76 340 duster overheating at idle

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Those fan blades are too far away from the radiator. You'd probably be better off putting a big 18/19 inch blade fan closer to the radiator just to pull air. Yes the proper shroud and fan combo will solve your issues with some more timing.

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Bewy said:
Hard to see from the pics, but looks like a BIG water pump pulley. WP pulley needs to be 10-20% smaller than the crank pulley; that speeds up water flow AND fan speed. Unless the pump pulley is smaller, you will be wasting time & resources trying to fix the problem.
If engine is stock, set initial timing to 15*.

More info here: www.stewartcomponents.com
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Alright so what size pulleys are everyone running?
My water pump pulley diameter is 6 1/2” across.
Crank pulley diameter measures 7 1/4”
I do have a smaller water pump pulley but it won’t work with my setup.
 
A smaller pump pulley might help, but I think you have something more fundamental going on. Could the rad be blocked? I do not rely on IR gun temp measurements. On an engine that is hot after running for awhile, everything is hot in the engine bay. So what exactly are you measuring?
 
Matthew Sermini said:
Hey everyone, this is a follow up thread from about a year or more back about my 76 duster overheating at idle. I’ve been busy so haven’t gotten around to my duster until recently. I finally got a temp gun and used it today to check out different spots on the engine bay while the engine was running/ shut off.

I have a picture of my engine bay setup right now and I know I need a fan shroud minimum. I’m prepared to redo the cooling system with a radiator and shroud from Mancini but I haven’t called them yet to order anything because I want to make sure I can get a radiator that will fit/ reccomendations from you guys.

Car was original /6 w ac. Currently has a 22” 3 row aluminum radiator (I don’t remember the brand). I’ve been told a 26” is the way to go but my core support opening is 22". I had bought a rock auto 26" 2 row open style radiator years ago and i remember it not fitting with the mounting locations. It was also probably junky anyways. I’d prefer an oem look for my car or period correct so I’d prefer to not put in electric fans or another aluminum radiator and shroud if possible. I know im asking a lot but that’s what I’m going for thanks.

I also just installed a new fan clutch from summit, on my Oem 7 blade fan and that has definitely helped slow the overheating issue but it is still present.
180° or 190° thermostat. I don’t remember exactly but was also planning on getting a 180° high flow. Timing might need to get backed off but I haven’t put the gun to it since I built the car and am planning on getting the engine tuned once I know the cooling system works as it should.

Temp numbers after driving around this morning then idling at my house for about 10 minutes until the temp gauge was creeping up

Upper hose: 170.9°
Lower hose: 171.1°
Thermostat housing: 207.6°

A minute or two after shutoff
Upper hose:186.9°
Lower hose: 182.4°
Thermostat housing: 221.7°
Center of intake: 241.3°

View attachment 1716278531

View attachment 1716278532
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Slant six cars got a much smaller opening in the rad yoke for the rad than the V8 cars. If you want to put a V8 in what was originally a /6 car, you need a bigger rad opening. So check the junkyards for a car originally equipped V8 and have them remove the rad yoke side panels. Install those in your car and put in the 26" rad with a shroud and a clutch driven fan. I'm surprised that you're surprised it's overheating. Putting a V8 in what was a /6 car, especially a built up V8 should give you exactly what you're seeing, every time.
 
340 A bodies got 22" radiators

440 B's got 22" radiators.

It's not the size of the radiator, it's most likely the tune up and lack of shroud.
 
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Start by getting a shroud, I am willing to bet that with just the proper shroud your issues will be over. Put a vacuum gauge to manifold vacuum and adjust your idle speed screws on the carburetor to the highest vacuum you see and then give the screws another 1/4 turn rich, make sure you have no vacuum leaks. I have seen cars running hot at idle because they were too lean on the idle circuit. You are having heat soak issues, make sure that you have a thick gasket under the carb like what was used with the stock thermoquads on the 340 engines, that will help mitigate some of the carb heat soak, you could also put some insulation on the fuel filter and fuel lines going to the carb.
 
HSDemon said:
Start by getting a shroud, I am willing to bet that with just the proper shroud your issues will be over. Put a vacuum gauge to manifold vacuum and adjust your idle speed screws on the carburetor to the highest vacuum you see and then give the screws another 1/4 turn rich, make sure you have no vacuum leaks. I have seen cars running hot at idle because they were too lean on the idle circuit. You are having heat soak issues, make sure that you have a thick gasket under the carb like what was used with the stock thermoquads on the 340 engines, that will help mitigate some of the carb heat soak, you could also put some insulation on the fuel filter and fuel lines going to the carb.
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I currently am running a carb spacer but insulating the fuel lines I’m sure will help too!
 
Hood to radiator support seal in place? Lower splash pan to radiator support seal in place? They help funnel the air through the radiator opening, not around it. And of course a shroud.
 
I would throw a 1/4” phenolic spacer under the carb and definitely a fan shroud.
 
AJ/FormS said:
FYI:
I run a stinking ancient, factory 318 rad circa 1973>75, in the factory slanty core support, with a 400plus hp 360 which will pull itself around all day at 550rpm and with 5 degrees of advance.
The rad in your pic will be fine, AFTER, you do the following;
1) install a shroud, and
2) Restrict the bypass hose,
3) move the clutch up closer to the rad, and
4) install a hi-flow pump, or at least an anti-cav. plate on the current one, and
Probably optional for you are;
5) If you have to, install a Thermostatic clutch or a direct-drive fan and
6) reduce the size of the w/p pully closer to the crankpully or slightly smaller than

Opinions;
1) A bigger rad, in your configuration, will NOT solve your problem.
2) After the system is functioning correctly, you can try a thermostatic clutch... but not a chance would I install a standard viscous-drive clutch, on an engine with a history of running hot..
3) as you have found out, measuring the hose temperature is meaningless.
4) IMO, This is an easy fix, and you will not need a bigger rad.

What you are actually looking for is a DIFFERENCE in temperature of close to 30 degrees, (minimum 20*) between the inlet side of the pump, and the outlet side of the stat house. Your system can't do it because;
1) not all the air that the fan is pulling, is coming thru the rad. Without the shroud, the air is just sneaking up from below, or simply recirculating the hot underhood air. The pump pulls whatever is easiest.
2) The pump is recirculating hot water from the bypass hose
3) the pump may be internally bypassing water around the open-sided vanes, and or, may only have a 6-blade impeller. and
4) if your pump-pully is BIGGER than the crank pulley, then it will be turning at fewer rpm than the crank. Most guys will tell you that this is a no-no. Whereas, I maintain that, with a pump that has the anti-cavitation 8 blade impeller, AND with a restricted bypass hose, I'd leave that change for last.

Other
>I would probably reroute that fuel line back to where it belongs, NOT running above the heater hoses and header. At idle, your header-pipes may run in the 400s, whereas the intake at 200 or so. Your IR gun cannot measure air-temp, lol.
>How close is your battery negative cable to getting into the fan?
> your KD spring looks a lil wonky to me.

Happy HotRodding
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I do not think moving the Clutch closer to the radiator is the answer. I once had a heating problem in my 340 reaching a temp of between 205 - 215 and I found that moving the fan back from the radiator allowed the fan to pull more air over more area of the radiator. My heating problem went away and will run between 175-180 and on a HOT day the highest I seen it reach sitting in traffic was around 195 and once when moving the temp drop quickly back down. I have a 180 degree thermostat along with a 22" shroud
 
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