500cc timing question

l have read that the 1985 500cc LC timing/engine was originally supposed to be set at 2.8mm BTDC.

Then it was advised to set at 2.2mm BTDC

l have read some set it at 1.5mm BTDC.

Questions,
ls there any performance differences between these settings?

ls there any jetting differences between these settings?

Cheers

I have also seen the recommendation to lower it from 2,8mm BTDC to 2,2mm BTDC to prevent kickbacks when starting.
It must affect the performance some when the timing is lowered 0,6mm (2,2mm BTDC) or 1,3mm (1,5mm BTDC).
When I retired my deceased Motoplat and replaced it with a programmable ignition did I use the ignition curve from the PVL system to start with, but set the base advance at start to 1 degree BTDC since I'm a coward
(My first 500cc twostroke)



It makes the bike nice and easy to start and I have the fully advanced curve over all the revs.
She now runs on a battery powered ignition that gives a good spark when started.
I have the jetting to sort out at the moment.

Mine was 2.5mm when I got it and had a kickback problem. I set it at 1.8mm and that helped the kickback issues alot which made it start easier. Didnt have to fiddle with kicker to get it "just right" so it didnt hurt you or the case/kicker gears.
I noticed no performance issues BUT I dont think I ride my 250 to its potential let alone the 500. Advanced timing-this will start a debate... Is more of a benefit at higher rpms and will build more HP. advanced timing is actually a detriment to an engine run at lower rpms with high load, short shifting-lugging out of corners. Reason is spark is happening further away from TDC and piston is still trying to cool from previous combustion causing detonation. Detonation damage can happen unheard, heh, thing is noisy anyway! Higher octaner helps as it has a higher ignition tempature to prevent pre-ignition on hot piston crown Slower timing will build better low end torque. My suggestion would be tailor timing to riding style. I dont race my 500, actually its down right now waiting my attention. I only have one because they are cool and I'm still a 15yr old at 43! When mine goes back together it will have a decompression valve to aid kick starting. I'm also doing some headwork to promote more efficient combustion, less chances of detonation. Ignition timing has to do alot with combustion efficiency, less efficient more timing (BTDC) I believe that is why alot of 500's had lower con rod bearing failures, fast timing and a low rpm engine. Just my 2c Scott

Bryll,
I would be very interested in the ignition you have. Please pass along the info. The thing that has me puzzled by your timing/rpm graph is the timing being retarded as rpm increases past 2K? On adjustable/programmable ignitions I have always had starting timing slow and increase to 1/4-1/3 of total engine rpm's depending on the load factor. I then have total (max) advance flat to max. engine rpm. I'm very curious why that curve reatrds timing as engine rpm increases past 2k on rpm? The rapid advance off of start rpm is common to aid the engine idling, which is light load and aids complete combustion=less unburned gasses polluting next combustion cycle. Would like to see what the effect of that timing curve would be on a dyno!

It's an ignition from Ignitech, I've installed it on fourstroke engines before and the curve you describe sounds like the ignition curve for a fourstroke engine.
(like this one for a XT600)


The curve I calculated my values from is this one from a PVL system for Husqvarna two stroke engines.


(picture from http://www.pvlspecialisten.se/teknik/TandSpankurva4000.jpg)

The Ignitech system is very easy to install and is actually working very good.
It's cheap too

I have just got my 85 500 back together after a good number of years..... I set mine to 1.5 BTC.. If I was a lot younger and quicker I would notice any power drop but as it is there's enough there and no kick backs...

Happy days..

Stu

The Ignitech system sounds pretty nice so far. Really tuneable.

I was wondering if anyone had actually made some chalk, soapstone, paint or the like where the rotor and a reference point is when you stick a pin in the little hole and then use a timing light to see if the spark is actually snapping there and if not how much error is in the method. Hopefully I remember the technique correctly as generally set it and forget it. If one had the cylinder off marking something on the ignition rotor at top center should be relatively easy and then going by degrees would be possible. Finding top with a dial indicator is kind of vague if I recall.

Fran