Rare problem - experience may help others

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Rare problem - experience may help others

Post by tobytj »

About 2 months ago my son came round in his coupe and when it pulled into the driveway I noted that it was running on three cylinders at idle. Asked him how long it had been like that and received the expected answer of “like what?”. It was only missing a cylinder on idle and was otherwise running beautifully. Pulled off some plug leads and quickly established number 4 had gone missing. The fact that it was cruising well eliminated head gasket and valves, crook plug lead and plug were possibility but since high vacuum cruising was OK probably not.
Spark plug on Number 4 was a lighter colour then the others indicating leaner mix on that cylinder. Aha!- manifold air leak says the front part of my brain. The booster draws on number 4 so checked that and the hose for leaks. Both had been recently renewed so no problem there. Sprayed some carby cleaner (flammable) on the manifold flange and momentarily. Right, crook manifold gasket says the front half of my brain, but the back half, that I ignored, said “why now after 7 years of running perfectly”.

Fix Attempt 1 – empty radiator, pulled off manifold, installed standard new gasket, put back together. Still running on three.

Fix Attempt 2 – same as above, thicker gasket and sealant. Still running on three.

Fix Attempt 3 – Must be a problem with the manifold face or a crack (Lynx single side draft manifold) says the front part of my brain, the back half doubts that this is problem that suddenly developed after 7 years. I have a spare! Same fix as above but with 45 dellorto mouted on a new manifold. Still running on three.

By this stage the weekend has come to a close and I am over it. Give car back to son. A month goes by.
Fix attempt 4 – same as above, check all surfaces again, file any potential clearance locations, with new gasket and super doper sealant. Tighten slightly, allow to cure overnight and torque to spec. Still running on 3.

Fix attempt 5 – spray the carby cleaner on the flange and again briefly runs on four. I am thinking crack in the head inlet port? Imperfection in head face?. The back part of my brain does not credit any of this but the front half grabs the surer doper sealant and smoothers the joints of the flange and head – leak now you bastard! Still running on three.

Dejectedly I resolve that the head will probably have to come off and be machined. So I drive the car back to my sons place, still cruising beautifully and note the temp gauge has not moved. Must have dislodged in the frenzy of manifold goo ing. Arrive at sons, pop the bonnet to reattach sensor wire and note the vacuum advance hose off. Strange I think, that should have made more impact than it did on top of the other leak. The vacuum advance is connected to the fitting for the booster on a take off for the vacuum gauge – so full vacuum set up not carby vacuum. I pinch the hose and engine note changes. Hmmm, goes the back of my brain. I place the line on the dizzy connection and suck – no resistance. Diaphragm in vacuum unit obviously stuffed.
Now the front half of my brain catches up. The advance diaphragm is right up against number 4 manifold port in a 4ZC1 so the carby cleaner I was testing with was being sucked into the hole in the Diaphragm and into the engine, not through a gap in the inlet manifold. There was never anything wrong with the manifold seal.

Fix attempt 6 – plug vacuum hose, adjust idle mixtures and speed screws, took 5 minutes. Running on four.
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Re: Rare problem - experience may help others

Post by Turbocoupe75 »

I feel your pain :)
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Re: Rare problem - experience may help others

Post by firebug »

Cool story. Always learning....
"Can't stop addicted to the shindig!..."
Peter Riedlinger
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Joined: Fri Jan 09, 2015 6:53 am

Re: Rare problem - experience may help others

Post by Peter Riedlinger »

Have a TC. I took the inlet manifold off and then replaced it. Temp gauge working well prior to manifold removal. Now gauge heads straight up to hot when I turn the ignition on. Checked wire to sensor unit from first multiple plug. Continuity okay. Is the problem in the wiring or the sender in the inlet manifold? Can I check the sender in situ? Also how can I check the wire to the sender. What voltage should it carry? The wire is yellow with a black trace and that fits with the circuit diagram in my Gregory's manual.
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