Cam problem...

Demorgan

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Hi! i have just finished rebuilding my engine from barrels upward and installed in frame, and was just making some final checks when i got suspicious about the timing, specifically the points opening and closing looked all wrong... upon closer investigation, it looks like the cam is 180degrees out... ie, the little locating pin that drives the advance mechanism and points DOWN when the left piston is at TDC on compression stroke..(and upward at TDC exhaust stroke) i KNOW that when i installed it i made sure it was upward and the pistons were at TDC, and surely its the cam that decides where in the cycle each piston is...therefore if the cam was right when i put it in, (ie, the notch pointing upward) and the left (and right) piston at TDC why is it not right now?? IF i had accidentally put it in upside down while engine is at TDC, it would mean the valve timing would be all out, but it isnt, they open and close all at the right time, its just the points are closing at 180degree from the 'F' mark on the rotor... am really confused! help!
i really dont want to have to open it all up again!!
and yes, i have searched for similar treads, which is why i am able to confirm the position of the cam by the driving pin behind the advance mechanism...

thanks!
 
Hi, Do you have the two corresponding lines on the advance rod and plate lined up?

I borrowed the below of Hughs page as it highlights the marks
 

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OK, so, if you walk the engine round its four strokes on the left pot (as in left sitting on the bike) and get the left pot to TDC on its compression stroke - where does the cam pin hole sit?

When you installed the cam, did you have the notch on the cam chain pulley vertical and the dot level with the top of the head. The notch denotes the cam is the correct way up as per the image below
 

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If the left pot is at TDC on compression stroke, the pin points downward.. But I def made sure both Pistons were at TDC and the notch pointing upward when I installed the cam!
 
I am not that familiar with standard crank, points ignition XS's however reading your post I can offer some clarification on some issues.

The cam has no effect on the pistons positon in the bore. The pistons positon is determined by the rotation of the crank.

The cam lobes determine the opening and closing events of the valves. The cam driven by the crank rotates at half the crankshaft speed (two revolutions of the crank to one revolution of the cam).

Once the cam is installed the valve opening and closing events relative to the crankshaft cannot change to any major extent under normal circumstances because they are connected together by the cam chain.

Finally this may be a complete red herring but the XS crank turns anti clockwise, try checking again ensuring that you are turning the crank in the correct direction.
 
Ah! I didn't realise it was anti-clockwise..however, it seems to make no difference.. The valves are all opening at the right times in relation to the Pistons, sucking-squashing-bangin--blowing, the advance mechanism correctly assembled. it's just the points closing 180degrees opposite the 'f' mark.. Which would be 90degrees out on the cam given 2:1 ratio? And yes, the driving pin on the cam is def pointing down when left pot is at TDC on compression stroke... Aargh!
 
Hi - turning it clockwise (while looking at the engine from the alternator side) you'd have seen the exhaust ports and inlet ports acting the opposite of each other. For example on the true exhaust stroke, turning the engine clockwise it would appear to be drawing through the exhaust port.

Just to be 100% turn the engine anticlockwise using the nut on the alternator end of the crank. Turn the engine untill you see the inlet port opening (the nearside port on the rear of the engine) and observe the piston through the spark plug hole - it should be on its downward (suck) stroke. Carry on slowly rotating and you should see the piston return up the bore (it's compression stroke)with no valve movement. When the piston reaches the top of its travel on that stroke - what postion is the hole?
 
Yes, I was using turning the nut on the alternator anti-clockwise and watchin the cycle, and all the valves were doing the right thing at the right time, BUT the driving pin on the end of the cam that is driving the advance unit/points is pointing down when left piston is at TDC on compression stroke, ie, both valves closed after, and after the intake stroke..
 
Your cam is installed correctly. Take a look at the cam pic in post #4. The top pair of lobes are for the right cylinder, the bottom pair for the left. The rocker arms ride on top of the cam. As pictured, which is how you install it during assembly, the right cylinder will be at TDC on it's compression stroke. The left cylinder will be at TDC on it's exhaust stroke. Rotating the crank another full revolution to bring the pistons to TDC again will put the left cylinder at TDC on it's compression stroke and turn the cam 1/2 turn. That will make the advance pin hole point straight down instead of up.

I think your problem may be in the assembly of the advance rod. The points cam on the left end and the little disc on the right end that ties into the advance unit both have locating pins of their own. They must align, point in the same direction, or your timing will be 180° off .....

AdvanceRod.jpg


I don't know why Yamaha did so but they drilled the pin holes all the way through the rod. That makes it possible (and quite common) to install the pins 180° off from one another. That throws the timing off and I'm guessing that's what happened here.
 
5twins, thank you...I will check that when I get home...! Yeah, I did notice that the holes went all the way through and thought that could really confuse someone, so maybe that's it..
 
The other thing is, you can't be checking for points opening and closing with just the naked eye. They start to open long before you'll actually see a gap. You need an electrical means to test for this.
 
"its just the points are closing at 180degree from the 'F' mark on the rotor... am really confused! help!"

Demorgan...................You don't understand how ignition systems work. What you see is correct.

The F mark is where the points must open, to allow the coil's magnetic field to collapse. Before that can happen, the points must close which starts current flow through the coil. Yamaha decided that points should close 93 degrees (of cam rotation) before the F mark. That 93 degrees is called dwell, and is the amount of time sufficient to build a strong magnetic field. Since the crank and camshaft are geared in a 2:1 ratio, that means the points will close at 186 degrees (of crank rotation) before the F mark.

Dwell can be different for various ignitions. For example, Pamcopete started with 90 degrees dwell, and later changed to 60 degrees dwell. That means with a Pamco, the magnet in the Pamco rotor will trigger the coil current to flow when the crankshaft is 120 degrees before the F mark.
 
Hello chaps! And thank you for all your advice.. I think, given all the above advice, everything is just fine...the reason I was conserned, was that the Haynes manual was telling me that the points should be doing something different from what I was seeing.. I have to admit the whole electrickery business leaves me scratching my head, I like to see bits of metal moving around, I can understand that! I'll take one last look and compare with what ye all said, strap on the carbs, open taps, lie back and think of England!
 
Hello chaps! And thank you for all your advice.. I think, given all the above advice, everything is just fine...the reason I was conserned, was that the Haynes manual was telling me that the points should be doing something different from what I was seeing.. I have to admit the whole electrickery business leaves me scratching my head, I like to see bits of metal moving around, I can understand that! I'll take one last look and compare with what ye all said, strap on the carbs, open taps, lie back and think of England!

Yes, points ignition can be confusing, and tend to need a lot of "fiddling". That's why many of us removed them and use electronic ignitions such as the Pamco. Just set once and forget.

England.................happy 90th birthday to the queen:D
 
Haha!! Thanks for the Birthday greetings to our esteemed unelected leader (though i love a bit of pagaentry!) Anyway, before we get into politics, yes, I intend to get myself shot of all that wobbly mechanical advance and points ASAP! Btw, is there a way of getting Pamco set over in the UK? Was gonna go Boyer which is 160quid, all in here..
 
Although, before I come across too republican (in the BRITISH way!), I should point out that our current Monarch, Auld Liz, was a wartime motorcycle dispatch rider, on old Brit iron, and is still a bit of a petrol head.. Just saying.
 
Indeed she did ;). Re your ignition, I've used the Boyer on a fair few bikes now and would highly recommend it. I'm using a Pamco on my new build fit the sole reason it's a 277 rephase.

 
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