The Stovebolt Site Forums The Shop Area Engine Shop What does the flapper on the exhaust manifold do?

On my '66 L-22 250 there is a flapper on the exhaust manifold. I understand that it heats the floor of the intake manifold.

What I'm wondering is what happens when the spring is missing. Is it in the cold or hot position when the flapper is horizontal and unloaded? What are the symptoms when it's not operating? How do you replace the spring?

The flapper looks to be permanently installed, so how do you replace and pre-load the spring? I'm assuming it's a bi-metal spring that is like the thermostat on my HVAC at home.

Thanks for the education

yep. heat riser . supposed to open as the exhaust gets the bi-metal spring hot. useless in my opinion. best thing to do is put it in the full open position and WHAM--WHAM----why you dirty #$%%$%^&* wham--wham-----------------------------tink----------------oh-oh! uh, ya know-ya just can't beat on cast iron like that! maybe best to remove the manifold and take the valve off, and leave the rod so's it don't leak. that's what i do. or, if it's a separate piece, have the exhaust shop remove it next time you need exhaust work, and make a new header pipe. if the stupid valve freezes in the semi-shut position it affects performance in a not so good way. high back pressure is hard on yer valves---and gets you funny looks at the dinner table. :rolleyes:

I recently found the heat riser flapper on my truck frozen in the shut position. And the spring was missing. According to the shop manual, when it is stuck in the "heat on" position, the result is poor performance, overheating and detonation. If it is stuck in the "heat off" position, the result is poor performance, particularly during warm up and low speeds. Assuming you have the same balance weight system that I do, the proper spring tension is one half turn of the spring from its relaxed position. You can test it with a decent heat gun but be sure the manifold is standing in the proper orientation if you are testing it off the truck or else the balance weight fouls up the test.

koolcar - You never cease to amaze me. Hit it with a hammer....is that how you install air conditioning too.

So what's the verdict...is it on or off when it's horizontal. I really don't want to pull the manifold, since I'm sure it's warped and won't seal back without running it across the shop floor 10 or 15 times.

I'm thinking this has something to do with my mid-range stumble. I'm thinking that when it's cold outside, the #3 and #4 cyl. are getting washed down with raw fuel. Tailpipe and plugs look rich, but the carb is set up a little lean at idle.

I'm about fed up with it. I've got a LS9 350 to put in it, its a high nickel, high tin casting (010, 020) with double-hump heads, but I like my 6. I just hate my fuel system and I hate to lose.

I'm open to any and all ideas...short of spending money

i have a collection of hammers that would amaze you. bluewrench applied to stud bolts, pull the header pipe---EASY WITH THAT RATCHET EUGENE! THEM STUDS BREAK EASY! actaully i like to heat the manifold and replace the studs---but that's me, anyhoo--check position of the scrap metal flapper---then do the deal. oh yeah---mid range whatzit. guess that means headers and the vette manifolds i got with no heat riser don't work. the concept of the riser was good in theory, but not very good in real life. were more of a pain than they were worth, and most guys, including me trashed the goofy things. different guys got different ideas--this was one of the first, and most feeble attempts at pollution control--heat the manifold to better evaporate the fuel---o.k.--why do racers use cool cans?

Can't recall which way it sits, but the balance weight is rigged so the flap is open when the weight is down.

The heat riser valve is used to redirect some hot exhaust gases back into the intake manifold to warm the engine up quicker. When the valve is in the horizontal (closed) position, it forces exhaust gases back through the intake manifold into the engine to warm it up quicker. (All of this happens at idle. At higher RPMs, the valve is forced open by the speed of the exhaust flow.

If the valve sticks in the closed position, the engine will not run well at all when it's warmed up. If it's stuck in the open position, it will take longer to warm up and may affect performance for the first few minutes the engine is running.

To answer your questions:
When the spring is missing, the valve will remain in the vertical position (wide open, because the spring's purpose is to maintain tension to hold the valve closed until the engine is warm.

The symptoms, when it's not operating will either be excessively long warmup times (if it's open) or poor performance and detonation when the engine is warmed up (if it's closed.)

You should notice a slot in the end of the rod that the valve pivots on. The spring fits in there, and the other end sits against a small rod-like projection on the side of the manifold. The spring is installed so that, when tense, it holds the valve closed.

Yes, the spring is a bi-metallic spring that expands unevenly when heated.

C1534

I wouldn't want it stuck in the closed position I've seen this crack manifolds. My 56 Buick service manual actually says to tap riser shaft with hammer lightly back and forth to free it so Koolcar ain't that far off. You can get lube from NAPA to keep it free.