dumb tdc ?

[84merc951]

a friend and just finished the assembly of his 302. the crank is on #1 tdc and before we where gonna adjust his valves we thought we would turn the crank a few times to see if everything is smooth and i noticed that as we were tunring the crank clockwise that the #1 cylinders exhuast valve opened up before the intake valve. is this correct as im having a brian fart. the #1 exhaust valve opened up at about half a turn of the crank. any help would be great

[85crownHPP]

I'm not too familiar with engine insides but that sounds right to me, if that TDC is after combustion, then the exhaust valve should be the first to open.

[jayh]

#1 at tdc should be between compression stroke and power stroke

so when you turn the motor the piston goes down on power stroke, no valve movement, after 180deg on the crank the piston comes up, exhaust stroke, ex valve should open, crank passes tdc again, piston goes down, intake stroke, int valve should open, 180deg later pistoncomes up compression stroke, should be no valve movement, plug fires, combustion stroke same process all over again

4 stroke engine starting at tdc- power, exhaust, intake, compression, power, exhaust, intake, compresiion, power, exhaust, intake compression, power, exhaust, and on and on and on

[84merc951]

thanks for the great info. for what ever reason i was thinkng the intake opens up first after tdc. how do people end up being 180 out somtimes.

[jayh]

thanks for the great info. for what ever reason i was thinkng the intake opens up first after tdc. how do people end up being 180 out somtimes.

the way that happens is when people just line up the timing pointer with the mark on the balancer without pulling #1 plug to make sure its on compression stroke so they end up #1 at tdc but they are between exhaust stroke and intake stroke instead of between compression stroke and power stroke. then the fuel doesnt get burned on power stroke so there is still fuel in the cylinder then the plug fires 180 off on intake stroke and there is still fuel in the hole so the explosion goes out the intake valve hence you get popping out of the carb or tb

understand?

timing gears are 2:1- 2 turns on the crank for 1 turn on the cam. its called 180 off because the rotor is pointing the opposite way

[Mercracer]

thanks for the great info. for what ever reason i was thinkng the intake opens up first after tdc. how do people end up being 180 out somtimes.

A quick review of the basics...
Intake, Compression, Power, Exhaust are the 4 strokes in that order. 2 strokes down and 2 strokes up of the piston. Your intake valve does open after TDC but it is TDC Exhaust stroke not compression stroke.
When you set the ignition BTDC you are causing the spark to fire just as the piston reaches TDC compression stroke. The expanding gasses from the ignited air/fuel mixture combustion is what causes the power stroke which is the piston being forced down.

[84merc951]

thanks i was just having a huge brian fart. at least we know where on tdc now.