Guru inputs much appreciated - my engine is a splash Ruby fitted with a newish ali head. After losing power, I checked the head on a flat plate (seemed fine but I ground it a little to be sure), changed the head gasket with extreme care and some gunk (previously just used a smear of grease). The valves looked fine. At first the engine wouldn't even run. In total bafflement, I changed all plugs for new ones. Now it runs but power is still way way down. Compressions are (1- 4)9 bar , 8 bar, 7 bar and 8 bar which surely indicates a problem. Shorting plugs to earth suggests that only 1 and 4 are doing any work. Any ideas?

John

John,

Did you loose pwer when you put the ally head on or has someting happend since putting the ally head on which could have caused your problem?

It is possible the head is still not sealing properly to the block. There is a simple test you can do.

Turn the engine over ( by hand so that one of the cylinders is at the bottom of the compression stroke.
Take out the adjacent plug ( or plugs ) then place a tube into the adjacent cylinder and put the otehr end to your ear. This is a listening tube. Get somone to turn the engine so that the adjacent cylinder ( the one with the sparkplug still in ) comes up on the compression stroke. If the gasket is leaking you will hear the compressed gas coming into the adjacent cylinder through the listening tube.

It sound complicated but in pratice it is very easy. Ideal tube is the clear nylon tube used for 'modern' car windscreen washers.

If the gasket is sound you sould not hear and hissing gasses. You should repeat this test for all cylinders.

You may need to take out all the head studs and check that the surface of the block is flat. You will be surprised how much metal can warp with age !

I use 'Copperslip' grease when bolting down an alloy head. You may find 'engineers blue' useful as well when checking mateing surfaces.

good luck

Going along with Malcolms suggestions, you could in theory check the inlet/exhaust valves the same sort of way, by listening at the exhaust and carb, although you cant always hear it. You can modify a spark plug with an airline fitting welded or brazed on and pressurise the cylinder to get better results, which is how posh garages do it when they do leakdown tests (also bubbles up through the rad).
I presume the ally head is good?? Apparently a lot of them are inferior to the late high comp head. If you have used copaslip you could easily just bolt the old head back on for a test.
Not wishing to teach you to suck eggs , but I presume all else is OK i.e ignition timing carb tappet settings etc etc?

Regards

Steve

John

Just re-read your post...

It only ran when you changed the plugs?
Are the plug threads and seats OK in the new ally head? Some may have inserts for the smaller plugs?

Steve

I fear I may be stating the obvious, but you say only one and four are doing anything. Are two and three connected the wrong way around?

I have been reluctant to make a contribution up till now, but, having converted the compression readings to PSI (131,116,102,116 PSI pots 1-4 respectively)so that I could understand what was going on I would have to say that while No. 3 is somewhat down on the others, it is not so dramatically so as to suggest a significant leak which would incapacitate the efforts of two cylinders in the way you have described.

A lot of people would like to be recording compression pressures above 100PSI in their A7's

If you have a "new manufacture" aluminium head of doubtful lineage, it might even be that the casting is such that the differences in compression are due to the differences in the casting of the actual sizes of the compression chambers! (Some of them seem to have earnt poor reputations)

I think Ian's advice to look elsewhere might be prudent. As well as checking that the leads are properly arranged to go from the distributor cap to the plugs in the correct order, have you checked that the leads are OK? They can go open circuit.

I am still unsure as to how your problerms arose.

Has the engine been previously running OK with this particular head installed?

Did the problem only arise following a change of plugs?

Did it just occur suddenly during use or, is there some other event which has lead up to the "breakdown?"

Best regards

Barry R.

Barry, Did you get the coil pics OK?

Hi John - totally irrelevant to your problem, I notice you say your engine is a splash Ruby. Are you using it in the Ulster I shipped to you, if so, how's the other engine? Cheers, Bill

First of all - many thanks for all the advice, much appreciated. This is a great forum.

I should have given the full story. The engine was fine with the ali head until I cooked it in the middle of nowhere following a hole in the rad. Following the rad repair, the car periodically blew hot water back from the rad cap, mostly into the face of my long suffering missus. My attempts to fix what seemed to be a blown gasket/warped head were what I described.

Reading the responses, it does seem most likely that I'm into an ignition problem - as Barry points out, the pressures, though different, are all good. I couldn't get to the car over the weekend but will do so next week. In the meantime, thanks again everyone.

Bill - great to hear from you, I'll reply by separate mail.

John

John,
Cooking an aluminum head is bad,bad news, that was the basic reason the hillman imp suffered 'bad press'. A brilliant engine, excellent power to weight ratio but an absolute pig once the engine had been overheated. All down to warped blocks and heads.

You have plenty of suggestions to work on, it would be nice to hear what the problem turns out to be.