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MG MGB GT V8 Factory Originals Technical - Four tuning/motor questions -- sorry for so much ignorance ...
My car (4.2, Carter 400 - pretty typical setup) seems to like to run rich. Fine, but it also consumes 93 octane gasoline at a prodigious rate. The tailpipe is coal-black, and the residue is a bit sooty. The A/F meter I installed tells me it's running rich - the needle is up against the rich side of the meter. But when I try even slightly leaner jets or needles the car pinks and flutters at cruise, and I know that is something to be avoided at all costs -- in addition to being annoying.
I just tried driving around with a vacuum gauge and I was a bit startled to see a wildy flucuating needle until I get up to about 1200 rpm. Is this symptomatic of a sticking valve, or is it just because the car has a fairly big cam? (I set the idle to 600, and the whole car burbles and shakes like a '60s muscle car, which is all in good fun.)
We had a thread a while back about Rover V8s with Carter carbs cutting on on sharp turns. One suggestion was that fuel pressure was too low. I installed a pressure guage in order to see, and I am getting exactly 6psi. Can I increase it without flooding the bowls (there is an adjusting screw back on the fuel pump that lets you go up to about 7.5psi)?
My car starts fine but cranks out a big plume of blue smoke when it is restarted from hot, after say 20 minutes at rest. The motor only has 8K miles on it. I assume this must mean the valve guides are leaking, though, right?
For a carb system, the fuel pressure may be on high side and may be causing probs. Try 3-3.5psi. You could try measing fuel supply aiming for 0.5pints(UK pints)/hp/hour.
If not enough flow consider changing float needle seat or bore of fuel lines.
The Jag SU pump is normally OK for Rover V8's.
|Recheck the timing. Over advanced timing causes pinking, that a very rich mixture can hide. However, the Rover V8 is amazingly tolerant of wide timing variations, much more so than the B-series engine.|
I moved from Greensboro NC to Colorado this winter and brought my car with me. A Carter 400
worked okay (but not great) on my Buick 215 in Greensboro, but miserably failed the smog test required
here before car registration (even on old cars like my '71). I ended up switching fuels, switching carbs
back to an Edelbrock 500 (because I had one and it was in better condition) and re-jetting both
primaries and secondaries.
For me, the biggest surprise was the fuel difference. I'd assumed the car would run cleaner on Ethanol
blended fuel, but my mechanic-friend insists if you tune for straight gasoline, then a tank of the Ethanol
blend will usually run rich. Moreover, it'll be difficult-to-impossible to tune with Ethanol fuel because the
performance will vary greatly with fuel temperature. My practical experience seems to match that.
I know in Greensboro they sell Ethanol during winter months and straight gas in warm months. Here
in Boulder County it's Ethanol year round, but we can buy straight gas just a few miles away in Weld
County. For my MG the difference was surprising. Just changing fuel smoothed up the idle by reducing
a strange "percolating" behaviour. I think you may find AFB (and Edelbrock) carbs are rather sensitive
to heat. It might help a little if you can find room for a phenolic risor block - I wish I had room for one! Perhaps intermittent droplets of unvaporized fuel can be correlated to your vacuum symptom?
A less obnoxious cam might help too. (Obnoxious is the word I'd use to describe mine... Am
I getting old or something?)
This thread was discussed between 25/05/2002 and 31/05/2002
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