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MG MGB Technical - milage trouble (and more)

Let me first preface my submission by stating that I am not a mechanic, shade tree mechanic and enthusiast yes! I am very much enjoying my first MG experience and if I wanted a cookie cutter miata I would have bought one.

I’ve had the car for about six months and in that time I’ve already gained much experience, but this problem has me a little stymied.

(I’ll try to be as clear as possible including the new language I’ve acquired)

I have a 1977 MGB, and I believe everything is stock. It has a Zenith Stromberg carburetor and all CA. (FED) smog equipment. I can see no modifications, I’ve spent hours comparing my parts to my Bentley manual, Moss and VB catalogues. Also, the car had only four thousand miles put on it in the past five years.

When I purchased the vehicle CA. requires a smog test, it did not pass. I also noticed poor fuel mileage. I was getting about 8-10 miles per gallon. Under the terms of the sale, the car would need to pass smog. We took the car to a local mechanic and he found the air manifold tubes clogged, a bad gulp valve and the timing misaligned at the drive dog on the distributor. Also one of the breather pipes had a make shift plug (I think this was the line off the oil separator but I personally did not see this). All the repairs cost the previous owner $600 and some change. I brought the car home and thought excellent, running good and looking good! (well for a few minutes any way)

About a week later I was filling up on gas and did my mileage calculation and ah oh…. Something was still not right. I was getting about 12 miles per gallon.

Upon investigation I found the breather pipe from the oil separator to the carburetor had a notch cut into it, facing downward as not to be seen (rubber hose). Apparently this helps the car pass smog by allowing fresh air in, not wanting to take it back to the same mechanic for obvious reasons. (mind you this was supposedly an expert British car mechanic.)

I replaced the breather hose and start of my odyssey began. Like magic the catalytic converter turned a sparkling hue of red, danger danger. (I have since learned that this can be common with the B’s)

The items that I have investigate and repaired are below, If anyone has anymore ideas about what direction I should be looking, PLEASE feel free!!!

Found the ELC (evaporative loss control) system pressurized, found the separation take dislodged 180 degrees, rotating forward and pointing backward. I believed this caused the fumes to collect in the tank and build up, at least I found the tank about half full with gas and not draining. Re-secured. I also found the charcoal canister had become open, screw fitting at the bottom, removed and retightened then re-installed. Checked the anti-run on valve for operation and it passed, although I do get run on now and then.

Rebuilt the carburetor, all seals and new needle, the old needle was worn and did have a flat spot. Not sure if there is a way to test the jet (size) as I can find no documentation as to the correct size of orifice. (did not replace the float, read article about a vibration that can cause overflow with worn pin and float fork, does not look worn but…) Also found the choke was stuck about half way though it’s travel. I cleaned all of these parts with particular care, refitted and set to spec. I’ve since tested two or three time to make sure it wasn’t “on” after engine warms. I’ve also adjusted the float height, missed that one on the first go around! I can get the carburetor to pass the 1/8th lift method of adjustment, mixture setting at idle. It idles ok for the most part, not super smooth but ok, I have noticed that the fuel pump tends to click pretty often, every three to four seconds will result in two clicks from the pump, sometimes four clicks. Also I get a little hesitation when the clicks happen? (Not a miss, just a little bump) At normal freeway speed 60-70 it run extremely smooth.

I’ve done the normal maintenance replacements, plugs, air filters (both), oil and filter, rotor and cap. By the way the later two were very worn. I set all of these to spec including the timing and valve clearance.

I’ve also looked for vacuum leaks, I’ve sprayed the engine down with no leaks found.

With all the work that that I’ve done I am able to get about 15-16 miles per gallon and the catalytic converter is still a little red. I’m not convinced that it’s fixed. I think I should be up around 25mpg and the cat should be dark.

The next steps (this weekend), check the fuel pressure at the carburetor inlet, I think it should be around 3-4 psi, I don’t have a Churchill nor do I think I will ever seen one! If someone could confirm that I would be most grateful. If the fuel pump passes I was thinking about checking each emission component?? Does anyone have any other idea’s??

Thank you,

Respectfully submitted,

Kevin Boldin
San Diego, CA.
KevinBoldin

Kevin, sounds like you've learned most everything
to know about the late 70's B fuel system/carb.

The converter should not be glowing red on the B --
this indicates the mixture is too rich. I found that
the lift the piston method with the Zenith
indicates a lean mixture, even though the mixture
is set correctly according to the CO meter.

To adjust the Zenith accurately, you really need
a CO meter (the gunson's works ok for me, but
its about $200) The CO should be 5.5% with the
air pump disconnected, air rail plugged, and
ELC hose disconnected from the port above
the carb fuel bowl. If set properly, the CO
will then be below 1% when the air pump is
disconnected.

Did you completely rebuild the choke with the
carb rebuild? The mixture will be way rich if
the o-ring on the choke metering needle fails.

If you can't get a Gunson's meter, try finding
a mechanic (doesn't have to be British-oriented)
with a smog machine who can double check the CO
and HCs for you.

The cat converter might be toast at this stage
if the rich running ocurred over a long period of
time. But a clogged cat would give you generally
poor performance and not a rich mixture.

Final thoughts: (1) If the carb is very worn the
jet will have gone oval resulting in a rich
mixture that can't be adjusted out. Check the archives for tips on replacing the jet. (2) Are
you sure the float isn't leaking?
Ronald

Yes, replaced the small o-ring on the choke needle. I'll have to go to a mechanic to get the CO tested. Kind of pricey for me after christmas :)

The cat got replaced when I bought it, forgot to mention that. Used, but could be toast after six months.. (Hope not)

The jet does have a little notch, I didn't see it at first, that's how small it is. I would assume that any elongation would not be good. I want to exhaust all other checks before attempting this! (I'm pretty confident but press fit stuff makes me think twice!)

Not 100% sure the floats not leaking, replaced the float needle valve but not exactly sure how to determine is I'm getting leakage? That's why I was going after the fuel pressure, if it's to high it would cause blow by?? I was thinking that If I checked the jet and needle while the engine is off I could see if the gas was over flowing through the jet??

Thank you for you help!

kb.
Kevin Boldin

If the float valve is leaking then the overflow will be via the vent pipe to the charcoal canister, not up though the jet. However if the vent pipe to the canister is blocked then you *will* get overflow up the jet, as even with a good float and valve the fuel level cannot rise to lift the float and shut off the valve. This should be obvious anyway, assuming an SU pump is fitted, by the pump continuing to click even if the engine has not been started yet. With the ignition on and the engine stopped, once the pump has filled the bowl and the float valve shut off, the pump should not click any more often than once every 30 secs. There are a couple of pump problems that can cause more rapid clicking, as well as fuel overflowing at the carb. Your mileage is pretty diabolical even for US standards, and to make you feel even worse I get low to mid 30s out of mine. I average 27 on a run even in the V8, which can drop to 11 round town, but have seen up to 34 on a very long motorway trip :o)
Paul Hunt

Kevin,

Have you looked at this web site, http://www.paulbunyan.net/~jasko/choke ? Normally the choke is the first culprit on the Z-S carbs when the converters running red hot.

Shoot me an Email at mgbpaul@yahoo.com and I'll give you my contact details. Perhaps I can help. Also recommend joining the Club at http://www.SanDiegoMGClub.org. We have almost monthly tech sessions that may be a help to you.

Cheers, Paul
Paul K

Kevin,
To answer your question regarding proper fuel pressure at the carburator: I don't know. But, I installed a pressure regulator, in the engine compartment, about 12" upstream from the carb inlet on my '71 model. The pres regulator has graduations you can set, between 1 and 5. Mine is set at '1', and runs just fine. I assume that corresponds to 'one psi'.

One thing you might want to check: ensure the gas tank isn't pressurizing. I learned this the hard way...car would 'quit', at really bad times, accompanied by fuel pouring out the charcoal cannister. Our Brit friends couldn't fully relate because apparently the British models have a vented cap, where the yank models vent to the charcoal cannister. Turns out, the vent pipe inside the tank had somehow fallen about 2 inches, where it would be submerged in fuel if I had more than 5/8's of a tank of gas. When the sun heated the air in the tank, the tank would pressurize resulting in poor running (well, it wouldn't run) and the aforementioned raw fuel venting into the engine compartment. Your symptoms are different than mine, but this is cheap to check, just by removing the cap and seeing if there's an unreasonable outrush of air. Just a thought. Best of luck

Dave
David Tye

This thread was discussed between 03/01/2004 and 04/01/2004

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