Camshaft Timing

[John L]

I have wanted to check this for some time now, the engine seems to run OK, but I dont have anything to compare it to.

So I have followed the original workshop manual, and reset the number 6 cylinder tappets to 0.040" and then turned the crank to TDC on number 1. But both tappets are tight at this position.

If I turn the crank back and come up again to TDC, the exhaust valve is still open, and when the inlet just starts to open, no clearance, the crankshaft is about 10 degress before TDC.

Couple of questions,

Does this mean that I havent got the original camshaft, as I have no idea what was fitted before,

Have I not done this step correctly?

Is there something else I can do to find out what sort of camshaft is fitted.

I have been setting the tappets to 0.010"

If I open up the 0.040" original tappet clearance on number 6, and then see if they have equal clearance at TDC, this would I presume mean that the camshaft would be timed correctly, but it must then have a different overlap, or duration?

 

I would much appreciate your comments and guidance please.

 

Regards John

[marvmul]

I have wanted to check this for some time now, the engine seems to run OK, but I dont have anything to compare it to.

So I have followed the original workshop manual, and reset the number 6 cylinder tappets to 0.040" and then turned the crank to TDC on number 1. But both tappets are tight at this position.

If I turn the crank back and come up again to TDC, the exhaust valve is still open, and when the inlet just starts to open, no clearance, the crankshaft is about 10 degress before TDC.

Couple of questions,

Does this mean that I havent got the original camshaft, as I have no idea what was fitted before,

Have I not done this step correctly?

Is there something else I can do to find out what sort of camshaft is fitted.

I have been setting the tappets to 0.010"

If I open up the 0.040" original tappet clearance on number 6, and then see if they have equal clearance at TDC, this would I presume mean that the camshaft would be timed correctly, but it must then have a different overlap, or duration?

 

I would much appreciate your comments and guidance please.

 

Regards John

What you are discribing is normal if the rotor is pointing at lead no 1 when the engine is at TDC and the valves of cyl. no 6 are both tight : they are 'in overlap'and a little opened.

When the cam timing is correct, you should find that, when turning the engine clockwise and then back again (towards TDC), the clearance of the exhaust should also disappear

at 10°, after TDC now, if the cam is symmetrical one. But this is an inaccurate way to check the cam timing, and it surely is not a correct way to fit a cam at the correct timing.

To fit a cam, you should find true TDC, and then turn the CRANK to the installation figure that is given for the CAM, then turn the cam so that the inlet valve no 1 is at maximum lift and then fit the distribution chain (with cam sprocket) without moving camshaft or crankshaft

[piman]

Hello Marvmul,

 

while it is not the normal way to time a cam (particularly a modified one) it is a method in my Mk 1 Triumph 2000 workshop manual (Standard Triumph issue) and is a quick and easy check. The point about accurate TDC applies to whatever method is used.

 

John, if the valves are tight on number 6, number 1 firing then open the gap equally some more until they both have clearance, then measure inlet and outlet to compare. What I don't know is what tolerance to expect except that the Triumph timing adjustment is accurate to about 4 degrees.

 

Alec

[marvmul]

Alec, I don't disagree with your arguement and both methods should give the same result in an ideal world, but you will find that the timing by installation figure is not always identical with the timing determined by the position of the cam in overlap with the same degrees of opening for both valves. This can be the case :

-when the cam is not a symmetrical one

-when you find that the cam, mounted exactly by the installation figure, will give a slightly different opening for inlet and exhaust valve when the crankshaft is at TDC

-when the manufacturer has found that the cam is performing better when the cam is advanced or retarded by some degrees.

I think that in these cases the timing should be determined by the 'installation figure' given with the cam. (why call it 'installation figure', when the cam should be mounted at another figure?)

[piman]

Hello Marvmul,

 

as I read the original post, John is looking to do a quick check and that method is the easiest.

 

Unless there is some obscure cam fitted, Triumph cams are symmetrical.

If this method indicates that there is an error then it will be necessary to get out a degree wheel, to see what the timing is. There are many ways of timing a cam, some call for degrees at maximum lift, others degrees when starting to lift (this needs an accurate tappet gap) and one car I had specified a dimension of lift at TDC. Whichever method there is room for error but I have yet to see any tolerance figure given.

 

For me I would initially do exactly as John is doing.

 

Alec

[John L]

Thank you for the replies... What confused me was the fact that I had to put nearly double if not more to get the tappets free at TDC... Well the out come would seem that that it is a std cam as it showed that with equal clearance the crank was at TDC.

Also with the correct tappet clearance of 0.010" the inlet was opening at about 35degrees BTDC.

I didnt actually do the whole cycle, I need to get a proper degree marker to make life easier and accurate.

Thanks again

John

[heliguy]

I thought my question might be relevant in this thread. After a bit of a delay, i am getting around to putting my engine back together. Its a CP series engine with an early TR5 PI camshaft. Should i just time the cam to the factory specs/marks, or has anyone found through experience over the years, that the car may run better with the cam timing slightly advanced/retarded? And if so, by how many degrees? The car will be driven "on the street" with a 150 HP PI system installed.

 

Thanks in advance,

 

Mitch

[Tom Fremont]

Hi Mitch,

 

You won't be necessarily be able to nail it spot on with the standard sprocket, so the conventional wisdom says to advance the cam ~ 2-3 degrees if possible so that it will pass through the sweet spot during the chain and sprocket wear life. There are only two positions available, so you have to make do as best you can. Note that there is some slop in the holes which will account for several degrees difference at opposing edges.

 

I use the same cam in my driver and couldn't be more pleased with its behaviour, set as described above.

[Guest ntc]

Toms quite correct here, the reason being most people time the camshaft and don't remember to allow for tappet clearance.