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MG Midget and Sprite Technical - Help with cam timing

Hi all, me again!

I just have a couple of questions regarding cam timing:

My DTI gauge arrived today so I thought I'd double check the cam timing as it had been installed on the dots and while the engine was out it made sense to have a look.

I used the method described in Vizard and the cam is reaching full lift on no 1 inlet (second pushrod from front of engine?) at 115* ATDC and it should be 107* ATDC. Does this mean the cam is too advanced?

The engine has run ok on this setting since I had it, could this be why I thought the cam was standard? Just checking as although I have checked and rechecked the figures but as I have never dealt with this before I wonder if I'm missing something.

Would it be ok to correct this with an offset key or would I need a vernier sprocket?

If I can use an offset key does it have to be specific for advancing or retarding the cam?

OK that more than a couple but I got on a roll!!!!

Many thanks
Mark
Mark Whitmore

I always time based on (inlet) valve open and close times... just like the cam spec says, and evening out any apparent error (maybe due to tappet clearance being slightly out...).

A
Anthony Cutler

Hi Anthony, the engine is currently out of the car and the head has been removed, I have set up by gauge onto the top of the block, inserted the pushrod and taken the measurement that way.

Can it be done this way or does the head need to be on?
Mark Whitmore

You are doing it just fine

Yes your cam can be that far out
Yes you need to time it better
First try what moving the cogs around does (how mutch is one tooth fwd or back)
Then see where you end up
Offset keys are just a number of degrees offset and can be used either way
Onno Könemann

One tooth is 18 degrees IIRC.
Dave O'Neill 2

I always set mine up just as you are doing. One tooth is 20 degrees, so will be too much. Offset keys work well and are much cheaper than verniers. Moss list an 8 deg key part no MGS108258 which is the one you want. If you can't find an 8 deg one, then a 7 or 9 would be close enough. (Unlike ignition timing, there is no absolute correct value for cam timing. If the cam timing is shifted a little from spec it will be better at some engine speeds and worse at others, rather than worse everywhere as with ignition.)

Make sure you recheck the timing after fitting the key. It works either way so it's easy to have brain fade and fit it the wrong way round!
Paul Walbran

Mark

You can put the dial-gauge onto the pushrod... but don't forget the pushrod operates without the additional gap of the 'tappet clearance' so will not match the manufacturers open/close spec... it will seem more 'wild' than the cam really is.

You need therefore to measure the open and close angle on the crank, and have the cam for the same error as 'rod is just starting to move / just about to stop moving the pushrod.

Or stick the head and 'gear on and time the valves as before.

A
Anthony Cutler

Great!!! thanks for all your help.

I will check it again tomorrow with the head and rocker gear in place just to be sure.

I have been doing a bit of digging around in the archives and would I be right in saying that I will be needing to advance the cam?

Also am I likely to notice much difference after setting it up correctly (if it is 8* out)?

Thanks
Mark
Mark Whitmore

BMC used a 'timing clearance' of .016" at the cam & other profiles are similar to this, so if you zero at .016" lift off the base circle you will be quite close to measuring the right spec in relation to valve opening and closing.

Theoretically, retarding the cam (your current setting is retarded - the valve is getting full lift later in the stroke) reduces low end torque and improves top end. Practice can sometimes be different. The advantage of a vernier is that it enables you to more simply try a number of different settings and optimise cam timing for your engine and your use pattern. This of course requires much time on a RR or a hand bit of road with a stopwatch or accelerometer.
Paul Walbran

Thank you all for the advice.

If I can just ask one more question that has me puzzled.

If I measure full lift on number 1 inlet, whether or not I take into account the gap between the rocker and the top of the valve, surely the valve will be fully open at the same number of degrees ATDC as measured on the protractor attached to the crank pully?

Sorry for all the questions, I like to get thing clear in my head.

Thanks for your patience
Mark
Mark Whitmore

Spot on Mark

Make sure you go a little before and the same little after max lift and split the difference for the actual timing. If you are measuring 115 and it should be 107 it is retarded and you need to advance. The engine will run flat when retarded. Usually one tooth is 18 degrees on a and b series. 106 degrees would be better than 108.

Peter
P Burgess

Hi Peter, I did the before and after and split the difference: 105* before, 125* after, giving me full lift @ 115* ATDC.

Many thanks, I can sleep soundly tonight without thoughts of cam timing going through my head!!!!

Mark
Mark Whitmore

Hi, just an update:

I picked up the relevent offset key this morning and after a slight hiccup (installing the key the wrong way!) it is now timed as it should be.

A big thanks to all the advice, both on this thread and also in the archives.

Thanks
Mark (awaiting next head scratcher!!)
Mark Whitmore

Mark

Good story

please tell me if the only thing you are doing to this engine is resetting the cam timing?

I tend to agree with Mark that we can all speculate on how advancing or retarding the cam may have on performance, however reality can be totally different. This is easy to explain and goes like this: Advancing the cam would get mixture in better at higher revs so power ought to be increased at the expense of torque further down the RPM range. Well that may sound right but there has to be a limit where no matter how far you advance the cam ultimate power will not be increased. so where is the limit? perhaps the cam designer may know! :)

Yours being retarded therefore ought to mean ultimate power will be down because the engine will not rev so well and bottom end torque should be higher. Well was it? seem to read that you were not impressed.!

Anyway when I have played with cam timing I have found no difference in performance with 2/3 degrees either side of design but can't comment on bigger differences so yours was substantially different. So assuming you change nothing else apart from resetting the cam it would be interesting to hear what your feelings are when you try the car again?
Bob Turbo Midget England

Hi Bob, having never driven another midget other than my own (which was slightly modified when I bought it), I presumed the cam was standard as I couldn't feel any increase in power (coming onto cam) but from what I have read this would be more noticible in a wilder cam? I have never owned a car that was modified in the engine dept so again my experience is rather limited?

The reason for checking the cam timing is that my gearbox required a rebuild and I thought whilst it was all out I may as well have a look and also broaden my knowledge a little to boot. There are no other mods being done other than the existing ones listed below.

The engine did seem reluctant to rev high and maybe for that reason I didn't do it that often!

Time will tell what the effects are but I will report back once I've driven it a bit.

The engine spec is:

1275 +0.030"
MG Metro cylinder head
Piper HP270 cam
HIF44 on Titan manifold
LCB manifold (to be replaced with Maniflow item
RC40 rear box.

This made 78bhp at the flywheel on Aldons RR (with twin HS'2). I had a look at the paperwork yesterday and max advance was set to 27*

I will get the car set up on a rolling road and may look at getting an electronic dizzy to suit the engine as I seem to remember from the archives that max advance should be 32*?

Thanks
Mark
Mark Whitmore

Max advance can be up to 32 degr
This means that in your engine max power could be at 28degr

Most of the times more ignition timing advance gives more power.
But going past 32 usualy yields no extra power.
H
However pinking is always a risk so start at 32 and see if it pinks if it does retard a little and try again

The only real way to find out is go to a RR
Onno Könemann

This thread was discussed between 10/03/2011 and 11/03/2011

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