E: Engine Valve Timing

SteveO

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Hi all. I'm back with another puzzler. I'm trying to replace the timing gear in my (very non-standard) Rapide. I've got a copy of the service notes, read them. I've also got Richardson and Stevens, but they aren't helping. You see the only timing marks I have, blobs of yellowing paint, when they are lined up make no sense. I'm looking at Fig. 56 on page 107 of Richardson's book, but my gear is completely back-to-front. At TDC on No 1 (rear) cylinder, my keyway on the mainshaft is at 5 o'clock, not 7 o'clock. The slots in the cam pinions instead of pointing to 11 o'clock (rear) and 6 o'clock (front) are transposed. My half-time pinion has a chipped tooth, which we surmise may have occurred when the tooth was centre popped to mark it, as there appears to be a round indent below the damage. One of the slots has a mark, but this is not the slot the key was in and if I put the pinion on the mainshaft, the broken tooth is nowhere near the paint mark on the idler. I'll try and post some pictures shortly, when they have uploaded from my phone.
 

timetraveller

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It is generally a waste of time, with an engine you do not know the history of, to try to use timing marks. The cams might not be standard or have been pushed into the pinions at any position. The correct way to do it is to use a dial gauge and degree plate and plot a graph of valve lift against engine rotation. That way you will find out what the cam profiles are and can then consider how best to time it. Do it for the whole cam profile in case you have a faulty base circle or whatever. If you lack the time or facilities to do that then arranging it so that there is equal lift on inlet and exhaust cams at about four degrees before top dead centre. This will get the bike running. If you do not have access to a dial gauge or degree plate then you need to find someone who has the time and patience to help.
 

vibrac

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I only ever use the 4 degree btdc method and two digital indicators I find that a better method than trying to detect what is the start or end of lift. the only other thing I use is a removable key in the HT pinion so I can try to get as near to 4 degrees on both pots (an impossible task)
 

Robert Watson

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I'm with Vibrac, I have done the 4 deg BTDC on engines with Mk1's, Mk2's, some unknown ones, and prewar 5X ones as well as a set of 105's. they have all run very well except the 105's, as that was only done a few weeks ago and is some time away from starting so the jury is still out!
 

ClassicBiker

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Steve,
I did my cams (MK III)at 4* BTDC and it worked a treat. I've included some links below where Vibrac educated me on doing it and where in appears in Tuning for Speed. One of the links show a simple tool I made for turning the cam against spring pressure. I had the added advantage that I was installing new cams and pinions and large idler and didn't need to replace the half time pinion. So I was able to get all the timing marks to align and didn't need to make any new ones on any of the gears.
So as far as I'm concerned the 4*BTDC is the method of choice. It's simple, expedient, and effective.


 

davidd

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Steve,

It depends a little on what you are trying to do and what tools you have. If you do not have digital dial indicators, then the 4 degree method is a little difficult to do. If you're trying to install timing gears and you want the marks to work like the Richardson figure shows them, then that is a little different. Although, it is important to understand that the Richardson figure shows the timing gear stopped at 4 degrees before tdc.

It would be good to know exactly what gears you want to replace and what cams you are using.

David
 

Monkeypants

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It might seem obvious, but I'll post it anyway.
It's a good idea to make your own set of marks once things are set, whether using the 4 degree method or the opening and closing method.
It was sure nice to have the marks there for easy reassembly after pulling my new engine apart for crank balancing.


Glen
 

vibrac

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Steve,

It depends a little on what you are trying to do and what tools you have. If you do not have digital dial indicators, then the 4 degree method is a little difficult to do. If you're trying to install timing gears and you want the marks to work like the Richardson figure shows them, then that is a little different. Although, it is important to understand that the Richardson figure shows the timing gear stopped at 4 degrees before tdc.

It would be good to know exactly what gears you want to replace and what cams you are using.

David
I have used the 4 degree method with a digital height guage first I set the measuring height of the closed valve to be the same by adding washers and shims to the shorter valve with placticene on top of valve cap once they are the same it's a question of measuring down the side of the spring as the valve opens or closes not ideal but doable especially on a comet when you can set the point first, lock cam and work backwards to the HT pinion.
 

Bill Thomas

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I am sure you know that if you turn the engine over, It's many turns before the marks line up again.
I do it by eye, Leave the adjusters out and just watch the pushrods tops as you turn the cam wheel back and forth by hand, I use a straight edge/ruler pointing from the middle of the spindle to between the pushrod tubes, With the slot in the camshaft end upwards on the back pot, With the idler gear not fitted.
 

roy the mechanic

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The 4 degrees method works well, if you can remove the valve caps. Last week I retimed a twin with the motor in the bike. I measured the position of the inlet rocker at tdc, then turned the motor to tdc at the overlap point, measured again, ended up with .107 lift rear, .111 front. It starts third kick cold, first kick hot The owner collected it sunday, returned 3hours later, "it goes really good, was cruising at 70 mph, next time i looked was going 90". I figure there is always more than one way to skin a cat. Provided the inlet is opened more than the exhaust at tdc you will have a reasonable runner.
 
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