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Air leak at inlet manifold 120-130 1967

posted by  Aidan on  Sun Jun 20 06:07 CST 2004
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last visit: Wed Oct 22 03:47 CST 2014
[RELATED]

Hi,

I've fitted a D cam, skimmed head, twin SUs and a 4 branch manifold with 2" exhaust to my B18A. After a lot of help from patient Brickboarders the major problems were sorted out and the car has been transformed. Its a real flyer now and has plenty of go above 4000 rpm where not much happened before. It picks up strongly from low revs in 4th as well and it also sounds great. The only problem is a rough idle which I've tried and tried to solve without success. I thought that I'd eliminated air leaks around the carbs but when I went back to basics yesterday and sprayed carb cleaner up under the heat shield the engine note changed. When I took off the carbs I noticed (and should have noticed before) that the exhaust manifold for the two centre exhaust ports sits higher than the alloy inlet manifold and (I think) prevents the inlet manifold from sealing properly. To test this I cut a washer in half to bring up the alloy surface underneath the stud washer for both studs at the base of the alloy manifold and put it all back together again. I got a smooth idle but after running the car for about 30 minutes the idle got rough again - I assume that the heat softened the halved washers and eased the seal at the base of the inlet manifold.

I've now ground down the exhaust manifold where the stud washer will sit so that it should be flush with the alloy manifold but I'm not convinced that the problem is solved. The alloy manifold only has two dedicated studs (at the top centre) and shares studs with the exhaust manifold at the top outer edges and at its base. This means that there is a more effective seal at the top than at the bottom of the inlet manifold(?) but thats the way the studs are positioned... To make matters a little worse the inlet manifold that I have has been worn at its base where the stud washer sits from overtightening - I've cleaned it up as best I can but its still a little rough.

So - is an air leak at the base of the inlet manifold a common problem and are there any tips for preventing them? Is it an option to use two gaskets or is that overkill? Given that the exhaust is steel and the inlet manifold is alloy, will they expand at different rates and cause more leaks as the engine heats up?

Any hints or tips would be much appreciated and apologies for the long post.

Thanks,

Aidan
--
1967 131, 1969 131






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