I am currently tidying up my engine a changing the cam to one a little less lumpy, I have a rollmaster timing chain and sprockets and was wondering what the reason for advancing or retarding a cam is?

1. Cams (reground ones) are not all the same due to grinding tolerances.
2. Standard sprockets have a keyway position tolerance too.
3. At one point a while back some stock sprockets were made to deliberately retard the cam, for better emissions.

Setting a sports cam up `dot to dot' will seldom give best power. The cam grinder specifies the cam timing, so ask him what it should be.
Often a cam is set up 2° or so advanced to allow for chain stretch in service.
This can be done with vernier sprockets of various types, or with an offset cam key.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

The good Doc has Nailed it once again (as always)
However, "Some" people also like to play with the cam timing with the intent to move the use-able power up or down the rev range, so with advance or retarding ability via the use of adjustable or vernier type cam gears makes it easy to have that "Play"

I`m not one to do this myself (Normally) but others i`ve known have used quite wild cams & tamed them somewhat for limited road / club race use,,, i myself see no point really,,, why not just use a more sensible (suitable cam) for the intended purpose of the particular vehicular propultion unit

so,,, there goes some more usless trivia for anyone interested

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

Vizard's big yellow book has a good explanation of how to do it

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did I tell you that I won a trophy?

When I had my F100 with a 302C motor I played around with the cam timing and it made a big difference to how the motor worked. If you went one way it moved the power up the rev range and if you went the other way it moved the power down the rev range. Moving the power down was what I needed for and F100 towing a HQ race car.

With the HQ race car some blokes would change the cam timing to suit different circuits. They claimed that Amaroo needed two degrees one way and Oran Pk needed two degrees the other way. I just left it in the middle so it didn't suit either circuit!

I'm going to be fitting a RE13T, what timing have other people used with this cam?

whatever cam timing GR tells you to use

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did I tell you that I won a trophy?