Question about timing on my Norton

On my 1971 Norton I have a question. With both pistons at TDC the left side intake and exhaust valves are both closed, indicating a compression stroke. At the same time the right side valves are both open about one sixtieth of an inch Is this correct? Should both the exhaust and intake valves be open? I thought only one valve would be open but not both. The timing marks are aligned and chain links are correct according to the shop manual. Wouldn't this cause blowback into the carburetors?

The period where both intake and exhaust valves are open is known as "overlap" Overlap is useful in using the outgoing exhaust pulse to suck in extra intake volume. The amount (in degrees) and timing of the overlap period is germaine to how this effect is used to increase power in a specific part of the powerband. This period of time when the intake valve begins to open but the exhaust valve is not quite closed is what you are seeing.

'Wouldn't this cause blowback into the carburetors?'
When the motor is running there are standing waves in both inlet and exhaust which prevent blowback at above the revs at which stable running occurs. When you kick the bike over, it only fires on one cylinder in the first instance, and the gas inertia created in the other inlet port starts the filling process for the other cylinder. You will notice that below a certain rev level the running is rough and above that, it is smooth. That level is known as the 'cam spot' and the inlet length and exhaust configuration combine with the cam timing (overlap) to determine at what revs it will occur.

acotrel said:

When the motor is running there are standing waves

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They might sometimes be referred to as 'standing waves', but they are in fact PULSED waves of gas,
with (relatively) long intervals of nothing much happening inbetween.
We diverge though.

Commandos have a relatively modern cam design (for the 1960s) with no real discernable 'cam spot',
it just pulls strongly from very low rpms all the way to near redline, if you actually try this.
(The Combat maybe less so, it does more get going after 5000 rpm apparently, but I've not owned a Combat).
If you watch the rockers - with the rocker covers removed - there is action earlier than you'd think on the exhaust and intake strokes.
Thats turning it over by hand, its rather quick once its running...

nortonisthebest said:

On my 1971 Norton I have a question.

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P.S. Have you had this bike running, and it ran well ?

Its not unknown for bikes that have been standing or with unknown history to have valves not seating very well, sticky or even held off their seats.
If you turn the engine over further, do all the valves actually shut, and also open and close fully in their cycles.
You can get the correct tappet clearances on all 4 valves, showing its all healthy and ready-to-go. ?

Both valves open at TDC is totally normal. What you should actually see is the intake valve ever so slightly more open than the exhaust, that is if the cam is properly timed.

Yes that is what is happening. According everyone it looks ok. The engine has been rebuilt with new super blend roller bearings, con rods, pistons and rings, head also rebuilt with new valves, guides and springs. I haven't ran it yet and I still need to put everything back into the frame. My biggest concern is rewiring it with a new wiring harness. I will be going with a Sparks electronic ignition and not sure how to wire it correctly. Any suggestions?

Its Like SPAGHETTI , but simpler , as its colour coded .

Main Color Purpose
Brown Hot all the time, unfused
Purple Hot all the time, fused
White Hot with ignition on, unfused
Green Hot with ignition on, fused

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for starters . then ; http://www.mgexp.com/article/lucas-colours.html

though you wont need half of that , unless its a space ship .

Rob a Mk 4 Zepher or look under the carpets in a Mercedes for Wires .
Best is Hillman hunter Wire & Fittings . Mustve gone upmarket in that dept .
And The Colour Codes are correct . Pre Euro Standard drivvle .

U.K. Lucas pre 75 are all the same as the link back to 1602 . :wink:

The intake-compression-power-exhaust cycle not a series of separate events, but rather each event is not complete and final before the next commences. Engines with more overlap tend to produce power at higher rpm. In reading about H-D's VR racing motor, on the dyno, it produced best power when the "stand-off" or cloud of air-fuel mixture blown back out through the intake was largest. So while the engine does not actually fire back through the intake, the breathing of the different cycles is not necessarily a one-way constant flow and can at certain points be reversed.

If your keen this link contains movies of computer simulations of the pressure waves in engine ducting (intake and exhausts) modeled by the late Professor Gordon Blair. A very keen motorcyclist and Professor of Engineering. The valves are the two small circles and the pressure in the cylinder head is modeled in the space between them. You can see it goes below atmospheric pressure at a certain point and during overlap this can pull the incoming charge into the cylinder. Pressure on the side is in bar. One bar being atmospheric pressure.

From memory the importance of overlap to optimize engine performance was first realized in the 1910s or 1920s.

http://www.profblairandassociates.com/RET.html