13 replies to this topic

[popenator]

Mini 1976 1000 Saloon

Metro 1275 engine, Stage 1 Kit, electronic ignition, 65DM4 distributor.

Hello, can anyone tell me what the correct timing figures should be for this engine with the stage 1 kit? The Mini Haynes doesnt tell me as standard for this type of engine.

I have timed it by ear, that is to say I have set the idle at 2000rpm, turned the dizzy until the revs stop rising, then backed the dizzy off by about 250rpm.

I have returned the idle speed back down to 900 and my timing is at about 14 degrees with or without the vacuum advance attached. And around 20-22 i.e. off the scale at 2000rpm

Any help would be great.

Thanks

[icklemini]

with or without the vacuum advance attached

at idle, the timing should be more advanced when the vac pipe is attached... as the vacuum at idle pulls on the vac advance... if this isnt happening then i would imagine your vac advance isnt working...

my timing is at about 14 degrees

Could be a little too high... but try it... hear for any 'pinking' from the engine when driving it under load...
Another way is to time it at about 32degs @3500 rpm (with vac disconnected) - will need a decent timing light to do that...

hth,
Dave

[popenator]

Thanks for the response

What could be done to confirm that the vac advance isnt working?

As for timing it at 32 degrees at 3500rpm. firstly, where did you get that figure from?

thanks

[Ethel]

Vac advance shouldn't kick in at idle, open the throttle a bit and you should notice a difference - or just suck on the pipe

Do you know what 1275 Metro engine it is? That nice Mr Guessworks has an engine number looker upper on his website, I can look in my Metro Haynes if you know.

[lrostoke]

If you search the net theres a fair few articles out there about tuning the A series, 34 - 35 degrees of total advance is apparently the optimum for the A series for power.
Its a starting point then you run it under load and check for pinking if this occurs then back off a couple of degrees at a time till the pinking stops. Which is pretty much were 32 degrees comes in allowing for modern unleaded fuels .

[lrostoke]

Vac advance shouldn't kick in at idle, open the throttle a bit and you should notice a difference - or just suck on the pipe

I'm not so sure thats true, We had this in a recent topic, I checked two carbs in my garage at the time and the vacuum port is on the manifold side of the buttlerfly valve, it isn't blocked when the valve is fully shut. Now to me with that valve shut is when you will be getting a fair bit of vacuum created.



Also why would you even bother disconnecting the vacuum pipe when setting timing if it had no effect.
Most timing figures specify a certain degree @ 800 - 1500 rpm depending on dizzy. Now tickover is around 900 - 1000

Also as a test with my timing set to 6 degrees @ 1000 rpm if I connect the vac this changes to 18 - 20 degrees.

[icklemini]

what lrostoke has said is correct...

[Ethel]

Well I can't argue if you've found a difference at idle. I'm quoting what's been explained to me and the theory makes sense. That an engine with no load will idle better with less advance.

There is always some air going past the throttle disc or the engine would stall, that'll be mostly at the top and bottom edges so the fact the hole isn't physically upstream of the disc doesn't mean it isn't upstream of the effects of manifold vacuum. The critical thing will be when it does get exposed to reduced pressure. Removing the vac pipe will ensure there's no advance, not all engines can be assumed to be set up correctly.

I note there's a hole in your throttle disc is that in place of a poppet valve? It could be that allows the throttle to be more shut at idle and is creating more depression over the vac take off hole.

[popenator]

Blimey chaps, this is a bit of a contentious issue!

When I remove the vacuum there is no notable change in the reading off the timing markers. Certainly nothing anywhere near going from 6 - 18

Are you pulling the end of the vaccum pipe off the dizzy or off the carb? Are you blocking any of the resulting orrifices i.e. the hole on the carb, or the dizzy, or the pipe itself.

The engine number is going to be tricky because some kind soul has rivetted a 99H Plate onto my block, if theres another way of getting the number, i'd be pleased to learn it.

Are we in agreement that sucking the vac pipe will prove whether vac advance is working? If so, what should the other end of the pipe be connected to. (I assume carb)

Thanks

[lrostoke]

If you want to prove the vac advance is working, disconnect from the carb end and suck on the pipe, If the engine is running and you are using a strobe you should see a difference.

the vac advance on the dizzys do operate at different vacuums, some need more than others before they kick in.

[popenator]

Thanks for this. I disconnected the vac tube from the carb and gave it a suck, and the timing mark changed so that proves its working.

I reconnected the end to the carb and disconnected the dizzy end and put my tongue on it and can feel myself being sucked into the carb, so vacuums working that end.

Should be all okay I guess.

Does anybody have any clues regarding discovering the true engine number?

Thanks

[Nightrain]

Seems to be some very mixed up people posting in this thread, cases of acquired knowledge but little understanding. Ignition Timing is all about Time really surprise surprise, it's to do with the time it takes for all the mixture to burn in the combustion chamber. Obviously it takes time for all the mixture to burn ie. right to the edges of the chamber if the spark happens at the wrong time it won't use all of the mixture effectively. Say it happened at TDC by the time all the mixture had been burn't the piston would already be on it's way down the bore. So therefore it is not been compressed fully so it won't be burn at it's optimum efficiently. As the rev's rise there is less time for the mixture to burn at the optimum compression because the piston spends less time at TDC. There are several ways that the combustion process can be speeded up so it happens at the right point. The one were concerned with is the Advance, this is were we start the combustion process before the piston is actually at TDC and then by the time it's at TDC the whole charge within the combustion chamber is alight and doing it's job. So the timing the engine needs at 1000rpms will be different to the timing the engine needs at 3000rpms this is where centrifugal timing comes in, this is controlled by the bob weights and springs. As the revs rise the centrifugal forces created by the spinning of the dizzy pushes the bob weights out, which in turn turns the base plate and alters the timming to give more advance. The springs control how fast the advance curve is altered by the bob weights.
The above works all well and good when you at full throttle and flat out, but and there always a but At small throttle openings the volumetric efficiency of the engine is a lot less, meaning the combustion chamber is not been filled as effectively. Because there's less mixture in the chamber it isn't compressed as much, giving a lower compression ratio, which in turn means the mixture burns a lot slower. This actually means we need to advance the ignition even more at part/light throttle openings because it takes longer to set alight all the charge and this is were a vacuum advance unit comes in. The vacuum on the inlet side created by the virtually closed throttle is used to swing the timing to give more advance. As we accelerate we open the throttle more decreasing the vacuum thus doing away with any advance created by it.

[lrostoke]

makes sense.

Something I'm curious about, if the max mechanical advance for peak performance on an A series is somewhere around 34 degrees @ 4000 rpm give or take.

If the vacuum is connected and you are say cruising at say 3500 - 4000 rpm, would the vac not over advance it.
I know if I watch my vacuum gauge it can settle to around 10 - 15" of vacuum when cruising.

I know some people who have had rolling road sessions have come away with the vac advance disconnected and just wondered if this was why ??

[Nightrain]

Something I'm curious about, if the max mechanical advance for peak performance on an A series is somewhere around 34 degrees @ 4000 rpm give or take.

If the vacuum is connected and you are say cruising at say 3500 - 4000 rpm, would the vac not over advance it.
I know if I watch my vacuum gauge it can settle to around 10 - 15" of vacuum when cruising.

I know some people who have had rolling road sessions have come away with the vac advance disconnected and just wondered if this was why ??

At 3500-4000 rpms the bob weights in the dizzy have taken over control of the engines advance and are usually against the stop pin. Meaning no further advance can take place.

Rolling road operators, usually disconnect the vacuum, because it's either leaking causing all sorts of other problems or there not concerned about economy. But mainly because it's easier !