On Tue, 1 Jul 1997, Dave Houser wrote:
> 1- We must have covered sparkplugs before but maybe not the modern day
> types now available(Is there a FAQ?). I've been running Champion N-9Y's
> since I've got 1/8" off the head, .040 overbore, 1-1/2" SU's, GJ
> needles, twin fuel pumps(ala MK. II TD), 4.3 MGA rear. I think I should
> be using a colder plug and want any recommendations you can offer. Old
> plugs look rich, with black deposits and white crud on top of electrode
> after a run back from Boston(200+ miles). Tail pipe pretty black, starts
> with very little choke, if any at all, jets all the way UP!
Hi, Dave. I suspect you already know some of this, but here it is anyway.
If plug fouling was the only symptom, I'd say your plugs might be too cold
a heat range. However the chokeless starts and the black tailpipe point
at an overly rich fuel/air mixture. That's a carburetor problem, not
a plug problem. Going to a hotter plug might keep your plugs cleaner, but
it won't solve the underlying problem.
Normally the engine should just barely run or not run at all with the jets
all the way up. If your jets are all the way up and your mixture is still
too rich, then something is very wrong with the carbs. Sounds to me like
your jets are either extremely worn or not the correct size to start with.
GJ needles combines with 0.090 jets are correct for H4 carbs on an XPAG.
Have you confirmed that the jets are actually 0.090? I've seen rebuild
kits containing 0.100 jets, which are meant for H6 (1-3/4") SUs. The
larger jet bore causes exactly the symptoms you described. Have you
confirmed that the needles are actually GJs?
Other things to check:
Do your jets drip when the car is at rest? If so, check the condition of
the cork jet seals. If they are shot, replace them. If the upper seals
leak, they allow fuel to be sucked into the venturi past the outside
diameter of the jets. This gives you a rich mixture that no amount of
adjustment can control.
Are the float levels correct? If too high, then the fuel level in the
jets is too high, giving you an overly rich mixture that can't be adjusted
away at the jets.
Are the float valves good? If not, your fuel pump will overfill the float
bowls, with the same result as above.
Are those twin fuel pumps of yours standard SU low-pressure pumps? They
should be. High-pressure SUs or aftermarket pumps with too high a
pressure rating, when mounted on the firewall, will sometimes force fuel
past closed float valves even if the valves are good. High-pressure pumps
are meant to be mounted back near the tank, not close to the carbs.
-----------------------------------------------------------------------
Chip Old 1948 M.G. TC TC6710 NEMGTR #2271
Cub Hill, Maryland 1962 Triumph TR4 CT3154LO (daily driver)
fold@mail.bcpl.lib.md.us
If cars had evolved as fast as computers have, by now they'd cost a
quarter, run for a year on a half-gallon of gas, and explode once a day.
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