timing, positive stop

[HxPaul]

I have never used the positive stop method myself as I always use a pencil or screwdriver in the spark plug hole and rotate anti clockwise until the piston hits the highest point . (I'm lazy) then I always use a degree wheel to mark and set advance

I'd be interested to know how much differnce in accuracy between the two methods .

I suspect the greatest potential improvement in timing and performance is likely to be in the final choice of advance setting.

It doesn't matter which method you use to get TDC,as long as you strobe the engine to the right firing point.In my case I always strobe my firing point to 17 degrees BTDC.

[MickYork]

Quote : It doesn't matter which method you use to get TDC,as long as you strobe the engine to the right firing point.In my case I always strobe my firing point to 17 degrees BTDC.

I think that's part of the discussion, which methods are the more accurate. A strobe will only work if your TDC and other marks are correct. Finding a accurate TDC is the critical measurement.

A dial gauge on the piston seems to be accepted as the most accurate, followed by the positive stop, which has been mentioned has more than one method.

[dscscotty]

Finding a accurate TDC is the critical measurement.

Absolutely, accurately finding TDC is critical to setting firing point e.g 17' BTDC. using both positive stop and dial gauge methods regularly my own preferred method is dial gauge, which can be a very accurate way of setting both TDC and timing when using mm's travel in relation to rotation/position of crankshaft BTDC.

[Sticky]

This might make it easier - the video starts at the relevant point. I made this for use with Casatronic but the principal is the same for any electronic ignition. Move it on to the 5-minute mark.

[a-teamlambretta]

This might make it easier - the video starts at the relevant point. I made this for use
with Casatronic but the principal is the same for any electronic ignition. Move it on to the 5-minute mark.

Just as described in my sketch

[johnnyXS]

nice video sticky ......and probably the most straightforward method of finding TDC .

I've always questioned the validity of trying to get TDC accurate to nanomillimeters or within a few minutes of arc when there is probably more than a whole degree lost in linkage losses in the sprockets and chain and points and big end small end bearings etc etc

The gains of setting TDC that accurately have negligable performance gains imho, compared to a simple pencil in the hole method. The TDC cut mark on the average magneto housing is usually 2-3 mm wide which could probably throw the TDC mark off by more than the difference gained between the 2x methods.

Of far greater importance to my mind is to set everything to correct tolerances and set the static advance at the most advantageous point BTDC along with fuel air requirements etc .

This issue crops up time and again over the years and I am determined to measure the difference to find out once and for all if there is any advantage to be gained and how much if any.
Nice little winter project as I check and reset my timing

[bsso78]

What has chain and sprockets got to do with TDC?
Im not sure you understand ignition timing and the effect it has on engine performance.
May I suggest A.Graham Bell's excellent book on two stroke tuning? It's a few years old now but still very relevant and will explain fully the performance gains to be made from igniting fuel at the correct time.

[johnnyXS]

What has chain and sprockets got to do with TDC?
Im not sure you understand ignition timing and the effect it has on engine performance.
May I suggest A.Graham Bell's excellent book on two stroke tuning? It's a few years old now but still very relevant and will explain fully the performance gains to be made from igniting fuel at the correct time.

thats me in my 4x stroke mode again answering questions in a 4x stroke overhead cam forum and a 2x stroke forum simultaneously.

The rest of what I said is correct though ......but you'd need to have a better understanding of ignition timing and performance.

Might I suggest that you read up on it .

AG Bells work on 2x and 4x stroke tuning is considered a bit dated these days to be honest.

[Steve J]

thats me in my 4x stroke mode again answering questions in a 4x stroke overhead cam forum and a 2x stroke forum simultaneously.

A prime example of why multi-tasking for men is not a good idea.....

[johnnyXS]

I can barely complete a single task these days without cocking it up let alone multitask

I think the point that I was trying to make ......badly....was that I think it is pretty pointless being obsessively 100% accurate over one one aspect without being equally meticulous about every other aspect of setting up and tuning an engine,..... especially an engine that is as inefficient as a standard Lambretta engine

[Steve J]

Yeah - that's the point - with a standard Lambretta engine, you might just get away with imprecise ignition timing, even with today's poor fuel quality. Increase the power output by kitting it, fitting an expansion pipe, big carb, reedvalve........ and you're into a situation where other factors come into play. At that stage, engine survival becomes much more dependent on everything being set as accurately as possible. Call me obsessive by all means, but I always use a dial gauge to find TDC & set points-based ignition, even on standard Ld engines with a 6.5:1 compression ratio...

[johnnyXS]

yeah me too Steve but I just think we sometimes need to take a step back and keep things in perspective .
Theres little point in my mind in setting your TDC to 15 seconds of a degree and then setting your spark timing out a degree or two or not setting the spark plug and points gap accurately.