Can Malfunctioning Manifold Heat Control Valve Cause Rough Idle? 67 Polara 383

[RogueOne]

The Manifold Heat Control Valve on my 67 Polara 383 wA/C seems to be missing the thermostat on the back of the exhaust manifold. I've been trying to locate some gremlins that have been causing a ROUGH IDLE. Compression is okay. I've checked for and fixed vacuum leaks, reset the timing, reset the a/f mixture screws. Carb is freshly re-built and I have a brand new gas tank, fuel pick-up and fuel lines. It's getting plenty of gas and spark. I've got a new coil, new spark plugs, spark wires, new distributor cap, new points and condenser. I'm starting to look at some more esoteric causes for the rough idle. Could this non functioning valve play any part? It seems to be in the 'open' position. Should I worry about this or is it okay to leave it non functioning?

 

[1970FuryConv]

The Manifold Heat Control Valve on my 67 Polara 383 wA/C seems to be missing the thermostat on the back of the exhaust manifold. I've been trying to locate some gremlins that have been causing a ROUGH IDLE. Compression is okay. I've checked for and fixed vacuum leaks, reset the timing, reset the a/f mixture screws. Carb is freshly re-built and I have a brand new gas tank, fuel pick-up and fuel lines. It's getting plenty of gas and spark. I've got a new coil, new spark plugs, spark wires, new distributor cap, new points and condenser. I'm starting to look at some more esoteric causes for the rough idle. Could this non functioning valve play any part? It seems to be in the 'open' position. Should I worry about this or is it okay to leave it non functioning?

Is it rough idle all the time, only when cold, or only when hot?

Open manifold heat valve should only cause rough idle what engine is cold

 

[Davea Lux]

A stuck heat riser will overheat and possibly boil the carb if it is stuck shut.

Dave

 

[CBODY67]

Just how "rough" is "rough"? These engines did not idle glass smooth, no matter what. I tried and never got there.

If the valve is in the open position, it might extend the time it takes the heat from the cast iron cyl heads to migrate into the cast iron intake manifold. Until that fully happens, unvaporized fuel might be in the bottom of the plenum of the intake manifold. Which might make it a bit "cold natured" and possibly extend the time it takes the choke to fully open. But not what I'd term a "rough idle".

To me, a "rough idle" can be the very early stages of a "burnt valve" or a cracked vac line somewhere. OR the carb idle mixture is out of whack.

Just some thoughts,
CBODY67

 

[cbarge]

Since you cannot plug in a scanner and read codes, Pull the plugs and read them to see which cylinders are lean or rich.

I recommend using a vacuum guage and timing light with advance feature at this point with a rough idle.
You can read the guage to see what's going on with rough idle.
When running perfectly, the goal is 18 to 20 inches of vacuum with the needle on the guage not jumping around or jittering.
When running you can pull one plug wire on/off one at a time.
If the gauge drops,cylinder is fine.
If you find a cylinder does NOT drop and the vacuum guage remains the same, that cylinder can be the culprit.
A cracked plug,a bad wire,etc
Set the advance on timing light to see where the initial timing is and also see where the full mechanical advance is (all in at 2800 or so RPM)
I run 10-12 degrees initial timing on my big blocks where they are the happiest with 32-34 degrees all in--which is nowhere near O>E specs.

Even with a rebuilt carb and fuel system,the idle circuit can still pick up dirt.
One idle screw may need more or less adjustment vs the other in order to run smooth.
By reading the vacuum gauge,as you turn the idle mixture screws the gauge will tell you if the engine needs more or less turns in/out.
If there is NO change,then the idle circuit is plugged.
Also if the dwell is not set correctly on the points,it too can cause a rough idle.
A stuck vacuum advance can also cause trouble. If the idle does not change when unplugging it,double check its operation and make sure it is using the correct vacuum port for the hose.
PLUS..there has been a pandemic of shitty condensors out there too.
Following factory specs for carb settings and timing with today's gas is a waste of time.
Corn fed gas run leaner and hotter so you may need to fatten up the mixture,possibly change the jets and metering rods and advance the timing more than the O.E. specs to avoid pinging and/or overheating.
You may have to run a spark plug with a cooler setting.

Some simple troubleshooting here,and hope this helps.
Cheers.

 

[RogueOne]

Is it rough idle all the time, only when cold, or only when hot?

Open manifold heat valve should only cause rough idle what engine is cold

It is idling rough regardless of hot or cold engine. That’s what’s got me puzzled.

 

[RogueOne]

A stuck heat riser will overheat and possibly boil the carb if it is stuck shut.

Dave

Good to know. It’s not stuck. It moves freely. The thermostat coil is there but I think it’s slipped off the stop pin.

 

[RogueOne]

Just how "rough" is "rough"? These engines did not idle glass smooth, no matter what. I tried and never got there.

If the valve is in the open position, it might extend the time it takes the heat from the cast iron cyl heads to migrate into the cast iron intake manifold. Until that fully happens, unvaporized fuel might be in the bottom of the plenum of the intake manifold. Which might make it a bit "cold natured" and possibly extend the time it takes the choke to fully open. But not what I'd term a "rough idle".

To me, a "rough idle" can be the very early stages of a "burnt valve" or a cracked vac line somewhere. OR the carb idle mixture is out of whack.

Just some thoughts,
CBODY67

I think it’s past acceptably rough even taking into account the age of the car. I’ve thought along the same lines that I was being too picky and I should just live with it but seeing the engine wobble and shudder non-rhythmically just doesn’t seem right. Listening to the tail pipe sounds like a bunch of empty cardboard boxes falling down the stairs. I’d be happy with the sound of just one cardboard box falling down the stairs.

 

[RogueOne]

Just how "rough" is "rough"? These engines did not idle glass smooth, no matter what. I tried and never got there.

If the valve is in the open position, it might extend the time it takes the heat from the cast iron cyl heads to migrate into the cast iron intake manifold. Until that fully happens, unvaporized fuel might be in the bottom of the plenum of the intake manifold. Which might make it a bit "cold natured" and possibly extend the time it takes the choke to fully open. But not what I'd term a "rough idle".

To me, a "rough idle" can be the very early stages of a "burnt valve" or a cracked vac line somewhere. OR the carb idle mixture is out of whack.

Just some thoughts,
CBODY67

I’ve gone over the vac lines a zillion times and adjusted the air/fuel screws up and down a bunch but the rough idle persists. I’m starting to lean toward it being a valve problem of some sort. I appreciate your thoughts. Thanks.

 

[RogueOne]

Since you cannot plug in a scanner and read codes, Pull the plugs and read them to see which cylinders are lean or rich.

I recommend using a vacuum guage and timing light with advance feature at this point with a rough idle.
You can read the guage to see what's going on with rough idle.
When running perfectly, the goal is 18 to 20 inches of vacuum with the needle on the guage not jumping around or jittering.
When running you can pull one plug wire on/off one at a time.
If the gauge drops,cylinder is fine.
If you find a cylinder does NOT drop and the vacuum guage remains the same, that cylinder can be the culprit.
A cracked plug,a bad wire,etc
Set the advance on timing light to see where the initial timing is and also see where the full mechanical advance is (all in at 2800 or so RPM)
I run 10-12 degrees initial timing on my big blocks where they are the happiest with 32-34 degrees all in--which is nowhere near O>E specs.

Even with a rebuilt carb and fuel system,the idle circuit can still pick up dirt.
One idle screw may need more or less adjustment vs the other in order to run smooth.
By reading the vacuum gauge,as you turn the idle mixture screws the gauge will tell you if the engine needs more or less turns in/out.
If there is NO change,then the idle circuit is plugged.
Also if the dwell is not set correctly on the points,it too can cause a rough idle.
A stuck vacuum advance can also cause trouble. If the idle does not change when unplugging it,double check its operation and make sure it is using the correct vacuum port for the hose.
PLUS..there has been a pandemic of shitty condensors out there too.
Following factory specs for carb settings and timing with today's gas is a waste of time.
Corn fed gas run leaner and hotter so you may need to fatten up the mixture,possibly change the jets and metering rods and advance the timing more than the O.E. specs to avoid pinging and/or overheating.
You may have to run a spark plug with a cooler setting.

Some simple troubleshooting here,and hope this helps.
Cheers.

I’m going to print this up and go through it step by step. You can’t go wrong with the basics. Thanks!

 

[CBODY67]

Does it get worse when in gear? Once you get it up to say 30mph or so, does it seem to smooth out with added speed on the road, but has much less power to go up hills and such while somewhat maintaining the speed from the bottom of the hill?

At the end of the tail pipe . . . if you might feel the pulses, are some stronger than others? If you might place a somewhat thick shop towsl at the end of the pipe, with a normal engine, the flow should keep the towel pushed away from the pipe, but if you watch very carefully (might even video it) you see the towel pulled back to the pipe, even cupping in to it, between pressure pulses, that is many times the evidence of an exhaust valve that is not sealing on the intake stroke and needs to be replaced. A burnt exhaust valve will not really hurt anything, but it needs to be replaced reasonably soon before it gets any worse.

Which means a valve job on that particular cyl head, new exhaust valve, and some attention to valve guide wear on that guide. When done, things will be smooth and powerful again.

A compression test with a screw-in tester can confirm the cylinders needing attention. SDing a cylinder balance test (where one cyl at a time is deactivated by pulling one plug wire at a time, either at the spark plug or at the distributor cap . . . carefully and you being well-insulated, electrically, so you don't get zapped!!!) . . . can also identify the weak cylinders. On the compression test, the good cyls will be a good bit above 100psi, but the bad ones will be more like 10-20psi, by comparison. In this case, it's not the numbers which are more important, it's how they relate to each other that is important . . . the gap between the good and bad ones.

Hope this might help,
CBODY67

 

[My three 67 Polaras]

Does it get worse when in gear? Once you get it up to say 30mph or so, does it seem to smooth out with added speed on the road, but has much less power to go up hills and such while somewhat maintaining the speed from the bottom of the hill?

At the end of the tail pipe . . . if you might feel the pulses, are some stronger than others? If you might place a somewhat thick shop towsl at the end of the pipe, with a normal engine, the flow should keep the towel pushed away from the pipe, but if you watch very carefully (might even video it) you see the towel pulled back to the pipe, even cupping in to it, between pressure pulses, that is many times the evidence of an exhaust valve that is not sealing on the intake stroke and needs to be replaced. A burnt exhaust valve will not really hurt anything, but it needs to be replaced reasonably soon before it gets any worse.

Which means a valve job on that particular cyl head, new exhaust valve, and some attention to valve guide wear on that guide. When done, things will be smooth and powerful again.

A compression test with a screw-in tester can confirm the cylinders needing attention. SDing a cylinder balance test (where one cyl at a time is deactivated by pulling one plug wire at a time, either at the spark plug or at the distributor cap . . . carefully and you being well-insulated, electrically, so you don't get zapped!!!) . . . can also identify the weak cylinders. On the compression test, the good cyls will be a good bit above 100psi, but the bad ones will be more like 10-20psi, by comparison. In this case, it's not the numbers which are more important, it's how they relate to each other that is important . . . the gap between the good and bad ones.

Hope this might help,
CBODY67

Good advice.

Funny I have had a couple of 67 Polara 2 barrels with these issues. First one had 141k on it, but the valves had been ground once before in the distant past. At that point I got it it needed a rering and another valve job.

2nd one had less than 90k and was well maintained. Lots of driveability issues. Worst was turning a corner on a left turn on a hot day. It would usually die half way through the intersection (giving a muffled pop through the carburetor) and I would use my momentum to carry me through and pull over to the side. Then I would say lots of prayers for it to start again. Most of the same symptoms of my 67 Polara 15 years prior to this one.

I was determined to get to the bottom of it and did a major tune-up and rebuilt the carburetor, including setting the float level. No change. I replaced the distributor with a remanufactured one. Replaced the fuel pump just as preventative maintenance. No change.

I tore the heads off the engine, flipped them over, and poured solvent in the combustion chambers. Three exhaust valves and one intake valve had heavy leakage. These engines really need hard exhaust seats installed for unleaded gas (and new exhaust valves). They surfaced them too, and replaced the guides. That ran about $650 at the machine shop in 2003.

No more problems.

I am not saying that is your problem. By all means, try the simple things first. But it might be helpful to post your compression test results for every cylinder. And it might also be beneficial to do a cylinder leakage test. Cylinder Leak Down Test - What Are The Results Telling You

 

[Loadrunner]

These engines did not idle glass smooth, no matter what.

In my mind they did idle glass smooth, properly tuned.

My balanced 383 in Roadrunner was so smooth it was weird, the 1st time I went for a drive in it and pushed it to near 7k - because it could do it with ease - and you couldn't feel anything.

I recommend it at least once, get an engine balanced. It will blow your mind.

Besides mechanical balancing, the cam has a lot to do with how lumpy the idle is, but with bone stock cam, it really should be smooth.

Cylinder imbalance perhaps?

 

[1970FuryConv]

It is idling rough regardless of hot or cold engine. That’s what’s got me puzzled.

It would be nice if you could try a known good carb to rule out any problems with the idle circuits in the carburetor.