Timing mildly tuned 948

[RogerRust]

I have finally got Sophie not to pink - 2 degrees after top dead centre static. TDC with the strobe and vacuum disconected. The weights are 13 degree ones so that gives me a total advance of about 19 degrees.

the problem is with it that retarded it doesn't want go really well.

Any ideas the mixture looks oK on the plugs and if I lift the piston it picks up then drops back.

[Peetee]

Have you got a 295 head? if so the recommendation after skimming a min of 60 thou is to get the equivalent packing under the rocker mounts to bring them back to height. It could be your are not getting ideal valve timing and lift or they aren't closing properly.

[RogerRust]

no its 12G940 with pocketed and relived valves. 1098 cam and HS4 carb with piper cross filter.

[bmcecosse]

2 degrees after TDC is hopeless! It will never run well like that. Are you sure it's pinking ? Try another dizzy - maybe that one is stuck - and/or the vacuum advance is not working. Your 13 degree weights will give 26 (engine) degrees advance - less the 2 deg after TDC = 24 degrees - definitely not enough. You may need to use best grade fuel - but the CR shouldn't be too bad on a 948 engine - just about 9.5/1 I would say. Don't worry about valve gear packing - the effect is miniscule - only worry about it for racing engines! I wonder if the cam timed in ok ? With that spec your engine should go very well indeed - maybe try to get a compression check - would be useful information.

[RogerRust]

I don't know how good my gauge is but it reads 150psi on each cylinder.

[bmcecosse]

Well - that's certainly not excessive (not wonderful either - but ok) and all 4 are the same, which is good. So it's not pinking because of stupidly high CR ! Must be dizzy problem - or are you trying to run on paraffin ?

[RogerRust]

proper leaded 4 star I'm afraid!
I have checked the distributor - no problem.
My only thought is I think my gauge under reads.
If it was the CR would retarding it stop the preignition

[Packedup]

I had a Triumph engine that always sounded like it was pinking unless I backed the timing off to the point where the car didn't have the power to roll over small pebbles.

Although I know the engine had been messed around with in the past, I have a feeling the pinking sound was really the start of big end failure. Backing the timing off meant the pistons weren't being walloped on each firing as much, so quietened things a little.

Most likely not your problem, and hopefully not of course!

[RogerRust]

just been rebored and reground with new shells and pistons etc.
I'm running out of ideas.

Howabout 4 star and octane booster?

[Packedup]

You mean like toulene? http://www.elektro.com/~audi/audi/toluene.html

Seriously, if you're sure you've got a pinking issue, it might be worth gradually adding more until the problem goes away. And if it doesn't start to worry, a lot!

Maybe avgas is worth a try, if it's still effectively 5 star leaded and you've a friendly airfield?

[Alec]

Hello Roger,

a couple of points,

I disagree with Peetee, shimming the rocker pedestal is wrong, it upsets the valve train geometry. It would only work if you skimmed the top of the head not the cylinder face of the head. The only problem would be if there had been so much skimmed off that you run out of rocjker adjustment and then need shorter pushrods.

Certainly it will never go well with the timing at 2 degrees ATDC. When you experienced the pinking, did it run on when you switched off?
I think I would be inclined to carefully check the operation of the mechanical advance. Check strobe timing at various revs, if you have a manual, you can get the advance\rpm details. Also run with the vacuum advance disconnected, it is basically an economy device and modifying the engine will mean that it is probably not correct any longer.

(Apparently it was the vaccum advance that destroyed so many MGA twin cam pistons by advancing the timing at high revs when it shouldn't have!)

Alec

[bmcecosse]

I doubt your gauge is under reading - but try it on another car. This head hasn't been skimmed (unless skimmed before you got it ?) - it's the valves that have been recessed (by 40 thou ??) and de-pocketed. So - it started life at 21.4cc and then the valve seating etc has probably added about 2 cc (maybe 3) making 23.4 cc which is v close to the original 24.5cc. So the CR will be only v slightly elevated - unless flat top pistons have been fitted. Are you sure the sound you hear is 'pinking' ? Not (as suggested) a mechanical noise ? I really don't believe it can be pinking unless the mechanical advance mech springs have beome detached or are v weak and immediately throwing the dizzy into full advance as soon as the engine starts turning - hence I suggest trying another dizzy asap. the vacuum unit only works when there is vacuum in the manifold - so only when the engine is making some revs and the throttle is closed - the idea being to advance the ignition under these low stress conditions and so have greater economy. The 'old wives' (sorry Alec - not you) stories of excessive advance from vac units are just that - and not true - they can't be - when the engine is working hard there is no(v little) vacuum and the vacuum unit rests easy. Just a point - what head gasket did you use (1275 I hope) and have you re-aligned the rockers on the shaft to bear directly on each valve ? Just trying to eliminate odd problems here. I really do think it must be mechanical noise - or faulty dizzy.

[RogerRust]

I don't think it can be mechanical noise because it has gone away now I have retatarded the timing.

I took the distributor off yesteday to check the weights both springs are on, but are probably not as tight as new. I have a spare but it has 15 degree weights so I don't want to fit that one! Today I will swap the springs and see if that helps.

I think the head was skimmed before I got it. Yes I did use the right 1275 gasket and realligned the rockers.

When I test the timing with a bulb across the points it gives 2 BFTC.

With the vacuum disconected at low tickover it reads about TDC

Incease engine revs and it goes up to about 15 BTDC

Turn the revs back down to tick over and connect the vacuum and it leaps up to about 10 BFTDC

Now increse revs and I get back to about 22 BTDC.

OK I'll go and recheck the bob weight springs and report back.

Roger

[Alec]

Hello Roger,

just be aware that ther should be two springs, a strong one nad a light one, the strong one will be loose as well normally, i.e. no tension.

BMCE, apparently it was true in the MGA's case due to some strange effect of the inlet manifold. I was not suggesting that it is likely or the problem in this case.

Alec

[RogerRust]

OK its a bit beeter now

4 degrees BFDC static 8 BFDC with no vac at tickover
about 24 with vac and revs

The sprigs needed an adjustment (the strong one never came into play even at full advance.

AND the cam was marked 13 degrees BUT had been filed to give more advance. I have put a 13 degree in off another dist and now it is behaving its self better.

When its run in I will book some time on a rolling road and get it set up correctly.

[bmcecosse]

I would use the 15 degree dizzy - remember thes are camshaft degrees and therefore X 2 for engine degrees. The vac only works at idle - and in the cruise - and ideally most good A series engines like 36/38 degrees total advance at high revs - so with 30 from the dizzy and 6/8 static it should be just right! I think your first dizzy was sticking - maybe part way advanced and then not advancing all it should when running. Use the best quality fuel (it gives better mpg anyway so cost balances out) and run the timing so it 'just' pinks v slightly when you put your foot down - but don't run it pinking all the time !

[Alec]

Hello Roger,

"The sprigs needed an adjustment (the strong one never came into play even at full advance."

What that means is that your distributor would go up to full advance at too low an engine speed. as the strong spring which controls the second stage (slower rate) advance isn't functioning.

Sounds as if you are getting somewhere. Just shows how such an apparently simple device is not as simple as it seems.

Alec

[bmcecosse]

In fact there are great power improvements to had from the A series with careful control of the ignition timing. lots of the Mini lads are using'Mega Jolt' fully mapped ignition systems. there is also a '123' dizzy which comes with a variety of advance curves which can be selected with a little switch on the side. Not cheap - but a 'once only' purchase!

[youngun]

does the use of electronic ignition improve engine performance in any way, as ive seen various kits advertised.
I will be looking to have a very similar set up on my 948 engine, i.e. 12g940 head, 1098 cam, single HS4 or HIF38.Would the addition of electronic ignition help?

[bmcecosse]

No - just stick with points - can be fixed easily when it goes wrong.