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MG MGB Technical - Rough Running 76B

Well summer has been around in Denver for about the past three months. Needless to say it hasn't been great from a running perspective. It seems that as soon as the motor gets warmed up. Temp = N the darn thing idles down to 5000 rpm's. It's almost like it's not getting enough gas or something. One thing to note is that my fuel pump seems to be clicking the whole time I drive when this happens to. Not a costant pumping but you can hear it. Also when I shut off the car and just have the ignition on every once in awhile you will hear the fuel pump clicking away. about every minute or two. I checked for fuel leaks and none to be found.
Josh

Could this be just another ZS problem?
Josh

>>>>>>darn thing idles down to 5000 rpm's
I think this is a typo? If this were true you'd almost be revving the nuts of the poor thing!
Cecil Kimber

yeah sorry 500 rpm. then I have to choke it to keep it going.
Josh

Straight off, it sounds as though you have a vacuum leak since you must choke the carb to keep it running. After looking for obvious disconnected hoses, squirt carb cleaner around the base of mating surfaces such as carb to manifold, manifold to head, hose connections etc. If there is a leak, the RPM will change dramatically, though momenterally as the carb cleaner "fills the vacuum void" before disapating.
Check wiring for shorting and "leakage". A good time is at night. Any breaks in the wire will treat you to a spark show in the darkness. A bad condensor will play games with you as well. Secondary coil wire is also a good suspect. As time goes on, the resistance builds up and reduces current through the dizzy.

These are checks you can perform with out a major outlay of cash. Let us know how you fare with this diagnostics. Best of luck.

cheers,

Gary
gary n. hansen

Josh. In addition to the above, the constant clicking of the fuel pump, assuming it is an SU and not a Facet, would indicate a problem in the fuel system. This could be either a problem in the fuel pump itself, a vacuum leak in the line going from the tank to the fuel pump or a clogged fuel pick up within the tank. To me, the main clue (clew in Brit speak, which we all should try to learn) is the fact that the fuel pump keeps clicking. An SU fuel pump will not do this unless there is a problem somewhere in the system. The fact that you are not seeing any fuel spilled out on the ground indicates, to me, that the problem is probably on the input side of the pump (fuel tank, fuel pick up, hose to pump, pump itself). Les
Les Bengtson

One thing to add on the fuel pump is that it doesn't do it all the time it just seemed to be doing it the other day after I was having the problems idleing. I started it up today it did the usual and clicked a few times and that was it.
Josh

Josh,
You might want to check if it drops to low revs only when the electric fan(s) are doing their job. The drain of electric when the fan motors kick in will cause a drop in ídle. This combined with interior fan, radio etc. might be the cause. I have the same problem and haven't found a solution yet. maybe low idle to begin with?
Carl

An SU pump only clicking 'every minute or two' with the ignition on but the engine stopped is fine. They should not click more frequently than once every 30 secs. If they do a fuel leak i.e. non-seating float valves, or the valves in the pump not seating, is indicated.

Do we know what carb(s) we are talking about? A manual choke is mentioned. SU chokes increase the idle speed, maybe the idle just needs to be set when the motor is fully warmed up and the choke is pushed home.
Paul Hunt

Hi Josh,
My '79B has been doing the very same thing these past two months. I think I may have figured out the cause last night. Do you have the "Heat shield" in place. (The one between the Cat converter and the carb.)Mine was falling apart so I removed it last fall when I rebuilt my ZS carb. Yesterday afternoon on my way home from work (outside temp 98 F.) the idle was dropping way down, so much that I had to "Heel/Toe" it when I came to stops. When I got home I popped the bonnet and felt the carb it was too hot to touch comfortably.

This could be your problem also, since you mentioned it only started happening this summer and only after the car has warmed up fully.

Does anyone know if these heat shields are still availabe. They are made of asbestos so I am thinking they are not made any longer. Is there a suitable repalcement for them?

John Fraioli '79B Denver Colorado
John F

To answer a few questions at a time. This is a ZS 175 with a manual choke conversion kit on it. It still has the heat shield on it but it most certainly was to hot to touch when I get done driving.
Josh

Another thing to check on the Z-S carb is if the metering
needle has popped out of the spring-loaded holder. If this
happens the carb will be much to lean, and can't be adjusted.

Also, if the needle is turned too far counter-clockwise, it
will pop out of the adjuster and can't be screwed back up --
hence also too lean.

I tend to go with Gary's air leak suggestion however. There
are so many places (EGR valve, Gulp valve, manifold gasket,
etc.) for a leak to occur that you're guaranteed to get one
every six months at least.
Ronald

John F,

Moss has a replacement heat shield of non-asbestos material. I used it when I rebuilt the engine in my 77 3 years ago, it works very well. I drive from San Diego to 29 Palms a couple times a month w/o vapor lock or any other issues. Go to http://www.mossmotors.com, click "MGB" and then click "Engine", it's under "Z-S Manifold, Air Cleaner, pc 113, part number 373-945.
Paul K

Thanks Paul,
I will add that to my shopping list for my next order,
(unless I can find something cheaper that I can make shift).

Running with out the shield sure can't be helping my low rough idleing condition, especially since I replaced my gutted out Cat. last fall with a new one.
John
John F

This thread was discussed between 16/07/2002 and 18/07/2002

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