Hi guys just finishing up putting my top end back together on my 75. I am checking the timing with a degree wheel just to make sure things are set right before putting engine back into frame. while at TDC there is a slight lag or dead spot in the piston travel although the intake valve is opening as soon as I move the rotor. Is this normal?? Thanks!
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You mean at TDC? then yes, the piston will appear to stop moving for a bit at TDC. When the crank pin reaches it's highest point in it's revolution it stops going up and starts going sideways. This only lasts a short time.
This stopping occurs just before TDC and restarts just after TDC. Referred to as piston dwell.
Leo
This stopping occurs just before TDC and restarts just after TDC. Referred to as piston dwell.
Leo
That's also why locating TDC by indexing (locating the midpoint between two piston positions of equal drop before and after TDC) is the preferred method.
Yes while at TDC, thanks for the response guys...I am curious though there does not seem to be an equal amount of "dwell" in each way you rotate the rotor. It seems to have more dwell when the rotor is rotated one way over the other. The only thing i can think of is being 1 tooth off on the cam but Im almost certain I set it correctly, thoughts? It just raises my concerns when checking with degree wheel that the intake is supposed to open 36 deg. before TDC and and it seems to be opening right away?
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What do you mean by "seems" and what point on your degree wheel is "right away?" You're either getting consistent readings from a dial indicator set up on a valve spring retainer or you're not, "seems" ain't in the picture!
Let's be very, very clear about something. Your camshaft has 36 teeth and turns at 1/2 crank speed, so 1 tooth at the camshaft=20* at the crankshaft.
Let's be very, very clear about something. Your camshaft has 36 teeth and turns at 1/2 crank speed, so 1 tooth at the camshaft=20* at the crankshaft.
Alright, at TDC I set my degree wheel at 0. I set an indicator on top of the left (rotor) side intake valve tappet. I start to rotate the rotor counter clockwise just like the service manual says and instantly the intake starts opening...Which to my understanding it shouldnt be opening until 36 deg BTDC. Besides a possible tooth being off on the cam, is there something else Im not seeing/doing correctly? Thanks for the input grizld1
Yamaha's service literature specifies valve lash set at .012" for inspection. Before you go farther, locate TDC by indexing, and set valve lash with your dial indicator; cupping at the valve stem can give you a false reading with a feeler gauge. If the readings you're getting now are close, your timing error is closer to 2 teeth than to 1.
If you're starting at TDC then you would see immediate movement because the valve is already part way into it's opening cycle. To see no movement, you would want to back the crank up to say 45° or 50° BTDC. Then start turning it forward slowly and the valve movement should commence around that 36° mark.
Good catch, 5twins! I hadn't thought he'd start looking for the valve to start opening after the specified point, but that's exactly what he says he did!
