The Stovebolt Site Forums The Shop Area Engine Shop Stock exhaust to intake mating issues/possible leak?

Hi everyone! Working on a 55 Cameo for an upcoming show. Had intake and exhaust leaks at the cylinder head, and the heat riser was leaking exhaust bad out of the shaft, so I pulled the whole thing, removed the whole heat riser flap and all, bolted it up with gasket material, and put it all back on with fresh gaskets. I couldn't really find any specific instructions for putting on the gasket between the intake and exhaust to mate them, or for torquing instructions, so I kind of just did it wrench tight in an X pattern. Well that's leaking exhaust pretty bad so either I did that wrong or else I didn't get the gasket surfaces smooth enough. If anyone has specific instructions for that gasket I would appreciate it. But that's not the main issue. It's not idling anymore, like at all. When the intake was leaking before it was idling rough but was still idling. I've been through the carburetor, changed all gaskets and can confirm it's metering fuel out of the idle circuit. So I know it's a vacuum issue, I just don't know where. Since I've sprayed starter fluid over every gasket area and no change, I'm starting to worry there's a crack somewhere in the intake. I miiight have dropped it about a foot on the ground when I was working on it, so that would be a possibility, but also what are the odds that removing the heat riser flap has overheated the intake and caused it to crack? I had heard of other people removing it so I didn't think anything of it but is that something I need to replace? Lots of questions I know. I'm a noob but I appreciate yalls input. Thanks!

One more thing is that the engine runs great as long as the throttle is open a little, so that tells me there's at least enough vacuum for that. I would think that would also rule out a vacuum leak big enough to shut down idle completely, but I could be wrong. Could just be that I missed an air passage or something when cleaning out the carb.

Have you confirmed that the gasket surfaces, both on the cylinder head and the manifolds, are flat and straight? Eliminating the heat riser valve was a BIG mistake- - - -it needs to be there and functioning properly. It's a pretty simple procedure to line ream the manifold holes a little oversize and fabricate a shaft that fits, which will last 3 days past forever- - - -especially if you make it out of stainless steel.

I'd suggest getting both manifolds milled flat, and leaving the bolts that hold them together slightly loose until both manifolds are snugged up tight against the head. Also, be sure the alignment rings between the head and the intake are in place, and aren't too thick- - - -they don't seal vacuum, but they do keep the manifold aligned so the gasket can seal. With a milled intake manifold, you'll need to grind a little off the rings to keep then from holding the manifold away from the head. Last suggestion- - - -buy and install a Remflex gasket- - - -it's about twice as thick as the original equipoment type, and does a really good job of sealing. Good luck!
Jerry

Edit: DO NOT bolt the ends of the exhaust manifold down. Those dowel pins in the head and oversize holes in the manifold are there for heat expansion. Bolting the ends of the manifold down tight will make it stress-crack eventually.

Thanks for the response! So I take it not having that flap can damage the intake? If I do add another shaft in though how do I prevent exhaust from leaking out of it like the last one?

Not having the flap in the exhaust manifold will not damage the intake manifold. The only thing it hurts is the cold engine drivability.

If you are interested in repairing/reinstalling your heat riser assembly, check out this 216 Exhaust Riser Repair conversation. Same process can be used on your 235. A good description and a couple of pictures to show what was done.

If you didn’t get a response from the carb cleaner check (not super unusual), I think it might be worth doing some additional diagnosis before pulling things apart.

Sometimes an unlit propane torch works better at locating a leak. You might have to temporarily remove the fan belt and run it that way for the duration of the test.

A vacuum leak that is bad enough to cause the terrible idle you are describing, should be one you can locate. If another round of tests still comes up empty….it’d be good to start exploring other theories. Ignition system would be the next place I’d look.