Triumph Repair: backfire through carb. on 1977 MG Midget

Question
QUESTION: Howard , you were a great help in the past when I had a problem with my TR6. I have a back fire in my 1977 MG Midget( 1500 cc which I believe is the same engine in a Spitfire?) through the carb. under load only and not at idle. Spark is good to all cylinders  I still replaced cap and rotor-it has electronic ignition. Timing set to specs. advance mechanism working established by timing light advancing when throttle up.  Valves adjusted. Compression is 140 in all cylinders. Carb.(Stromberg) rebuilt. New gas filter and pump. New fuel. Vacuum is 13 post butter fly  on carb. Sprayed for vacuum leak  at  top of intake manifold- no signs of leak. Carb. set to almost max rich. One last thing; when you pull spark plug wire off of plug at #1 or #4 the engine will die, but when you pull wire off #2 or #3 you only get a small drop in RPMs? What is up? thanks for your help, Gabe

ANSWER: Hi Gabe,

Your symptoms don't pinpoint the problem but if this car was brought to me in a dealership I would have to do this.

First I would examine the distributor cap very closely with a strong light. Look for carbon trails from the inside posts to other posts. The same on the outside of the cap.

The reason is that #1 nor #4 wire has any method of killing the other 3 cylinders what so ever. Electricity will go the easiest rout and when you remove the plug wire of #1 you force the current to look for another path to get to ground and if a cap is even slightly dirty the next easiest rout could be the next plug which could be on ist's intake stroke and thus get a mild explosion back into the intake thus destroying any vacuum at that second thus killing the engine.

Here is a test for that. attach a small wire to the top of #1 and #4 plug wire and attach the plug wire to each plug being carefull to keep these test wires from shorting on to any ground. Now start the engine and this time do not pull the plug wire to test #1 or #4. Now take an insulated plires or just slip two pieces of rubber hose over two finger so you can take the test wires one at a time and short the test lead to the head to kill each cylinder one at a time and if that makes the engine still run on the other three cylinders You have proved that the distributor cap is carbon tracked and bad.

We do this with a scope in jsut seconds in the dealership. But most people don't have a scope even in small shops.

If the test wire test still kills the engine you need to pressurize each combustion chanber with shop air pressure 120 PSI up. To do this get a fitting (from any mechanic's tool truck) The fitting adapts your air hose to the spark plug hole. You put the #1 piston on TDC and put the car in 4th gear and brake on. Then apply the shop air pressure to #1 cylinder and leave it on and open the throttle and witht the air cleaner off listen with a hose in you ear in the carburetor for any slight hissing sound. Now do the same at the tail pipe. Then remove #2 spark plug and listen in the #2 plug hole with the hose. This is a 100% test of any valve leakage or the head gasket between #1 and #2 cylinder.

Reset the crank so #4 is TDC and repete the test for #4 the same way.

Their are some basic facts and that is that neither #1 nor #4 plug wires have any possible affect on the other cylinders unless your removing the wires forces the spark to jump someplace else.

By the way never remove a plug wire with the engine running as you can destroy electrical components elseware. This is especially true on cars with electronic ignition.

Let me know,

Howard

---------- FOLLOW-UP ----------

QUESTION: Howard thanks for the quick response. I did buy a new cap. After replacing the cap the symptoms were the same as with the old distributor cap. Not sure why I would need to do the 120 psi test if my compression was 140 in all cylinders? Is that similar to a leak down test? Could there be  be a vacuum leak under the intake manifold that I can't see creating a lean mixture in cylinders #2 and #3?  Thanks, Gabe

Answer
Even if you have a good compression test it don't mean that there is no blow-by into the next cylinder. Only a pressure test will tell for sure. Or some bluw-by into the intake manifold. You must look at it under the clear fact that there is NOTHING in a normal engine that can make it kill the other 3 cylinders when you kill either #1 or #4.

So you must look at it and ask WHY ? Then look at everything that can do that. You can have 140 PSI on a compression test and still have blow-by. You can't write a cylinder off as "OK" just because it has a 140 PSI on a compression test.

An intake manifold leak is possible on #2 & #3 and some are difficult to spot. One method I used was better then spraying around the intke. It is dangerous though but it sometimes will find a leak that other methods will not. Put the car outside and have a fire extinuisher handy and spray Butane (not lit) around the intake. As a gas it can access under the intake that you can't access with any liqid spray.

I liked the compressed air in the combustion chamber because it checks everything and it is a 100% valid test. Except the test for rings because even a new engine will Hiss into the crankcase with 120+ air applied to the combustion chamber. I even found minor leaks that showed no symptoms at all. But they would have in time.

Howard