Poorish compression and piston leak by
- Thread starter nkt267
- Start date
Never heard what compression ratio piston were you running . This greatly affects what ign advance gives the best results and least tendency to detonate . A too lean mixture promotes detonation , watch the plug color .
Pinking is usually detected by a feeling of roughness and a sound like breaking glass , best to listen with your helmet removed . Start off with a main jet a couple sizes larger than std . and fresh clean plugs of the correct heat range to better detect the color .
Sid .
Pinking is usually detected by a feeling of roughness and a sound like breaking glass , best to listen with your helmet removed . Start off with a main jet a couple sizes larger than std . and fresh clean plugs of the correct heat range to better detect the color .
Sid .
I am running an 8:1 at the moment because when I changed the piston that's all they had in standard size.
I have been running a larger main jet for quite a while as when the piston failed last time I had been running hard 2 up. My thoughts at the time were that a combination of weak mixture,slipped ignition timing and a lower than normal gear ratio caused my first catastrophic piston failure.
When I rebuilt the engine I fitted a new big end, Carrillo rod,piston muff and liner. Since then I have upped the gear ratio to standard,it doesn't go as well now though, and fairly carefully run the engine in.
So I was suprised to find that the barrell was as scored as it is.The compression was ok but not like my original Comet where you could stand on the kickstart without it going over compression. My 1932 Sunbeam will not go over compression without the valve lifter, by comparison you could kickstart the Comet without the lifter which was why in the end I stripped it down.It would still cruise at 65-70 without much fuss, it just goes to show how relatively bullet proof a Comet engine is..john
I have been running a larger main jet for quite a while as when the piston failed last time I had been running hard 2 up. My thoughts at the time were that a combination of weak mixture,slipped ignition timing and a lower than normal gear ratio caused my first catastrophic piston failure.
When I rebuilt the engine I fitted a new big end, Carrillo rod,piston muff and liner. Since then I have upped the gear ratio to standard,it doesn't go as well now though, and fairly carefully run the engine in.
So I was suprised to find that the barrell was as scored as it is.The compression was ok but not like my original Comet where you could stand on the kickstart without it going over compression. My 1932 Sunbeam will not go over compression without the valve lifter, by comparison you could kickstart the Comet without the lifter which was why in the end I stripped it down.It would still cruise at 65-70 without much fuss, it just goes to show how relatively bullet proof a Comet engine is..john
8 to 1 is a nice compromise , our Meteor is very strong on this ratio but is ported out to 32mm with the same size carb and manifold , and other tweaks that matter . Will exceed the ton , very smooth and strong running . Super easy starter . Runs a magneto and a SuperTrapp . And a double speed oil pump .
Why do you think the ATD is misbehaving ? Describe please .
Is your gas cap vent enlarged ? Are your taps big bore , and do you open both at speeds above 65. Or only open one ?
All extremely important if you are a hard runner . These tips if ignored can burn pistons . Believe it . Sid .
Why do you think the ATD is misbehaving ? Describe please .
Is your gas cap vent enlarged ? Are your taps big bore , and do you open both at speeds above 65. Or only open one ?
All extremely important if you are a hard runner . These tips if ignored can burn pistons . Believe it . Sid .
More . Please state which carburetor and silencer is used , and if an air filter is used . All have great enfluence over power production . And what is the highest speed even seen on a flat surface . Note that all Vincents , both single and twin come std. Overgeared . A 48 tooth rear sprocket really wakes up a twin , and a 46 enlivens a Comet .
Ridden two up on a Comet definetly taxes it considerably and limits it's performance and one must accept that fact , trying to squeeze a quart out of a pint plot is rarely rewarding . Like cylinders , two scoops of ice cream is always more fun . Sid .
Ridden two up on a Comet definetly taxes it considerably and limits it's performance and one must accept that fact , trying to squeeze a quart out of a pint plot is rarely rewarding . Like cylinders , two scoops of ice cream is always more fun . Sid .
Quite right Sid. As a teenager I laboured under the dillusion that a smaller rear sprocket would make my Comet faster. The bike was transformed after fitting a 52tooth sprocket; much better acelleration and top speed, even improved fuel consumption as the engine could now rev freely.