QuestionQUESTION: Howard
Back again with a new problem with my 1980 US Spec MGB LE. I wrote to you recently about the car overheating. I haven’t found a definite cause yet but in trying to return the car to original configuration have encountered another problem.
The car isfitted with the original single Zenith Stromberg carb but had been retrofitted with a manual choke conversion. This conversion wasn’t working well and I decided to replace it with the original water choke when I rebuilt the carb. I rebuilt a choke and calibrated it per John Twist’s video. I installed it on the carb and went to hook up the water lines and found that the fitting from the back of the head was missing. Obviously the head had been replaced with one from an earlier model that did not have this port.
I located a correct used head and had it reconditioned by a machine shop (that I trusted from past projects) including magna fluxing for cracks and new hardened valve seats for unleaded fuel.
I noticed after putting everything back together that the engine idled roughly. I was able to pull the #1 and #2 spark plug wires off during idle with no change in idle speed or roughness. Since I’ve read many of your answers on this forum I decided to go about the diagnosis in a logical way.
Compression:
I ran a compression test on all cylinders and got #1-130, #2-130, #3-125, #4-122.
Since the numbers seemed ok and the variance was less than 10% I moved on to fire.
Spark:
The car had been retrofitted with a Mallory dual point distributor. I checked the engine timing with a timing light and adjusted the timing to 10BTDC. I also checked for spark on all 4 cylinders with the light.
I put a spark plug against the block and checked for and got a good blue arc. Not trusting the Mallory distributor due to some internet research, I first replaced the condenser with a typical General Motors condenser, no improvement. Then I removed the entire distributor and wires from my 1971 MGBGT (which runs great) and installed it in the 1980. No change. I verified and re-gapped all spark plugs and eventually replaced the Champion plugs with NGK. No change.
Verified firing order end to end on plug wires, 1342 counter clockwise.
At this point, still no change. Cylinders #1 and #2 are not contributing at idle speed.
Fuel:
I installed an inline fuel pressure gauge and regulator. When the ignition is on, the fuel system is charged to about 2.5 psi whether the engine is running or not. I believe this should be acceptable. I didn’t measure the fuel flow but reasoned that if it is able to maintain pressure while running, there is no problem with fuel pump throughput.
Since this car has a single carb and the two rear cylinders are running, I tried to concentrate on fuel related items that might affect the front two cylinders and not the back two.
I checked for vacuum leaks using both the starting fluid and propane torch methods and found only some small leaks on hoses that I subsequently repaired.
I checked the manifold vacuum with a gauge and it read between 7 and 9 inches of mercury and oscillated back and forth with the speed of the engine. My understanding is that this engine should develop about 20 inches of vacuum if running correctly. It might be coincidence but 9 inches of vacuum is about half of what I should see. And only half of the cylinders are contributing at idle.
The oscillation in vacuum leads me to believe that the problem is related to specific cylinders (#1 and #2). Although I was not able to find a vacuum leak on the intake port during my test, I reasoned that such a leak would cause the symptoms I was experiencing.
Suspecting a leak at the intake port area for cylinders #1 and #2 on the intake gasket, I removed the manifold and replaced the gasket and retorqued to 30 ft-lbs. Started it up and no change.
The only thing I can think of is an internal fault or crack in the cylinder head allowing ambient air to be drawn into cylinders #1 and #2 instead of the air/fuel mixture from the carb?
Sorry for the long description but this one has me stumped. I’m thinking that the used head that I had reworked by the machine shop had an internal issue in the intake port that was not caught during the reconditioning process. Your thoughts?
ANSWER: Hi Dave,
You didn't tell me if it would pick up the other two cylinders when you bring up the RPM. Also one other thing you should do is to remove the valve cover and rotate the engine (not running)and watch the amount of valve action of all 8 valves and see if they all depress the valve springs about the same. While you are there double check valve lash on the valves of #1 and #2 cylinder.
One other thing to do is put a short bolt in the spark plug cap of number 1 or 2 and hold it close to the head 1/8 inch and have someone spin the engine over to see if it physically jumps a good spark (not with a spark plug#
Using a spark plug is ok to check fire but making the spark jump 1/8 inch puts a strain on that circuit and if there is a problem the spark will take an easier rout if one is available. #DO NOT TRY TO MAKE IT JUMP MORE THEN 1/8 INCH)
Your method of a "Cylinder Kill test" to find a dead cylinder is dangerous to the ignition system. The reason is that when you pull a plug wire away from a plug the field built up in the secondary winding of the coil will look for some place to go since you removed it's method to unload. Often that easier place is down the coil tower to the primary circuit which if it is an electronic system can fry it. When the old points system was used the worst that could happen is it might jump inside the coil and damage a coil but most of the time it does no harm.
But on electronic ignition you should use a thin point like a 12v test light and probe the rubber cover on the spark plug. It will not normally even burn out the test light bulb but to be safe just attach a clipped test lead to the point and to a good ground.
The cylinder kill test was correct to test for a dead cylinder.
I doubt any head crack can kill one or two cylinders. It is suspicious that you have two dead cylinders that are next to each other so do the compression test again but this time test #1 with the spark plug in #2 and then again with the spark plug out of #2 to see if there is any difference between the two tests (throttle open on both tests)
A single carb system can not pick which cylinder it wants to feed. But do keep in mind that the late MGB Stromberg carburetor was set so lean by the factory that it requires the stock air filter to supply extra vacuum to pull fuel from the jet. Even when these cars were new you could have one idling fine and just open the front of the air filter and pull the air filter out and the engine would not run.
Because of BLM and this design of the intake and the exhaust manifold the earlier exhaust manifolds and the intake manifolds with the dual SU carburetors (up to 1974)have become gold.
Howard
---------- FOLLOW-UP ----------
QUESTION: Howard
No worries on frying the electronic ignition. It was removed by the previous owner and replaced with a Mallory dual point system. My cylinder kill testing was with the Mallory and later with a Lucas 25D out of my other MGB.
I did the compression test as you suggested. Compression was the same on #1 cylinder with and without spark plug in #2. Both around 120 psi with throttle open. Engine wasn't fully warmed up yet. (but was already missing at idle). It always misses at idle whether cold or warm. It's difficult to say for sure but the miss seems to go away as the engine speed is increased. The car is very driveable with good power and response. It just idles very roughly and cylinder kill indicates that #1 and #2 are not contributing at idle.
AnswerTry this, after it is warmed up and idling rough slowly start to restrict the air flow into the intake with your hand. Do it in a progression and note if at any time as you slowly restrict more and more of the air supply it speeds up any at all or smoothes out any. If it does you have a very lean condition for some reason. An intake manifold leak is the more common cause. Especially when it affects specific cylinders.
If it does speed up or smooth out any, you need to test for a leak and the most common is a gasket between the head and manifold and a good method is to use a propane torch (not lit) spraying the gas around the intake. (keep fire equipment handy incase the propane lights up) Some mechanics just squirt oil on the joints but that is difficult to access under the bottom. The use of a combustible gas makes the engine smooth out and speed up also thus confirming the cutting off the air supply test. Also pinch off all vacuum hoses to see if they are supplying the air leak.
Let me know,
Howard
