Good evening folks.

I'm in the process of rebuilding an American import 1974 Ty 250 which turned out to have a 320 piston and barrel,

it had a fair amount of piston slap so it's been rebored by Nigel Birkett to second oversize and is now up and running.

However, it suffers from pre-detonation, a leaded additive for the fuel has helped but it still bangs away sometimes and

is fitted with a black coloured 'JT'carb'.

Would anyone be able to point me in the right direction and confirm what carb and jets it should have as well as how

I stop the detonation?

Many thanks.

Pete.

If that is a TK carby (that came on the 1973/74 TY250) that you have, I can give you the jetting specs, but they will probably not be right for the 320 motor

i'm sorry but i don't know the answer to your problem but i just thought i would let you know that someone as broken a ty 250 and is selling all of the parts on ebay, the engine parts aren't any good to you but their could be other parts that you are looking for.

Would have thought Nigel was a good place to start looking/asking

The description of the problem is confusing

There is no Pre detonation, there are at least two forms of abnormal combustion

Pre ignition: where the fuel mix is ignited by a source other than the spark plug, this is similar to having incorrect ignition timing, it could be a glowing bit of head gasket or carbon etc, sometimes the fuel is ignited by both the plug and the other source and the flame fronts collide

Detonation: the fuel mix does not burn evenly from the spark plug out like it should, it explodes (no the mix is not supposed to explode), this is sometimes due to incorrect ignition timing but most often it's excessive compression or lean condition

It's kind of hard to tell the difference between the two but detonation is far more common

Does it happen only when the engine is full hot, does it only happen at partial throttle etc etc

First thing to check is the timing. Remember that bike has points and a condenser. If you do not know that last time they were replaced it probably makes sense to replace them and set the timing correctly first.

The leaded additive is intended to lubricate the valve and valve seats in 4 stroke engines so you really don't need it in the TY. I would check if the additive is flammable by itself. If it is not, then adding it will have the effect of leaning out the mixture. If it is flammable, it should have little or no effect on lean or rich mixture.

More information will help us give you better answers.

When is it making the noise?

Idle?

mid-range RPM?

Full throttle?

Acceleration or deceleration?

First thing to check is the timing. Remember that bike has points and a condenser. If you do not know that last time they were replaced it probably makes sense to replace them and set the timing correctly first.

The leaded additive is intended to lubricate the valve and valve seats in 4 stroke engines so you really don't need it in the TY. I would check if the additive is flammable by itself. If it is not, then adding it will have the effect of leaning out the mixture. If it is flammable, it should have little or no effect on lean or rich mixture.

More information will help us give you better answers.

When is it making the noise?

Idle?

mid-range RPM?

Full throttle?

Acceleration or deceleration?

Most of the additives that I have seen in the UK also have an octane booster as older engines were designed for real fuel, probably worth trying.

An octane booster would certainly reduce pre-ignition (pinging or pinking). Pre-ignition usually happens due to one or more of 3 causes:

1) timing too far advanced

2) Too low octane of fuel

3) high compression.

Yes Ross the lead was there principally to raise the octane level but we have other things to do that now and most folk ran on two star in any case. If all is correct timing and carb wise then super unleaded should cure the problem. It did on my Beta anyway.

Thanks for the replies:

0007: I'm not sure if it's pre-ignition or detonation but it makes a harsh knocking noise especially when the engine is under load and warm, on initial start up from cold it's fine but within a couple of minutes it starts and is there at any throttle opening apart from very low gentle revs with virtually no engine load but if I put the choke on when warm the problem almost disappears.

feetupfun: I'd be grateful if you could let me know the jetting specs for the TK carb at least I'd have a base to work from, thanks.

mickwren: I appreciate Nigel has a vast knowledge of virtually anything trials related and has previously sorted bike problems for me but he's such a busy and sought after bloke I thought I'd give him a break and put the question to a wider audience.

that's_a_five, b40rt and 2stroke4stroke: I'll renew the points and condenser and check the timing, I've been told by a previous 320 owner that adding a second head gasket to reduce the compression will also help so I'll try that too.

toetoe: I'll have a look, thanks for the info.

Thanks again for the advice folks.

Thanks for the replies:

feetupfun: I'd be grateful if you could let me know the jetting specs for the TK carb at least I'd have a base to work from, thanks.

Teikei Y26P as fitted to the first model TY250 (now known as TY250A)

main No 112

Needle Jet S-85

Jet needle 5C9Z

Clip position 3

cutaway No 3

Pilot jet No 50

Air jet 2.5

Starter jet No 90

Air screw 1.5 turns

Float level 21 +/- 1 mm

When I built one of these 320 motors I bought the "kit" from Craig and then put all the internals in it from a TY 250A engine. I bought the Mikuni VM26 and used a 35 pilot and 290 main! The main jet increase seemed excessive but was spot on. It could well be that your carb has the original jetting and the main is far too small. The problems you speak of could simply be fuel starvation in the upper rev ranges.

Tony

In your earlier post you stated " if I put the choke on when warm the problem almost disappears." So I think you have answered your own question. Good luck sorting it out. Let us have some piccys sometime too.