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MG MGB Technical - dies without choke

the car is a 74.
the cars starts fine, but when the choke is pushed all the way it stalls and dies. even with the choke on it sounds like it is running rough. If you take the front 2 plug wires off, while the car is running, they show spark but it doesn't sound like the engine changes. If the 3rd plug wire is removed the engine drops. The fuel filter doesn't appear to have a lot of gas in it, but there is a flow.

Any suggestions would be appreciated
rick baranyi

Had a similar issue with my 79. Turned out to be vacuum leaks. Check vacuum hoses, IE: distributor, brake booster, crankcase venting hoses, fuel emission hoses, etc.

Check tightness on the carburetor mounting bolts and I/E manifold to head nuts. Mine have loosened up over time. The biggest culprit was the 4 manifold-to-head nuts.

Let us know how it works out


gary hansen

Good advice from Gary. As these cars get older, the slow running screws on each carb tend to work loose and can slowly unwind, it could be that a simple re-adjustment is needed. A worn dizzi can produce erratic timing and therefore running speed. In some engines the vacuum advance diaphragm is a weakness.
Good luck.
Roger Walker

I took off the hose in front of the front carb and put my finger over the stem from the carb and the idle picked up. The hose is the one that attaches to a metal hose,runs over the air cleaner and reattached to the rear carb
rick baranyi

Are 2 and 3 reversed? It's not unusual to find one plug lead shorted out doesn't make much difference to the idle speed, but two making no difference would be much more unusual. That's the first thing to resolve.

When were the carbs set up? Could be way out of whack and unbalanced i.e. weak meaning it needs choke to get a usable mixture.

I'm confused a little by your description in your second post. Are your carbs twin HIFs? The only hose between the carbs in that area I'm aware of is the crankcase ventilation system, which comes off a port near the flange that bolts to the manifold on each carb. There should be a Y-piece that interconnects the hoses from both carbs - no metal pipe, short hose from the front carb and long from the rear - to the front tappet chest cover. That interconnecting hose also runs over manifold side of the carbs, not the air-cleaners. Pictures?

If that port is open to atmosphere it does noticeably weaken the mixture and lower the idle, so if removing the hose and blocking the carb port causes the idle to pick up compared to with the hose connected, the implication is that that hose is open to atmosphere somewhere.

Paul Hunt

Paul you have the right part. Any idea where the leak could be
rick baranyi

Anywhere from the carb ports back. I.e. the hoses to the Y-piece, cracked Y-piece, hose to tappet chest cover, cracked tappet chest cover, or anything letting air into the rocker cover or crankcase that shouldn't be.

As a 74 you should have a sealed oil filler cap, if that is vented or its seal is faulty it will weaken the mixture.

The rocker cover should have a port on the back connected to the charcoal cannister, and hoses from that go to the carb overflow/vent ports and tank. If any of those hoses have a leak, or the gaskets for the rocker and tappet covers are leaking, they will let air in. The tank hose may go via a separator, with another hose to the tank itself, so those can cause it as well. The fuel filler cap should be sealed as well, so if that is vented or the seal is leaking, or the top of the tank is rusted out, that will weaken the mixture.

It's a matter of disconnecting those hoses in various places, in a logical and progressive sequence, and blocking the suction side as you did with the carb ports, to see if that makes the same difference to the idle. If it does then the problem lies further out from the carbs. If it doesn't then you have probably passed the fault i.e. it lies between you and the carbs.

Remember a fault in the hoses to one carb hose will weaken the mixture in both, so you need to check both hoses for both carbs.
Paul Hunt

Thank you Paul
while starting to check for the leaks, the pressure coming out of the fuel filter was checked. it was 2.2. could that be part of the problem?
rick baranyi

No, 2.2psi is fine. However that's not to say that the float valves aren't leaking a bit which will raise the level of fuel in the float chamber and jet, and does affect mixture.

That would normally mean they will eventually overflow, which could take some time depending on the rate of leakage, but with the charcoal cannister it can take a very long time for fuel to drip from the bottom of the cannister, so remove the hoses from the overflow/vent ports first as it will appear there much sooner. One way of checking is to turn on the ignition but don't start the engine, and time how long you get between fuel pump clicks - assuming an SU-style pump. Disconnect the coil during this test to prevent it from overheating. The pump shouldn't click more than once every 30 secs, in practice it should be longer than that, and could be several minutes. Also clicking could be caused by the non-return valve in the pump inlet leaking back slightly, as well as a float valve leaking.

As long as it's more than once every 30 secs any leakage even through the float valves should be absorbed in normal running. I recently discovered mine were leaking very slightly, even though it was a minute or more between clicks. I only discovered it when testing something that needed the ignition to be on for a long time (coil disconnected) and eventually heard the overflow dripping.
Paul Hunt

This thread was discussed between 24/09/2015 and 28/09/2015

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