I am wondering if I can block, or partially block the (exhaust valve) heat riser, or just leave it open. I bought a manifold and it came w/o one. All the pieces to one add up to over $250. I could only salvage a few pieces of the old one. Any input would be appreciated.

 

I take it you bought a used exhaust manifold that had the heat riser "flapper" removed. I didn't know that you could even buy the flapper parts separately, but sounds like you found a place. That would be dumb for $250, as a whole new manifold costs about $170 from advanceauto.

If you are in a warm climate, you could simply block off the heat riser alltogether. Just use a piece of sheet metal, not too thick, and trace out the pattern where the exhaust manifold bolts to the intake. If you are in a colder climate, that may not work. Of course, there are numerous solutions to carb icing that the guys with headers have to employ. But that awfully complicated to overcome your problem here.

Leaving the heat riser passage wide oepn is not a good idea, as you will get overheating problems with the carb.

You MAY be able to talk someone out of the flapper and shaft from an old cracked exhaust manifold. The stock manifold are pretty poor design and tend to crack and or freeze up the flapper. If you decide to go that route, let me know, as I might have one laying around. Might bave gotten scraped recently with the high iron prices though.

later.....

 

I actually bought it new. It was only $70, but it didnt come with the heat riser. I tried to salvage the pieces from the old one but the bushings were gone. The big bushing is $85 alone. Unfortunately I live in maine and cold weather is a big part of the year. What will happen if I just leave it open. What will overheating of the carb do?

 

I'm sure you can find somebody to fabricate things for you

 

The carb overheating thing -- I don't really know. My dad's 240 had the riser valve stuck open for years. As I recall, sometimes it would refuse to start back up after it was good and hot -- the base of the carb was so hot that it boils out the gas. If you know what to do (pump pump pump) you could finally get her to go.

A bigger issue is efficiency -- you are not doing yourself any favor running a red hot (well not really) carb. The fuel air mix will have less oxygen becuase of the heated mix. That's why you see all the attention made to cold air intakes etc.

What carb are you running? One option is to get an aircleaner with the cold/hot air flapper (I don't know proper name) but anyhow, you bolt this shield around the part of the exhaust manifold, then a flexible tube goes up to the air intake horn of the air cleaner. In the horn, there is a thermally activated flapper. Moves at about 110 degrees. This was fords later addition to the exhaust riser approach, and both were in place for years. Anyhow, you get pretty stabile 110 degree air going through the carb this way, which prevents icing, and isn't too hot either.

If you went the air cleaner "stove" approach, you could block off the riser as I described above. The aircleaners in question usually were found atop Carter YF or YFA carbs, I think. They are pretty common -- I got mine off ebay for $15. Junk yards ought to have them too. People thought they had something to do with pollution controls in the 70's as they appeared about the same time -- but they don't. Of course, some fellas just have to get rid of them and swap on a chome aftermarket aircleaner.

If you are already running a YF carb, then that's a good option for you. Maybe you even already that type of aircleaner. In which case, just verify that its all working, maybe replace the flexible tube, as it is probably shot, and use a heat gun to verify that the flapper in the air horn is working.

Carb icing occurs at cool temps (not deep cold) when the relative humidity is high. Sort of spring and fall fog at 30-35 degrees in your part of the world.

Too bad you just didn't go with EFI cast manifolds -- you can get them used for $60, and they are almost as good as headers. Their only drawback is the fact that they don't have any provision for the heat riser, making heated carb spacers, or air cleaner intake "stoves" like I describe necessary. But you have that problem anyhow. And the EFI headers require a custom exhaust.

Good luck