1300 head

[daveyl]

Just thought I'd share this little bit of nonsense. I have a 1098 engine fitted with a 1300 inch and 3/4 su, big bore exhaust and a Big-valve 1300 metro head.
It 'pinks' like a piano no matter where I set the timing (mostly top gear). And so I've just ignored it. Anyhoo, I had a spare standard 1300 marina head and fitted that instead (no reason, just wanted to test it). Well, what a tranformation! Maybe not as quick (very marginal) accelerating through the gears but..big improvement in top gear. It has more oomph and torque and will pull the 5th gear +3.6 diff far better. 70 mph at 3300 revs on the flat.
The only thing I can put this down to is that the marina head looks hardly to have been skimmed where as the 'big-valve' defintely has, maybe too much. The big-valve I polished and ported it, really put some effort in and turns out I needn't have bothered! Any thoughts? Shame, might be unusable now, although I may try it on my 1300 engine (if I ever get it re-built) and see how it goes.
Anybody had the same problem?

[chrisryder]

very interesting, when you took the head off, were there any signs of damage to block deck or exhaust valve faces? did you pocket the deck of the block to clear the exhaust valves or did you just fit it without?

[daveyl]

Not inspected the head thoroughly as yet. But yes the the deck is pocketed and the old gasket looked in good shape.
I only drew the conclusion about the possible 'over-skim', as when I fitted the marina head I had to hugely adjust the tappet adjusters to close the gap.
Thanks, I will inspect the head in more depth and see if there is anything in that.

[MarkyB]

Try measuring the total thickness of the removed head with a digital calliper gauge.
There is a number, that I can't remember off the top of my head, for a standard head.

[bmcecosse]

The big valves are really just TOO big for a 1098 engine. The standard 940 valve sizes are 'just right' - and again, the standard chamber size of 21.4cc is 'just right' for the 1098, the head should not be skimmed - the CR will be too high = massive pinking/detonation as you have experienced. All heads start life at 2.750" - any less is the amount that has been skimmed off. The big valve head will be fine on a 1275 engine - with dished pistons........

[Matt]

standard head is 2.75" deep (give or take 2-3 thou)

[daveyl]

Thanks for the info. chaps. The head is 1.73" thick so, probably ok. The valves and head surface appear in good nick; i'll take the valves out at some point and check them and re-seat.

[chrisryder]

hindsight is a wonderfull thing but a compression test could have given an indication of whether it had too high a compression ratio.

The head is 1.73" thick

i hope you mean 2.73. An inch is a lot to skim off a head!

[bmcecosse]

I wouldn't have thought a 20 skim would make much difference - especially if the chambers have been 'polished' up a bit..and the exhaust valves sunk in slightly.

[daveyl]

Haha. Yes sorry, 2.73. It's run the same way for about 18 months from the re-build. Did a compression test probably 6 months ago and no change at 165lb (good oil pressue too) from that new build. It's always been lumpy on idle and I've tried electronic ignition (the cheap one) and then new 45d dissy, swapped to a 1.5 inch carb, mini sports coil (I like this by the way), all to no avail. But..it is fitted with a slightly up- tuned road cam.
It's a small irritation, as it goes very well above that. Actually I'm thinking of going backwards in mods and trying the mini manifold combo and 'pea-shooter' down pipe with 1.5in SU and see how that goes.
Would try the cam, but too much effort, as the oil pump is matched to the cam. However, I'll probably try a re-tune now it's not pinking. Never tried a AAA needle tho.
There is a reason for all this, as I want to get it just right before the engine is replaced and moved on.