Cam at TDC but pistons at the bottom

[badassjamie]

This engine I got from a po wouldn't start despite good spark, fuel, and everything else looking fine and so upon further examination, I discovered the cam shaft slot when at TDC, the pistons were at the bottom of the stroke, or at the bottom of the case. I observed this by looking thru the plug hole.

I'm assuming the Cam is 180 degrees out? Is it just a matter of repositioning the cam with pistons at TDC?

Thanks

 

[XSLeo]

If the TDC marks on the stator and rotor line up and the pistons are not at TDC the cam won't matter.
Watch for the piston to come up as you turn the engine over. When you see it in the plug hole insert a plastic drinking straw in the plug hole so one end is on the piston. Now as you turn the engine the straw will move down as the piston comes up.
With the straw at it lowest point check the timing marks on the rotor and stator. Do they line up at TDC? If they do then look at how the cam lines up. The notch on the cam on the left side next to the sprocket should point straight up.
On the right side of the sprocket there is a dot or dimple out by the teeth on the sprocket, this should line up with the sealing surface of the head where the rocker box sets.
Page 57 in the Clymer books shows how the marks line up. Page 61 in the Haynes book.
Leo

 

[Brian902]

perhaps this will help

http://www.650motorcycles.com/CamTiming.html

 

[badassjamie]

It has a PMA and is not marked for TDC. When the Cam notch is pointing straight up, the pistons are as low as they will go.

If I can't depend on the PMA to indicate TDC then I needed to look for the notch in the cam, which didn't make sense because the pistons were at the bottom of the jugs.

Therefore I'm led to believe the Cam was installed with the pistons at the bottom of the stroke and should be adjusted for when they are at the top?

 

[smokedyerstang]

That's what I'm thinkin. Pain in the ass. And was the pma on it when you got it?

 

[smokedyerstang]

If youve got the stock charging parts you could throw em back on to get tdc, then put the pma back on and mark it..

 

[badassjamie]

I was going to do that, but I want to know if the cam is just out 180 with the pistons at the bottom. As in Bottom dead center!?
The PMA was already in but I had the Pamco wired to the green coils and battery just to hear it run. I had known good carbs and every variable was perfect but the engine would not run and it confused the hell out of me, so then that when I started to look at the position of the timing and found the index notch on the cam to be straight up when the pistons are at the bottom, not the top.
Make sense?

 

[5twins]

How can you see that? It's inside the motor and not visible when assembled. All you can see with it assembled is the pin hole in the end of the cam from Brian's link.

 

[badassjamie]

Yes, I'm looking at the pin of course

 

[Brian902]

Bad Ass,

Try this:
- remove both plugs
- turn engine counter clockwise as viewed from the left side, as per the normal rotation of the engine
- watch for intake valve rocker to move down, opening the valve and keep turning until the rocker rises again, closing the valve
- now put your straw down the left plug hole, same side as the PMA, of course, and turn the crank until you feel the piston rising and is at it's highest point. you can go back and forth a bit to make sure, but at this point both the intake valve and exhaust valve rockers should be loose, in other words both valves closed.
- you are at top dead center or at least close enough for a "rough" verification
- now look at my link above and see if you are close

- next step would be to properly mark your PMA for TDC and timing marks and truly verify your timing

Good Luck.

 

[peanut]

great answer Brian..... its going to very useful to me shortly Thanks

 

[Brian902]

Bad Ass,

I made up this template to work with my PMA.

With the left side valve adjusters "removed", so the valves hardly move, don't want them hitting my piston stop.

I use a piston stop, degree wheel and homemade pointer to find TDC.

Then I will be re-marking my PMA and side cover, which I haven't done yet, so I'll have valid marks from then on. Making a new 4to5mm key so it's repeatable every time I remove my PMA.

Made the degree wheel using TurboCad but can't find a way to export it in 1:1 scale so others can use. I would have to mail you a print copy. Not a very good solution. Paper copy fits tight and sits there perfectly.

 

[grizld1]

Never mind.