TZR 50 carb setup

[TZR 50/80]

Hi everyone, this is my first post and I need some help with my TZR 50, it has a polini 80cc bore kit, technigas expansion chamber, bored out reed block with polini carbon reeds, high spec inlet manifold, 21mm TNT carb with a polini sponge air filter.

I've only just put the carb on, previously was the standard 16mm, it went fine but I always knew it could do more as the powerband was tiny and right at the top of the rev range, now with the bigger carb the powerband is much bigger (and a bit more powerfull) but a lot lower down the rev range and it starts to run out of puff at about 9250rpm whereas before it could pull to 10250, if anyone could sugest anything I would appreciate it alot, thanks! : )

[triton john]

Sounds like you have done everything at once instead of doing it stage by stage and getting each bit right before moving onto the next.

Have you got loads of spare jets and slides ? Have you done plug chops with the engine under load ? With modern fuels it is harder to get a decent plug reading but it can point you in the right direction.

I would start with the smaller carb and see what is going on and maybe try a different cutaway and main jets and needle position. When you have it runing better with this set up - cos you will - go onto the bigger carb and do the same and maybe go down a tooth on the front to bring it on the pipe earlier. There are basics when doing this sort of thing but it takes a lot of time, patience, experience and money to get things right.

[TZR 50/80]

Hi, thanks for the reply, I have not done everything quickly I've had the bike for about 14 months and been building it up slowly and had the old carb on it forever, which was running fine but just wasn't quite big enough and was the last thing I needed to upgrade.

I have plenty of jets to mess around with, also I was wondering which direction does the jet size have to go when upgrading to a bigger carb, larger or smaller, also the 16mm had the airscrew on 6 turns out, whereas the 21mm, at the moment only has 1.5 turns but the jet is quite a lot smaller, so do you think it might just need more air and petrol? : )

p.s No way am I going a tooth smaller on the front sprocket, it actually needs a tooth bigger for higher top speed (but only after its running right)

[Airhead]

6 turns out is just plain wrong! 1 1/2 is more like it, jet numbers increase as the jet gets bigger. plug chops are the order of the day...are you familiar with them?

[TZR 50/80]

yes I am, I think, is it when you ride it flat out for a little while then pull the clutch in and kill the bike immediately, then check the colour of the plug, I just dont know how long to ride it for before killing it thats all.

[Airhead]

hills are best to load the engine, full throttle 1/4 to 1/2 mile would be enough, try to pick a safe road without too much traffic in front or behind or at least a quiet time. Do you know the jet sizes that are in each of the carbs?

[TZR 50/80]

The main jet in the 16mm is a 118 and the 21mm has a 85.

[Airhead]

the thing is though are you copmaring like with like, have the jets the same thread and are the carbs the same make, it does rather sound like the 85 is too small and if this is the case you are seriously risking a holed piston or seizure so treat with extreme caution doing a plug chop , i would be tempted to inspect the two jets for starters and fit the one with the bigger orifice ASAP, then take it from there

[TZR 50/80]

My Dad seems to think it could be the spark plug gap, and that it needs a bigger one to give it more top but we've done this before on the old carb with no results, do you think it could be a factor? thanks in advance!

[Airhead]

in a word NO...sorry dad! more likely a jetting issue. I cant stress enough you need to act with caution to prevent serious engine damage...fit the bigger jet right away and continue to plug chop etc and not use the bike until you sort it

[TZR 50/80]

HI,

Both the jets are a 5mm thread and both carbs are dellorto, the 85 seems to be the only thing the 21mm runs on, I have tried the 118 in the 21mm, the bike wouldnt even pull me up a small hill, then tried a 95 and it ran much better, then the 85 even better, but only up to about 9250rpm then it stops developing power and just starts screeming its head off!

[Airhead]

and is the throttle needle position also adjustable, with a clip and choice of grooves ?

[TZR 50/80]

Yes, how would you recommend adjusting it, I always get a bit lost when it comes to needles and what it does when you move it

[Airhead]

ok lifting the needle UP ie moving the clip DOWN with make richer and vise-versa

[Airhead]

you need to find the best main jet size though first and the only way to do that is full throttle plug chops with caution, I said 1/4 to 1/2 mile but at first try the 1/4 in case its lean and the plug shows white, member wild foamy had a 50 to 80 big bore on his DT, might be worth seeing how he jetted for that one?

[TZR 50/80]

Ok i'll give everything a go, hopefully it'll be good news after that.

Thanks for your help.

[wild foamy]

OG: my DT had a 60 bore kit, and i remember my first big bore kit ceised up miles from anywhere, leaving me to push it 3 miles uphill to my garage in the darkness.

as mentioned in the email i sent you, my DT was running the original carb but with a 100 main jet, higher needle and a cone filter, but if im honest i think even that was too lean,

its better to start with too big a main jet (or "rich") and work it down until it runs right than it is to start lean (not enough fuel) as running lean causes overheating, holed pistons, scored bores and eventually ceising up (see story at the top of this post)

[triton john]

Definately go with the biggest jet you have and take it from there. Using different cutaways will also help in different areas. Get the book Tuning for Speed by Phil Irving it is available on line somewhere to view as the book is about £45 and deals with just about everything regarding bike engines. Do not be put off that the book deals with older bikes as an engine is an engine the practices are the same now as when this book was first published. Phil Irving was the man behind Vincent and this book realy is a must have for anyone wanting to build a good engine.

[icedragon]

Sorry to jump posts but did you sort out your issue as I'm facing a similar problem.

[Airhead]

i doubt he will be back mate but we'll see