The mid year AFB carburetors have a part called idle compensator valve. Looking down on top of the carb you can see most of it in the secondary venture area. It's shaped like a small cross and it has something to do with vacuum. The man that rebuilt my carb left it out and just plugged the hole. Is this item needed.
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Re: Idle compensator valve, does your AFB carb have one
It's called a hot idle compensator. It has a bimetallic strip that will cause it to open at high temperature to lean an overrich mixture due to excess vaporization from the fuel bowls.
If it opens idle speed should increase. It was never clear to me if the one on my 327/340 ever opened up, so it may not make much difference in normal driving other than low speed in extremely hot weather.
There have been several discussions on this device by me and Stu Fox. IIRC he tested his in a pan of hot water to see when it opended up, but I think the test was inconclusive.
Duke -
Re: Idle compensator valve, does your AFB carb have one
If you have one in a Rochester they sure work; install one upside down and you'll find out. Car runs great until it warms up...then kaput.
Ask me how I know...Comment
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Re: Idle compensator valve, does your AFB carb have one
You can live without a hot idle compensator. Troublesome part and very hard to find. The '63 to '65 Fi units use them. Typically I include the part. I test them with a match and then use compressed air to cool them off. If they seal they are OK. The RPM is supposed to increase when the valve opens of course. But not by much. IN an FI car if it goes up 100 RPM that's a lot.
You didn't ask and this question is not about Fi's but be very careful buyin a NOS hot idle compensator. Study it closely when you get it. You see Pontiac used them a lot in the RP carbs and the old FI's. But Pontiac used two configurations. l is different from the Corvette.
One of the Pontiac ones will interchange with the Corvette and in fact RP used the same part number for both in later years. You know the old Chevy counter parts. One that fits all. Welllll that was a screwup or should I say an error.
Pontiac made a HIC valve that was not flat It was curved. Very hard to see but if you use one on an FI unit expect to never be able to idle the car down.
Story: When we re-restored a good part of my 63, the Little white car I restored the FI unit and quickly took it to my pal Dave Cardina. He was restoring the engine compartment and chassis. His buddy rebuild the engine. We could not get my own car to idle down. How embarrasing was that!!! And the car was due to be in Marlborough for a regional in one week. 750 miles away so it was.
Turned out that dummy me used a curved Pontiac HIC. Dave lives about 135 miles from me and I had zero parts. So we blocked the valve off with homemade thick gasket. Car idled down to 900 with no problem.
For export restorations I never leave the pesky HIC on. Invites problems.
Leave it off Chester. JDComment
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Re: Idle compensator valve, does your AFB carb have one
Fuel bowls are always vented into the upper throat of the carb so that if the air cleaner restricts and lowers the pressure in the air intake, the air pressure pushing down on the fuel is also reduced. A good way to maintain more constant fuel mix ratios.
The problem with that is that on a very hot day when the fuel leaving the tank may be close to, or exceed 100 degrees, the heat from the idling engine can raise the fuel temp to the point that it will start to vaporize in the bowl. If allowed to vent normally through the bowl vent in the throat of the carb you have fuel added to the mix that wasn't metered through the orifices intended to control idle mixture. You have an overly rich idle mixture.
This is what is known as "heat soak/percolation" The remedy was to externally vent the bowl when temperatures reached the point that the fuel would vapourize in the bowl.
So, the hot idle compensator, or idle vent as it is also known, is intended to open a vent to atmosphere outside the airstream when temps are high enough that fuel will vapourize in the bowl. Without it, on hot days you will get poor idle quality with stalling and you may notice some increased cranking to start after it does stall.
SteveComment
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Re: Idle compensator valve, does your AFB carb have one
As Duke referred to;I tested one in a pan of hot water on the stove. The test was inconclusive because once the water starts to boil it obscures your view of the valve and seat.
I ran hot summer tests with mine off and plugged for awhile, but could not experience any better or worse running of my beast. My reason for wanting to remove it was that it appeared as though it restricted the secondary ventures with the large flat tabs. Similar restricting tabs can be seen on the primary clusters on all 3720 series AFB's. The 3460 series carbs on 1963 Vettes do not have the tabs in the primary ventures. 3460s also have "vapor vents" in both primary's and secondary's (little holes above the butterfly's through to the exterior of the carb body) that 3720 series do not have.
I have since learned through reading the AFB books that Carter used the Tabs to overcome an intake manifold deficiency on L-75 engines. Apparently, no such deficiency was noted with the L-76 aluminum manifold, on 1963's anyway.
While led on the subject of comparing 3460's with 3720's, besides some minor linkage differences and of course air and fuel jetting, the secondary air valve weights on 3460's are lighter by several grams. Bet you needed to know that, but the point being is that the 3460's are more for the SHP engines than the 3720's are. So, for anyone contemplating replacing their 3460 or 3461 with a 3720 or 3721, be aware that you will loose some slight performance edge (big deal, right?).
Stu FoxComment
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Re: Idle compensator valve, does your AFB carb have one
Try testing it like you do a thermostat. Stick a piece of string between the lever and the seat and hang it in your pot. Put a thermometer in the water and record the temp at which it falls off the string.
SteveComment
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