Detonation ?

[MikeB]

Gents

3 litre SC, 1981 Euro standard spec apart from 40 Weber IDAs and SSI early exhaust system. Just performed a top end rebuild. Std cams timed in at 1.6 mm. running on Super Unleaded

When I set the ignition timing to 5 deg BTDC at 950 revs, the car idles well and lifts well, but then under hard acceleration I am getting a hard knocking noise that I believe is detonation from 2500 revs to 5000.

If I set the timing to approx 5 deg ATDC the knocking noise disappears but the car then has intermittent spitting out of the exhaust below 2500.

The only thing I have changed since prior to rebuild is the the cam timing, which I have advanced to the above setting. The car drove well before with no knocking. The ignition timing was retarded on the 5 deg std setting. Have rechecked the valve settings to ensure there isn't a tight gap

So would the cam timing effect the low end running and produce the spitting?

[Cortina]

Mike I don't exactly understand what you have done !

You say you have advanced the cam timing ? ... Why ?

If cams are std - then why would you not use correct cam timing ?

Ign timing after tdc must be all wrong . I have not looked up , but it must be x degrees before TDC ?

Go back to cam timing - answer that one first and we can take from there - ring if you want

cheers

Alan

[MikeB]

Alan

There is a range of cam settings for the SC, 1.4 - 1.7 mm iirc, so I have set the cam to the advanced end of the range to give a slight emphasis to the lower end of the torque curve.

Have done further investigations this evening, and am suspecting an inefficient idle circuit on Nr 5 cylinder, as turning the idle screw in or out has no effect on the idle revs and pulling the plug lead had a similar non effect. Have checked the plug and it seems to be working, so have blown out the idle circuit and will check if there is any improvement tomorrow.

[Cortina]

OK .. I thought you had done something radical !!! ... that would not affect running to the extent you describe.

Keep me posted , I have a degree in cam timing (sort of )

PS - have entered the Kirkistown sprint on Sat to give the borrowed Cortina a chuck about (pre Circuit )

[MikeB]

God luck, Alan

I'm looking to Craigantlet at the start of May, hopefully get this sorted soon