ET: Engine (Twin) Cam timing -- again

[Robert Watson]

The last three engine I set up I set the cam timing as close as I could with crossover of the valves at 4 deg BTDC. They have all run well at that. The A Rapide with 5X cams, TTR with 5X cam and C rapide with Robinson stellated MK11 cams, but I never thought to check any of them with the opening and closing as outlined in “The Books”.


So I am messing with my Comet that requires some timing chest attention and have just replaced the cam with a Mk2 (Stock one out and a Robinson stellite one in) and set it up at 4 degrees, and for entertainment checked the opening and closings at 10 thou lift.

I get


In open at 44 BTDC (Richardson says Mk1 at 40-42 and lightning at 55)

In close at 83 ATDC (Richardson says Mk1 at 60-64 and lightning at 68)


Ex open at 46 BBDC (Richardson says Mk1 at 72-70 and lightning at 73)

Ex close at 62 ATDC (Richardson says Mk1 at 28-33 and lightning at 50)


I see if I move the cam back about 10 deg I would get 54, 73, 56, and 52, somewhat closer to Mk2 specs. But crossover would be 14 deg BTDC.


Anyone got any thoughts??

 

[Martyn Goodwin]

Here are some cam plots that I have created myself from in house measurements. The Mk1 cam came from V3 products (Neal Videan) while the Mk2 is a Terry Prince cam. Your replacement is one of Mr Robinsons - I wonder what he specifies as the settings for his cams as they may well be different than those produced 50+ years back at the factory.

Martyn

 

[litnman]

Martyn,
Are your graphs made with a digital camshaft profiler?

 

[Martyn Goodwin]

Martyn,
Are your graphs made with a digital camshaft profiler?

No - made by taking readings from a timing disk attached to the mainshaft on the drive side and also pair of dial indicators bearing on the push rod cups in the cam followers. See the item in OVR #39 for info on how I did it.

M

 

[Martyn Goodwin]

No - made by taking readings from a timing disk attached to the mainshaft on the drive side and also pair of dial indicators bearing on the push rod cups in the cam followers. See the item in OVR #39 for info on how I did it.

M

should add - I put the data into Microsoft Excel then used its graph functions

 

[Robert Watson]

OK, just to follow up..... It is a Mk2 cam. (book numbers shown in brackets)

at .010 lift with equal lift set at 4 deg BTDC + lift A .165

Readings taken on top of the adjusters on the outboard end of the rockers.

In Open 58 BTDC (55)
In Close 68 ABDC (69)

Ex Open 73 BBDC (61)
Ex Close 47 ATDC (50)

seems to run just fine.

 

[Martyn Goodwin]

As I inferred earlier. There are a number of sources today for 'new' cams for our bikes but my experience is no 2 are alike. Example - Terry Price produces at least 4 'variants' of Mk2 cams and not one of those is anything like a Stevenage Mk2. I would expect the same of the other styles. Different makers each using the same designations (Mk1, Mk2 etc) produce different cam profiles.

All you can do is take measurements from whatever cams you can get then compare them.

As to cam timing equal lift @ 4BTDC is a good place to start. After that is down to dyno tuning - a tedious and expensive path.

 

[davidd]

I always find it difficult to compare Vincent cams as the seem to be measured at differing heights. The original Phil Irving calculations were measured at 0.005". The MegaCycle cams are measured at 0.050". Harley, and most others, measure their cams at 0.40". I think of a Mk1 as a 30-44-44-30 cam measured at 0.050". The Mk2 as a 37-51-51-37, also at the same height.

If you look at Martyn's first graph you can see easily that the cross-over or equal lift occurs just before TDC, which would be the vertical "0" axis. You can see that the equal lift (crossed lines) occur just before TDC at 4 degrees, or so. If you look down at the 3rd party Mk2 you can see that the crossover is nearer TDC (which can be moved), but what is most noticeable is that the graph lines show a lot of wobbling compared to the lovely Mk1 graph lines.

If you look at the wobbly lines where the valves are opening and closing you can see why you have to measure the cam at 0.050" instead of 0.05" on the lift scale. The exhaust has a lump right at 0.030". If you are trying to compare cams, you would like to stay away from the anomalies or they may skew your numbers.

When you use a cam machine you do not have a connecting rod or a piston.

As a result, you cannot set the degree wheel at TDC because you have no idea where that is. The best you can do is turn the cam until you get equal lift at both valves. You can then set the degree wheel at zero or 4 degrees before zero knowing that this cross-over point is now your only reference.

I think this machine is something Roy Robertson used to check cams.

David

 

[Vincent Brake]

i measure with only the small springs, as i found out it makes a bit of difference, than again head (hat) off n on....

after the 4 deg tdc setting, i always check inlet closing points, make a bit difference in compression, and smoother running

 

[Little Honda]

Here are some cam plots that I have created myself from in house measurements. The Mk1 cam came from V3 products (Neal Videan) while the Mk2 is a Terry Prince cam. Your replacement is one of Mr Robinsons - I wonder what he specifies as the settings for his cams as they may well be different than those produced 50+ years back at the factory.

Martyn

View attachment 16760

Are your exhaust cams higher than inlets?