Dieseling is caused by a burnable mixture, compression and a source of ignition.
Hot spots are usually the cause of dieseling in our cars.
Make sure you have the right plugs. I use a 6 range NGK for most brit
applications. A 5 is a bit warmer and some cars can get by with it. 7
is rather cold, and could lead to fouling.
Other hot spots are sharp edges and corners. When the head was done,
was it milled? If so, you need to break the sharp edges with a little
sanding wheel or a scotch-brite wheel or even a little needle file.
Doesn't take much, like .005" radius, to eliminate those hot spots. Any
sharp bits sticking out where they added seats? smooth 'em off. Same
with the spark plugs. Sometimes where they cut the electrode off, there
is a tiny little corner on the electode that sticks out and will glow.
Sometimes on over-bored engines, the headgasket can hang out a bit and
be a hot spot. Not so much a problem on the XPAGs, but there it is.
Also bear in mind that your nice new head probably gave you a tad more
compression, with new valves sitting high up in new seats, and probably
a light skim cut to make sure it is flat.
A lean mixture will lead to higher temperatures and can lead to
dieselling. Be sure that the mixture is right. In the same vein, be
sure your cooling system is clean and efficient.
Did you actually compute your compression? Since every XPAG head has
had a different machining, rebuilding, repairing, etc, history, you
can't just go by the book and know what your compression is. Borrow a
burette and actually measure the volume and you will know for sure.
Same with the block. How much has been milled of in past rebuilds?
Measure the deck height and you will know. Normal XPAG is what, 7.2:1?
MkII a bit higher. So what? By now, every engine is like a hot rod,
they have been cut so many times. Not too long ago, I built a TF engine
for a fellow that had been cut so many times that, even with 2 head
gaskets, it was at 10.1:1. I had to rework the combustion chambers to
get it down to a driveable 9.3:1. He still has to run good gas, but it
scoots and doesn't overheat or knock. But without measureing, we would
never have known and would have had to pull the engine to figure out
what was going on. And probably would have had the extra expense of
buying a new head or doing a bunch of rework. But measuring it all
before saved his bacon.
Timing can lead to dieselling, but it is a rarity. Most often too much
advance will lead to knocking, which is like dieseling, but during
high-load running. The two flame fronts, from the plug and from the hot
spot, colide and make a noise. If your timing is right and the engine is
running well, your timing will likely be between 5 and 15 degrees BTDC.
If you are still running at 0, like the books says, you are wasting
power. Modern fuels hardly even compare to the wild range of stuff that
used to pass for flammables, but they do like more advance. Still, more
than 15 and you are probably knocking under load. Depending upon the
advance in the "can", your compression ratio and cam choice, you will
want a total advance of about 30-33 degrees, all of it in by about
2600-2800. Get a good dial-back timing light and see what is really
going on.
If you really are having problems, you might want to look deeper. Where
is the cam set? An advanced camshaft can help cause dieseling. A flaky
ignition switch that doesn't completely shut off will do it. Bleed-back
from a mis-wired charging system? More of an alternator problem, but it
can happen. Dieseling is a pain in the a**, and shouldn't happen. When
it does, there is always a cuplrit. Good hunting!!
Cheers..Andy
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