On Mon, 3 Apr 1995, Jon N Swanson wrote:
> the other 3 plugs and there was no perceptible change in running). The car
> runs rough on #2, will only run with the choke full-on, and occasionally
> backfires out the forward carburetor. After running on #2 for a while,
> I checked the plugs and found #1 and #2 dry, #3 and #4 wet with gas.
>
Backfiring out the forward carb indicates a lean mixture. Needing the
choke indicates either a lean mixture or too much advance. A dry plug in
a cylinder which is not firing indicates a >lean< mixture (like, all air?)
Wet plugs on the rear set indicates a really excessively rich mixture
(flooding condition) or no spark at all.
> I have SU carburetors, I think they are HS4's, there is a metal # tag on one
> that says AUD135R. There is an aftermarket electric fuel pump, the float
An aftermarket pump better be in the 3.5 to 4.5 psi range for SU carbs.
Some of them go up to 9 psi... you'd need a fuel pressure regulator for
those.
> bowls are both full of gas.
>
No doubt!
> My compression tests out at 110, 115, 115, 120
Not a problem.
> New parts: Distributor cap, rotor, plugs (gapped at .025), plug wires,
>points,
> condenser. The dwell is set for a .015 point gap, verified at close to 60
> degrees while sputtering along on #2 only.
>
Depending on which distributor is being used, it's 60 +- 3 or 51. It
should run either way.
> What is the best thing I can do with these carburetors of unknown pedigree?
> Should I attempt to clean them? How? Rebuild them myself? Is there
>someplace
> that rebuilds them reasonably? Should I give up and get an aftermarket
> Weber (if a reasonably priced one exists?) Am I even on the right track?
> I'm not trying to turn this car into some sort of high performance monster,
Yes, rebuild them yourself! It's mildly complicated but not a really big
deal if you are somewhat organized. Buy a rebuild kit for front and for
rear. I wouldn't put Webers on since that would add another set of
variables. This is a good idea for any carbs that have been sitting
around for years. I'd put in new floats while I was in there.
A first ditch effort would be to squirt carb cleaner/starting fluid in
them while cranking. You may get some of the other cylinders to fire off
this way. I bet the carbs are both filthy with lacquer, though.
Fuel delivery problems, Tests, and Solutions (generic)
P) Aftermarket fuel pump is too strong, forcing fuel through the needle
valve on the rear carb. Float is sunk. Crud is blocking needle valve.
Worn needle valve.
T) See if fuel pours out the overflow vents (hose that goes to the
charcoal canister when fuel pump is on but you aren't cranking the engine.
S) Use SU or proper Facet fuel pump, or add fuel pressure regulator to
existing pump. Dismantle carb and check for fuel in float (note: it can
be SO full you can't tell. The float should be very light, with no
bubbles in it <g>). Rebuild carb.
P) Air leak in front carb or front section of manifold. This can be
caused by improper "detox", loose manifold nuts, bad gasket, or a
thickness difference between exhaust and intake manifolds.
T) plug holes and see if "idle" changes. Rock the carb/intake assembly
up and down and see if it moves (it shouldn't). Spray Brakleen around
the gaskets, and at the throttle shaft, to see if the backfiring ceases.
S) Make sure all holes are plugged (what are you doing with crankcase
ventilation at this point?). Replace gasket, tighten nuts if possible.
If you are using an HIF intake manifold with an old exhaust manifold, the
flange thicknesses will differ by 1/8" or so! We made custom spacers for
this, but you could just cut washers in half. Rebuild carb with new
throttle shafts.
P) SU piston frozen all the way down or all the way up
T) lift the piston with your finger. It should move smoothly but against
a fairly firm resistance (if you have 20W carb damper oil like I have).
Drop it... it should drop quickly and smoothly to the bottom.
S) Corrosion and crud? More likely a badly aligned needle. Rebuild carb.
May need replacement needles if they are scratched.
P) Piston goes up but needle stays in place.
T) If you disconnect the throttle linkage and run exclusively on one
carb, and it runs BETTER, that's an indicator. (This happened on the
Spitfire and drove me nutty).
S) Remove needle and piston. Screw the needle properly back into the piston.
P) Mixture control is totally out of adjustment. Probably in two
different ways.
T) Lift-pin tests make no sense at all.
S) Screw adjustment nut/screw all the way tight, and then back off two turns.
A tool you might want to invest in at this point is a Gunson Colourtune.
This is a sparkplug with a quartz window in it, so you can see both spark
and flame color. This will tell you 1) are the plugs really grounded and
firing IN THE SPARKPLUG HOLES (they could be corroded) and 2) What's
going on in the cylinder(s) which is/are firing?
John M. Trindle | jtrindle@tsquare.com | Tidewater Sports Car Club
'73 MGB DSP | '69 Spitfire E Stock | '88 RX-7 C Stock
Home Page: http://www.widomaker.com/~trindle
"How can you be in two places at once, when you're not anywhere at all?
- Firesign Theater"
|