High compression getting me down

[dgently]

The twincam in my +2 survived an overheating episode in early Summer when I drove without a fanbelt, but the hesitation and rough running between 1000 and 2000 has gotten worse.

My engine is the standard 105bhp. I replaced the points, my advance measures bang on 10 degrees. My idle is steady, I set the mixture when the engine was hot - it idles around 600 rpm when cold and 1000 when warm.

No leaks, oil is clear. Have been meaning to check for low compression since the reheating episode just to be sure. Finally got hold of a compression meter and was shocked to find 200 - 220 psi on all four cylinders. Not what I expected. After the last rebuild, it was 160 across the board.

Then started listening for pinking, and heard it yesterday going up a hill. Quite faint, not easy to hear above the normal cacophony of squeaks and rattles. Engine has always started well, got a bit more stubborn recently. It has never "run on" after switch off.

Here come the questions!

1. Is my high compression likely related to the overheating episode, or is my head just coked up?
2. I understand I should retard the timing a little to avoid pinking. Should I rotate the distributor clockwise or anticlockwise? What would be a sensible first try - from 10 to 8?

I'm worries that I could be looking at a rebuild I can ill afford, and would welcome guidance

Thanks in advance

[neilsjuke]

If your timing is too advanced this will cause pinking and overheating are the the bog weights working you will need a strobe to check them.
Neil

[types26/36]

Finally got hold of a compression meter and was shocked to find 200 - 220 psi on all four cylinders. Not what I expected. After the last rebuild, it was 160 across the board.

1. Is my high compression likely related to the overheating episode, or is my head just coked up?
2. I understand I should retard the timing a little to avoid pinking. Should I rotate the distributor clockwise or anticlockwise? What would be a sensible first try - from 10 to 8?

Maybe I'm missing something but I fail to see how overheating would increase the compression if anything it would decrease it and I cant see how a little carbon could increase the compression by 60 psi
Think I would veryify the compression with a different compression tester,
As regards the pinking.....try fuel from a different source and/or company
and make sure your servo (if fitted) is not allowing brake fluid to be drawn into the engine.
Retard is always the direction the cam turns which is anti clockwise in the case of a T.C.

[types26/36]

the bog weights

The" BOG" weights I know he's in Ireland but really

[ill_will]

From reading comments (by people who know a lot more than me!) on various car forums, it sounds like the absolute pressure values indicated by a lot of compression testers aren't to be trusted.

Have you tried another gauge? Or have you tried the gauge on another car, to see if the readings look plausible? If you live anywhere near me (cambridge) you're welcome to use mine. 20 psi range sounds like quite a lot, though. Were readings from the same cylinder consistent?

Significant variations in compression could feasibly cause rough running problems: could you try driving for a bit at the 'worst' rpm and then pull the plugs? you might be able to work out if one of the cylinders has a lean mis-fire perhaps.

good luck sorting this out. I had/have endless problems with an alfa 1750 gtv which has a misfire in the same range. Compression is even though, but intake vacuum changes a fair bit from cylinder to cylinder. I've got my money on a worn exhaust camshaft, which has measureable differences between lobe sizes. It runs sweet above 1500 though, so i'm not going to muck about with it.

will

[dgently]

I took the reading twice, and double-checked the meter against a Saab reading 130, which the owner said was bang on.

I have no reason to link the overheating to the high compression, except the hazy amateur notion that I might have softened the head and somehow (evenly!) shrank the combustion chamber of all four cylinders.

Even though I grew up in one, I don't know what a bog "weight" is isjavascript:emoticon(':)')
javascript:emoticon(':)')

Silly I'm sure, but I'm just anxious about that reading.

[peterako]

I would 'kill' for compression figures like yours!

Might be worth getting Paul (Cobban) or John (Miller) to have a look at it if you can.

John Larkin may know someone in Dublin who could look. Not sure who John uses for engine issues.

Sean Murray has similar compression for his Twincam after HE rebuilt it and is very happy.

I'll chat to another +2 owner in Wicklow and see if he can help.

Nearly every engine that has gone into Paul Cobban's workshop has come out singing like a symphony....even when it didn't go in for engine work!

If you are coming down this way, let me know. At the very least I'll meet you for a coffee

Take care,
Peter

[dgently]

I would 'kill' for compression figures like yours!

Sean Murray has similar compression for his Twincam after HE rebuilt it and is very happy.

But would he be happy if his compression had since climbed by 25%? It was rebuilt to stock, and read 160 across all four

If you are coming down this way, let me know. At the very least I'll meet you for a coffee

Thanks Peter that's very decent of you. Since my post I've retarded the ignition to good effect so far. The feedback here is clear, the first thing I need to do it test again with a different meter.

I could be making a fuss about a simple timing issue.

Thanks to all for the help and advice