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Twin Valve Timing


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What's the best way to check the valve timing on a twin with the engine in the frame (can't reach the upper valve caps/covers).

The cams are Gary Robinson's 105.
Timing is stated as I.O. 47 degrees btdc - lift 5 thou with 2 thou nip.

I checked it previously with a dial gauge on the pushrod end of the rockers, but I suspect that it's not quite right as the engine is popping and spitting and the exhaust has a very harsh tone.

Is the 'on the rock' method suitable? if so can someone talk me through it?



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I always check at the top of the valve stem.
I seem to remember someone saying that checking at the rocker ends gives an incorrect reading,but how much out i have no idea.
Mk2's have the same effect on the exhaust note and a Vin,especially a Comet ,can get a pretty staccatto exhaust note.
I think you may have to wait for someone who has used the 105 cams for some sensible advice..John


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try checking for crossover at 4 to 6 degrees BTDC exhaust stroke, exhaust closing intake opening,with crossover being the point of equal lift.
It shouldn't matter if you are checking off the cam at the rocker or in at the valve, as long as you do same for all. Equal lift is equal lift' no matter what. This figure seems to hold for almost all cams in all engines, small block Chevvies, Briggs and Strattons, Vincents and so on.


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I have never liked the Vincent way of measuring cam timing. Measuring 5 thou or 2 thou seems so wrong. I have almost always used dial indicators on the rocker arm, mostly because the ufm was on. The ratio of the rocker should be 1:1. I think for most it is difficult to have brackets that hold the dial indicator properly and the rocker does "rock", as opposed to the top of the valve stem. But, knowing this should help keep you on track.

The diagram in Richardson's showing all the pinion marks lined up was done at 4 degrees BTDC on the exhaust stroke. I use digital dial indicators because it is easier to spot the crossover point. If you can do this and the crossover point is good, the inlet should be opening at 47 with the weird instructions. I would just run through the strokes again and record the openings and closings at 0.040" and 0.050" so you have a record of what the timing is according to industry standards. You may not care, but this will allow you to compare your cam timing to all other cam timings.

I think it was ABCD that posted some good information on the 105 when I incorrectly stated it was the lobe separation angle. As I remember it is the maximum lift that occurs at 105 degrees.


roy the mechanic

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Popping and spitting can be caused by incorrect carburation. A harsh and angry exhaust sound could well be cam timing. On the "thing" in the picture (on the left) I spent ages trying to achieve the timing figures for the T P V mk4's. Gave up, went for equal lift at 10 degrees before t d c. It has a nasty, angry exhaust note, but I'm getting used to it, it goes so well! Makes the old rap feel like a moped! Between 4-6 sounds like sensible for a road bike. There is no mention in either Richardson or K T B on this subject.


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If you are going to measure cam timing from the rocker caps you should nip up all the pushrods first to remove an variations. You should do the same when measuring from the valve caps too.

Vincent Brake

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I set it for equal lift indeed, but also check inlet close point to be similar front/ rear,
i overheard once a good story on it, as it has to do with the amount of compression one gets in each cylinder
and compression has great influence with the speed the mixture burns.
i guess it can cause big vibrations, when they are diffrent.


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I have never liked the Vincent way of measuring cam timing. Measuring 5 thou or 2 thou seems so wrong.

2 - 5 thou "nip"
Then set the dials to Zero.
Any flex in the rocker arm/s (+ follower+ cam spindle play + whatever else comes into the equation (pushrod compression?)) is then eliminated.
Set timing disc to 0 once "nip" has been taken up.
This is especially important when measuring on the rocker arms (although I was taught to follow same practice when reading directly off the valve stem as well)

roy the mechanic

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The basic idea of equal lift before t d c is to ensure that the inlet valve is further open when at t d c . Hence on racers and hot-rods it is important to ensure sufficient valve to piston clearance! An example is Aston Martin 6cylinder, the setting at t d c is inlet open 110thou, exhaust 90 thuo open. If you wind it back , equal lift will occur at some point before t d c . As an aside, 50's and 60's Rover cars were set-up on the exhaust peak at 105 degrees. I wonder if this is connected to Mr Robinsons designation.

Martyn Goodwin

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I have taken the advise of PEI in Tuning for Speed by measuring the valve lift (dial indicator on the top of the valve) as outlined in the chapter "Improved Cylinder Filling"; it is important that the steady plate be in place and that all lash be taken out of the valve train when taking measurements. It was a time consuming task but was very informative, especially after entering the data into Microsoft excel and producing a number of detailed graphs. My cam is a recently purchased (less than 10,000 miles use) Mk.1 and below is the overview graph of the results of my measurements. Detailed examination of the data shows that my cam is pretty close to the mid point of the specification contained in Richardson. It is interesting to note that the point of equal lift is 4 degrees BTDC.

The main advantage as far as I see it is, once you have obtained like information about the cam(s) in YOUR bike - remember no 2 cams are identical - you can then make subsequent changes/adjustments to cam timing by measuring just the exhaust valve - which on a Comet can be done without the need to lift the UFM .

If anyone wants to see all the details of MY Mk 1 cam (remember yours will be different) send me a PM with your email address and I will send you the excel file.

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