MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - LE is still running lean


This is the only Zenith carb equipped B I've ever had that ran lean. I rebuilt the carb myself, had the autochoke redone, replaced vacuum lines, checked the fuel pump, and still can't figure out why this thing is running lean. The mixture is set to full rich ( all the way clockwise with the adjusting tool), and the car still backfires through the carb a bit when cold. It runs fine when it warms up, but I can tell it's not rich enough. BTW, I also replaced the cylinder head with one that I had redone. The airpump was removed before I bought it but all other emissions gear besides the cat is there.

Any ideas? I also checked the float height again, which was fine.

Thanks,
Dave
David Plantz

Is the head ported? You may be drwing in more air than the carb can handle in its current configuration.
Jeff Schlemmer


Hi Jeff,

No, I didn't have it ported. It was checked for cracks and had new guides put in. I should also add that I'm running the stock air cleaner.

Thanks,
Dave
David Plantz

Dave. Do you have a relationship with a mechanic who has an exhaust gas analyzer? If so, it might be worth testing the exhaust. Backfiring is caused by unburned fuel, but the root cause may be either a too weak mixture (not burning properly and unburned fuel building up in the exhaust) or a too rich mixture (not all the fuel is burned and the unburned fuel builds up). Knowing which is your problem would be a good first step.

Les
Les Bengtson


Hi Les,

I know of a British car shop about 2 hours away that should have an analyzer. That's where I was planning to take the car if I couldn't get this figured out. One thing I noticed was that there's never any black smoke out of the tail pipe.

My plan was to switch this car to HIF 4's, but I really don't have the money now as I'm in college. I was hoping to make due with the Zenith for now.

Thanks,
Dave
David Plantz

Dave. Sounds like an excellent plan to me. I have had Z-S carb which ran quite well. However, they could not be adjusted to meet our required emissions test standards. My first choice would also be the twin SU carbs. But, what I have is the DGV.

A trip to a professional, one having some experience with the Z-S and a selection of needles available, might be a wise investment.

I well remember my poor student days. I think it is a plot to force college graduates to be "successful" (i.e. wealthy) in life.

Good luck. Les
Les Bengtson

Dave, As an interested outsider from "downunder", at the risk of asking the obvious, have you checked the ignition timing, sounds like it could be too far advanced.
Regards, Owen.
O.T. Daley


Hi Owen,

I did check the timing, twice. Following the advice of fellow BB members, I set the timing at idle to 12 BTDC ( think this was the right setting, but can't remember).

Thanks,
Dave
Dave Plantz

Your right Dave, 12BTDC is ok, the recomended is 14 deg. for high compression engines and 10 for LC, if you checked it with a strobe light you would have needed to remove the vacuum advance line from the distributor, otherwise the result would be retarded ignition, which could also present the symptoms you've described.
Regards again,
Owen
O.T. Daley

Still sounds like an air leak, although you
noted you replaced the vacuum lines. Also try

1. Manifold gasket or loose manifold nuts.

2. Leaking smog valve (esp. the gulp valve.)

Ronald

David,

How does the car idle? Does it race? I had a problem with my '68 bgt, in which it ran very lean and idled not less then 2000 and then got higher as it heated up. The problem was the way the previous owner rigged up the crank breather hoses when the emissions stuff was removed. On my car, there was supposed to be several hoses and valves coming off the top of the intake manifold. All of these were removed by the previous owner and a single hose was run from the crankcase breather to the top of the intake manifold. This allowed the engine to suck lots of air through the engine and into the intake. The result was a very lean mixture and then ever I tried to make it richer, it idled faster.

Here is a quick test to see if its an air leak from the emissions stuff. remove all hoses and valves from the top of the intake manifold. Put a hose on each fitting on the manifold and put a plug or dowel in the other end of the hose. Use a hose clamp to seal the hose to the manifold and dowel. This should force all air going in to the engine to go through the carb. If that fixes it, check....
Gulp valve and any other emissions valve, oil filler cap, all emissions hoses for cracks.

-Steve
Steven J. Korotky

Check th evaccuum advance & retard. I had the same symptns and eventually founf that the diaphragm in the A + R was split, letting air into the manifold.
R L Tinkler

This thread was discussed between 12/04/2006 and 17/04/2006

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.