Per, when I got my 51 Hollywood with the 254 8cyl. It had a cracked manifold right near the heat riser valve. I replaced it with an NOS manifold and eliminated the riser with a steel block-off plate. The heat riser was frozen and I believe that caused the manifold to crack.

Hello Per, hope you had a great Holiday ! As Ed pointed out, removing the heat riser is a good thing, especially on the 8 cyl. engines. The carb. is mounted on the exhaust manifold and my opinion, doesn't need anymore heat , especially with todays modern, hot burning fuels.! If you look closely at the heat riser shaft, there was/is a 3/32nds cotter pin in it. That pin should be parallel with the manifold, indicating the heat riser is in the open position. If it isn't, and is at 90 degrees,[pointing to the carb.] it is in the closed position. This is directing the rear 4 cylinders exhaust around the heat passage at the carb. The volume of exhaust being squeezed around that is not a big issue as long as the engine is idling, but once the rpm's come up, it creates a restriction, thus causing the exhaust gas looking for a place to escape. The manifold gasket usually is the first place, then lets fresh, oxygen rich air in and continues to do damage. I have guessed this is the reason it pops the top off the studs with the nuts still attached, which I'm sure you have seen numerous times in the past. We always plan to restore the manifolds on all the Hudson engines we build, and ALWAYS address the heat riser. Usually, big 6 engines you can get the heat riser to free up, if it isn't already loose, or I should say, loose as can be, missing the bushings in either side completely. Again, this is an exhaust leak, especially heard when you are accelerating. We do not block them off, but simply cut the heat riser flap out, drive out the small bushings in either side, usually worn almost in 2 pieces ,if not missing completely, drill the hole where the bushings were with a 7/16th's bit. It is only 1/16th. from that size anyway, once the bushing is removed. install a 7/16th's short bolt in either side, use a torque prevailing metal nut. This makes it nice a quite under the hood without that annoying exhaust leak. We also ALWAYS make sure we remove the plugs located around the manifold/ carb base and clean the exhaust passages and ports from all old carbon. This actually allows the manifold to breath, which actually allows the manifold to cool when the engine is running. If you look under the hood at this area when the hood is open on cars, it is usually very evident the heat riser is stuck, or not working properly when you see the paint is all blistered off the manifold in the carb area. This again, has/is a domino effect causing running issues, especially when it has been running for awhile, vapor lock frequently, hard starts after a stop for fuel, etc..... Modern fuel doesn't need any more help creating excessive heat on our old engines.
Twin-H manifolds are kind of the opposite. Because of their design, the heat passages become somewhat plugged around the base of the carb pods with carbon. A few years ago, I had one that I couldn't get it to stop vapor locking, only after I would drive it for a couple of hours at highway speeds ? It really had me stumped, so I removed the manifolds, separated the manifolds, convinced the manifolds had something wrong with them?. After taken them apart, I found the passages were completely full of carbon, and as far as I can tell, it would take a few hours of solid highway speeds to make the carbon build up to actually start glowing, kind of like charcoal in your B/Q grill. Once it was glowing, it would overheat the manifold and continue to burn like charcoal, even after the engine was shut off. Of course, if you think there was going to be any fuel left in the carbs after it sit for 10 minutes, you would only be foolish. We clean all the passages as best as possible, re-assembled the manifolds back together, which can sometimes be very difficult to make them mate together again without a little more work. Sorry for the long explanation about restoring manifolds, when your original question was just the heat riser, just felt the need. Looking forward to 2017 and driving our HET cars !!!

[quote="Doug Wildrick" post=18795]
Twin-H manifolds are kind of the opposite. Because of their design, the heat passages become somewhat plugged around the base of the carb pods with carbon. A few years ago, I had one that I couldn't get it to stop vapor locking, only after I would drive it for a couple of hours at highway speeds ? It really had me stumped, so I removed the manifolds, separated the manifolds, convinced the manifolds had something wrong with them?. After taken them apart, I found the passages were completely full of carbon, and as far as I can tell, it would take a few hours of solid highway speeds to make the carbon build up to actually start glowing, kind of like charcoal in your B/Q grill. Once it was glowing, it would overheat the manifold and continue to burn like charcoal, even after the engine was shut off. Of course, if you think there was going to be any fuel left in the carbs after it sit for 10 minutes, you would only be foolish. We clean all the passages as best as possible, re-assembled the manifolds back together, which can sometimes be very difficult to make them mate together again without a little more work. Sorry for the long explanation about restoring manifolds, when your original question was just the heat riser, just felt the need. Looking forward to 2017 and driving our HET cars !!!
Doug, on the twin H manifold, are you saying to pull the expansion type plugs and clean the passages? I guess you could then tap them for a pipe plug that would be fairly easy to remove in the future?

Yes Kerry, remove the expansion plugs under each carb.pod. You need to purchase new plugs though, because there is not enough metal to install pipe plugs. If you have ever seen the Twin-H manifolds that are all cracked and split apart under the carb base, where these plugs are, it is because the passages are or just about plugged, causing them to sweat. The condensation inside this area starts corroding away the bottom. If it then freezes, it breaks it all out. When removing these plugs, try pulling them out Versus driving them inward. The machined round hole for each plug , has a very thin seat, and if corroded badly inside, can break away the seat for the new plug to seal properly and cause more problems. We have seen these corroded badly enough that after we have installed new plugs, we had to then braze the plugs into place to make them seal again. After you have restored all the parts and pieces, and have mated the 2 together, you should have them planed for trueness.

Doug, thank you for the detailed information. I took a look today at two eights that are waiting for my attention, a '47 and a '51. The '47 has the cotter pin pointing directly at the carburetor. The '51 doesn't have the cotter pin any more, but it would also be pointing at the carburetor. There are acorns in the '51 intake manifold that have to come out! Now I'll know to pay attention to carbon deposits too.