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MG MGB Technical - Why does the RPM's drop!

I have checked the archives. I tried the suggestions, but I still on occassion have the same problem. Once my car is at normal operating I can drive the car stop for traffic and the idle is perfect. I can drive futher and stop at traffic, and the rpm's just drop off. I always catch it by pushing the throttle and bring the idle back up. I can drive again and stop for traffic and no problem at idle. It seems like this happens whenever it wants to or else it works just fine. When I pull the car into the garage it always idles like normal.

There has to be a reasonable answer as to why it only does this when it wants to. It does happen every time I take out the car and on a 30 mile trip this will happen about 3 times at idle and all the other times at idle it is ok.

My car is total rebuilt with all new plugs, wires, coil, ignition, fuel pump, gas tank, evaporator cannister, and everything else is new.

Any other ideas I can check for.

Ray. Help on this one.
Ray 1977mgb

check the vac. line to your dizzy and the dizzy itself.
May be you should have a look upon Mat Kimmins site and think upon EDIS ignition. The dizzy you are running on your S/C car is fine for normal use but does not realy meet the demand of the engine you built.


Vacuum line is great and so is the dizzy. I can run 0-70mph in 12.5 second with no hesitation. Even after a fast run my idle will stay at 950 rpm. Than if I drive a little hit a stop and idle the rpms will just sudenly drop. Why? Than I can drive some more and stop and the idle stays strong.

Not ready for the EDIS yet. Like to solve problems first before getting into new ones.

Ray 1977mgb

Same problem (sort off) came up with mine.

Fast uneven idle, but a blip would get it back to a normal idle.

The butterflys were not centred properly on the shafts, and one was jamming just before the idle position.

ian F
Ian Fraser

Ray - it may depend on what you mean by "drop". Mine tend to wander as well. Sometimes it's lower when the clutch is in, as opposed to in neutral. Posters on this BBS say that it's due to drag within the clutch mechanism- part of that slow slide to eventual replacement. I notice that how hot the car may be will affect idle speed as well. Too hot will drop it as well. I set my idle higher than most posters - about 1000 rpm. When it drags, it'll drop to about 800. If you set it at 750-800, when it drops you will in danger of stalling.

I can't say why it drops specifically - but I do know why modern cars have smooth idles - they're programmed that way. In order to meet emissions regs, the car MUST operate at certain parameters. Modern systems in fuel injection and ignition timing are co-ordinated to operate everyhting in a certain manner. Carburetors are in many ways better ways of operating car motors as car motors - but not necessarily as emission control devices. Due to a myriad of reasons your idle may wander, and I'm sure you will find the reason(s) and deal with them. But if you're comparing our cars (even yours - is this the supercharged one?) to modern cars, ours will tend to be analog in a digital world.

An explanation, perhaps, just not much help.
John Z


I will check your butterfly theory soon.

Just to be on the safe side I had an extra fuel pump so I changed that tonight. It made a slight difference, but I still get that drop.

John, My idle is at 1000 rpm also. With the lights on, radio on, and fan on it drops to 950. Not bad. When I put the clucth in will drop down to 900 at idle and than pick back up and stay at 950. Again this is what I want.

Than for no reason at all I will come to a stop and the idle will drop as low as 600 rpm. My car does not stall it just drops lower than usuall, than the next stop it will be ok again. More like a back and forth affect.

There has to be something else doing this and I will find out sooner or later but there are some who may have had the same problem and found a fix.

If not than I will keep playing with other alternatives until I pin point this drop. It would be nice to have this same 1000 rpm idle and drop only to 900 with everything on and the clutch in consistantly or am I asking to much.

Ray 1977mgb


The HIF 44 has a bimetallic spring for hight adjustment of the jet. May be some kind of better heat-shilding of the float chamber might work.
Wrapping the header helps a lot. I tried this on my GT V8 and it works well.

I have fabricated special stainless heat shields for the twin HIF 6 in my ST stage 6 roadster and it helps to protect from the heatemission of the exhaust header, although not really cures this problem with the ideling when hot.

You should also check whether the drop of rpm is incororated in the cooling sequence of the electric fan, as ideling rpm drops when it is swiched on.
I made this experience with my roadster, fitted with an 55 amps alternater and the largest battey that fits the 1975 battery compartment.


Ray, You might try seeing if your carb piston is hanging up when coming to an idle as this can cause a situation like this. Ray '67 SC Roadster

On a supercharged engine such as yours all the fuel has to go through an egg beater and then around some corners and up to the cylinder head.

A hot system could cause the air to expand and there be less energy getting into the engine when the throttle is closed.

Another possibility could be that some of the fuel is condensing out in low flow conditions and the lean mixture is slowing your idle.

Under load the supercharger will compress air and get hot. Under light load/partial vacuum, the supercharger could be below ambient air temperature - the fuel could partially condense to a liquid in the blower.

It's probably a problem with your throttle linkage.


Barry Parkinson

Ralph, Ray and Barry,

Some very good points to look at. I have the Moss Heat shield and this works really good in keeping the carburetor cooler.

I do need to check the piston and see if it is hanging and also the butterflys. I had it off the other day and all seemed to work properly but I should check it with the engine running by taking off the air filter.

Barry had me thinking about the ambient temperature. With Samoa always in the 90's year round I just might need to make a air box to get a little cooler air from outside rather than inside the engine bay. I never use the heater fan so I am seriously thinking of removing it and building the air box there and use the front cowl for the incoming air.

Thanks for the suggestions,

Ray 1977mgb

Check you have the correct free play in the throttle cable. It could be that the idle speed is being maintained by the throttle cable and not the idle screws, then after idling for a while things get hotter and the cable expands, and at the next movement of the throttle the the butterflies are allowed to close a little bit further than before. Could also be sticking cable, butterflies, spindles, pistons, etc.
Paul Hunt 2

This thread was discussed between 14/06/2006 and 16/06/2006

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