "How to fit a 600 into a 3tj".

Cool cheers.

This one is cat motor, loom, carbs & TPS, 3tj airbox modded to similar volume as the cat one (I have a cat one available too)

Another update on the FZR 600 3TJ after the Rd 2 Bemsee Thunderbike sport @ Cadwell:

Moving the radiator up and back appears to have solved the issue of the front wheel making contact with the rad under braking which is great news, I was able to compete without mechanical issues of any sort.

I now have three further issues to resolve before I think the bike will reach it's potential. The extra power of the thundercat motor coupled with the nature of Cadwell park caused numerous tankslapping moments. The bike will lift the front wheel out of chicanes/slow corners and when it lands causes the bike to go into an oscillation which is only cured by letting off the throttle. It will also tankslap quite badly off the start line which makes getting away quickly difficult. More worringly I got into a major tankslap just before the braking zone of Park corner. I''m guessing it does this as the slight rise in the track unweights the front and as I was going past a back marker at the time I pulled the bike to the right before I normally would. I think a steering damper and extra compression damping on the rear will cure this though.

Gearing is another problem. I ran 14:46 at Cadwell and never got into top gear down either straight but that may be because I was a bit tentative with the throttle coming onto each straight. Elsewhere the gearing was okay but that's down to the bikes ability to pull like a train from as little as 6,000rpm all the way to 12,000rpm.

Final problem is the bike makes too much power! I was 5hp over the power to weight limit so I will need to restrict the top end. I could add weight but I feel that that goes against the whole point of this bike. Unfortunately I was checked after the race and so the bike's first race win was disqualified. I was a little gutted but I the bike still managed a second place with full wets on a dry track against other bikes on full drys ( I know poor tyre choice I'm afraid). However I knew I would be called for a post race check as soon as I opened the throttle on park straight and the bike went past everything else like they were standing still.

Once these mods are done I think the bike will be extremely competitive.

PS I am still trying to get to grips with how to effectively launch a Yamaha clutch. So far the most effective method seems to be to have the bike at 3,000rpm off the start, let the clutch fully out and then just drive off the throttle. It's the only way I have managed to avoid the on/off nature of the clutch resulting in repeated wheelies

If you used a couple of links in the chain with a smaller rear sprocket you could effectively extend the swingarm length which might calm it down a little ... ?

Yeah, it's an idea. I've got about 2cm left of adjustment. I may add 4 or 5 links, put a 48 rear sprocket on to try to use top gear and then hopefully have enough to slide the wheel fully back. It'll be interesting to see the effect on the handling. I really need a trackday to try all these things out as it's a bit hectic doing it at race meets whilst running two classes

You can cut the back of the swingarm a little so the spindle can move back too.

Another update on the FZR600 3TJ:
I had the bike out again at the Bemsee Brands GP meet in the Thunderbike Sport Class. Since it's last outing I have added a steering damper to the bike and added more compression at the rear and less rebound damping on the front in an attempt to control the tankslapping.

The changes have made a world of difference, the bike handles well and never gives a hint of tank slapping. The front still goes light off the throttle but it's totally controlled. I have had to put a baffle in the exhaust to limit the power to 88hp (it makes 92hp on Mark Dent's dyno) and now carry 12l of fuel to ensure I still comply with the power to weight limit. The results from the meeting don't really do the bike justice as I'm still trying to get used to the bike. It feels very different to my zxr400 and needs to be ridden in a different manner in order to make the most of the extra power. The bike managed a 4th and 3 x 5th places against R6s, 749s and GSXRs but more importantly for me it had the third fastest laptime of the weekend in it's class at 1min 37.7s ( the TBS lap record had stood at 1min 37.4 coming into the meeting) despite running on a set of scrub tyre given to me. I think that a new set of tyres coupled with a stronger front brake could easily see the bike get into the 1min 36s. Next step will be to strip the front calipers and replace brake fluid


I have finally figured out the clutch on the yam. The secret seems to be to ensure that the bike is not too hot before trying to launch it. Once hot the clutch becomes grabby and is terrible but if I can keep the temp down it's much better. So, arrive late to collecting or switch the engine off whilst waiting to go out