E: Engine BTH Electronic Mag Timing Comet -Q?

[Jez Nemeth]

Like the second suggestion.
if there's enough degree range of movement on the Mag fine tuning on the slots on the roadside...bit more me that.

 

[Jez Nemeth]

Martyn, don't know if worth mentioning, standard paper gasket on cylinder, no compression plate if that makes a difference either way?

 

[Martyn Goodwin]

Martyn, don't know if worth mentioning, standard paper gasket on cylinder, no compression plate if that makes a difference either way?

Same setup as me

 

[davidd]

Jeze has a 8:1 piston fitted. My experience with that compression ratio in my Comet was that 36 BDTC full advance resulted in heavy detonation when under load - I now run 32 BTDC and seat of the pants, the engine is both smoother and stronger.

Martyn,

You may be correct. Jez is running a 105 cam and I don't remember what you are running. Much depends on the fuel, cam and piston together. Jez would be running around a 7.8:1 dynamic compression ratio, so he would not be losing too much from the static CR 8:1 number on the box. In the US, you can run 9:1 dynamic CR on 92 octane premium safely, so my guess would be that he would be well within the limits of the fuel and the cam assuming that he has some selection of fuels.

It is relatively easy to set your timing so that it does not knock by retarding the ignition. It is almost impossible to set your timing for maximum power without a dyno. The difference between the two is quite a bit of power.

I was still gaining power when I ran out of "slot" at 19 degrees, so I left it there. I had a static CR of 11.23:1 with a dynamic CR of 8.1:1.

Your point is well taken that the test rides should be done with an ear to engine sounds.

David

 

[BigEd]

Like the second suggestion.
if there's enough degree range of movement on the Mag fine-tuning on the slots on the roadside...bit more me that.

You should have enough movement to see which way works best, i.e. more or less advance. I f you find that you get to the end of the slots and still improving then you cand remove the magneto gear, move the magneto in the direction you were finding best and test again.
In most cases, I wouldn't have thought that you would need that much adjustment but as Davidd points out this could be dependant on your engine specification. e.g. Davidd may have been using twin plugs.
Be consistent, e.g. find TDC the same way each time, preferably using a stop through the plug hole and a degree disc, turn the engine in the same direction up to the stop point created by the timing rod in the magneto and don't alter anything else, e.g. jetting as you have introduce another variable and will not know which one made it better/worse.
Make a note of what you've done for reference later.
If you are able to check your timing with a strobe it makes it easier to know where you are from a firing degree point of view. Take your reading once the degree disc shows that the magneto has stopped advancing.

 

[vibrac]

With the crap petrol nowadays unless you are religious with your filling stations (I have managed last year with Esso Premium only but you need to know where the pumps are) otherwise fine tuning to a degree or so can all go out the window when you buy some supermarket crap. No wonder modern cars need a couple of computers and the resources a chemical factory so the half asleep pilots can use modern petrol without thinking about it.
But if you start in the mid 30 degrees you have plenty of slot room

 

[macvette]

With the crap petrol nowadays unless you are religious with your filling stations (I have managed last year with Esso Premium only but you need to know where the pumps are) otherwise fine tuning to a degree or so can all go out the window when you buy some supermarket crap. No wonder modern cars need a couple of computers and the resources a chemical factory so the half asleep pilots can use modern petrol without thinking about it.
But if you start in the mid 30 degrees you have plenty of slot room

Sorry to say this but the "crap supermarket fuel" is a myth. There are six refineries operating in the UK. They supply fuel to their own outlets and supermarkets. The octane rating of all grades both own brands and supermarket is the same so 95 octane or 99 octane is the same in either outlet. These ratings are tested post production by the refineries using variable compression single cylinder test engines. So the choice of branded vs supermarket fuels of the same octane rating will not affect ignition timing or performance on road bikes of the age of ours.
I didnt work for supermarkets or oil refiners, but I did spend 40 yrs in the oil and gas industry engineering,building and comissioning these facilites including some of the biggest in the world so this isnt something I"ve googled.
Sorry, didnt mean to hijack the thread. The basic message is its the octane rating of fuel and compression ratio of the motor which determines the ignition timing.

 

[TouringGodet]

Similar here in the States, there are only so many refineries, even the major fuel brands can come from the same refinery. But many of the fuel vendors have their own recipe for the additives. Can the additives affect engine timing requirements?

 

[macvette]

Similar here in the States, there are only so many refineries, even the major fuel brands can come from the same refinery. But many of the fuel vendors have their own recipe for the additives. Can the additives affect engine timing requirements?

Not really, fuel octane is what determines timing. Imagine two fuels with the same octane rating, one branded and one from a supermarket, both will have additives in small percentages. The branded or premium fuel will have more types of compounds which are said to preserve engine efficiency longer by cleaning the fuel systems, which is probably true in strictly controlled test conditions but this is difficult to demonstrate in real world driving conditions. The difference in octane ratings is due to the blend of hydrocarbons in the fuel.

 

[Robert Watson]

I do remember about 30 years ago when the "PCB in transformer oil" was a major environmental matter that one of the local "cheaper" fuel suppliers was adding a small amount (I am sure for a large fee) of the waste into his supply chain. Got caught, and punished, although I suspect not nearly as much as he would be today. I think that was probably one of the better sales tool for the big oil companies.