Car in bits again...
AO Silver Member
Join Date: Feb 2007
Location: Hampshire, UK
Hi Brit - there is one other possible factor at work, and that is the quality of the fuel. If you are using stock timing, then I assume that you are running your engine on 97 octane super unleaded, as a minimum? Alfa boxer was originally designed to run on 4-star leaded fuel, with a minimum octane rating of 98 RON. Super unleaded is as near as we can get to that now in the UK, and that is 97 RON. If you are running on something equivalent to our standard 95 octane fuel (equivalent to 3-star in old money), then you need to back off the ignition timing 3° because the 95 octane does not resist detonation as effectively as 97 octane.
I think you have mentioned in the past the variable quality of fuel over there, could that be the problem?
K&Ns can lean out the mixture slightly, but I thought you had gone up one size on the jets to compensate? That's all that K&N advise you to do.
A little bit of jitter is to be expected at maximum advance, so what you have seen suggests the trigger electronics and mechanical advance mechanism are all in good order. If it was spark scatter the timing would be jumping all over the place, particularly at high rpm.
If it does turn out to be fuel related, you can either try the various octane booster potions on the market (most of which have no effect), or you can limit the maximum mechanical advance by fitting stronger advance springs, or even re-profiling the bob weights to limit their maximum travel. This way you get to keep the 8° BTDC timing at low rpm (which preserves the idle quality) but don't run into detonation at the top end.
Or you could just retard the timing by 3°. The MSD should help to smooth out the idle when the timing is 5° BTDC, and your mechanical advance won't go beyond 31°.