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MG Midget and Sprite Technical - Why best with no vacuum advance?

This is a bit of a long running saga (apologies for mentioning it again). I started an earlier thread wondering whether I was getting pre-ignition or just some sort of mechanical noise (gearbox, little end bearing etc). To follow up, it was definitely pre ignition. I found that retarding the distributor significantly helped get rid of the pre-ignition rattle that occur at 3-3.5K revs. Or so I thought. In fact what cured it was accidently leaving off the vacuum advance connection. Now I find I can advance my ignition significantly without the problem reoccuring. What I don't understand is why. Surely when accelerating hard (that's when the problem occurs) there is NO vacuum advance so what difference would removing it make? The vacuum take off is from the nipple on the Hif44 just behind the throttle butterfly so should be 'normal'. I ran around 100 miles without vacuum advance but used around 5 gallons of fuel (as expected fuel consumption was high in this condition) so I'm not keen to continue but do not see an alternative. Any explanation of what is going on.
Chris H (1970 Midget 1275)

On face value I would agree with your synopsis but guess that the following was also contributing:

was it running lean as a result of the Vac tube being off?..and that caused the pinking...retarding the timing masked the pinking but significantly reduced the part throttle torque (retarding base setting and no Vac Advance) meaning that you drove the car with more throttle hence more fuel was consumed?

I would suggest a plug clean or change and a tune and carb balance from base settings, if it was pinking to the extent that it sounds like then the plugs may be iffy. (I've seen this on other tuned engines though not A-series)

HTH

Rob
robnrrugby

Running with the vac pipe disconnected on a ported vac carb like HIF44 would make it run lean, since air is being introduced into the inlet on the engine side of the jet. Normally you'd do more MPG and increase tendency to pink as Rob says.

Are you using a high CR head? When you go above 9.5:1 you can enter the realm of needing a lower rate of advance for the 2-3.5K rev range (esp if you have a near standard duration cam say 260 degrees or less). Aldon do a 100AYV dissy with slight stronger springs for this. (I used one on my tuned 1275 to good effect.)

The ported carb has slightly different vacuum charas compared with the take-off on the manifold. With manifold vac, the vac behaves as you suggest; the harder the engine is working, the less vac. Quite simple.

But the ported vac take-off is just the air-side of the carb butterfly at idle. Hence no vac at idle. Then, as the throttle is increasingly opened, the butterfly in effect sweeps past the 'vac' take off and exposes it to more of the vacuum in the inlet (which of course drops as throttle opened, as you know). Now another effect comes into play: as the air flowing past the 'vac' take off speeds up, the Bernouilli effect says that this represents a vacuum too. So even on full throttle with little manifold vac, there is a residual vac from this effect.

Later 1275s had ported carbs, so you have to believe the dissy mech advance and vac units allowed for this.

I'd gradually retard the dissy in measured steps (with vac connected - you don't want to eschew this economy device for road use) until all traces of detonation (bad!) and pinking disappear. Then find out what your static timing figure is.

If it's around the normal(ish) 5 to 7 degrees BTDC, then that's fine. If you find it at TDC or worse, retarded, then you've got some thinking to do, based on:

- check mixture; too lean might promote pinking but would not normally cause detonation
- too high CR means you need dissy remap
- use higher octane fuel...

Tell us how you get on.

Sorry for long post.

A
Anthony Cutler

I should have said in my post that the vacuum pipe was blocked off because I originally disconnected it to set the timing (and just forgot to reconnect). Therefore it remained blocked and there would be no weakening of the mixture.

I have an MG Metro head and I am wondering if it has been skimmed too much in the past (thus raising the CR). As you will see from my earlier post I run very cold plugs (BPR8ES), only use super unleaded plus octane booster. I have a 123 distributor set on '4' which gives 30 degrees max advance but only a shallow increase in advance (just 16 degrees at 2000 rpm). I tried the 123 because I had the same problem with an Aldon AYV dizzy and a custom made jobby before that. I did find that disconnecting the vacuum to the Aldon had the same beneficial effect.

Could there be any connection between this problem and fitting 1.5 high-lift rockers. I do not recollect the problem until after I fitted these (but my memory may be playing tricks)?

Chris
Chris H (1970 Midget 1275)

The 123 does provide some extra ideas:

- is there a setting for reduced mid range advance (less than 16 degrees?)?

- or retard the stactic timing to zero, and use reduced mid range whilst allowing (say) 38 degrees max at top end.

This would give less timing at bottom/mid, but allow the engine to do its stuff over 5K. You get the idea.

If no detontation, increase the static by 2 degrees at a time and check carefully; remember to reduce the max setting by same number when you're done.

A
Anthony Cutler

Thanks Anthony - there are some interesting ideas here. I think I have selected the most conservative advance curve but I'll try your suggestion relating to greater max advance but retarding the static timing. Incidently, how can I check the static timing with electronic ignition (I don't have any points to probe so I always do it dynamically now). Is it low tension lead to earth and turn the dizzy till the voltmeter kicks.
Chris H (1970 Midget 1275)

let me just notice that 30 degrees max advance is a bit low i always noticed the best results at 32 degrees

my 1380 with a 12 to 1 compresion ratio runs happly with ron 98 at max 32 advance without vac (non on the dissy)
so with the compresion i think you should be ok
but what is you bore size and what pistons do you run that helps to give a guestimate cr

now one question from me if you want to run with vac advance but run a weber (so non on the carb)
where do you get tha vacuem from can it be taken from just one manifold pipe (there is a tapped hole in just 1 of the 2)
Onno Könemann

Chris I do not like the "ported vacuum" at all as it does act the same as described by Ant.

I always use a vacuum pipe connected to the inlet manifold and then I know that the vacuum only acts as a fuel saving device.

As you will now be aware that 32 degrees full in is the figure you need for ignition timing, hopefully the curve you should use ought to not give too much advance at midrange when you time he engine to this figure.

Unlike Ant I have never had pre ignition problems with compression ratios of up to 11.5 to 1 even with 95 octane fuel but I have always used a vac advance directly onto the manifold.
Robert (Bob) Midget Turbo

Hi Bob

I agree that manifold vac is a better gauge of how hard the engine is working. I'm trying to think of the advantages of ported vac, since it means

- low advance on idle and low throttle, where it would appear to hurt economy

- rising advance as the throttle progressively opened (OK, provided advance doesn't lead to pinking)

- low vac on trailing throttle (hurts economy)

- some vac on WOT, which may lead to over-advance

Curious; must say I never had a problem with it. But there must have been a reason for change, e.g. since the vac is never max manifold vac, it might have allowed for finer control of advance under certain conditions.

Chris

You can use bulb or voltmeter and wait for volt change as you suggest.

A
Anthony Cutler

Hey chris,

Peter Burgess gave me a heads up on high lift rockers (1.5) that I have not read about or heard of but makes alot of scence

He advised that with 1.5 rockers and the SW5 you need to advance the cam an additional 5 degrees beyound the 106-108...you said your advance issue started when you installed the 1.5 rockers....

The other thought you might look into is the aldon dissy if it has a combo mechanical/vacume advance.... I learned last week that my duel point mallory was a combo dissy, which is rather rare, the vac advance is good from idle to around 1500 rpms and then the mecanical kicks in at 1350 rpm and takes it the rest of the way to the top of the rpm range...I cant remember the sugnifigants of this system, but worth looking into, other then gives good low end power while making more fuel efficiant, it would certianly answer why you getting advance with the vac hose capped ad disconnected.

Prop

Prop

Thanks for all the comments and suggestions - food for thought. I have had a great idea (famous last words!). In my box of sundries I have found a spacer for a HIF44 with a vacuum connection. I will fit that close to the manifold (at least well behind the carb) to give a vacuum takeoff from well behind the carb. I'm slightly concerned the hole into the 'manifold' is less than the diameter of a pin (its tiny). Is that OK or should I open it out?
Chris
Chris H (1970 Midget 1275)

Chris

The extra spacer is good, since it will give more room for the butterfly / airflow in the manifold plenum; used 2 spacers on my HS6 installation.

It doesn't matter if the hole is small, since they is very little instantaneous airflow (the volume of the bellows) and no net airflow.

Note this config is not eactly the same as manifold vac, since there is airflow past the 'outlet'.

Tell us how you get on.

A
Anthony Cutler

how thick is the spacer...enjoy the side benifit of extra torque.

Prop...ya gots to love a spacer for cheap power...prop
Prop

Anthony - you are a genius!!! Following one of your suggestions, I set my 123 dizzy to a 36 degree advance curve and then retarded everything by 4 degrees (giving max advance of 32 degrees albeit at slightly higher revs - approx 5k rpm). This seems to have reduced the advance in mid-range but given additional advance at the top end. A short test run (usually sufficient to tell if the problem still exits) suggests no more preignition and the engine runs sweetly. Only time - and some miles - will tell if this is the best solution. I'm going to fit the spacer with the vacuum bleed but not at present. I'll see how things shake out. Many thanks for the suggestions. Going for a more advanced curve was just the opposite of what I had thought to do. Isn't this BB great!
Chris
Chris H (1970 Midget 1275)

if i am correct you 123 should have an 33 or 32 degree setting.
have you tryed those?
in escence i am asking why the elaborate 36-4 method?

but enjoy the midget!
Onno Könemann

Onno. The reason for retarding the ignition is that I get preignition at the 32 degree advance curves. By selecting the 36 degree curves and retarding by 4 degrees means it is 4 degrees retarded across the entire rev range. At mid range the ignition is less advanced that it would be with the standard setting (in this case 14 degrees at 2000 rpm instead of 18 degrees when it was knocking). Chris
Chris H (1970 Midget 1275)

It sounds like there is misfire or crossfire when the vacuum advances the timing. The rotor is passing the distributor spark contact causing the spark to jump to the next contact. If you are running electronic switching in place of points then adjust the slotted wheel so the spark jumps from the rotor to the correct distributor contact.

I have an old distributor cap that I cut almost in half so I can see where the rotor is in relation to the distributor contact.

Anyway... just another area you may want to check.

Bob
RJR Ritter

This thread was discussed between 24/06/2009 and 01/07/2009

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