Running problems with new purchase

Hello all, just bought a Thundercat so thought I would join the forum to say Hi and ask a couple of questions.

There are a couple of issues that I was hoping someone could help with. The first is the bike runs rough between 2000 and 3500 rpm, this happens in all gears and only under load, it seems like it is missing or it is a fueling problem. In addition to this I have noticed that occasionally there is the 3000 rpm TPS error on the rev counter but this is intermittent whereas the rough running is always present.

I checked the TPS as per the Haynes manual and got the following results:

Black and Blue – 4.71K Ohm

Black Yellow – 1.74K Ohm (Closed Throttle)

Black Yellow – 3.81K Ohm (Full Throttle)

The Black Yellow results appear to be out of spec according to the manual, would you agree that it is faulty and do you think this could cause the rough running, if not what else do you think I should look at?

The final issue is that the temp gauge needle only just moves off the minimum reading, is this normal, I would have thought that it should move more than this, I am guessing this could be a temp sender problem?

Thanks in advance for any help that you can provide, Cheers Nick.

Hi, welcome.

Can't help with the TPS, the temp gauge sounds about right - quite normal to run what appears to be too cold, on the other hand when they do warm up (hot days, standing in traffic a long while etc) the opposite seems true, will get up to about 104c before the fan kicks in.

I would say the TPS is your problem; it certainly needs sorting.

Temp guage: normal!

Thanks for the quick replies, I will get a TPS ordered, at least the temp gauge is Ok.

My tps is doing that and the temp gauge is fine. Let me know how you get on and how easy it is lol

Have you got any running problems with yours?

I just checked the price of a new TPS, it is £260

Not sure if this is the same thing but I have a feeling of deja vue here - I just spent a whole w/e recently trying to rectify a problem where my cat would not pickup smoothly between 2-3K. No matter how slowly I opened the throttle, it was like an on off switch 2k or 3k but not able to get anywhere in the range in between.

I suspected a problem with the idle mixture, as at small throttle openings the idle jets still play a part in the fuelling - however after messing about for hours trying different settings I decided that this was not causing the problem.

I moved onto the TPS and found that the readings were not as expected, so I decided to try and get this right. Now a word of warning here to anyone attempting this - if you decide to displace the carbs from the manifolds so that you can get to the TPS screws make sure you plug the inlet manifold with some rag/kitchen roll before unscrewing the sensor.

Stupidly, I decided to take the sensor completely off and in the process managed to drop the small bit of plastic that couples the TPS to the carbs



And yes.... Murphys Law strikes again, guess where it went.....



Can you see it....



Well to cut a long story short, and to remove a lot of expletives, I managed to extract with with a hoover and some wine making plastic pipe - Phew.

Anyway back to the TPS.....
I did have a problem where I could not set the correctly calculated value as per the haynes manual - it was a long way out and there just wasn't enough adjustment to get within spec - it was then that I realised that I had the TPS upside down (or 180° out) the wire should come out the top, not the bottom. So after correcting this I was able to get the value almost perfect... but alas the 2k-3K problem was still there !!!!

I did read somewhere that the TPS sensor on the cat simply allows the ECU to choose between 2 or 3 different engine maps, so it is probably just used to determine if the throttle is nearly closed, wide open, or somewhere in-between.

I'd be interested to know if anyone else is seeing this issue, as at a track day recently I checked out another thundercat and found that it too had the same problem.

Maybe it's a feature rather than a problem

Crikey Bob - Well recovered.

TBH - I'm not sure whether mine suffers this or not - maybe I just keep it above 3.5k all the time.

I guess (when working) my bike could be a little bit sluggish but I couldn't say I even thought it of it a problem. Bit like going up a hill in a car and not having enough oomph and having to change down a gear??

This job looks like too many spanners for me on the Haynes scale of spanners anyway.

Have these cats ALWAYS been like this (rough problem 2 - 3.5k) or have they developed it?

I guess it it either
a) This is a 'feature' to a greater or lesser extent on all cats and you're just picky or darn unlucky.
b) Unfortunately your cat has a specific fault with the TPS or some other fancy gizmo.

Maybe there are some local Thundercat biker(s) that could meet up with you to compare notes.

Good Luck
Rosco

Flippin Heck Bob that was close, were you getting the TPS error on the rev counter?

I have decided that I am going to start from scratch before I start tinkering with the TPS. I checked the valve clearances and they were good, the carbs have been stripped and cleaned, coils and plug cap resistance is fine, am going to change the air filter and plugs and balance the carbs and hope one of these fixes it, if not I will have a play with the TPS. I did notice that the Main jet was a 124 where the manual says 155 however I think I am correct in saying that the Main jet is more to do with top end so am going to rule this out for now. The mixture screw was also different sitting 1 3/4 turns out rather than 1 3/8 but not sure how much difference that will make.

Yes it sure was, closer than you realise - you can just see it in the pictures, the valves were partially open. Any wider and it would have dropped into the cylinder....

It would be worth you checking that the TPS is not upside down, wires should be coming out the top, not the bottom.

I seem to have gone through all of the same steps as yourself, unfortunately I didn't check the jet sizes, but I think you are correct the main jet shouldn't affect the low end.

With regards the mixture screws I tried everything from about 1 to 3 turns, and although it made some difference it was very difficult to detect. I have even toyed with welding some bosses in the down pipes and then using a lambda sensor to get the mixture correct for each cylinder separately.

I'm sure this is curable, We'll just have to keep looking.

Just my personal opinion, but I think asking a Thundercat to pull from 2,000 rpm is a bit unfair.

When I first got the 'Cat, it was my first 4-cylinder bike after many years on cars, and before that old Brit machines. So I too used to keep the rev counter near the bottom end. But in fact the engine's designed to work at much higher revs. The valve overlaps needed to get good performance at the top end mean that it can't work efficiently at the bottom. So not only does it struggle to run smoothly, things like fuel consumption will suffer as well, since a lot of the incoming fuel/air mixture will go straight through and down the exhaust pipe. And there is a school of thought that says pulling from low revs can knock out your crank bearings too. That's not to say you have to rev it to the redline all the time, just keep it in its "happy zone".

Forget the actual numbers for a minute, pretend the rev counter is calibrated as a percentage of redline revs. So on the 'Cat, 6,500 revs would be 50% etc. In my experience, most road-going 4-cylinder petrol engines, whether for bikes or cars, have a useable rev range of about 4:1, so the minimum revs you should be using are 25%, = 3,250 on the 'Cat, about 30 mph in 3rd gear. For comparison, the average car has the same useable range, 4:1, but the numbers on the rev counter are about half as much.

Rambling on a bit further, though it's not really relevant to this, Diesel car engines only have a useable rev range of 3:1 (typically 1,500 - 4,500), as do V-twin bike engines (3,500 - 10,500), and single bike engines can be as low as 2:1 - my MT03 needs to be doing somewhere near 3,500 before you can give it much throttle, and the redline is at 7,500 (by which time the bike has all but shaken itself to bits - still good fun, though).

Well that's the bike back together, have just been out for a ride and it is running so much better. I didn't mess with the TPS in the end, I thought I would only touch that if there is still a problem after doing the rest of the stuff. I didn't notice the 3K error but as this only happens very occasionally I am not going to worry about it for now.

All I need is a heat wave and I will be sorted.

nicko600 wrote:Well that's the bike back together, have just been out for a ride and it is running so much better. I didn't mess with the TPS in the end, I thought I would only touch that if there is still a problem after doing the rest of the stuff. I didn't notice the 3K error but as this only happens very occasionally I am not going to worry about it for now.

All I need is a heat wave and I will be sorted.

Nicko, what did you end up doing ?

Bob.

Because of the cost of the TPS i decided to start with the basics.

Checked the valve clearances which were fine.
Changed the plugs.
Checked the coils and plug caps which were fine.
Stripped and cleaned the carbs.
Reset the mixture screw.
Balanced the carbs.
Replaced the airfilter.

I took it out for a longer run on Saturday and if I am honest it isn't 100%. I occasionally noticed a rough spot at about 3K but it is so much better than it was and is almost unnoticeable.

The rough spot was really only noticeable when in town traffic in 3rd gear and can be easily avoided by being in 2nd as Bobh mentioned. To be honest I don't ever remember previous 600cc bikes having the same issue pulling from 3rd in the same situation but I have been away from road bikes for 3 years and anyway I can live with it.

I might have a play with the TPS adjustment sometime as it is still out with the specs in the manual but for now it will do.

I wonder has anyone else noticed this rough spot at around 3k revs in third?
When I am fit and well again I'll get out on the bike and see if mine has a similar thing, as it could just be something with all bikes (unlikely but ya never know) Will report back with my findings soon

Mine has suddenly developed one around 3 500 RPM. It wasn't right when I bought it, but seems a 'good blast' to Big Mac last year sorted it out and has been fine since, till ..

A few weeks ago I thought it wasn't running right, seemed to go OK but didn't sound as good as it should. That deteriorated to only running on 3 cylinders. That could have been a leaking vacuum pipe and/or a faulty plug. Mechy stripped the carbs, cleaned and balanced. It back on 4 now, but still doesn't sound right - almost as though it's still only on 3 cyl, but now there is no loss of power. It clears off around 4k RPM.

I've been thinking my problem is gunge in the carb, even though the mechy cleaned it out. Put some IPA in yesterday and took it for a ride - maybe wishful thinking but it is slightly better, but still not 100%.

Mechy has seen a few Thundercat's all with the same problem (was wondering if he's not the common denominator), however he worked on my first 'Cat for a few years and I never had that issue.

Might not be the same issue though.

Mine pulls clean and strong throughout the rev range and in any gear. I wouldn't say it is normal.

went out on my cat today, no flat spot on mine, but was a bit stuffy when I started it up but after a good warm up then a bit of a thrash its running sweet as a nut again