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MG MGB Technical - Random Engine Miss

When I purchased my 72B last May, and prior to working on it at all, the engine had a random miss fire.

While driving along, warmed up OD or without OD - and under steady throttle the engine would miss for a second or so. Most like a hiccup. Not a continuous miss and there was no loud noise like a backfire. Just a simple cough or hiccup - and on it went.

It was more random in the beginning and has progressed to the point that I can anticipate that it will happen sometime on each drive.

Over the past year I've replaced point & cond with the Pentroix (sp?) system & replaced cap and rotor at same time, did a valve job this spring, replaced melted white wires under dash that ran from coil to tach last year, installed fuel filter before fuel pump and I've changed this recently too - and other numerous non-engine things.

One cannot force the cough or hiccup to happen. It can happen just cruising along at city speeds of 20-30 mph at 2000-3000 rpm or on the highway speeds of 50-65 mph at 2500-3500 rpm.

I can go up steep hills, in 4th gear at 25mph or so and accelerate without a miss or hesitation; so it takes a load well.

Occassionally the miss is harder, like a real drop in power and then lunges (sp?) back to performance. The miss is definetely physically noticeable.

I do not know how old the coil is nor the spark plug wires. The car does have an after market fuel pump (brass block) mounted by the heater motor, but not sure how old it is. I will be moving this to near battery as I get vapor lock symptoms when shutting it off when hot.

The car starts perfect. But may hiccup occassionally when cold but this goes away with a little pull on the choke. It may be me but this cold run hiccup doesn't seem the same as the driving along hiccup.

I am anticipating that now that it happens each time I drive it, instead of just once in a while; that I may find out what the problem is while setting on the side of the road.

Any ideas on what is wrong and what additional parts I should start storing in the trunk?

R.W Anderson

Small miss is usually caused by too lean mixture. I think ignition timing can also cause this(too retarded?). If there is a large miss where the engine lunges back, then you might have a fuel delivery problem.
willieL

had this happen to mine recently. Noticed the tach dropped each time. Found out electrical connections at the starter had worked loose.
kra Karl

Cutting out then restarting *without* a backfire ever does imply a fuel problem rather than ignition. However with the 'reservoir' of fuel in the float chambers if the pump stops you wouldn't get an instantaneous loss of power, more like a fade out as the chamber empty and the mixture changes, and a fade back in again as the pump cuts back in. Also, in a less than perfect world, one carb is likely to empty before the other which will give a stuttering before complete power loss when the second carb empties. However the pump could be stopping and starting all the time, but if it is stopped for less time than it takes to significantly lower the fuel levels in the float chamber you wouldn't even notice it.

Check fuel delivery by removing a pipe from a carb and directing it into a container and turning on the ignition. It should deliver *at least* one Imperial pint per minute, in a steady and consistent stream with negligible bubbles. In practice a good SU pump should deliver nearer to two Imperial pints per minute. If the pump *is* stopping and starting too quickly to always be noticed when driving, it shouldn't take too long to see it happen in this test. But it still could take some time and some quantity before it happens.

But the symptoms do point more to an ignition problem. If there is a problem in the ignition LT circuit this *will* show up on the tach, but an HT problem won't. However HT problems are more likely to show up under load like acceleration and climbing hills than cruising on the flat, and are usually accompanied by a backfire on restart when the unburnt fuel in the exhaust ignites.

As far as the other symptoms go, what is the general state of tune of the ignition and fuel systems?
Paul Hunt 2

I've since run my problem past some local MG shop mechanics and they are "leaning" towards electrical. I have not thought to watch the tach when the hiccups happen, but will try.

As far as "state of tune" - I think the car starts fine. It does take pulling out the choke 1/2 or more and then it fires up immediately, very seldom does it require a 2-5 sec crank to start. Of course choke must stay on as I slowly drive off. If I try driving off immediately it will require more choke or it will actually backfire or pop through carbs, at least it sounds like it is an up front pop, not a tailpipe pop.

I also think it is getting worse, as a couple days ago it wasn't the usual hard miss/hiccup it was a series of hiccups that can shake the car. Not like a simple running out of fuel and slowing down or an electrical short turning things off and the on again. It is similar in feel to hitting small bumps in the road (no I'm running over things!) - a definite shaking of car.

Tonight I may change timing back to standard 10 degrees or so, as a month or more ago I tried that recommended timing at 4000 rpm or so which resulted in an at idle timing of 16 degrees or so. I may also turn carbs back rich 1 flat, as I leaned them 1 flat several weeks ago.

Although some tech tips suggest the timing may still be too retarded, not too advanced. I do have plenty of steep hills in town to keep advancing timing, drive up a steep hill, etc., etc., until it pings and retard back a tad.

But this may only alter general performance, not correct a random hiccup. I'm on my 3rd fuel filter, (trying out different locations - not because they were dirty) so I don't think its a dirt thing, but I'll take tops of float bowls again just to check.

But this hiccup has been there since I bought car, regardless of all the repairs I've done.

I have a feeling if I keep driving it locally, (too nervous to go too far) something is eventually going to get so bad it will be obvious what the problem is.

Thanks for assistance so far.

BobA
R.W Anderson

Bob,

My bet is on the grounding wire of the groundplate in the ignition.

Because of the constant moving groundplate it will break in time.

Do use a flexible wire to replace it (if it is faulty)
Willem van der Veer

Mixture or timing on their own are very unlikely to cause the intermittent cutting-out you are getting, they regularly affect running under specific and reproducible conditions. The wire Willem mentions will show up on the tach, as will the 'points' wires where they go through the distributor body, although that is more likely to affect older points wiring rather then new pertronix.
Paul Hunt 2

Are you running with non standard air filters? SOunds like it's goin weak on a trailing throttle, but if you on pancake or K & N's filters you will have to put richer needles in. Let me know.
Allan Reeling

Since nobody has mentioned it I would suggest you check the carb pistons for free movement by lifting the pistons, checking for resistence and binding. This was the cause of a recent problem that I encountered. Ray
RAY

Sticking pistons won't cause an erratic missfire under steady throttle conditions as they aren't being asked to move anyway.
Paul Hunt 2

I had a similar problem, turned out to be the ignition switch.
Neil
Neil22

This will also show on the tach, and additionally as the ignition warning light coming on. The warning light is the way of diagnosing the difference between loss of power to *all* the ignition circuits, and a break in the coil/points circuit.
Paul Hunt 2

Low piston damper oil will allow the piston to rise rapidly giving a very lean mixture. This, in turn, will produce a loss in power momentarally. Ray
RAY

Pistons rising too quickly will indeed cause hesitation upon rapid opening of the throttle, but it will not happen at steady state as Paul says and it doesn't feel like a sudden drop in power, more like a failure to respond to the throttle. Sticking pistons restrict air and fuel, so much the same effect. Fuel starvation generally (so far as it is not TOTAL) has a similar feel, the engine goes flat.

Try connecting a meter across the ignition switch and jiggling the key to see if there is a change in resistance.

Neil
Neil22


I had the same annoying proplem it was driving me crazy. I had gone thru the ignition and carbs twice. finially I removed the intake manifold and could see the gasket was going bad. Since I had replaced it not that long ago when the head was redone I had the intake checked for warpage and found it several thousands off. A machine shop took care of that for me and it has been fine ever since.
Charles O'Brien

A fellow MG club member informed me lat night that on his drive through Wisconsin this last weekend, that he too encountered the "famed" hiccup or irratic loss of power. He however noticed the tach dropping in sync with the hiccup. He pulled off the road and started checking wires. He found that wire "inside" the connection at the tach was the problem. Fixed wire, and hiccup was fixed. I may just run a new wire and new ends to remove this possibility.

But this repair will have to wait as I'm off to Montana (Bitterroot) by plane to hike for a week - with hopefully no car repairs to be had.

This is a bit optomistic though, as I may be borrowing a friends Jaguar to get around in. He used to have 2 and at least 1 usually ran - now he only has 1, so I may be hiking on the highways as well as in the hills.

Thanks for all the discussion. I was amazed at how this discussion picked up life.

BobA
R.W Anderson

This thread was discussed between 04/09/2007 and 19/09/2007

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