Pinking problems with new cylinder head

Post by **johncannon** » Tue Sep 26, 2006 8:13 pm

I have recently fitted a 12G295 cylinder head (modified with lead-free exhaust valves), but cannot get the car to run properly without pinking. I had 60 thou skimmed off the head as recommended, but no matter what I do I cannot get it to run effectively.

I have changed Carb Needles, Distributor etc but to no avail. Am I missing something?

When I retard the ignition it does not run smoothly. On accelerating, I see a power dip for 1 second before the power kicks in.

Any advice appreciated

Post by **ColinP** » Wed Sep 27, 2006 10:43 am

By skimming the head you have raised the compression ratio. This will require higher octane fuel to prevent the pre-ignition of the fuel/air mix - pinking.

As fara as I'm aware, it's only the fuel octane rating that will cure this (if the timing's set correctly) - you might need to use one of the higher octane brands, or use an octane booster (additive)

Cheers,

Colin

Post by **Willie** » Wed Sep 27, 2006 4:04 pm

If octane booster is not practical then you MUST gradually retard the ignition
timing until the pinking stops, otherwise you will damage the engine.

Post by **Peetee** » Wed Sep 27, 2006 4:38 pm

By skimming the head you have raised the compression ratio

It is generally felt that a 60thou skim of a 12G295 head is necessary to restore standard compression when fitted to a Minor engine.

but cannot get the car to run properly without pinking

perhaps the head had already been skimmed?

Post by **kotemaori** » Fri Sep 29, 2006 5:51 am

measure the combustion chamber, sounds like the heads been skimmed before, or you may have hi comp pistons fitted

Post by **Dominic** » Fri Sep 29, 2006 7:17 am

I always use the higher octane fuel as a matter of course. I get an appreciable improvement in MPG that far outweighs the extra cost of the fuel. I also get a much smoother tickover, and a noticeable "liveliness" of the acceleration. This is a standard 1098 with unleaded head off the shelf from MGM spares. (No idea how much, if any, it has been skimmed)

Post by **kotemaori** » Fri Sep 29, 2006 8:38 am

I can't say I've ever encountered "lead free exhaust valves", usually hardened valve seats are fitted, Stellite being the type I'm familiar with

Post by **chrisd87** » Fri Sep 29, 2006 10:39 am

You could try fitting two head gaskets to lower the compression a bit.

Post by **bmcecosse** » Fri Sep 29, 2006 8:57 pm

NO - don't do 2 gaskets !! You must stop the pinking - so first move is indeed the best grade of petrol. Your 60 thou skim will only have brought the CR back to normal for a 1098 engine. You must check the vacuum advance and the mechanical advance in the dizzy - both these systems MUST be working correctly for good performance. I suspect the vacuum advance is leaking - this forces too much static advance to be used for good idling and then leads to pinking. Normally when starting the engine the static advance would be only a few degrees - as soon as it fires up a good vacuum is drawn in the manifold and the vacuum advance pulls the timing up by up to 10 degrees (varies with spec of vacuum unit) - and good idling is obtained. This also makes for a more 'lively' engine - when the throttle is opened the vac advance lingers on for a second or so and the engine picks up nicely - the centrifugal advance then takes over as engine speed increases giving up to 25/30 degrees running advance - depending on the dizzy spec. If no vacuum advance exists because the diaphragm is burst - then the owner dials in up to 15 degrees 'static' advance to get good idling and pick up - but as engine speed rises the mechanical advance still operates adding it's 20/25 degrees giving far too much running advance - and pinking!! So - suck on the little pipe to the dizzy - you should be able to pull a vacuum and the plate on the dizzy should move if all is well. No vacuum (and therefore no movement) indicates burst diaphragm - they only last 10/15 years max. Sadly they are not cheap !!

Post by **chrisd87** » Sun Oct 01, 2006 5:54 pm

I replaced my vac advance a few months ago, it was only £15.

If you find that the head had already been skimmed then as a last resort you might be able to fit low comp pistons to bring the CR back down to a reasonable level.

Post by **bmcecosse** » Sun Oct 01, 2006 8:33 pm

If the head has indeed been 'twice skimmed' (unlikely because it would have broken into the oil way!!) - then you have scope to de-shroud the valves with a grindstone in leccy drill - and at the same time drop the CR. But really - I sure it's a timing problem, not excessive CR.
Last new vac unit I got was £2 at Biggar classic show - but they seem to go up to £25 or so on ebay, so your £15 was ok to!!

Post by **johncannon** » Tue Oct 03, 2006 6:54 pm

I do not believe the head had been pre-skimmed -I measured the thickness of the head against manufacturer dimensions.

I have tried changing the distributor –the one on it defiantly had a blown diaphragm, but the new one was ok. I could not get it to move though with suction? Should it be very easy to achieve?

Post by **bmcecosse** » Tue Oct 03, 2006 8:41 pm

Yes - even my feeble lungs can draw a vacuum that obviously moves the plate. Maybe it's stuck - but the diaphragm is still ok. Or other possibility is that the connecting link has come away from the baseplate - it's only hooked over a pin - should be obvious with careful inspection on the bench.

Post by **IslipMinor** » Tue Oct 03, 2006 11:22 pm

I would disconnect the vacuum pipe and set the timing to the correct static setting. Try it to see if there is any pinking. If there is then retard the timing by 5° and try again, if OK, then advance up gradually until it pinks, and then retard until it doesn't! If a 5° retard does not solve it, I would do a back-to-basics check on the ignition settings.

Next connect a good timing light and check that the mechanical advance is working correctly - total should be about 30°. Maximum is 32°.

If this is still OK, then re-connect the vacuum tube and test it. The vacuum has a small effect on idle, most effect on mid-speed range and light throttle and none at all at full throttle.

If, when you open the throttle quickly, you get a short amount of pinking, but it goes away 'by itself' as you continue to accelerate, this is called 'burst detonation' and you should check the vacuum unit. So long as it is only for a very short time, can be ignored, but more than 1or 2 seconds means a new vacuum unit.

Post by **bmcecosse** » Wed Oct 04, 2006 7:16 am

But you DO need the vacuum advance to 'brighten' the engine. So make sure it's working with a physical check. And yes - I get just a slight tinkle as I put my foot down - this is the vacuum unit still holding the advance for a very short time - without this the engine will seem 'dull' and slightly slow to respond. But at least you did find the diaphragm was burst in the original dizzy - so provided this one can be made to move it will be bound to help matters!

Post by **johncannon** » Thu Jun 14, 2007 10:30 pm

I though i had solve this problem with putting 98 unleaded in, but having driven it around france for the last couple of weeks it has returned. It pinks when under load, and partiuculalr badly on hills. I keep takin my foot off to stop it but it is very undrivable like this

Seeing as all the issues started when i exchanged from a standard head to the 12g295, i am assuming i may have increased the compression ratio? Is is there anyway i can easily lower it? Are thicker head gaskits available?

Alternitivly has anyone tried an octane booster?

I am wondering if my distributer is at fault. The vaccuum advance works, but i am not convinced the advane and retard mechanism will function correctly after 30 years + sevice?

Has anyone got any thoughts?

Post by **RogerRust** » Fri Jun 15, 2007 5:28 am

I had a similar problem when I changed to a skimmed 12G940 head

please see;

http://www.morrisminoroc.co.uk/index.ph ... ic&t=15404

Post by **bmcecosse** » Fri Jun 15, 2007 8:06 am

You MUST check out the dizzy - that's where the problem lies. When you suck the vacuum you must see the parts move - if they don't move then either prob the mechanism to unstick it - or get a new vacuum unit. To test mechanical advance - twist the rotor arm and it should feel springy and return. When the head was skimmed - were all the sharp edges removed from around the combustion chamber ? And can you remember what thickness the head was /is before/after the skimming ? I would have started at 2.750 " thickness. You should also check what plugs you are using - and consider changing to a harder plug - say Champion N6Y (or whatever the modern equivalent is). And of course - only use the very best grade of fuel. Perhaps the mixture also needs attention - what colour are the plugs when you take them out after a good long run ? Should be biscuit/beige colour - if white then far too weak.

Post by **IslipMinor** » Fri Jun 15, 2007 8:59 am

Check the maximum advance you are getting. Disconnect the vacuum and check the mechanical advance with a strobe at 3,500/4,000 rpm - it must not be more than 33/34°. Usually 32° is the best.

If the mechanical advance is correct, leave the vacuum disconnected and test on a hill to see if it still pinks - it should not! I run 10.5:1 CR on Shell Optimax and have no pinking at all.

What plugs and carburettor are you using and is the mixture set correctly? You will need a richer needle in the carb with a 12G295 head.