I thought I'd send some observations that I have made over the 10k miles I
now have on my V8. Some of this may be helpful and some not, but here goes.
When I put my engine together I had a Carter 625cfm AFB carb that my dad had
bought from JCWhitney. By any reasoning, this is way too much carb for this
small a motor. It was calibrated for a chevy according to the model number.
I have it mounted on a 4bbl manifold with no identifying marks on it, so I
can't say who makes it. The carb ran nearly perfect out of the box, I may
have been able to run it one stage lean but I never bothered trying. I
routinely turned 19 to 20 mpg with a T-10 and stock rear end. No matter how
much time I spent in the throttle it never really affected gas mileage. The
car had a lot of top end with headers and cool air intake. Torque was never
lacking since the B is so light. The problem that I did however encounter
is the car could be hard to start, especially when hot. I think the cause
here is that the air velocity was so low that it wasn't atomizing the fuel
well. For the same reason, I had a lot of hot idle problems, these became
severe when the water temp hit 190deg or higher, this was never a car that
you could get out of and run into the house to get something because it
would always stall. The part that I don't understand is that the primary
and secondary bores, and primary and secondary venturies are the same
dimensions as the 500cfm model which seems to be the hot ticket on these
engines, so I don't completely get the problem but what can I say. The carb
has an air valve on the secondaries that will only pull open as much as the
engine requires, at full throttle the valve will pull all the way open.
Recently I changed out the carb and intake for the stock rochester 2bbl and
manifold that I got with the motor. You can really feel this combination
suffocating. When I tip into the throttle it feels like I am towing a boat.
When I get all the way into the throttle it moves pretty well, but not like
the 4bbl. It behaves a lot like a stock B, when you get hard into it, it
makes a lot of noise but doesn't have the go to match. Idle is very stable
and will hover somewhere near 4-500RPM. I did have to bump it up a bit
because my 17lb flywheel is just too light to hold steady idle this low, it
starts to oscillate between 300 and 600 with that light a flywheel. I'm
really shocked at how much more linear throttle response is with this combo.
The biggest, unexpected, difference is that I get the same mileage, maybe 20
to 21 if I'm gentle; but I pay a bigger price for getting hard into the
throttle even though it lacks a secondary; it does have a power valve but I
don't really understand how they work at this point. I did manage one tank
at 23 and I think some of this problem is due to the less then perfect
behavior of the choke. The carb is from an automatic equipped car, which if
I remember correctly is jetted leaner then a manual tranny car. Either way,
there was some power sacrificed to safety, as when I floor it I can smell
unburned gas in the exhaust. I think the carb is throwing extra gas at the
engine to prevent detonation in the event that the car is floored in the
wrong gear. Lastly, when the choke is functioning properly, the car starts
instantly hot or cold, and will idle indefinitely.
Enough about carbs, this is something I hope people will find useful. When
I set up my Howe HTOB they recommended setting a slight preload, I seem to
recall that I ended up with 10 thou preload. Within 6000miles I had clutch
slippage under acceleration. For a while I feared the worst, but it
occurred to me that the preload on the HTOB would increase as the clutch
wore and might be causing the problem. I pulled one shim, which as I recall
was 30 thou. I decided that even though it is only a one hour job to
reshim, I would pull one more shim just to cover me in the future. The
change did solve that problem, and had another unexpected benefit. I have
heard a lot of complaints about the excessively high pedal effort required
to operate the HTOBs and creating a clearance at the bearing reduced this.
It is still not light but, by creating some clearance here, it seems to have
moved the clutch actuation into a sweeter portion of the pedal / master
cylinder geometry.
I'm sure that there is more, but that is all that comes to mind right now.
James Nazarian
71 MGB Tourer
71 MGBGT V8
85 Dodge Ram
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