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MG Midget and Sprite Technical - camshaft timing

Hi, following on from my 'fuel?' thread recently, it appears my camshaft is 20 degrees retarded, which I understand equates to 1 tooth on either cam or crankshaft cog.
My question is: Is there an easy way to move 1 of the cogs 1 tooth, whilst keeping the other in position?
Also, as the cam is retarded, would I move the cam cog forwards or the crankshaft cog backwards? Is one way easier than the other?
All help gratefully received.
1275 engine.
Thanks,

Andrew
aj robinson

All the manuals I've read on the A series just say line up the dots on the two cogs. If it's still not correct then you need a vernier type camshaft cog to get it more precise. It sounds like whoever assembled your engine was not very precise.

On my engine (that someone else assembled) the distributor drive was incorrectly positioned so my dizzy is in a funny position but it all works OK.

Rob
Rob aka MG Moneypit

To advance the cam you want to move it counter clock wise...to retard the crank you would move it clock wise

prop
Prop and the Blackhole Midget

To advance the cam, you move it clockwise.

You need to pull both gears and turn the cam sprocket one tooth within the chain, which will give you 18.

If you want to be really accurate, you'll need an offset woodruff key, or vernier gear, but for general road use, 2 is near enough...and certainly a lot closer than it was.
Dave O'Neill 2

Dave,

I agree with the clockwise to advance, but there are 40T on the cam sprocket, which gives 9 per tooth, so for 20 doesn't it need moving 2 teeth = 18?

Andrew,

You say that 'it appears my camshaft is 20 retarded' - how has this been identified? As said above, line up the dots with standard timing sprockets and you will be pretty well right, unless you have a reprofiled cam - do you?

Richard
Richard Wale

Richard as Paul explains on the fuel thread, one tooth is 18 degrees on either cam or crank when measured.
Best method of retiming the cam if it is one tooth out is to get engine to TDC number one and see if dots are close on cam and crank gears, if not rotate 180 degrees. The marks will either be spot on if the problem is the cam or one tooth out of build error. Either way mark the part of the tooth on the cam gear which will take the gear clockwise by exactly one tooth. Keep engine at TDC, slip gears off enough to get cam wheel off camshaft, turn clockwise one link on chain, refit and check marks again. 18 degrees is a lot even on a repro cam and, to date,I have only seen this problem when it is a build error
Peter
Peter Burgess Tuning

Andrew, do you know what method was used to determine the valve timing? The book method for a standard midget cam is quite quick and easy to do, so you could re-check yourself:
Set no 1 inlet tappet clearance to 21 thou, then turn the until the clearance just goes. That point should be 5 deg BTDC.
(Rather than setting to 21 thou, I use 25 thou and then insert a 4 thou feeler, as I have found it easier to determine when the feeler blade is nipped rather than try and work out the exact point at which clearance disappears by rocking the rocker.)
Paul Walbran

Thanks everyone.
Richard/Paul: Identified using a steel rule and spirit level method (I will try and upload the details).
Done by an old school engine tuner with an excellent reputation; he's made a massive difference to my car.
No idea if I have a reprofiled cam or not.
Peter: I suspect build error or previous owner 'fiddling' because the car has always felt a bit 'top end' and lacking in mid range. The recent tune up has made a huge difference, and my guy reckons it could be even better if the camshaft was advanced.
aj robinson

hope this image works

aj robinson

Thanks Andrew ... as per the note at the top of that page, the method works well for symetrical cams - which will include symmetry in timing as well as lobe profile.

In the case of the Midget, it's a bit of a trap as the timing isn't symetrical - in two ways: The duration of the inlet and exhaust lobes are different (252 deg exhaust, 230 deg inlet), and the timing of the lobe centres is different inlet to exhaust (inlet peak at 110 deg, exhaust peak at 105 deg). So around the overlap period that's being measured, the inlet opens only 5 deg before TDC whereas the exhaust stays open until 21 deg ATDC.

Assuming the opening and closing profiles are the same (which IIRC they are) then the point at which both valves are open the same amount is 13 deg ATDC, not at TDC itself.

Given the amount of work involved in re-timing, I would suggest you do the check I outlined in my earlier post, it's quick, easy and accurate, and is also the official workshop manual way of doing it and maybe save the frustration of getting the chain cover off and finding there's nothing wrong.
Paul Walbran

Paul

I'd never seen that method of checking valve timing in the WSM - until now.

It is remarkably simple, as you say.
Dave O'Neill 2

With what you say Paul, if the cam is std Midget then the cam is retarded around 7 degrees which would make more sense. A figure one could well see with repro cams. 20 always seemed too many as the car would run as sick as a parrot, we have had a fair few where folk have been one tooth out.
Peter
Peter Burgess Tuning

Have you seen this OP. . . .
Might help you, its in two parts

https://www.youtube.com/watch?v=rad7cdJmT6k&list=PL1DF21E39347AB8EA&index=9

P Bentley

http://www.youtube.com/watch?v=rad7cdJmT6k&list=PL1DF21E39347AB8EA&index=9
Prop and the Blackhole Midget

Thanks everyone, that is certainly food for thought!
I can live with 7 degrees, particularly as the car is running sweet!
aj robinson

"20 always seemed too many as the car would run as sick as a parrot" Exactly! One of my customers brought a new (to him) race car to me for tuning after finding it was not up to expectations. I found the cam was about 15 deg retarded (offset key the wrong way round) and sorted it out. The car went 5 seconds a lap better! But it was so much faster that he didn't adjust his braking distance enough and ended up in the wall and losing the rest of the season!
Paul Walbran

Hi Paul this puts into graphical form exactly what you say above, lemon back to rocket with one tooth on the cam.
A perfect example of 'one tooth out' Race Magnette'. 72.9 bhp power line with 18 degree retard! 121 bhp line with cam as it was before the quick rebuild and 129 with hi lift rockers.
Andrews difference will not be as marked as the cam is so mild and much more tolerant of 'wrong' timing.
Peter

Peter Burgess Tuning

Hi Peter, ineteresting effect of the roller rockers there on the midrange, what was the cam duration & overlap?
Paul Walbran

Paul - "(Rather than setting to 21 thou, I use 25 thou and then insert a 4 thou feeler, as I have found it easier to determine when the feeler blade is nipped rather than try and work out the exact point at which clearance disappears by rocking the rocker.)"

I was taught to rotate the pushrod. When you can't there is no clearance.

Rob
Rob aka MG Moneypit

Hi Paul
It was an STR 930 mini profile designed by Mike Garton on a B shaft. Modern variant of the old 770/649 profile. 50/70/74/45.
Peter
Peter Burgess Tuning

What is the correct advance for an old BMC 731 cam? I got it from a friend, so it came without any info. 1098 engine, standard 12G295 head.
Bernie Higginson

Peter

did you mis type a number above ... im getting a 57% improvement for advancing just 1 degree

prop
Prop and the Blackhole Midget

731 should have full lift no 1 inlet at 110 deg.
However, it can be worth experimenting with it, I found a useful midrange gain from bringing that forward to 104-106 deg in some engines.
Paul Walbran

18 degrees, one tooth on the cam Prop.
Peter
Peter Burgess Tuning

Thanks Paul. The engine is coming out this winter for a gearbox rebuild, so I can have a play with it then.
Bernie Higginson

'I found a useful midrange gain from bringing that forward to 104-106 deg in some engines'

Yes definitely.

After many years with a Kent MD276, I fitted a Piper 270 (also 276 duration, but more lift) into our 1380 and set it 4 advanced (same as the Kent), and was very pleased with the performance and driveability.

When I had to build a replacement 1380 2 years ago, I spoke to Piper and they said to time it 'straight up', which I did.

The 2 engines are identical spec, apart from the block, which was the reason for the replacement, but driving the new one is quite different. Much more noticeably 'cammy', so if I do need to do anything to it, I will reset it to 4 advanced.

Unfortunately I never had the first version with the Piper 270 on a rolling road, so any comparison now is with the Kent 276.

Richard
Richard Wale

This thread was discussed between 15/08/2016 and 18/08/2016

MG Midget and Sprite Technical index

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