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MG MGB Technical - Weber Idle Speed & Vacuum

I have a 1980 MGB with a Weber DGV carb with a K&N air filter installed by the previous owner, and the carb has a few problems. At times it's just not responsive to acceleration and needs a second nudge on the gas pedal to get over the initial sluggishness and sometimes it backfires at the carb. The backfiring seem to decrease (completely?) as the car warms-up. Also, at idle the RPMs slowly decrease below 500, but if I bump the break quickly the RPMs increase to 700 or so.

I have little doubt that it is running lean, but I checked the jet sizes with Brit-Tech, and it has the recommended jet sizes. Since the carb responds to break pumping at idle, I'm thinking I have a vacuum issue.


I disconnected the vacuum hose from the Cannon header, and the car started much quicker than normal and idled at 1000 RPM. If I plug the hose, the old problem return. When attached to the servo, I could not blow air into the hose.

I noticed that the breather hose is just running down the side of the engine (as recommened by Brit-Tech) instead of running to the air cleaner. Could this be the issue????


What could be the issue?
(Should I adjust the idle screw even if I feel there's a vacuum issue?)







David Emerson

David,

There is a vacuum hose connected to the header, or intake manifold?

By disconnecting the hose you a letting more air in at idle. Is the choke backing off completely? I had a 32/36 DVG on my car and now I am with the 38/38. Very satisfied with them both. I could dig up the tuning stuff I got with the carb and fax it or e-mail to you. Brit-tek have been very helpful.

Shareef
Shareef Hassan

David,

The hose you disconnected may be the vacuum supply for the brake servo; I assume you meant that it was attached to the intake manifold and not the exhaust header. If you have good brake power you probably are not loosing vacuum there. Is the vacuum advance line routed from the base of the DGV through the Transmission Controled Spark Advance switch (usually atop the brake pedal box)and then to the distributor? How is your anti-run-on valve plumbed into the DGV system? Vacuum leaks could occur on any of those lines. Also you can try to re-torque the manifold to head bolts.

Now to your symptoms. The idle variation could be caused by something as simple as too weak a throttle return spring. The accelorator linkage is not the best arrangement and it is difficult to find a good place to attach springs that won't interfere with the works. What main idle jet are you using? If you are using the 50 you could try to go up to a 55 or 60. Redline Weber has a good article on setting the DGV idle on their website: www.webercarburetors.com then go to the tech notes.

Backfiring and sluggishness are most likely timing issues. You may be too far advanced for your lean mixture.

Doug
D. Cook

Shareef/D,

Yes, I did mean to state that the vacuum hose is attached to the intake manifold, and it is the vacuum line running to the servo.

Regarding breaking power: I'm new to the MG world, but the car seems to take too long to break. This past weekend I replaced the rotors and pads and they are not broken in yet, so I don't know if this problem is corrected or not. (Side note: A local MG mechanic recently bled the break lines while doing some rear axle work, and I don't think it's an "air in the lines" issue only because the breaks where the same before and after his work.)

As part of a tune-up for emmissions, the mechanic also set the timing to factory spec and replaced the points but these change had little or no effect on the carb issue.

I was wondering if the servo was the source of the carb problem, but the break peddle firms-up after killing the engine and drops a bit when the engine is restarted.


Few questions at this point:
1) Given the test described above, should I rule-out servo issues?
2) Since bumping the breaks restores the idle speed,
it seems like the idling issue is a symptom of another issue, so I've avoid changing it until now.
Should I attempt to adjust the idle at the carb anyway?

I'll check on the idle jet size, and plumbing on the vacuum lines this evening and update the post


Thanks for any and all help!
David Emerson

David,

If the timing was set to original "specifications" it may not be adequate for your car as it is now equiped. The original specs were for the Zenith/Stromberg carb with a catalytic converter and other emissions devices. If I am reading correctly you now have a DGV with a "free-flow" exhaust (Cannon/Peco?)and no cat. Does the car still have the air pump and rail?

A good investment for checking vacuum problems is a vacuum gauge available from local auto parts stores for about $20. A recent post by Daniel Wong on this BBS suggested that you should have 16 to 18 inches of vacuum on the manifold at idle.

I would not rule out the servo yet, but further testing is indicated. Most of us didn't buy our MGs for their powerfull brakes, they will feel spongier than modern cars. You shouldn't notice any fade on a stop from about 60 mph if the brakes are OK.

Doug
D. Cook

This thread was discussed on 27/04/2004

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