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MG MGB Technical - MGB Missfire

My 1970 BGT, 50,641 miles from new, misfires under load at 2,500 revs in all gears. I have changed the whole distributor and leads. Rebuilt the carbs and checked the fuel pump for flow rate. It appears to be fuel starvation. Any ideas???
Bruce Mansell

Did you check or replace the ignition coil?
John H

Yes I changed the ignition coil
Gary Kinslow '69B roadster Pale Primrose

The compressions are 170 psi per cylinder
Gary Kinslow '69B roadster Pale Primrose

I'd recommend much more diagnosis, changing the distributor, leads, coil etc. - if it was in a vain effort to fix this problem - is expensive and frustrating.

What makes you say fuel starvation? What is the tach doing when the misfire starts? What happens if you pull the choke? Does it happen just revving the engine? Light loading? Or only heavy loading? Did this start happening suddenly by itself? Gradually over time? Only after doing something like rebuilding the carbs? You mention things like this but unless we know the order of things happening, and what the reult of your fuel delivery tests were, we can't target the response. What *is* the pump delivery? Should deliver *at least* one Imperial pint per minute with a pipe removed from a carb and directed into a container, with minimal bubbles.

Paul Hunt 2

One thought... the ignition goes through the Tachometer, so don't discount that connection when you are trouble shooting.

warmly,
dave
Dave Braun

Hi Paul and dave
The tach is steady when the car misfires. It sounds like a Suburu Imprezza when misfiring. Pulling out the choke makes no difference. I can free rev the engine to about 4,500 revs before you get a bit of a misfire. It started happening after the car was left for 12 months without starting and there was unleaded fuel with additive in the tank. The carbs were rebuilt after the trouble started. The fuel delivery test delivered over a pint a fuel a minute with no bubbles.It also happens under light load and in all circumstances you cannot get through the problem
Many thanks Bruce
Bruce Mansell

Paul & Dave
I have also changed the head
Regards
Bruce
Bruce Mansell

You mention thast you changed the distributor - why not look at the distributor that was on the car - verify that it is the correcto ne for the car and check the condition of the advance plates - if you pull the plate out of the distributor, they should be even to each other - not cocked at an angle - in other words the top plate should be parallel (sp) to the bottom plate.

I think you problem is with the vacuum advance or the centrifigal advance. Check both distributors and make sure you have the correct advance unit and correct advance cam for your engine.


Jeff
J Delk

The trouble starting after the 12 month sit and unleaded fuel conditioner in the tank has to be a conincidence.

It sounds most likely to be electrical. I agree with the advance questions Jeff raises. Furthermore, a miss won't necessarily show up on the tach; but are the two white wires securely fastened to the tach?

Have you put a timing light on the wires to see if you can identify which cylinder(s) is(are) missing?

Valves set carefully at (typ.) 15 thou?

dave
Dave Braun

Hi Paul and Dave
The distributor that was used first was a new one fitted with optronic ignition with no vacuum pipe. The second was the original with points and vacuum pipe.
I have tried a timing light on all cylinders but the light is constant for all. I will check the tach wires and the tappets are set to 15 thou.
I have a new 123 Electronic Ignition distributor to fit together with new engine breather pipes and rubber fuel pipes. I will let you know the results.
Thanks again for your help
Regards
Bruce
Bruce Mansell

Bruce: Not to come across like a moron, but reading through this thread, no where is it mentioned about the spark plugs. Try a fresh set. I am a little suspect of your fuel also. Try feeding fresh fuel remotely and see if that changes anything.

Let us know how things progress with your problem solving.

cheers

Gary :>{D
79 mgb
gnhansen

If the tach is steady when this misfire occurs you can forget the low-tension side of the circuit i.e. points etc. in the distributor. Must be HT either cap, rotor, leads or plugs, or fuel. My first test with any misfire or non-starter is always to connect a timing light (inductive pickup type) to the coil lead and plug leads and watch the flashes as it is the quickest and easiest thing to do (after looking at the tach for a misfire). If the flashes get irregular with the misfire on any of the leads then it is an HT problem. If the plug leads are erratic but the coil lead is OK it is the cap or rotor. If the coil lead is erratic it is the coil. If the flashes on all HT leads are steady then it could still be plugs (change them, even if new, or refit the old ones if you only changed them as a matter of course), or fuel. With that delivery from the pump it isn't fuel starvation from that source, but could be a carb problem. Get the misfire happening and turn off the ignition. If you are running come to a halt, then turn on the ignition again and if the pump chatters away it is one of the float valves sticking closed or something similar stopping fuel filling the chamber. Do the same test again but don't turn on the ignition when stopped, remove the float chamber lids instead and see if one or other of the fuel levels is low. If no pump chatter then it could still be a blockage in the jet feed pipe from the bottom of the float chamber, the sealing washers can break up and bits block the jet pipe. Remember the float chambers are full of fuel if removing the jet pipe in-situ, and blow air through from the jet end.
Paul Hunt 2

Bruce - I would use the standard dist on the car and hook up your timing light and check that your ignition is advancing according to spec for your engine. It may be that your advance is off at the higher rpms. I couldnt tell from your message if you had checked the advance or just checked for tyhe spark on the various wires. My vote is still for an advance problem -

Good luck with finding the problem - these things will cause you to chase your tail for weeks sometimes,

Jeff
J Delk

I've changed the distributor today but as yet have not tried the car out due to the rain. What I did notice was a radio suppressor attached to the points side of the coil which I have now removed. Could this be the problem. Let's hope so.
Bruce
Bruce Mansell

I doubt it, the only thing it could do is go short-circuit, which would fry the white wire.
Paul Hunt 2

Maybe a long shot but I want to share this with you,...
Your problem seems to be te same I had early this year, looking at all sorts of possible and maybe possible causes for this misfiring? In the end it where a critical low fuellevel in the floatbowl of the carbs. I had giving them a good maintenance that winter before and acidently?? chanced the level setting of both carbs. after resetting the floatlevel se performed fine again.

greeting Jacob
Jacob Halma

I have changed to the 123 distributor and also changed the flexible fuel pipes under the bonnet together with the engine breather pipes. I have also removed the radio suppressor from the coil. The revs have increased to 4500 rpm in 1st gear but the misfire still persists albeit at 3500 rpm in other gears. After 3 miles the engine then went onto 2 cylinders; on investigation I found that the front needle valve in the float chamber had stuck closed. It was a new assembly. I changed the needle valve assembly again and tried again but the misfire was still present. Jacob how do you change the float level on an HS4 when it all appears to be unadjustable (plastic float). Who knows anything about the pointless su fuel pump as I think the problem is in this area?
Regards
Bruce
bruce Mansell

Washers under the valve is the way to adjust float height, but it isn't critical anyway.

Did you do the fuel delivery test? At least one Imperial pint per minute at the carbs with negligible bubbles? My V8 came with a 'pointless' (very apt) pump, was fine for quite some time then started short-stroking and causing fuel starvation. Even though it is a rubber bumper and hence the electrical end in the boot and I could *see* it happening nothing I did made any difference, I just had to wait until it decided to start working properly again. After 2 or 3 bouts of this I junked it and fitted a refurbished points type pump and have had no more problems.

Putting the supposed disadvantage of having to do something with the points every 20 years or so but being able to get home by thumping the pump, against not having any points to maintain but being stranded when it fails, the points pump gets my vote every time.
Paul Hunt 2

Thanks Paul
Yes I did the fuel delivery test and got at least a pint in the minute. The flow appeared to slow down a little in the middle of the test! I have a relatively new points SU Pump which I shall fit and try. Was your pointless pump an SU pump?
Regards
Bruce
bruce Mansell

Not an SU but the plastic bodied one (Moprod?) which is sold by certain large parts suppliers. Wouldn't have a pointless SU either.
Paul Hunt 2

Bruce,...
Attached to the float is a brass tab (SU HIF), bending this a little bit will affect the float level. Doing this according the Haynes workshop manual (page 81) will give you a good setting.
Also how to chance or to check a float level setting on a HS4 is explaned in this workshop book (page 76).

Jacob
Jacob Halma

This thread was discussed between 17/10/2006 and 29/10/2006

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