ignition timing.

[Jg92]

Was wondering what people were running for ignition timings on modified t500 engines.
Tr500 spec porting and chambers, squish heads, borderline unacceptably high compression ratio...

[Zunspec4]

Hi Jg,

What fuel are you using as it will impact upon timing. If it is premium unleaded I would start at 2.4mm BTDC and develop from there. If you are using a leaded race fuel your engine will tolerate more advance. I would still advise to start at a conservative BTDC figure and work from there, ideally on a dyno where you can see the affects of changing the ignition and jetting.

Cheers Geoff

[Jg92]

Hi Geoff

I was hoping to be able to use 94 aki pump fuel.
Apparently that is what the PO was running in it, but the squish clearance and compression ratios leave much to be desired. I suppose i will start on the conservative side of things.

Thanks

Jordan

[Zunspec4]

Hi Jg,

Modern unleaded pump fuel burns faster than old style leaded petrol, hence the general need to run less advance than before. I would recommend that you get the best "premium" pump fuel. In the UK I use Shell Advance which is about 97RON, although I then mix it 50:50 with 100LL AvGas.

Squish - I would keep it to a minimum of 1.00mm, my race engines run about 1.2mm

C/R - My heads have been worked for a 24cc combustion chamber volume which is conservative but keeps detonation at bay.

Cheers Geoff

[Jg92]

Hi Geoff

The problem i have is that the squish is around 1.6mm at the moment, and the combustion chamber holds 18cc at this clearance.

While the previous owner claims they ran pump fuel, i’m a bit sceptical about that with a 15.1:1 compression ratio.

That being said, i’ll try to err on the side of caution when doing things.

Cheers

Jordan

[Alan H]

2 stroke compression is measured from closure of the exhaust port. Not bdc. Use a pressure gauge in the plug hole and see what psi you get. 'Normal' will be about 120-125psi. Skimmed head/barrels/different porting could be over 150, but that's getting high for longevity.