rotax 503 timing question

[andy c]

i've just been setting my timing, (nippondenso cdi) and i noticed that the
ignition starts to progressively retard as high revs are reached - i set it
to 2mm btdc at 2000rpm, but at 4000 revs it starts to retard and at full
revs has retarded to approx 1.3mm btdc

i would have expected the plugs to have to fire *earlier* at higher revs to
allow time for the fuel combustion to spread -
why would it have to fire later as revs increase? or is my CDI faulty ?

many thanks,
andy

[andy c]

not sure - check that the coil is earthed properly, mine was fractured from
its bracket and i had to ground it with some meaty cable -
maybe disconnect the cdi and dab some volts (car battery?) momentarily
across the coil to see if your plugs spark then - if they do then it's
starting to look like the cdi (might be worth pulling the flywheel to check
for anything obvious first though)

good luck, and check if your timing does what mine does if you get it going
:-)
best,
andy

> A question for you though, have you had problems with the cdi on your 503?
> Just acquired a scat with a 503 and it has stopped running to apparent no
> spark, see thread "Rotax ignition" please.
> Eric Holt in Calgary
>
>

[Tony Westwood]

This site may be of help in diagnosing your Ignition problems
http://www.ultralightnews.com/enginemaintenance/maintenance/dcditiming.htm
Best of luck
Tony
"andy c" <a...@nospammm.com> wrote in message

news:102038583...@iapetus.uk.clara.net...

[andy c]

thnx, but wrong ignition
afaik there aren't any sites detailing the rotax nippondenso ignition

can anyone offer any explanation for why the ignition should progressively
retard at higher revs as described ?
many thanks, andy

"Tony Westwood" <tny...@ozemail.com.au> wrote in message
news:%auA8.838$Q85....@ozemail.com.au...

[Ralph DuBose]

"andy c" <a...@nospammm.com> wrote in message news:<102043852...@demeter.uk.clara.net>...

> thnx, but wrong ignition
> afaik there aren't any sites detailing the rotax nippondenso ignition
>
> can anyone offer any explanation for why the ignition should progressively
> retard at higher revs as described ?
> many thanks, andy
>

Limiting ignition advance is something that is done when the engine
designer has a lively fear of detonation/excessive pressure. A high
performance two stroke piston crown would melt thru very qickly if
pre-ignition were superimposed onto high cylinder loading. Early
turbocharged, non intercooled race car engines were set up with no
ignition timing advance at all. That was the only way to get thru a
long race. With 2 strk engines, survival of the piston depends on
limiting the temp. of combustion at high power situations because the
is not enough ways to cool it otherwise. That is why they are jetted
to get exremely rich at peak power..

[andy c]

thanks ralph,
mine is certainly rich at full revs - it uses 1 gallon every 5 mins running
flat out
do think my cdi must be faulty to be retarding the ignition as revs
increase?

[andy c]

eric, is this your ignition ?
http://216.37.204.202/Skidoo_OEM/Skidoo.asp?Type=13&A=24&B=7
if so check the generator/ coil etc. resistances and i'll compare them to
mine
you can email me at andyspost AT hotmail.com if you like
andy

> Andy,
> So would I, normal automotive timing advances as revs advance as far as I
> know, will be interested in an informed reply.

[Rick Holt]

andy c <a...@nospammm.com> wrote in message

news:102046539...@demeter.uk.clara.net...

> eric, is this your ignition ?
> http://216.37.204.202/Skidoo_OEM/Skidoo.asp?Type=13&A=24&B=7
> if so check the generator/ coil etc. resistances and i'll compare them to
> mine
> you can email me at andyspost AT hotmail.com if you like
> andy
>
Andy,

Looks like mine. Will be at the shop tomorrow and will e-mail after that
what I find. Thanks for link and your offer.
Eric Holt