7 replies to this topic

[alpder]

Getting pinking on a recently built engine. 1310 A+ / 12g940 lead-free / HIF38-red spring / Kent 246 - which I believe is not much different from an MG Metro cam? The engineer who did the rebore and skim told me he would aim for 10.5:1 (and as a complete noob I said "whatever" - should've said aim for 9.5 or 10 I guess.)

 

I can drive like a granny, and get no pinking at all. So if the only answer is head off then I can leave the fix until salt-on-the-roads winter-time. Right now just want to get some ideas about what's the best answer, please.

 

Symptoms are: floor it coming off a roundabout (especially uphill or when towing)... and when the rpms hit 3500 the pinking starts just like a switch has been pressed and continues all the way up the rev range. Ease off slightly and the pinking stops. Floor it again and after a second or so (the delay caused while the carb piston lifts?) the pinking starts again.

 

What are my options...

Better fuel... doesn't seem to make any difference using E5 or E10.

 

Change ignition timing... already tried a 2-degree retard and makes no observable difference.

 

Try to richen the needle at the thin end? (This needle was hand-made on a rolling road so it's not a case of 'just' buying a different needle.)

 

Have the distributor curve changed or buy a more appropriate distributor?

 

Or is the only possible answer to have the head off and increase the volumes?

Edited by alpder, Today, 09:26 AM.

[68+86auto]

What is the current timing and advance curve?

[ACDodd]

Too much compression. Artificially retarding the ignition to ‘get around that’ is futile, it looses more power than having compression ratio a little too low and using more ignition timing.
That said, what ignition timing are you using across the reve range, maybe you are just using way too much anyway?

Ac

Edited by ACDodd, Today, 09:40 AM.

[alpder]

Sorry - should have said. I believe it's the original Metro 65DM4. Mileage unknown. Very unlikely to have been modded or swapped-out - the rest of the engine was standard and equally "patina'd" with a layer of oily muck when acquired.

 

Timing was set by the rolling road crew. When I got home I checked it with a basic strobe at idle (no vac) I saw they'd set 8 BTDC. I can't measure rpms but picking up the throttle a smidge doesn't change that reading.

[alpder]

Too much compression. Artificially retarding the ignition to ‘get around that’ is futile, it looses more power than having compression ratio a little too low and using more ignition timing.
That said, what ignition timing are you using across the reve range, maybe you are just using way too much anyway?

Ac

Thanks AC... figured this was the most likely answer I'd be hearing :-(

 

Head off this winter, then. What CR do you reckon I should aim for, if E10 is my preferred fuel?

[ACDodd]

9.75:1 max.

Ac

[alpder]

Aim for 9.5 then.

 

A quick calc tells me I'd need +4cc (per cylinder). Is this easy to do in the chambers without - say - reseating the valves deeper or drastically changing the gasflow in the chamber? (Unfortunately - and I have no idea why - the engineer skimmed the block, not the head, when increasing the CR. I just went with what he said as a complete noob. So now just slapping a spare head onto the engine won't undo the change.)

 

I notice +4cc is almost exactly what I'd get with two gaskets. (Cue a fiery debate and possibly even my expulsion from the forum for just mentioning it...)

Edited by alpder, Today, 11:03 AM.

[ACDodd]

What pistons are fitted?

Ac