Shavarsh HalfDork
9/1/22 1:15 p.m.

Hello All,

I have been thinking about alternative ways to keep a carburetor at a relatively stable temperature in a hot engine bay to aid with tuning consistency. My current setup (summit carb with thick plastic spacer) will run quite rich or lean (particularly on the idle circuit) depending on how hot the engine bay is. The established fix for this condition is to use an electric fuel pump with a return line so fresh "cool" fuel is entering the carb to keep it from overheating. I started thinking that keeping the carb at a stable temperature (no matter what it is) can be achieved in other ways. I've been thinking about ideas including engine bay fans, fuel coolers, and cooled baseplates. In my research I found that there have been heated/cooled carb baseplates used mostly for cold climate truck applications. 


My question is this:

Does anyone have first hand experience with a carb baseplate with water (from the cooling system or closed loop) circulating through it? I'd like to hear about the advantages/disadvantages you have experienced. If its a known bad idea, I'd like to know before building one to test.

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
9/1/22 1:23 p.m.

It might help knowing what the engine and carburetor is.  Qjet on an Olds 403 is a lot different than 4 Webers on a 4-cyl brit

For many engines it is a non-issue since the manifold will be the same temperature as the coolant after a few minutes.

For most carburetors it's a non issue because the choke circuit can be tuned to properly provide the right fuel.

Abnormally cooling or heating the carburetor might take it outside the engineered parameters for choke enrichment tuning.

I think what I'm saying is... if there is something out of whack with your fuel ratio, it's not due to a lack of a heated spacer, it's due to the fact that the carb isn't tuned correctly.

MiniDave Reader
9/1/22 2:10 p.m.

All good points.

On my classic Minis I run the heater core hot water thru the intake manifold, controlled by the heater valve which is before the manifold  - my theory is if it's cold enough to need the heater it's cold enough to need to heat the manifold to prevent carb icing or fuel dropout in the manifold. Frankly, I have no idea if it works this way or not as I can't seem to tell any difference in the running that couldn't just be based on temperature difference.

I do know some cars are pretty sensitive to intake icing over - my VW used to freeze up on cold damp days if I forgot to change the preheat lever in winter. (There was a lever on the intake that would switch over to preheated air from the exhaust)

Shavarsh HalfDork
9/1/22 5:00 p.m.

That's a cool setup miniDave!

Curtis, this is a summit brand 500cfm (autolite 4100 style) carb on the 302 in my pinto with a performer 289 intake. I'm curious what you think might be causing the carburetor to lean out when getting hot other than the fuel overheating in the bowls/idle circuit. The tune is very close at 190° F operating temperature however for awhile after hot start or while idling in heavy traffic it will lean out on the idle circuit.


I mentioned the electric fuel pump/return style regulator because I realize there are established ways to improve this behavior, but I'm having fun thinking of additional possible solutions. I believe that if the temperature of the carb body itself can be controlled it will return more consistent behavior.

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
9/1/22 5:54 p.m.

In reply to Shavarsh :

It's possible that your fuel is getting hot, but two things give me pause.  1) there are billions of windsors out there that don't have this problem, and 2) unless it's boiling, it wouldn't be causing a lean condition.

Is the dizzy hooked to ported or manifold vac?  Stock cam, or aftermarket?  What is your vacuum reading at idle?  Initial timing?  Compression ratio?  I don't know your experience level, but start with the basics.  It's really easy to get the timing and idle a bit off.  If the idle screw is just a bit too high, it uncovers the transition slot in the venturi.  That makes a lean condition on throttle tip-in.  Could be a weak fuel pump letting it not get enough fuel to support anything more than idle.

Edit:  I love fun engineering projects.  I won't poo-poo that idea.

Mr_Asa GRM+ Memberand UltimaDork
9/1/22 6:09 p.m.

That setup wasn't used just on cold weather applications, they got used all over the place.  Just about every stock I6 Mustang I've seen used it.

Shavarsh HalfDork
9/1/22 6:59 p.m.

In reply to Mr_Asa :

You're right, here is one I found for a 64-66 Galaxy:

Picture 4 of 10

Berck Reader
9/2/22 10:03 a.m.

I think those water blocks are more about preventing carb ice on cold days than they are about providing any meaningful cooling.  I wonder if you're actually vaporizing the fuel in the float bowl? I'd first focus on keeping the fuel delivery cooler--check the routing of the fuel lines to keep them as far away from heat sources as possible.

Mr_Asa GRM+ Memberand UltimaDork
9/2/22 10:14 a.m.

In reply to Berck :

It isnt about carb icing or carb cooling.  It is about getting the system to as close to a steady state that a car engine can be in order to reliably make the same power no matter the scenario.

You keep the carb at the same temp and you are going to get (more) predictable vaporization. 

Not about cooling.  Not about heating.

GameboyRMH GRM+ Memberand MegaDork
9/2/22 10:36 a.m.

Lots of factory vehicles have coolant running through the intake manifold or a carb spacer, but that's done almost entirely to eliminate carb icing. People looking to add power would often disable these systems to reduce intake temps (with questionable results).

That said, I think you'd be treating the symptom rather than the disease if you add heat just to fix the inconsistency with this carb, something else is wrong that's causing the mixture to vary wildly with temperature.

Berck Reader
9/2/22 3:59 p.m.

In reply to Mr_Asa :

Okay, great.  Never mind that "same temp" implies both heating and cooling.  Either way, I don't think one is going to solve the OP's problem, which sounds a lot more like overheated fuel...  Carburetors do a great job of metering liquid fuel, not such a great job of metering fuel vapor.

Shavarsh HalfDork
9/2/22 4:24 p.m.

Let me clarify a bit. There isn't a significant swing in the rest of the tune, mostly the idle circuit. The carburetor is set up "properly" to my knowledge with a "square" length of transition slot uncovered. If I drive for a long distance after hot start the idle circuit will behave again, I assume due to enough fuel flowing to cool the carb body (although I could be wrong here). The fuel bowls are not emptying (no vapor lock) although there is likely some boiling going on in the fuel pump and line. I'm not so much interested in a "fix my car for me" type discussion, just wondering if there is some first hand experience on the board showing benefits or detriments of keeping the carburetor at a controlled temperature (likely coolant temperature). 

Berck Reader
9/2/22 4:56 p.m.

Ahh, got it.  My understanding is that these appeared in emissions timeframes for exactly the reasons stated.  But if you're not able to get the idle circuit adjusted so that it runs correctly after a hot start, that sounds farther out of range than one of these could possibly help with.  

I'm not suggesting vapor lock (which would imply fuel boiling in/before the fuel pump) but rather vaporizing in the carb before it's metered.  I've never personally had this happen with a car, but have experienced it with an airplane on auto fuel (which has a higher RVP than 100LL).  Made it run too rich at idle and too lean at full power.

Toyota solved the solved the hot start idle problem in the Land Cruiser by installing an electric fan that points directly at the carb that ran on a timer for 10 minutes after you killed the ignition.  One of the kludgiest factory solutions I've ever seen, but it does work.

jfryjfry SuperDork
9/2/22 4:59 p.m.

I have a feeling if the engine bay is hot, the engine is hot. And if they are hot, then the carb is hot already.  And as the engine cools, the carb would cool and the engine bay would cool. 

but, in pursuit of, you know, the science, try logging it with an infrared thermometer.   Log coolant temp, head temp, carb body temp, and maybe firewall temp under different circumstances and see if the carb's temp fluctuates much more than the coolant temp as everything else heats and cools. 


Mr_Asa GRM+ Memberand UltimaDork
9/2/22 6:57 p.m.
Berck said:

In reply to Mr_Asa :

Okay, great.  Never mind that "same temp" implies both heating and cooling.  Either way, I don't think one is going to solve the OP's problem

I didn't imply anything, my friend.  I flat out said that it does both.
It is possible that it won't solve the problem, but I've seen it solve similar problems.  It depends on what the underlying issue actually is.

Intakes don't get cooling, cylinders do.  Intakes, especially cast iron, retain a lot of heat and have no way to shed that heat.  An aluminum spacer with an aluminum carb on it and coolant running through it is going to do a huge job of preventing issues with heat soak even when the engine isn't running.

mopar67440 New Reader
12/5/23 1:29 p.m.

Hey Shavarsh ,

Were you able to get the answers or a direction to your post?

I'm looking to do the same thing. Use a carb spacer under a Holley 4150 with water circulating thru it and a cooler in front of the radiator to maintain a cooler, more consistent carb temp. Not trying to add heat. Trying to remove it! I already implemented many safeguards including a heat shield & 1" phenolic spacer, plus insulated lines & elec fuel pump.

The issue I'm trying to address is A/F is right where I want it until it's been running for over an hr or really hot days where A/F starts leaning out. The cooler would bring more consistency to the A/F.



I plan on measuring primary float bowl fuel temps before and after running, then implement my idea and check fuel temps again.



Shavarsh HalfDork
12/5/23 2:50 p.m.

In reply to mopar67440 :

I ended up trying a 3 barb fuel filter meant to prevent vapor lock. This didn't solve the issue, although it did have a noticeable effect in slowing down bad behavior. Are you using a return style regulator? That would have been my next step. 

Please follow up and let us know what you find.

I punted, and am in the process of converting to fuel injection. I also think a decent bandaid solution would be to install a manual choke cable, to be able to lower afr when the car warms up too much.


Edit: you can also look into the "carb cheater" by thunderhead289 on youtube for another approach.

Noddaz GRM+ Memberand PowerDork
12/5/23 7:15 p.m.

Perhaps something like this may help:


03Panther PowerDork
12/5/23 8:03 p.m.

The old fords had the water spacer long before emissions started hard core. 
I agree with Mr Asa on its use for stable temp (both heating and cooling)

His setup sounds exactly like the fords they were on; us young "hot rodders" always replaced 'em with a plain spacer. So I don't know if it'll help... but I'd try it and see. 

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