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MG MGB Technical - timing advanced?

hi everyone. i have the 1970 gt with 24d dizzy.since having the car i never thought it run quite right with it holding back and not running right like points gap not correct. changed points and condenser too many times to remember and set up timing with strobe light at 14 @ 600 revs and always run the same. fitted electronic ignition module re timed and still run the same. took dizzy out and had checked and all ok. when i put it back and timed again still the same so left dizzy loose advanced by turning clockwise and it ran better, advanced a bit more and now runs fine.when i put timing light back on to see what it reads its off the timing scale at 600 revs. can anyone explain this and is it ok.

thanks in (advance). no pun intended. bob.
Bob Taylor

Hi Bob,

It does sound a bit strange but I have had similar problems with other cars. I had a Land Rover Discovery that wouldn't start unless the timing was seriously maladjusted, but this was a fuel injected car with all sorts of engine management computers causing complications.

Although in theory your car is very advanced, if it's not pinking then in reality it can't be far off.

Have your tried all the normal tests like compression and mixture?

Mark

M D Howard

hi mark. havnt checked compression, engine fully rebuilt 1k ago but had the problem before, running sweet now. checked and adjusted the mixture and all plugs the right colour.
bob.
Bob Taylor

Possibly the vibration damper has spun and that leaves the timing mark off.
SANDY SANDERS

i had a similar problem after my rebuild, turned out the cam timing was 180 degrees out, so the dizzy had to be timed way off!!! maybe checking the cam timing on yours may help?
A P New

Cam timing? Do you mean crank timing?

It was normal for many of these engines to run better when advanced to more than spec, as the spec took into account a whole series of worst-case scenarios for components as well as a safety factor. These days with unleaded even the higher octanes many won't even reach the spec figures without pinking, so the rule of thumb has to be (on high compression engines anway) advance as far as you can just short of pinking at any combination of throttle, revs and load.

But I would check the TDC marks as has been suggested. You can get a rough idea by removing No1. plug and peering in the hole until the piston is sort of stationary between just going down with the crank turned one way and just going down the other.
Paul Hunt 2010

This method of accurately finding TDC was posted on the midget/Sprite board by Deborah Evans...

Make up a 'stop' from a piece of bar stock machined so it will fit into a spark plug hole (ideally thread it so it screws into the hole) such that it will prevent the piston rising to TDC (alternatively get an old spark plug and weld an extension onto the electrode).

With the stop in place, s,lowly rotate the engine by hand until the piston encounters the stop (and the engine can no longer be rotated). MARK the pulley against your timing pointer.

Now rotate the engine the opposite way until the piston can rise no further, Again MARK the pulley against your timing pointer.


TDC is the point equidistant between these two marks and can be marked as such.
Dave O'Neill 2

Bob, thank goodness you're here!

I have a similar problem(?) with a 70 GT. At 600-700 RPM idle, my timing by strobe light is about 20 degrees BTDC. It runs very sluggishly below this.

No pinging (using 98 RON unleaded) at all regardless of how I load the engine at this mad setting. I have done a TDC check using a piston stop and it is spot on. Car runs very well. Did a compression test a month or so ago and it was 160 158 157 160 (1,2,3,4).

I am not game to advance it any further. Really weird.

John
J Tait

This may be daft as I've no time to check the manuals...

The timing marks (and therefore on the pulley too) on the single row (18V) timing cover are in a different place to the duplex (pre 18V).

The former are at about 10 o'clock from memory, while the latter are at about 7 o'clock.

But the later cover has a better seal (lip seal rather than felt). So someone may have mixed and matched at some point during the rebuild or a previous rebuild...? I've an 18V engine with a duplex chain for example.

The difference between 7 and 10 o'clock positions is about 45 degrees, and given that the cam turns at half crank speed, that would give 22 degrees out - in the ballpark?

Someone smart will no doubt point out that the rotation is opposite and so I am totally wrong.. but hey, worth a guess?

N
Neil22

First I do have a Dizzy problem on my model 25, in that the vacumm advance is not working, but mechanical advance appears to work. Weeks or a month plus ago, upon discovery that dizzy did not have a functioning vacuum advance, I experimented with timing at 600-800 rpm.

Car runs amaziningly better at around 20 BTDC, up from where it was around 10 BTDC. I've locked it down to this for now. I'm also running 10% ethanol in MN.

I still haven't double checked the timing of 20 at idle to what it is at around 4000 rpm. There is in theory a maximum it should get up to, something around 35 degrees I think. Someone I'm sure will clarify this point. I keep forgetting the numbers.

Also, on my 72B with matching era engine, the timing mark or timing plate is mounted around 10 o'clock with 5 degree intervals marked out. I'm just guessing at the 20 degrees by eye balling it just off the timing mark plate.

I have a feeling that even after I get the dizzy fixed this August, that at idle with vacuum disconnected I may still find that 20 degrees is better than 10 degrees.
R.W Anderson

As the timing is measured on the crank pulley the divide by two for dizzie degrees doesn't apply, and the early and late TDC marks are 153 degrees apart, I can't see it running at all on the wrong one.

Depend on the distributor but vacuum can add anything from 6 to 4 degrees, so upping from 10 to 20 on a distributor with no vacuum advance making a noticeable improvement is distinctly possible. Whilst vacuum advance is primarily a cruising economy aid it does also make a noticeable difference to part-throttle acceleration.
Paul Hunt 2010

This thread was discussed between 03/07/2010 and 06/07/2010

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