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MG MGA - Diagnosis HELP!!!!

Hi folks. I'm trying to diagnosis an ongoing issue.

I have been experiencing intermittent backfiring through the carburators.

It seems to happen for no reason and stops for no reason.

I'll be pulling along and pop, pop I let off gas pedal and it tends to disappear. It can reappear again or it can just go on. Associated with this time period is a noticable drop in power.

Once when this occured I pulled off and car dieseled pretty noticably which it has never done before or since.

Otherwise the car runs like a top and idles fine. I've checked the fuel flow and it spits out well more than 1 quart in a minute.

It has electronic ignition that displays no other problems.

Do you think this is an intermittent valve issue?

If so how do I test this. If not what else could it be?
T McCarthy

Are you overheating? If so it could be a valve is sticking from getting too hot, high head temps can also cause dieseling. An intermittant lean condition can also cause what you are seeing, check for stickey float valves gunk in the float bowls, and vacuum leaks in the intake system. Ignition system failure when hot can also be a common problem due to faulty coil, condensor, or electronic ignition. The problem with electronic ignition is that a fault of this type can be very hard to find as it won't die and stay dead.The only way to check it is with a new unit or put your old points back in for a test
John H

John,

Yes it tends to do this when warmed, however the car runs a steady 185 degrees and will backfire when tooling along and air is flowing through the engine bay presumably keeping it cool.
T McCarthy

T. Backfiring through the carbs is caused by an intake valve being open (pathway from the carb, through the intake manifold, through the cylinder head passage, through the open valve, to the cylinder), or a valve/valve seat is damaged, or, something is causing one or more valves not to fully seat when the cylinder is being fired.

First thing to run is a warm engine compression check. Instructions for the basic compression check can be found in the tech articles on my website, www.custompistols.com/ under the MG section. This will tell us if there is a basic problem with the valves and valve seats.

Next thing to do is a running compression check, if you have one of the screw in type compression checkers. Disconnect one spark plug lead, pull the spark plug, install the compression checker, have an assistant start the engine, then, read the compression. Should be between 80 and 90 psi. Release the pressure through the valve on the gauge and let it pressurize again. Should still be between 80-90 psi. Do this for each cylinder, replacing the spark plug and wire on the last cylinder checked and and removing the spark plug and wire on the cylinder to be checked. Keep track of your readings. If there is a problem with the intake valve sticking, it should be noted by the running compression check in two ways. First, the running compression will be significantly lower; second, if the backfire is common, it will go away when you check the cylinder which is causing the back fire.

If the compression checks do not identify a problem, check out the ignition system. There is an article on testing the spark plug wires on my website. If you have a cap problem, or wires that have weak insulation and cross each other, two cylinders may be firing at the same time, the primary cylinder (i.e. the one intended to receive the spark) and another cylinder which is being fired on the intake stroke or early in the compression stroke before the intake valve for that cylinder has fully closed.

The last possible cause is the timing is inconsistent and one cylinder is being fired too early, before the intake valve is fully closed. The timing for cylinders one and four can be checked by simply hooking up a timing light and observing where the light is flashing. Timing for cylinders two and three is observed by making a temporary timing mark 180 degrees opposite the factory mark and using it to observe when those cylinders are firing.

One of these tests should help you establish what the problem is.

Les
Les Bengtson

Perfect.

Thanks as always Les!

I'll report back

Tysen
T McCarthy

I have had this experience for two different reasons. Once was when the engine had a very bad case of valve seat recession, and the rocker arm clearance kept going away preventing an intake valve from closing completely. When it was first apparent backfiring through the carburetor would occur under high load conditions, fast running or pulling up a long hill. The increased heat in the head would cause some thermal expansion of the valve parts to aggravate the situation.

The other time was with a freshly rebuild head having bronze valve guides with insufficient valve stem clearance. Heat from hard running under heavy load would expand the guide wall thickness to constrict on the valve stem, causing the valve to hang open. This was more common on the exhaust valves but could happen occasionally with an intake valve.
Barney Gaylord

Another clue?

I drove the car to work today without a single problem. The ONLY difference I can think of is that its 10 degrees cooler today and 20% less humid.
T McCarthy

Tysen,
You might try taking a look at the distributor cap for signs of arcing (carbon tracks). Cross firing has been known to occurr because of a faulty distributor cap.
Good luck,
GTF
G. Foster

This thread was discussed between 25/06/2007 and 28/06/2007

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This thread is from the archive. The Live MG MGA BBS is active now.