heat riser toast

[Keith M.]

Well...for once I am not seeking advice...just wanting to share that I took a little detour today to remove exhaust initiated by wanting to make differential removal easier. Discovered, despite what an arguably useless thing it is, that heat riser was totally dysfunctional due to wear on the part that should be stopping on the pin. Nothing that money can't fix by picking up a NOS one on fleabay.

Also discovered that the exhaust hanger bracket that sits under the tranny had LH rear basically cut off/short. Again, nothing that money can't fix. Gotta love restoring a 54 YO car. No pics on that as I won't have it out til i raise up the tranny to get it out.

[Mike T.]

Re: heat riser toast

Keith - Glad you found an easy item to fix...at least compared to all the other components on a Corvette.
When I got my NOM 66 L72, and got around to tweaking a few things, it was running well but had a bit of a problem when I'd
get it back home after a short drive. It was not overheating (checked with both, the dash gauge and an IR gun) but I'd hear some 'bubbling'
going on in the carb after shutdown and noticed the rear bowl was leaking out fuel into the venturies. I had already checked the heat riser
and found it to be looking good and not frozen so had discounted that but when nothing else rose to the top of the list, went back
and looked at it again. Turns out while it moved freely when cold and sprung back to the closed position when closed...after a decent warmup
the dang thing was still closed yet could be opened up by pushing with a long screwdriver. When the engine got warmed up, that heat riser
was still closed, causing the pass side exhaust to back up through the intake and heating up the carb. Until I run across a replacement, I wired it open and for now, problem solved.

[Keith M.]

Re: heat riser toast

ya...I don't totally understand how the thing works..but don't really need to...i mean...i know in concept. Meaning...when things heat up that spring will expand and cause the valve to close...opposite of what one would expect. just a dumass thing with an easy $$ fix.

[Mike T.]

Re: heat riser toast

Keith - Well, it's actually the other way around. When cooled, that spring holds the damper (and it's counterweight) in a closed position, to help push some of that heated exhaust back up through the head and intake and out the other side, to get the engine up to operating temp.
When the spring heats up, it's 'supposed' to relax and the counterweight then drops, opening the valve.

[Mark E.]

Re: heat riser toast

Don't mess with heat risers. They're troublesome and unnecessary. Avoid the troubles of a plugged up exhaust, leaks, rattles and overheating by replacing it with a spacer. And the next time you remove the intake, use a gasket that blocks the riser passages.

[Keith M.]

Re: heat riser toast

Don't mess with heat risers. They're troublesome and unnecessary. Avoid the troubles of a plugged up exhaust, leaks, rattles and overheating by replacing it with a spacer. And the next time you remove the intake, use a gasket that blocks the riser passages.

Well...I was wondering about that. My intake and exhaust manifolds are not original. I thought the heat riser valve had something to do with divorced choke. There is nothing connecting the two on my car right now. So I guess my question is...what riser passages?? What are they and where are they?

Separately...if you look at my first picture...the spring is holding the valve pinned open..despite the weight...which is where it would stay until the car warmed up..which would put pressure to unwind the spring and leave it exactly where it is..pinned open....Maybe this valve is just orders of magnitude F'ed up. In response to Mike's post...a coil spring retracts when cold and expands when hot. Mine is already pinned open so ...no where else to go. In theory...the weight should put pressure due to gravity on the spring to keep the valve closed. Then...when temps rise...spring expands from heat overcoming gravitational force of the weight and opens the valve. Can't happen with what my valve shows.

[Mark E.]

Re: heat riser toast

Well...I was wondering about that. My intake and exhaust manifolds are not original. I thought the heat riser valve had something to do with divorced choke. There is nothing connecting the two on my car right now. So I guess my question is...what riser passages?? What are they and where are they?

Separately...if you look at my first picture...the spring is holding the valve pinned open..despite the weight...which is where it would stay until the car warmed up..which would put pressure to unwind the spring and leave it exactly where it is..pinned open....Maybe this valve is just orders of magnitude F'ed up. In response to Mike's post...a coil spring retracts when cold and expands when hot. Mine is already pinned open so ...no where else to go. In theory...the weight should put pressure due to gravity on the spring to keep the valve closed. Then...when temps rise...spring expands from heat overcoming gravitational force of the weight and opens the valve. Can't happen with what my valve shows.

When the riser is closed, exhaust gases are forced through a passage that goes up th RH cylinder head, into and across the intake manifold under the carburetor, then into the LH head and exhaust manifold. This warms up the carb and engine to open the choke quickly for emissions and drivability during cold starts. You can block this passage as it goes between the heads and intake to keep the carb cooler.

When cold, the tension of the thermostatic spring in the riser keep the valve closed. As it heats up, the spring expands and loses tension. As the weight overcomes the lessor spring tension, the valve opens.

[Keith M.]

Re: heat riser toast

When the riser is closed, exhaust gases are forced through a passage that goes up th RH cylinder head, into and across the intake manifold under the carburetor, then into the LH head and exhaust manifold. This warms up the carb and engine to open the choke quickly for emissions and drivability during cold starts. You can block this passage as it goes between the heads and intake to keep the carb cooler.

When cold, the tension of the thermostatic spring in the riser keep the valve closed. As it heats up, the spring expands and loses tension. As the weight overcomes the lessor spring tension, the valve opens.

Mark,
Thanks for that detailed explanation. Very interesting. I already ordered a NOS heat riser so it will be interesting to check it out when it comes. I think the one off my car is totally messed up and non functioning...fortunately appears to be stuck in an open position.

[Bill B.]

Re: heat riser toast

Keith,

When you install the new one, make sure it is oriented correctly .. that is, when installed, if you push the weight down towards the ground, the valve opens up. If you release it, the weight pops back up and closes the valve.

Someone before you may have installed the hear riser upside down, thus never opening no matter what the exhaust system temp was.

[David M.]

Re: heat riser toast

[QUOTE=Mark Edmondson (22468);930303]Don't mess with heat risers. They're troublesome and unnecessary. Avoid the troubles of a plugged up exhaust, leaks, rattles and overheating by replacing it with a spacer. And the next time you remove the intake, use a gasket that blocks the riser passages.[/QUOT

I second Mark. Ive eliminated the flap in both of my 68s. I blocked off the intake crossovers too. No more hard hot starts and the engines are much more responsive on hot days. No more burnt paint on the intake. This modification if done creatively is undetectable.

Neither car gets driven in the winter. The 442 has the divorced choke which I've tweaked to work. The Vette had an electric choke conversion.

[Michael J.]

Re: heat riser toast

After replacing the one on my '65 L79 3 times, and it still will not open and stay open when hot, I just wired it open. These things are pretty useless anyway it seems. I would have removed it and put in a spacer, but that wasn't worth the trouble. If I have it judged again, I'll remove the wire.

[Keith M.]

Re: heat riser toast

Thanks for all the input. I found a NOS one that I am going to install. Part of the decision on which way to go maybe depends on what climate you live in?? Either way...its an easy change to put in a spacer if it ends up causing me hassle.

[John F.]

Re: heat riser toast

Soak the old one in EvapOrust. They clean up pretty well and can be salvaged.

[Keith M.]

Re: heat riser toast

Thanks for all the input. I found a NOS one that I am going to install. Part of the decision on which way to go maybe depends on what climate you live in?? Either way...its an easy change to put in a spacer if it ends up causing me hassle.

I would...but it is defective beyond worth repairing. See 1st post and pics above. There are two stop points on the arm attached to the weight. The one that stops when cold is worn through so the spring pivots the weight beyond the designed stop point and where it should never go....leaving it in open position...but open 180 degrees off where it should be when normal open. So as my engine heated up...coil spring contracts...eventually moving the valve closed and as the spring continues to heat it should move it to open again 180 degrees opposite of where it started..until it hits the other side of the stop. So to me the riser is FUBAR...53 yo spring on top of everything else going on with this. A new paperweight or landfill food.

[Philip A.]

Re: heat riser toast

Clean the original one up. Then weld the shaft in the open position on the inside so its not noticeable. And now you have what look like an original equipment HR without the heat riser troubles.