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MG MGA - Sudden Engine Roughness

While driving at a sustained 50 mph or so, the car suddenly developed a running roughness. The standard smooth exhaust noise turned into a distinct "PRRRRRRRBT" noise and there was a very noticable Ticking coming from the engine.

I pulled into a gas station. I checked the oil (just 'cause)which was fine. I also pulled the distributor cap and cleaned it a little.

I started it back up and it was still running the same, I decided to limp it back to the house as I had my 11 y.o. neice with me. While driving it suddenly cleared up.

This has happened once before while driving at speed on the highway, it also stopped after about 3 minutes.

I'm focusing on the ingition system as the culprit.

Does this sound like anything you all have experienced before?
Tysen

Dirt in the fuel line possibly?

Or a vapour lock? Others may have more detailed ideas.

Richard.
Richard Ross

Thanks for the thought.

I've focused away from the fuel system. The car never lost speed nor died from the issue (though it did lose some acceleration)and changes to gas pedal and the choke didn't change how the engine was running. It just ran rough up and down the acceleration curve.....
Tysen

Dizzy loose?
JohnB

Tysen

My ten pennyworth is that you are on the right track. 90% of these type of problems have for me been electrical even though I often try to convince myself that 'it must be fuel'.

My betting is that one of your plugs stopped firing for a while - running on 3. Intermittent faults can be a b****r to find. Contact breaker gap closing up would rate high on my list and is a fairly easy one to check out.

Steve
Steve Gyles

Hmmmm thought so.

Prolly time to convert to electronic ignition.
Tysen

With the nasty exhaust noise, I guess a sticky exhaust valve not closing completely when the combustion chamber gets hot. Most likely to happen during a high speed run, or prolonged uphill with extra throttle.

Is this a relatively fresh rebuilt engine with new valve guides? Bronze guides maybe?
Barney Gaylord

Barney,

Thanks for weighing in. No its not a fresh rebuild, it was rebuilt about 8 years ago with 6,000 miles or so since.

Any way I can check out the sticky valve theory?
Tysen

Tysen. You mention taking the car out for a drive, developing the problem, shutting down the engine, restarting the engine and the problem was still there. Believe it or not, this is a good thing--from the troubleshooting perspective. It means you can stop the engine, hook up one of the screw in style compression testers and do a "running compression check" of the engine.

There is a basic article on the compression check on my website at www.custompistols.com/ which might be of general interest. I have not yet put together an article on the running compression check (maybe Barney has?) as I am still researching the subject.

The basic test, however, is easy to describe and would suit your purposes quite well. Recruit an assistant (your niece perhaps?) and take the car out for a drive until the symptoms develop. Pull into a parking lot, or other safe area to perform the test. Put the transmission into neutral and the handbrake on. Pull the number one spark plug and install the line to the compression tester. Have your assistant start the engine. Read the initial reading on the tester, release pressure and read again. Write down your readings as you do this check on each cylinder. All of the running compression readings should be about 90 PSI, considerably lower than what you get on a "standard" compression check. If you have a stuck valve, the running compression reading for that cylinder will be significantly lower than it will be for a fully functional cylinder.

On the ignition side, check the wire that grounds the base plate to the distributor body. If it is beginning to go bad, you will have symptoms similar to what you describe.

Les
Les Bengtson

You could try cleaning your ignition wires by wiping down with a rag wetted with a bit of WD-40, and check the connections to all the plugs at the plug caps.

Check the plugs themselves, is one a different color or appearance than the others?

As with what Les says, someone told me that if you drive at highway speeds for a while, then cut the engine and coast to the side of the road, then get out and pull the plugs to check for color, you will get a better idea of what's going on when the engine is running under power (rather than just a problem with the idle settings, for ex.). You could do this whether or not it is developing the problem at the time (which may never happen again, for all we know).

(As far as a sticky valve, on my bikes I have always added a couple of capfuls of MMO to the oil to help with the top end -- just because. Same with the lawnmowers. Couldn't hurt here.)

Sorry if this isn't on the money -- just thinking out loud.
AJ Mail

Had a similar problem just after purchased my 60 roadster that had not run in some time. Check for contamination in float bowls. Replace fuel filter as well. Good luck! I'm still chasing my gremlin (Motor Stuble)
skip

It's very common for the distributor hold down bolt to loosen. Before you do anything else, grab the dizzy and try to turn it.
Fred H

I had a similar experience due to a condenser that began to fail as it got hot. If the car starts to do this more frequently, suggect that you change the condenser.

Ira
Ira Spector (PA)

I can't believe what I'm reading.

A group of us went to Gatlinbujrg. On the way home, in that dreadful heat, the engine of a TF started to splutter.

On the road, we changed the fuel pump for a new one; we changed all four spark plugs for 4 new ones; we installed a complete, brand new pertronix; we changed the cap with wires; rotor; a brand new coil; we shorted the ignition directly to the battery - ditto the old and the new fuel pump(s). Wasn't dirty gas, as others inclufing my TF also filled at the same time.

The only thing we didn't change was the complete distributor (we had one), because we didn't have a timing light.

We came PREPARED but it didn't matter didly-squat, because it didn't make any difference. With new plugs, the engine would run for a minute (or less) and then 2 plugs would foul to black.

We adjusted the SUs, 'till we were blue in the face.

On a cold engine, it starts right up (with clean plugs) and runs cleanly for a few minutes, then it starts all over again.

The symptons you have described, exactly describe out problem, but you've (all, collectively) provoked some excellent thoughts particularly a stickey valve. Thanks Barney for that one!

To make a long story short, the TF came home (500 miles) on a tow bar.

I'll keep the BB folks informed. We may all benefit from this - whatever it is!

Gord Clark
Rockburn, Qué.
Gordon A. Clark

Same thing happened to me last week. I finally got some time yesterday afternoon to take a good look at it.

I started the engine and then pulled one plug wire at a time. Pulling #3 resulted in no drop in engine rpm. I suspected a burned exhaust valve. Compression check showed equal readings in each cylinder (160 - I suspect my gage is out of cal). I swapped the #3 and #4 spark plugs and then the problem moved to #4 cylinder when the plug wires were pulled. I replaced all 4 plugs (saving the good 3 for an emergency) and the car ran beautifully.

I believe that I caused the premature failure of the plug by periodically pulling and cleaning them. Probably cracked the plug during reinstallation. Leasson learned - "if it ain't broke...".

Don Carlberg
D. R. Carlberg

Gordon'' I bet you a fresh snowball that you have either a sunken float or a poor float valve. When two plugs foul out like you described it can only be attributed to surplus fuel.
Sandy
Sandy Sanders

One more thought to throw into the mix - I had this same problem on a 1982 Honda Accord -- turned out one of the plugs has a piece of carbonised grit stuck in the gap.
Mark Lambert

Hey Mark - Saw your beautiful A at Palo Alto againg this year. Did you win the class (again)?!
skip

This thread was discussed between 26/06/2006 and 16/07/2006

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