Comet badly tuned or just underpowered?

[clevtrev]

The engine will not run more than a few seconds with pre-ignition. The only
way to control pre-ignition is just keep any pre-ignition sources at bay.
Spark plugs should be carefully matched to the recommended heat range.
Racers use cold spark plugs and relatively rich mixtures. Spark plug heat
range is also affected by coolant temperatures. A marginal heat range plug
can induce pre-ignition because of an overheated head (high coolant
temperature or inadequate flow). Also, a loose plug can't reject sufficient
heat through its seat. A marginal heat range plug running lean (suddenly?)
can cause pre-ignition.
Passenger car engine designers face a dilemma. Spark plugs must cold start
at -40 degrees F. (which calls for hot plugs that resist fouling) yet be
capable of extended WOT operation (which calls for cold plugs and maximum
heat transfer to the cylinder head).
Here is how spark plug effectiveness or "pre-ignition" testing is done at
WOT. Plug tip/gap temperature is measured with a blocking diode and a small
battery supplying current through a milliamp meter to the spark plug
terminal. The secondary voltage cannot come backwards up the wire because
the large blocking diode prevents it.
As the spark plug tip heats up, it tends to ionize the gap and small levels
of current will flow from the battery as indicated by the milliamp gauge.
The engine is run under load and the gauges are closely watched. Through
experience techni-cians learn what to expect from the gauges. Typically,
very light activity, just a few milliamps of current, is observed across the
spark plug gap. In instances where the spark plug tip/gap gets hot enough to
act as an ignition source the mil-liamp current flow suddenly jumps off
scale. When that hap-pens, instant power reduction is necessary to avoid
major en-gine damage.
Back in the 80s, running engines that made half a horsepower per cubic inch,
we could artificially and safely induce pre-ignition by using too hot of a
plug and leaning out the mixture. We could determine how close we were by
watching the gauges and had plenty of time (seconds) to power down, before
any damage occurred.
With the Northstar making over 1 HP per cubic inch, at 6000 RPM, if the
needles move from nominal, you just failed the engine. It's that quick! When
you disassemble the engine, you'll find definite evidence of damage. It
might be just melted spark plugs. But pre-ignition happens that quick in
high output engines. There is very little time to react.
If cold starts and plug fouling are not a major worry run very cold spark
plugs. A typical case of very cold plug application is a NASCAR type engine.
Because the prime pre-ignition source is eliminated engine tuners can lean
out the mixture (some) for maximum fuel economy and add a lot of spark
advance for power and even risk some levels of detonation. Those plugs are
terrible for cold starting and emissions and they would foul up while you
were idling around town but for running at full throttle at 8000 RPM, they
function fine. They eliminate a variable that could induce pre-ignition.
Engine developers run very cold spark plugs to avoid the risk of getting
into pre-ignition during engine mapping of air/fuel and spark advance,
Production engine calibration requires that we have much hotter spark plugs
for cold startability and fouling resistance. To avoid pre-ignition we then
compensate by making sure the fuel/air calibration is rich enough to keep
the spark plugs cool at high loads and at high temperatures, so that they
don't induce pre-ignition.
Consider the Northstar engine. If you do a full throttle 0-60 blast, the
engine will likely run up to 6000 RPM at a 11.5:1 or 12:1 air fuel ratio.
But under sustained load, at about 20 seconds, that air fuel ratio is
richened up by the PCM to about 10:1. That is done to keep the spark plugs
cool, as well as the piston crowns cool. That richness is necessary if you
are running under continuous WOT load. A slight penalty in horsepower and
fuel economy is the result. To get the maximum acceleration out of the
engine, you can actually lean it out, but under full load, it has to go back
to rich. Higher specific output engines are much more sensitive to
pre-ignition damage because they are turning more RPM, they are generating a
lot more heat and they are burning more fuel. Plugs have a tendency to get
hot at that high specific output and reaction time to damage is minimal.
A carburetor set up for a drag racer would never work on a NASCAR or stock
car engine because it would overheat and cause pre-ignition. But on the drag
strip for 8 or 10 seconds, pre-ignition never has time to occur, so
dragsters can get away with it. Differences in tuning for those two
different types of engine applications are dramatic. That's why a drag race
engine would make a poor choice for an aircraft engine.
MUDDY WATER
There is a situation called detonation induced pre-ignition. I don't want to
sound like double speak here but it does happen. Imagine an engine under
heavy load starting to detonate. Detonation continues for a long period of
time. The plug heats up because the pressure spikes break down the
protective boundary layer of gas surrounding the electrodes. The plug
temperature suddenly starts to elevate unnaturally, to the point when it
becomes a glow plug and induces pre-ignition. When the engine fails, I
categorize that result as "detonation induced pre-ignition." There would not
have been any danger of pre-ignition if the detonation had not occurred.
Damage attributed to both detonation and pre-ignition would be evident.
Typically, that is what we see in passenger car engines. The engines will
typically live for long periods of time under detonation. In fact, we
actually run a lot of piston tests where we run the engine at the torque
peak, induce moderate levels of detonation deliberately. Based on our
resulting production design, the piston should pass those tests without any
problem; the pistons should be robust enough to survive. If, however, under
circumstances due to overheating or poor fuel, the spark plug tip overheats
and induces pre-ignition, it's obviously not going to survive. If we see a
failure, it probably is a detonation induced pre-ignition situation.
I would urge any experimenter to be cautious using automotive based engines
in other applications. In general, engines producing .5 HP/in3 (typical
air-cooled aircraft engines) can be forgiving (as leaning to peak EGT,
etc.). But at 1.0 HP/in3 (very typical of many high performance automotive
conversions) the window for calibration induced engine damage is much less
forgiving. Start out rich, retarded and with cold plugs and watch the EGTs!
Hopefully this discussion will serve as a thought starter. I welcome any
communication on this subject. Every application is unique so beware of
blanket statements as many variables affect these processes.

 

[gwild]

Thanks to all for the pointers.

Further specifics of the bike as far as I can remember:
Compression: stock or only slightly higher if I remember correctly
Plug: BPR5E1X (Iridium as recommended by the forum)
Rear sproket: stock
Silencer: Burgess straight through
Piston clearances: as honed by a well-known member of the VMCC based in Bristol.
Passenger and myself: not yet privilege to enough years of beer drinking to nuture proper weight behind us.
Magneto: was rebuilt at the same time as my build, only 2 years/ 1500 miles ago by very capable Oxford member who is very professional so I would be surprised if it was going off.
ATD: rebuilt by Roy Price at the same time.
Fuel: the normal cheap stuff

I will follow consensus and check and set ignition timing to 34 BTDC following Tom's advice probably Sunday.

When at speeds around 60 MPH the engine seems to be at very high revs with an almost homogenous tone, should I be worried? Once again this is my inexperience knowing what it should sound like.

Thanks again for your kind suggestions, I will report back.

Gwild

 

[Black Flash]

While you are working at your bike, I suggest you clean out your carb and blow through the mainjet. Also check carefully that your carb is really vertical looking from the back. a slightly turned carb with its offset float chamber can greatly affect the the petrol level, thereby enriching or leaning out the mixture.
I don't know if plugreading is still possible with this crap they sell to us as fuel today.

Bernd

 

[Howard]

Hi Gwild

Probably a red herring, but high revs/low speed could be slipping clutch ... overheating ... pre ignition/detonation ( Sorry Trev, I just opened this thread, it's Monday morning and I need to be more awake to read your technical jargon - but it does look interesting, and I will read it.

Other than that, I'm inclined to agree with the ignition timing lobby.

H

 

[gwild]

Thanks ClevTrev for the interesting and informative article.
For a beginner like me it is extremely helpful.

In one section it mentions that 'pinging' (from post spark detonation) has a characteristic tone of 6400 Hz.
I used a digital function generator from the web to generate a tone +- the quality of my headphones, and I can convince myself that the sound emitted from my engine seems in roughly the same ballpark.
With this positive result I checked the timing and found it firing rather later than 34 degrees BTDC. I re-timed, re-assembled, and the bike fired up by gently leaning on the lever.
Positive result- the bike started easier, idled easier, and best of all felt twice as powerful- before 50 was about the limit, now 65 uphill isn't much of a challenge.

There's still a slight metallic noise which I couldn't hear before, but then this IS a Vincent even if only half a one.

Question: from a plug test learnt from theory, my plug looks similar to this. Should I richen the mixture slighly as this was a short run on a coldish day? If so, where should I start?

thanks to all for pointers, I'm amazed what a different it's made!

gwild

 

[Tom Gaynor]

I preserved from Cycle magazine an article on plug reading by Kevin Cameron, he of TZ / Kenny Roberts / Yamaha factory fame. When I finally got round to reading it I found that I'd been looking in the wrong place on plugs for years. One ought to look at the bottom of the annulus that surrounds the centre core.
I'll see if I can find it.

When my Manx was running best, the plugs were a dirty white. Certainly not "light brown", which was far too rich. I don't think a Vincent is anything like as picky, but on standard jets (180 or 190, I forget) my plugs are black on the side electrode, and dirty white everywhere else. This isn't a proper "plug chop", killing the motor at full throttle, but just pulling them when I get back home after a run. I'm pretty sure a plug chop (i.e. no time spent on the pilot jet) would show dirty white all over. I think modern fuels make "classic" readings obsolete (and have a hazy recollection that I've read as much) and just as "black is the new white" in fashion statements, white is the new light brown in plug colours. Of course if you have an air-lever, you can find out for sure. If at sustained full throttle closing the air-lever slightly increases your speed, you were too weak. Temperature does make a difference: my Shadow splutters and back-fires for the first mile or two this time of year if I open the air-levers too soon, in summer I only need the air-levers to start. It would be fair to say I aim to run it lean (keeps 'em keen...) but I've been running that way for the last 10,000 miles, and it'll cruise at 80-odd, and top 100 quite easily when desired. On private roads, it goes without saying.
Since comparisons are always useful, I rode what I thought was a pretty sparky Comet yesterday, and it would hit 65 in third and 70 in top quite easily on a country road. Take a tooth off the gearbox sprocket, or a couple of teeth off the rear sprocket and it would probably have been a lot quicker. It is after all geared to do 90, at which I reckon you'd have to be flat on the tank.
The answer to most Vincent mechanical noise is to wear earplugs...

 

[Bill Thomas]

Hello Gwild, I have not seen anybody recomend a spark plug with an R in it for a Mag', It's not good. Still you seem to have found the timing out so you are on the right track. All Vins are overgeared so two up it's best to buzz it in third, To get your speed ,Then take top, Good luck Bill.

 

[b'knighted]

Go Wild,

Sorry to criticise but you really mustn't think of the Comet as only half a Vincent. As you are a beginner it may not be obvious that the Comet is so much more but, although it is only 499cc, instead of 998cc, its performance is considerably greater than half the performance of a twin. It is only missing one cylinder and head, its still got all the other good points.
Notably, when you get it sorted, it should be able to approach 85mph in third, one or two up. Mine did that regularly but never exceeded that in top, which I'm sure relates to the overgearing. Not many twins can boast double that speed. It should return better fuel consumption figures than a twin and will stop considerably better. It will be easier to throw about on winding roads and in traffic.
In case no one has pointed out to you it is closer to Black Shadow spec than Rapide and although it uses a front head it has a rear cam..
If the postwar twin did not exist the Comet would shine in comparison with its contemporary 500cc competition. The good points of Comets only get overlooked because on the twin they are so famous..

Good luck with it and when the time comes try to add a twin to your stable rather than replacing the Comet with one. They go well together.


Cheers,

 

[gwild]

Two up again this weekend and there was still some of the pinging noise if I accelerated hard from low speed, but otherwise better power - cruising at 60 or so seemed possible, but not much more.
Is it of the opinion of the gathered assembly that I should further hunt for solutions?

One further observation: when chasing a twin yesterday at around 50mph, at a certain throttle opening the bike seemed to choke and slow down until I throttled back below that position. I opened the throttle again, the bike slowed, so I opened further and the bike seemed to suck through the problem and then increase in speed as normal. Is this normal behaviour? Does this indicate just a temporarily blocked jet, or something more sinister?

A few additional notes relating to your posts above:
Tom, you said "If at sustained full throttle closing the air-lever slightly increases your speed, you were too weak". Regretfully I forgot to check this, but what is the mechanism governing this? I naively assumed closing air lever reduces air thus more petrol thus slower

Tom again, you also said " Take a tooth off the gearbox sprocket, or a couple of teeth off the rear sprocket...". Wouldn't this have opposite effects? It seems to on my bicycle which spins slower and therefore is easier to see.

Bill, now you say it I can't find the reference. I'll have another look. Still, seems to start nicely now even when warm.

b'knighted please do not worry, I was being facetious- it is easy to forget that tone of voice does not carry well down the internets. Having attempted to sync carbs on a twin I know first hand some of the single's advantages, and am very proud of my bike I built myself even if I still need tutoring in some aspects.

gwild

 

[Tom Gaynor]

The air lever drops an obstruction into the air flow, reducing it. It has no effect on the fuel. So if the motor goes faster when you close the air-lever, it needed a little less air. Ergo, it was too weak before. Note however that this has to be done carefully, slowly, because if you close it completely at full throttle, it'll choke itself to death.
Despair not: this is not rocket science, but it isn't simple, either. Amal's tuning procedure is still the best way to set up an Amal carb, but they make it sound more black and white than it is.
Many British bikes were overgeared. This gave a high theoretical top speed, useful for marketing, and perhaps great economy at moderate speeds, but acceleration suffered. International Nortons would do a calculated 125 mph. Yeah, sure, on the downhill run to Creg, with a following wind. On the flat they wouldn't pull anything like that. Trying to keep this simple, power delivery is not constant. If max power is at 6000 rpm, and at 6000 rpm in third, let us say your speed is 70 mph, and that power is exactly that required to overcome resistance, dropping 1500 revs into top drops the power, and the bike will slow down. Or won't go any faster. If however it picks up 5 or 10 mph on a downhill stretch, you'll often hold that: the downhill put you further up the power curve. When one revs it to the max before changing up, you're trying to get far enough up the power curve in the next higher gear to have some in hand to go faster. Easier maybe to think of a bicycle: you might go faster up a hill in a lower gear. With madame on the carrier it may need to be lower still. Taking a tooth off the gearbox sprocket lowers ALL the gears.
I used to mark the twistgrip so that I knew whether I was on 1/4, 1/2 or 3/4 throttle. Then I knew which part of the carb I had to adjust. Choking at 1/2 throttle means the needle is too high (rich) or low (weak). Easy to find out...
In fact, the biggest thing you've achieved, that many devoutly desire, is a bike that starts easily. Contrary to received wisdom the most important thing about a bike is not acceleration, road-holding, steering, or top speed, it's starting. Because if it won't start, nothing else matters. And 65 two-up isn't all THAT shabby.