E: Engine Ignition Advance

van drenth

Well Known and Active Forum User
VOC Member
Got the same problem, noted at rebuild that the holes in de cilinder muff where to "small"for
the cilinder studs,
Drilled them bigger , AND problem solved.
Muff could expand better when really hot.
Jan van Drenth
 

Martyn Goodwin

Well Known and Active Forum User
Non-VOC Member
Got the same problem, noted at rebuild that the holes in de cilinder muff where to "small"for
the cilinder studs,
Drilled them bigger , AND problem solved.
Muff could expand better when really hot.
Jan van Drenth
Thanks for the tip Jan. Next time I have the muff off I will check carefully.

I wonder if any one else has had this issue?

Martyn
 

oexing

Well Known and Active Forum User
VOC Member
Hi MartynG,
looking at your photos I wouldn´t blame the fuel or ignition or some such matter for doing that sort of disaster. My guess would be a substandard piston ring material that broke at higher revs and eating into the grooves. Your engine does not seem to be that highly stressed from installed components to have this sort of effects I´d think.

Vic
 

greg brillus

Well Known and Active Forum User
VOC Member
The piston shown is a classic case of detonation........And very much yes a combination of fuel and ignition. I'm surprised that you cannot hear the engine pinking away.......I've ridden plenty of Comets and twins that are pinking and it is easy to hear........it is most likely a case of too lean a fuel mixture for the throttle position held and too much advance for that given engine speed.......Nothing wrong with the piston or rings........Just the set up.........Also running a Comet at 75 MPH continuously is probably asking a bit much of it........If you are going to do this, you need to know that the tune is correct, just like i am doing at present on the racer........I don't have any spare pistons, so I can't take any chances. if you just guess at what you are doing, then you are taking a risk.
 

oexing

Well Known and Active Forum User
VOC Member
Hmmm, at 7.2 c.r. and 98 fuel detonation ?? I can´t see any traces of detonation or melting on the piston top, just pretty black so it was well below critical temperatures I´d think. And lean on fuel - does not look like it but maybe an effect of limping home . 75 mph too much for a Comet - really ? Our 1935 Guzzi, all cast iron, does 70 all day long, on motorways if it had to. So my guess, there were several bits of piston ring broken which wrecked the top groove ??

Vic
 

Nigel Spaxman

Well Known and Active Forum User
VOC Member
Those look to be really nicely made pistons. It does look like it was running rich and I wouldn't have thought running rich would ever cause any harm to pistons. I have had some very well used Triumph 650 hepolite pistons do that. One time I had a piston ruined because one of my cylinder liners moved down. It was on a Triumph with an aluminum big bore kit. The kit wasn't very well engineered, the problem was the liners didn't have much of a lip a the top and the aluminum used for the casting seemed pretty soft. The head gasket pushed the liner down slightly. This caused it to pinch in which pushed the rings down ruining the piston. I fixed it by making liners with a much larger lip. Vincents don't have that problem. Make sure your bore is straight and there is no lip at the top. Make sure also that your liners are not to tight a fit where they fit into the recess in the head.
 

greg brillus

Well Known and Active Forum User
VOC Member
What kind of normal loading could possibly go through a piston and it's rings to cause them to break up........I cannot imagine a manufacture with the experience an knowledge of CP or any other top quality piston having that kind of issue.......Not all symptoms of detonation melt holes though piston crowns, the shock load from the flame fronts hitting each other is what has overloaded and caused the rings to break........In many cases the actual ring lands break off. This is also a symptom of detonation at lower engine speed and loading thus causing eventual damage rather than something more catastrophic like a hole melted in the piston crown from a lean condition whilst the throttle is held wide open.
 

Gerry Clarke

Well Known and Active Forum User
VOC Member
In relation to the cylinder stud holes issue mentioned above.

I had cause to rebore two years ago. In the process I ordered a new muff and found on reassembly that I couldn't seat the cylinder head onto the spigot - the head just wouldn't travel all the way down the cylinder studs. Rather than the holes being too small, it seemed to me that the studs were being drawn slightly together and the pitch between the stud holes in the cylinder muff was .020" less than on my old, believed original, cylinder muff. I then did what I should have done in the first place, which was to have my old cylinder bored out. As expected, the head went on perfectly well then and we were back on the road.

The Spares Co. were very good about this and refunded me, but said they couldn't see any issue with the cylinder holes on whatever slave system they used to check. I was surprised that it only seemed to happen to me.

Gerry
 

Martyn Goodwin

Well Known and Active Forum User
Non-VOC Member
It seems to me that the piston crown , especially in the area near the exhaust valve, became overheated and 'plastic' allowing the ring groves to distort and eventually for the end of the top ring to break through into the combustion space right under the exhaust. At the time my ignition at full advance was 36 BTDC.

The replacement setup, same make of piston, new muff n liner with 0.0045" piston/bore clearance was rebuilt but this time with ignition at full advance 34 BTDC. After around 8,000 miles noticed compression was a tad low so pulled the barrel only to find the same thing was happening - distortion of the ring groves - but not as severe as earlier.

Again new piston, this time 0.020" oversize and rebore with 0.005" clearance and fitted a decompression plate under the muff. Still with 34 BTDC - result was a surprise seat-of-the-pants slight but noticeable improvement in overall performance.

It was only recently that as I explained in post #3 that I have now changed the ignition to give full advance at 30 BTDC, full retard at 4 BTDC. Went for a short run yesterday (COVID rules mean I cannot travel more than 5 km from my home) and was very pleased with the performance, definitely improved torque on the hills.

I am running the original magneto. I stripped the ATD down, added metal to the fixed ears then filed the added metal back to give the advance range I wanted being 13 at the magneto that yields 26 at the crank. The hardest bit of all is setting the timing as the ATD tends to move as you tighten the retaining bolt. Patience and swearing helps!

Martyn
 
Top