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MG MGB Technical - What is Wrong?

My car will not start. The ignition system is working and fuel is reaching the sparkplugs. It was running and then it just wouldn't start. It turns over but does not even attempt to start. Any ideas? Please help!
Layne

Layne-
If fuel is getting to the carburetors, then it's probably the ignition. Check to be sure that all of your connections at the distributor and coil are both clean and tight.
Steve S.

Same thing happened to me after I adjusted my points today. THe wife and I got in the car and it just turned and turned. Then I remembered I left the distro cap off. :/

Maybe I shouldn't admit that in this crowd. :(
Steve Simmons

Layne, You say the ignition system is working and fuel is reaching the plugs. To help us help you would you mind telling us how you have determined the the ignition is working? Any additional details such as year of car and does it have electronic ignition or points will help. With the information you have provided anything we say is only a wild guess. Thanks, Clifton
Clifton Gordon

Wet, "fuely" sparkplugs means that the plugs aren't
firing.

Some Lucas distributor rotors have been known to
develope an internal short from time-to-time. You
cannot determine this by just looking at the parts.

Try switching to another distributor rotor & cap.

Recheck the center distributor wire.
Daniel Wong

Once on a trip to Blackhawk raceway in Wisconsin, the TR3 I was riding in, died on the freeway. The owner, [since deceased] had spare parts and installed another distributor. It still wouldn't start, until we discovered the rotor sitting on the fender.
Then there was the time I rebuilt a customers '67 'B' engine. On the breakin/road test, it too died.
Oh no, what could I done wrong.
[I don't know why I always blame myself]
Well, for whatever reason, it was the rotor again. Go figure!
Safety Fast
Dwight
Dwight McCullough

Wet plugs, and a strong smell of fuel around the car, usually means it is flooded, which will happen with long periods of cranking. Once that happens you need to crank with no choke and a wide open throttle to clear it. As Clifton says, in what way is the ignition working? Checking that a spark occurs at each plug is one thing, but is it at the right time i.e. has the distributor slipped? It is also possible for a marginal cap/rotor to break down when faced with trying to spark the plug under compression, which results a higher HT voltage, but not when sparking in free air. A timing light clipped on to each plug lead, most of the 12v type with inductive pick-ups anyway, will only flash if HT is reaching the plug and the plug is not breaking down i.e. the apark *is* jumping the gap in the cylinder. The older series type of timing light will flash even if the plug is breaking down. But four plugs all deciding to break down at the same time is pretty unlikely ... unless you know better, Scully?
Paul Hunt

The car is a '77. It has a point type distributor, which has appearently been changed from electronic. The plugs will fire in the open air, I have not checked them under compression. The spark appears white however instead of blue but is very bright, I'm not sure what that might indicate. The plugs do get covered with fuel after a while of cranking. The coil is new and the plugs are new.
Layne

I finally got the car started by disconecting the crancase vent hose, thus allowing a huge amount of air to bypass the carburettor. I also tied the choke in the off position. It idles pretty good with the hose off though still a little rich, but will not rev up past 1700 rpm. Should I try to adjust the mixture untill it runs with the hose on, or is something else at fault here? I don't understand why it would run and then suddenly wouldn't. I checked the diaphram in the carb and its not torn. Its a zenith single carb.
Layne

Sounds like you're running very rich. Is your float stuck? Does the fuel pump run continuously? What were you doing prior to the aforementioned fault? Just jacking the rear of my car was enough to cause the float to stick the other day.
R. L Carleen

The float was not stuck when I checked it, I'll check it again. The fuel pump seems to run any time the key is on, I assumed the pressure shut it off when the float closed. Is that correct?
Layne

The fuel pump clicking at all times may not be a big issue. Mine has done that for 3 years and 10,000 miles of daily driving. I think mine is an air leak somewhere that I can't seem to locate, but it doesn't leak any fuel nor is there ever a gas smell.

I have an mga with an mgb engine with mga carbs. I know nothing about the single carb set-up, but I don't think many people run them. You sound like a pretty good mechanic and I bet you could install a set of HS4s in 1 day.
Fred H

A leaky non-return valve in the pump can cause continuous clicking. It is likely to reduce fuel delivery rate, but still be within the requirements of the carbs. A leak on the suction side would cause bubbles to be delivered to the carb when pumping, but the pump should still stop with the engine stationary.

If disconnecting the crancase vent hose allowed the car to start it sounds like a problem in the emissions plumbing and/or anti-runon valve, but with it off you will be getting a massive air-leak and weak mixture. As a temporary measure see if the engine runs any better with that hose plugged. If it does then you need to check the various hoses and components for blockages. Remove the petrol filler cap to check you can blow through the tank hose.

If the big port on the bottom of the valve is blocked, either by the valve being operated when it shouldn't be or by debris, it will allow carb vacuum to suck the fuel out of the jet(s) and so prevent running. However that should also stop the plugs getting wet with fuel during cranking.

The valve should only operate with a click when the engine is running and the ignition is switched off. A couple of seconds after the engine has come to a halt it should click again as the valve releases. If it clicks when you turn on the ignition the oil pressure switch is stuck closed, or the grey/yellow to it is shorting to ground.

Locate the pipe that goes from the carb float chamber to the top of the canister and remove it from the canister. With the ignition on and engine stopped does fuel come out of there? If so the float valve is not shutting off the fuel supply hence the continuous clicking.

If that is OK have a look at the jet (easy on an SU with the air filter off, dunno about a Zenith) and see if you have fuel pumping up out of there with the ignition on. If so, remove the aforementioned tube from the carb and if the pumping then stops that pipe is blocked.
Paul Hunt

The car is running almost right now. Here's what happened: It flooded and wouldn't start, I took the top off the carb to check the diaphram and put the air piston back with the two holes in the bottom facing away from the manifold. This meant the air piston could not open, which caused excessive flooding until I removed the crancase hose and restricted the RPM after I removed the hose. After I discovered this I put the piston in with both holes facing the manifold and it flooded so now one hole is facing the manifold and the other is off to the side a little. I need to know exactly how the piston is supposed to be oriented as I can't find anything to positively locate it and the Hayne's manual is no help. The top on the carb was also turned 90 degrees from where it belongs but I don't think that affected it. I may consider the dual carb conversion but new ones are way too expensive, what would a used set cost me? Is there any difference in gas milage between the single and dual setups?
Layne

The carbs are made so that the cover appears to be 90 degrees off relative to the other. This prevents the mis-match of Forward and Rear.
Dan

Dan, I only have one carb. Somebody please tell me which way the air piston needs to be facing!
Layne

This thread was discussed between 04/08/2003 and 15/08/2003

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