motor getting **** HOT!
Posted: Fri Nov 13, 2015 8:00 pm
More of a technical question really, here goes.
I've spent the last couple of weekends putting together (more like throwing together from spare parts) a motor that i can use for (ahem) "High Speed Seagulling"
Based on a silver century block with slightly older crankcases (CP model as it had the best crank bushes with the least amount of play in them) and an LLS cylinderhead (mid compression type if that's the right terminology) and a seagull 4 con-rod with a new gudgeon pin.
A basic pre-strip down compression measurement found 50psi in the combustion side of the cylinder. HOWEVER, piston and port alignment at BDC were so far out , there was a gap of around 1 to 1.5mm between the transfer slot on the piston and the port window itself. What to do....
i carefully skimmed the crankcase down by the same 1-1.5mm measurement and now the piston lines up perfectly while at BDC.(with base gasket fitted)
By doing this it's also allowed the piston to move further up the bore and in doing so has increased the compression at BOTH ends of the cylinder. Combustion chamber compression is now 75 psi (cold) but after a short run it rises to near 85psi (hot) and crankcase pressure seems a bit higher too. Also due to the "skimming" when the exhaust port is fully open, the piston now covers this slightly by about 2mm.
Now, the real problem with this motor is that it runs too hot for my liking. Just by touching the crankcase after 2 mins running in the bin (no prop and in neutral so the motor isn't loaded up) it's "scorchio hot". There's plenty of water coming out from the "pee hole", waterjacket has had the usual treatment so it's near as damn it spotlessly clean.
I've also had to alter the base plate timing just to get it to run anything like acceptable. (wipac gold flywheel with points not the cdi type) The basic static timing was 48 degrees BTDC (standard setting using the original dimple location.) To get it to run now i have to move the timing to near 20 degrees BTDC. ( is this too much or not enough?) It's running with an Amal 416 carb at the moment with plenty of room for improvement later on if i need it.
Have i overdone it with the compression value? Does the timing need to be backed off a smidge? Am i running too lean or too rich with the fuel? ( i don't think so as the plug tip is fairly dry with greyish deposits which tells me i must be somewhere near right with the fuel side of things)
How can i get the temperature to come down in the cylinder and keep a reasonably reliable motor without having it seize up at high(ish) revs? Having spent a few hours going through some of the "books" i'm still at a bit of a loss.
How hot do these things get when they're running at over 5500rpm's for hours on end? (i havn't got a tachometer yet but i'd guess it's running close to that maybe a bit more)
I'm all ears
HELP.
Jon
ps be gentle with me, i'd normally work on 102's but this is something different for me just to "play" with. (nudge wink)
I've spent the last couple of weekends putting together (more like throwing together from spare parts) a motor that i can use for (ahem) "High Speed Seagulling"
Based on a silver century block with slightly older crankcases (CP model as it had the best crank bushes with the least amount of play in them) and an LLS cylinderhead (mid compression type if that's the right terminology) and a seagull 4 con-rod with a new gudgeon pin.
A basic pre-strip down compression measurement found 50psi in the combustion side of the cylinder. HOWEVER, piston and port alignment at BDC were so far out , there was a gap of around 1 to 1.5mm between the transfer slot on the piston and the port window itself. What to do....
i carefully skimmed the crankcase down by the same 1-1.5mm measurement and now the piston lines up perfectly while at BDC.(with base gasket fitted)
By doing this it's also allowed the piston to move further up the bore and in doing so has increased the compression at BOTH ends of the cylinder. Combustion chamber compression is now 75 psi (cold) but after a short run it rises to near 85psi (hot) and crankcase pressure seems a bit higher too. Also due to the "skimming" when the exhaust port is fully open, the piston now covers this slightly by about 2mm.
Now, the real problem with this motor is that it runs too hot for my liking. Just by touching the crankcase after 2 mins running in the bin (no prop and in neutral so the motor isn't loaded up) it's "scorchio hot". There's plenty of water coming out from the "pee hole", waterjacket has had the usual treatment so it's near as damn it spotlessly clean.
I've also had to alter the base plate timing just to get it to run anything like acceptable. (wipac gold flywheel with points not the cdi type) The basic static timing was 48 degrees BTDC (standard setting using the original dimple location.) To get it to run now i have to move the timing to near 20 degrees BTDC. ( is this too much or not enough?) It's running with an Amal 416 carb at the moment with plenty of room for improvement later on if i need it.
Have i overdone it with the compression value? Does the timing need to be backed off a smidge? Am i running too lean or too rich with the fuel? ( i don't think so as the plug tip is fairly dry with greyish deposits which tells me i must be somewhere near right with the fuel side of things)
How can i get the temperature to come down in the cylinder and keep a reasonably reliable motor without having it seize up at high(ish) revs? Having spent a few hours going through some of the "books" i'm still at a bit of a loss.
How hot do these things get when they're running at over 5500rpm's for hours on end? (i havn't got a tachometer yet but i'd guess it's running close to that maybe a bit more)
I'm all ears
HELP.
Jon
ps be gentle with me, i'd normally work on 102's but this is something different for me just to "play" with. (nudge wink)