E: Engine Comet Ignition Timing 2018

[davidd]

Using 34 degrees on a modern BTH on a comet ( I had to smile at what is "reasonably stock" in USA David) should give about 4 degrees at full
retard shouldn't it?

Tim,

I used the word "stock" to mean "popular" as I tend to think that most riders use specs very close to David T's. It seemed too presumptuous to use the 19 degrees I used on the racer, which I thought was still advanced a bit. It is difficult to get a street Comet owner to check the timing on a dyno.

I think the method Robert uses is the best non-dyno approach, but it is likely to result in the engine being advanced because you are backing off the advance until you get the good behaviour. I am just guessing that this is still too advanced. It is unlikely to do any harm, but maybe it keeps the engine too warm? I am not sure if you can set the minimum timing with the best torque without a dyno.

The racer does not have a KS, but I always ran NGK B9's, which are pretty cold, with the BT-H. It was an excellent starter, but I was using rollers or bump starting. I never worried about starting, it would spring to life.

David

 

[oexing]

Nothing mysterious about spark plugs, you just have to pick the suitable grade for your way of driving. Any brand will do nicely provided the plug reaches its temperature to burn itself clean, so forget all handbook numbers or recommendations by other bikers. You will have a look at your spark plugs to decide on the appropriate grade to keep them clean.
As to ignition advance curves, a Comet engine is not more special as any other engine, medium performance level. So why not look at any other types to see what works for them. Cooling should definitely be no factor taking into account that it has alu head, alu cylinder and reasonable finning. The combustion chamber is not too bad, in fact more modern than many contemporary types with high domed pistons in small bores of long stroke types, so no complaints here. To me a range from about 5 degrees retard and mid 30s for advanced should be quite good with higher c.r. , a very mild engine with less turbulence in the mixture will need nearer 40 degrees. A twin spark head will be happy with 30 degrees or less. To repeat myself, the familiar numbers 4 to 34 look very reasonable and plausible but I heavily question full advance way before reaching 3500 rpm. Look at any other more modern engines , not necessarily ones with the old Lucas ATDs that leave a lot to be desired. Too much advance at a too early stage will produce a hard running engine when the maximum combustion pressure meets the piston in its upward motion, nothing gained with this, possibly just excessive heat . I would do some trials with some springs in the ATD that are slightly stronger to get full advance at min. 3500 rpm, maybe better 4000 rpm. A Vincent engine is no low power side valver with a c. r. of 5 that will tolerate anything you do.

Vic

 

[vibrac]

What plugs are you running with the BTH. I had B7 series in the Black B and it had the same parentage as you are experiencing. Swapped them out for 6's and it became a one kick start almost every time.. Had the exact same issue on Tony's 636 Comet. The BTH does not like cold plugs

I have a new B6EV at the moment and waiting for a Champion N5c strange thing is it starts easily on the rollers or the Foxley starter and it runs fine on the road and goes down to a tickover I have checked the timing at 34 degrees but it is not starting on the kickstart the compression is not high, head is about 80cc on a 600 cylinder so about 8.5 :1 I remember reading about rust in BTH but next job is to swap out a coil I do see a faint spark at the plug (but thats no way to check a BTH!) and I dont think there is an easy way to check a coil but as I say it runs sweet as a nut once it starts perhaps its my old leg....

 

[vibrac]

What plugs are you running with the BTH. I had B7 series in the Black B and it had the same parentage as you are experiencing. Swapped them out for 6's and it became a one kick start almost every time.. Had the exact same issue on Tony's 636 Comet. The BTH does not like cold plugs

I have a B6ES at the moment and waiting for a Champion N5c to be delivered It starts fine on my rollers and also the foxley then runs well, rides fine, and goes down to tickover but it will not start on the kick start I have checked the timing at 34degrees and the head volume is around 80cc on a 600 capacity so about 8.5 .
I can occasionaly see a spark at the plug but I know thats no proof on a BTH. Next job is to swap out a coil but I dont know any way to test one off bike I also remember something about rust in a BTH,
A thought occurs to me also that perhaps the pilot is to rich (28mm VM Mikuni) i seem to remember that more than 1.5 turns out on the screw means a smaller pilot jet I will do some more reading the plug is sooty
Hey Ho,something is just out of kilter but once running its as sweet as a nut, perhaps its my old leg....

 

[Bill Thomas]

A standard Comet @ 4000 revs =About 64 mph, A lot of people would never get full advance, If we did what you say. Cheers Bill.

 

[greg brillus]

Just take the alloy end cap off the BTH and look at the small pole ends of the coils, if there is any signs of rust or similar, buff it off gently with a dremel. Just make sure the rotor arm with the magnet on it is out of the way and be careful not to pinch the wires that pass through the small cutout at the lower rear of the cap when you re-install it. This turned a Shadow from a non starter to first kick again.

 

[oexing]

Bill,
the purpose of a bike is not to reach full advance at any chosen speed but the target is to get an engine that is happy with its carburation and ignition to operate without undue heat, harshness and vibrations . I cannot tell if 4000 rpm may be optimal for full advance at around 34 degrees or maybe better set the springs for 3500 rpm . That has to be tested on the road or rolling road if you get access. And the setting depends on the state of tune for that particular engine. Anyway the diagram further up that shows 2700 rpm full advance with all available springs and bobweights for the old Lucas ATD is waaay off the marks for modern knowledge. That was OK for old sidevalvers and real low tuned ohv engines. But one model of ATD cannot possibly fit for all types of engines around in their very different state of tune - definitely. It is also a matter of thinking out of the box , look at other machinery and draw one´s conclusions. One need not accept "original" facts and ideas for being the optimum, very often just a result of conditions and knowledges available then.

Vic

 

[davidd]

I should have also mentioned that David T is running a single plug new BT-H, so a very typical ignition. The piston was changed to a 10.5:1 and the subsequent dyno run set the ignition at 28 degrees.

Many racers run at full advance, including me, but I could not tell you that it is hard on the engine from the performance.

I avoided mentioning ATD's because I think it can be a problem with no good answer, but it is interesting to see how owners deal with it.

Bill, you are good on that 64 mph at 4000. Of course, maybe never getting to full advance is a good thing!

David

 

[greg brillus]

Just look at the change in fuels from back then to now. Slower burn low octane verses faster burn higher octane, so the advance rate should be different. I'm with Vic on his thoughts, and this is probably more of a noticeable issue on a single that is working harder than a lazier twin.

 

[Robert Watson]

Actually Greg generally speaking higher octane fuel burns slower which helps if you have a "knock" problem, but that is not so for all fuels. and every refinery/blender makes it differently....

If I recall there is an "expert" in this field that lurks from time to time...