QuestionQUESTION: I discovered a leak at the intake manifold between N01 & No2 cylinder. I removed the manifold and replaced the gasket and torqued the nuts to 15 ftlbs. It still leaked so removed it and had the manifold machined flat as it was a little warped, reinstalled and torqued to 15 ftlbs. It still leaks at the same location. The manifold is not cracked and the head does not appear to have any cracks. Is there a sealant I should use? What else can I do. I sprayed the area around the manifold and carburator with throttle body spray and the only area that appears to have a leak is the manifold.
ANSWER: Hi Allen,
What kind of car is this?
Howard
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QUESTION: Sorry, I got ahead of myself. It is a 1978 MGB, Zenith Stromberg carb and cast iron manifold.
ANSWER: Hi Allen,
That car is notorious for running lean as they were still trying to meet emission standards but if there is a intake leak you have to be doing something wrong as two flat surfaces and a new gasket don't leak unless there is a crack (which I would think you would have spotted already) I would examine the thickness of the surfaces that the hold down washers hit to see if they are different thickness at the exhaust and intake points, especially since you had the intake resurfaced. My guess is that the nuts and washers are holding the exhaust manifold and barely touching the intake manifold.
Even when that car was right it was so lean that it just about would not run with the air filter out. The vacuum formed in the intake due to the restriction of the filter was needed to get fuel out of the jet. All of the MGBs of that era had lean running problems and most owners purchased 67 and earlier B intake and exhaust manifolds with the twin SU carbs to correct them.
Check the thickness of the exhaust and intake manifold at the washer contact points.
Howard
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QUESTION: Since the one intake runner feeds both No1 & No2 cylinder where it leaks both would be running lean with a rough idle and poor vacuum reading. I get a fluctuating reading of about 13-14in Hg at idle. But if I increase the RPM above idle to 1500 or 2000 I still feel a miss in the engine. The small leak at the manifold should not influence this because of the volume of air going into the engine or does it? I also pulled the plug wire off No1 while it was idling with very little drop in rpm. No2 did the same thing whereas No3 and 4 there was a very definite drop in idle to almost stall. Does this confirm the lean mixture in 1 & 2 or is it spark related. I will add shims under the flat washer to improve the seal and see if that makes a difference. Thanks
AnswerAllen,
While working is several Dealerships over the years I would often get symptoms from customers "It does this when I do this and it does that if I do this etc, etc. I learned early to smile and nod my head while letting all the symptoms go in one ear and out the other on to the floor. Because I found it as useless information in 99.9% of the time.
You are correct that a vacuum leak is of little consequence at higher RPM. But you are jumping to incorrect conclusions on the cylinder kill test.
If you brought your car to me and told me all that, I would smile and nod my head and let all you told me go through my head and out the other side on the floor and I know that some may say it is because there is nothing between my ears to decipher what was said.
But! I learned to ONLY diagnose a problem by testing and the first test is either a compression test or better still a leak down test (if you have a "Leak Down tester" available) If not, a compression test will do. (throttle open and at least 4 or 5 revolutions of the crank and all the plugs out)
In your case I would get one of those fittings to put in the spark plug hole that adapts to a line compressed air and test each cylinder with all the plugs out. Put #1 at TDC of the compression stroke, In 4th gear and hand brake on when testing it and put a piece of hose in #2 plug hole and listen in the hose for any leakage between #1 and #2 cylinders. Do all four that way.
If everything is ok then proceed to the ignition. The best is a scope but if you don't have access to a scope just set the timing to specs and remove the plugs again and put all four on each plug wire and lay them on a metal part of the engine and spin it over and watch each plug spark to see that they all are a thick spark and blue in color (not in sun light)
By then you will have found the problem so It is no use looking at the carburetor as it don't know which cylinder it is feeding nor when. Only combustion chambers and ignition systems know that.
let me know,
Howard
