Thinking yet again ,changed it to 33 and will see how it goes , my built sheet from the factory shows 44 ,thats a big chage .
ATD Advance Timing Curve?
- Thread starter cinquecento
- Start date
When your "A" was built the fuel of the time probably took a long time to get burning so it was "fired" earlier. The current petrols seem to burn faster needing less advance.
33 is an excellent setting to choose with modern fuels and compression of around 7.5:1, should result in a smooth running motor. BUT I recall you are running 9:1 pistons, so even 33 may be a tad high. Others with similar compression to your motor are running with full advance around 28 to 29 degrees.
When on the road, with the engine under load, such as when pulling hard up a hill if you detect any sign of pinging/detonation then reduce the advance by 2 degrees (down to 31) and road test again. Keep doing this (reducing the advance) till the pinging has abated. Detonation or pinging is a sure way to destroy the piston and also induce premature failure of the big end bearings. It also means, that with the detonation happening early, the motor is NOT producing its best power.
Dropping a gear seemed to get me up the hill with out changing the best advance for my bike when I wasn't putting abnormal loads on the ol girl.
Of course Martyns way would be the safest and only a small power loss would occur.
Of course Martyns way would be the safest and only a small power loss would occur.
Chanky is NOT correct in regard to jetting and he has incorrectly made this assertion a few times in the past.
At first lean running was suspected as the cause of my piston failure but the root cause was eventually and correctly diagnosed as detonation caused by way too much ignition advance.
Since the rebuild my bike has covered over 3,000 miles with leaner (not richer as asserted by Chanky) jetting than when the piston failed - BUT The ignition advance has been reduced from 38 BTDC down to 31 BTDC. For a short time I actually ran with 28 BTDC then started gradually advancing the ignition to the point where I could JUST detect pinking/detonation when under load hauling uphill then I backed it off 2 degrees to settle on the 31 BTDC that I now use
For the record, before the piston failure I was running a 210 main and a 106 needle jet. Today I am running a 210 main and a 105 needle jet in my Amal Mk1 Premier 30mm carb. CP 7.2:1 piston and a K&N air filter. I use ONLY 98 RON fuel.
This information has also been entered into the other threads by myself.
In the last 3 days my Comet set up as described, with me aboard, has covered in excess of 600 miles at an average speed of 56 MPH - with absolutely no issues.
Lets not confuse detonation with pre-ignition, which are two different animals. Detonation occurs when there is NORMAL ignition , a kernel of the fuel air mixture starts to burn. As the burn increases ,in size the acceleration of the burn also increases. If the heat in the combustion chamber is too high due to load and other things ,the leading edge of the burn ball splinters into fingers of burning air fuel mixture. These fingers are burning at an accelerated speed over the burn ball. There are pockets of unburned fuel air mixture between the fingers of highly accelerated finger fuel air ratio burn. This compresses the unburned fuel air mixture to a point of EXPLOSION. This is not a burn ,but is the same as a stick of dynamite, the explosion exceeding the speed of sound . There are now two flame fronts , the expanding burning ball with fingers, and the explosion flame front which when they meet cause a supersonic clash heard as a metallic sound or pinging referred to as detonation. As this is a shock wave and has but a small heat factor , an engine can survive many thousands of miles ,enduring minor detonation. Pre -ignition is self explanatory, something in the combustion is hot enough to ignite the air /fuel mixture ,
a sparkplug ,having too hot of a heat range for the fuel air mixture "the leaner the mixture the hotter the combustion chamber temperature" becomes a glow plug giving up its purpose to provide ignition at a predetermined point , other things that can cause pre- ignition are, glowing bits of carbon, and a read hot exhaust valve. A proper fuel/ air ratio, determined by the carb jetting, does Not ensure against the most destructive cause of piston failure , which is a too lean air/ fuel mixture. The engine can hole a piston in just a few seconds, from running great, too disaster, it is swift and unforgiving. The temperature of the fuel burn ,goes from happy to sudden death, the burn becomes highly oxygenized, and turns into a blow torch, easily melting the piston instantly. When inspecting for the cause of piston failure, if the piston shows signs of cracks, broken rings and ring lands, or caved in piston crowns, the failure was due to extreme detonation . If the piston shows signs of melted metal, it was run on too lean of a fuel mixture, usually caused by foreign matter in the fuel system, (DIRT) , momentarily plugging the main jet, just long enough to ruin your day.
a sparkplug ,having too hot of a heat range for the fuel air mixture "the leaner the mixture the hotter the combustion chamber temperature" becomes a glow plug giving up its purpose to provide ignition at a predetermined point , other things that can cause pre- ignition are, glowing bits of carbon, and a read hot exhaust valve. A proper fuel/ air ratio, determined by the carb jetting, does Not ensure against the most destructive cause of piston failure , which is a too lean air/ fuel mixture. The engine can hole a piston in just a few seconds, from running great, too disaster, it is swift and unforgiving. The temperature of the fuel burn ,goes from happy to sudden death, the burn becomes highly oxygenized, and turns into a blow torch, easily melting the piston instantly. When inspecting for the cause of piston failure, if the piston shows signs of cracks, broken rings and ring lands, or caved in piston crowns, the failure was due to extreme detonation . If the piston shows signs of melted metal, it was run on too lean of a fuel mixture, usually caused by foreign matter in the fuel system, (DIRT) , momentarily plugging the main jet, just long enough to ruin your day.
A lot of old car engines use to suffer from Pre ignition, The easy fix for manufacturers was to install a solenoid in the carburetor in the idle circuit to stop the fuel flow......I think it worked very well. I liked your description on detonation Max......Is it true that when an engine detonates that the cylinder pressure raises from a normal 600 to 900 PSI to well over 2200...? or is this the temperature....?...Cheers.......Greg.
Excellent bit of explanation Max. Here are 3 photos of the failed piston from my Comet. No sign of melting of the piston - The top compression ring groove has actually opened up, a section of the top ring has broken off and been wrenched upwards and out of the groove - I assume it eventually went out the exhaust. That bit of broken ring scored the bore beyond repair and also badly damaged the exhaust valve and seat.
The second compression ring groove has collapsed trapping the ring and preventing it from applying any pressure on the bore. It was only the oil ring that kept the motor running! And it ran in this condition (photos) for around 400 miles till I got back home (it was the last leg of a 3 day 1,300 mile trip) and was able to strip down the motor and start repair work. In case you wondered, its (or was) a CP piston. Oil use during those last 400 miles was high due to the excessive crankcase pressures resulting from the failure of both compression rings.
I was advised that the light grey/white ash deposits on the piston crown indicated that the mixture (thus the carb jetting) was just about spot on. The reason I moved down a size on the needle jet was to reduce fouling of the spark plug at moderate speeds/throttle openings.
The second compression ring groove has collapsed trapping the ring and preventing it from applying any pressure on the bore. It was only the oil ring that kept the motor running! And it ran in this condition (photos) for around 400 miles till I got back home (it was the last leg of a 3 day 1,300 mile trip) and was able to strip down the motor and start repair work. In case you wondered, its (or was) a CP piston. Oil use during those last 400 miles was high due to the excessive crankcase pressures resulting from the failure of both compression rings.
I was advised that the light grey/white ash deposits on the piston crown indicated that the mixture (thus the carb jetting) was just about spot on. The reason I moved down a size on the needle jet was to reduce fouling of the spark plug at moderate speeds/throttle openings.
So, just nicely run in then!!