Topic: Hot Idler Compensator

[ChrisV289]

Was at my friends shop again and he had a 289 sitting there out of a Dearborn car that had the original setup still intact. Thought I'd post it.

[CharlesTurner]

Not seen very often these days, think I have one somewhere too.

[Hipo giddyup]

I have one too and have no idea what it's purpose is... Comments?? It's off of an early 65'...

[ChrisV289]

According to Bob's book "it opened slightly under high engine compartment temps causing improvement in idle stability."  Says it was on AC cars, so did all AC cars come with this? The car it came out of did not have a compressor on it but the holes in the firewall were punched out...

[J_Speegle]

Think there are a few pictures of these posted on the site up through  1967 applications.

[Murf]

The hot idle compensator is simply a bi-metal strip connected to a rubber stopper that opens when the temp in the engine compartment exceeds a certain value.  When it opens it creates a vacuum leak in the PCV hose and increases the idle speed - the thinking was that the increased speed would allow the fan to draw more air through the radiator and cool down the engine.  In reality it really did not help much and in fact was pretty touchy about seating properly after being opened by the bi-metallic strip and caused lots of erratic idle problems when in use. Most were replaced by a short piece of copper tube or sometimes the strip was wrapped in black tape to keep it from opening the little stopper.  They were included on all factory equipped AC cars in at least some of 65 and I think in all of 66 but today are pretty few and far between.  When they work they are just marginal in effect  but mostly they were discarded due complaints from customers.  I still have one in use on a 66 and with an understanding of what it does, how it does it and a little tender loving maintenance it works as it should.  They were, along with a clutch type fan, a good indication of a real factory equipped AC car. 

[67gta289]

A 67 variety from 7F01C129. This was an AC car.

[WT8095]

This one was on a '68 390GT motor I picked up. Engine code was 322, date 7M. I don't know if the engine was out of a Mustang, though. It appears to be an original installation - the ends of the hose were very cleanly cut. Note the lack of hose clamps though. The hose DID have clamps at the PCV and the metal tube.

The bimetal strip was taped by a P.O., it was either broken or not functioning properly.

[ChrisV289]

How early were these installed on Mustangs?  And is it something judges should be looking for when judging a car that has AC?  I don't show it listed on the judging sheets.

[lancelot66]

I had no idea our car should have had one of these until I read through Bob Mannel's book and this was described. No doubt, it was tossed by a PO or mechanic. Murf, you couldn't have described the purpose and function any better. Very well done. So I spent a few months on eBay and finally found an original setup with hoses, clamps and the "Carburetor" on the underside. Will go on the engine when I get it all back together. Kinda at Winter standstill in an unheated garage, so no painting, gasket sealing, etc. until we get some warmth here in the NW.

[69cobrajetrugae2]

Here is a 4100 C4AF-DH, FORD PART NUMBER C3AZ-9510-AK.

I am not a carburetor specialist so when I go through a unit I make an effort to understand the operation and test as many circuits as possible. 

In the sixties there was a problem with fuel percolating out of the carb and flooding the engine in hot weather. Perhaps the reid vapor pressure of straight run gasoline of the era was partly to blame, I don't know.

I noticed the idle compensator valve and traced the circuit. It seemed to me that it would begin to open at 135F and fully open at 195F when  a car was turned off in hot weather when heat soak would boil off the fuel in the float bowl and flood the engine upon a restarting the vehicle.

The purpose of the valve as I see it is as follows.

1. Open and help vent the vapors of the raw fuel to the air cleaner as it changes state in the intake manifold when the car was turned off and sat in the parking lot.

2. It would remain open when the car is started to add additional air to lean out the now, very rich, air fuel ratio. The airflow of the running engine would cool the valve well below the 135F to 195F where it opened and close it.

It doesn't make sense that it would open when the engine is running to cool it off.  Not to me anyway.

First of all, the additional air would lean out the engine since the air is coming straight down a non metered hole which would make the engine run even hotter, some of the cooling effect of the fuel is removed in the now lean condition.

In addition, speeding the engine up to cool it off is like a dog chasing it's tale. The engine is burning more fuel and the automatic transmission is engaging creating a load and more heat to the cooler which is in the radiator. Advancing the timing would do a much better job of removing heat.

Lastly, the valve on my carburetor begins to open at 135F and is fully open at 195F. A running engine is pulling in outside air which is partially heated by the engine compartment, but should be well below 135F. If this valve was designed as a hot idle increaser the control range would need to be much closer to avoid nuisance leaks. If a engine is overheating or is in danger of overheating, this little valve isn't going to do anything to help the situation.

[WT8095]

The primary purpose of the compensator is to correct an overly rich mixture during a hot idle condition. When idling (i.e. the car is not moving), airflow through the radiator and into the engine compartment is reduced, which means the engine is running warmer and the underhood temperature is higher. Hotter air under the hood means hotter air into the carb. Under these conditions fuel atomizes more readily, leading to an excessively rich condition. Adding a little bit of extra air helps compensate for the condition. That's the design intent, anyway - how effective it is can be debated.

[69cobrajetrugae2]

Whatever the thing does, the one on my carb cracks open at 135F and is fully open at 195F.

Dude, one of us needs some coffee.  First you say rich, then lean, you almost got me confused.

I believe that the fuel is metered into the primary's by way of the venturi effect. Hotter air is less dense therefore it will pull less fuel out of the carburetor, but the air fuel ratio is the same, the engine is simply unable to pull as much air mass in as a cold day. I don't understand how atomization leans out the air fuel ratio. I don't understand how creating a vacuum leak will richen the mixture.

 I remember seeing lots of cars with the hoods up in the summer at high altitude rest stops as a kid in the sixties. My 428 has done it a couple of times, I smelled a whiff of fuel and she didn't start with a snap as she normally does.

[WT8095]

Dude, one of us needs some coffee.  First you say rich, then lean, you almost got me confused.

My error, I meant to say rich, not lean. I'll correct it in my post.

[69cobrajetrugae2]

Correction, a lean burn misfire is rich as the lean mixture was sent out unburned. You are correct that lean can be rich with that situation.

The 67 Impala wagon with the 327 275 HP Dad bought had a high altitude devise on the carburetor, I wonder how it worked?

Dad was convinced that it worked while climbing through Bishop one summer.

I found this on a quadrajet discussion

Some applications from 1967 to the late 1970s used a hot idle compensator that was located in the rear of the float bowl (our feature car had the place in the casting, but was not equipped with one). It was used to compensate for an overly rich mixture from excessive fuel vaporization in high-heat conditions. A bimetallic spring that responded to temperature would then open a port and introduce fresh air into the idle circuit to lean the air/fuel ratio. .