My carbs suffer from heat soak when I'm in traffic too long.

I've already installed (good) heat shielding, carb phenolic spacers, etc. I believe the issue is that when I'm sitting in traffic too long, there simply isn't good air circulation in the engine bay, the carbs are sucking in really hot air from just under the hood. It's ironic given all that room in there with the slant....

As another data point, when I switched to dual 10" electric fans on the radiator, the engine stayed nice and cool, but the heat build up in the engine bay quickly flooded my carbs. Switching back to mechanical fan cooled the carbs better.

So, I'm wondering if I install a cold air induction system will that keep my carbs cool from the inside? If there is cool air running through the carbs all the time, it should keep the gas in the bowls from boiling in traffic. Does this make any sense? Otherwise I'll need to figure out some fans or something to keep that air moving when I'm in traffic. I'd much rather cool them from the inside and not have to use fans.

I'm going to try it, but I'm wondering if anyone has experience with using cold air induction to address a gas boiling issue. It doesn't help that today's gas is more volatile, lower boiling, than gas of ye olde times.

brian

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P.S.: I've already designed a cowl induction system (in my head) that doesn't require cutting the hood (I hate hood scoops) and I'm going to implement it within the next few months. Meanwhile, i have to avoid sitting in traffic jams for more than 10-15 minutes....

b

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A few general responses here ......

I use a pair of tubes to the air cleaner, off the firewall side of the cool air box at the base of the windshield, for a cooler air charge. Not sure if it's helping with heat soak, as I no longer connect the manifolds together either.
The resultant occasional gas fumes on starting in hot weather may not be acceptable to you.

I've gotten good results in engine compartment air flow with some holes at the rear of both vertical fender well walls to let air out the back better.

This only relates to carb heat indirectly, but I've gotten my best results in overall engine temp control with a combination of a three row radiator and re-balancing the system by deleting the bypass hose in favor of a 3/16" hole in the top rim of the T-stat.

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Duct your air intake to the front of the engine bay. That way you get cool air to the carbs, without gassing yourself and creating a severe fire hazard as you very easily can with a homemade cowl induction setup.

Also, what's your fuel line material and routing from the fuel pump to the carbs? What kind of carbs are these? And if you're running at least half a stock exhaust manifold, does the heat control valve work correctly?

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The difference between the volume of air two 10" electric fans move and what the factory stock belt driven fan moves is a large. Add the fact that electric fans don't always spin which slows or stops any fresh cool air entering engine compartment when car is standing still.

The factory fan moves enough air though engine compartment to keep it cool enough to prevent hot fuel problems when stopped in traffic.

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First, thank you for all the good feedback, very helpful.

I haven't foreclosed getting cool air from the front, but the design I had in mind for that option was a bit more complex. I was wondering about gas fumes in the cowl.

I use a small block v8 radiator and a side emanating thermostat housing, so my top radiator hose crosses in front of my fan; I believe this blocks some airflow to the carbs. I had used a goose neck custom thermostat housing that positioned my top hose above the air flow - I'll go back to that over the side directed thermostat housing. Also I can fabricate a fan shroud that has a divider to direct airflow at the carbs more. Do you have pics of the holes you describe to get more airflow in the engine bay?

Dan, I use two 32/34 DFT Webers. The issue is that the power valve in these uses a spring and membrane, rather than a check ball type valve. I did some reading on this. With the Webers that use a plastic check ball, folks swap for a metal ball and this apparently keeps the hot gas from boiling out of the bowl. For mine, I could try a stronger spring. Still, I think the answer is better cooling. I can't run higher fuel pressure to overcome this as the Webers run at 5 psi or less. I have heat shielded my way to more traffic time, but ultimately I need to keep them cooler.

Brian

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Not just toxic and noxious for vehicle occupants to breathe, but think about the blower motor—the unshielded blower motor that sits there generating a little stream of sparks between the brushes and the commutator.
Great...so what's your fuel line material and routing from the fuel pump to the carbs? And if you're running at least half a stock exhaust manifold, does the heat control valve work correctly?

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Dan, thanks for adding the blower motor point. I should've noted that I don't use the factory blower system.

Here's the only shot I have that includes even half of one of the holes mentioned (upper right border). Sorry, I didn't photo that mod when I did it.

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I use an electric fuel pump, to a pressure regulator on the passenger fender, well away from heat, then insulated lines to the carbs (SS braided over rubber with teflon gas line in the center). the gas lines pass over my alternator bracket, around the front of the valve cover and over my heat shield and to the carbs, so they are insulated well and generally away from heat.

Here is a pic showing how the fuel line runs from a filter up to a small in line pressure gauge and then into the fuel pressure regulator, splits into two lines, goes over the alternator bracket, around the corner of the valve cover, and to the heat shield and carbs. I suppose where they are suspended above the front most arm of the dutra dual would be where they would see the most heat, but I've put a heat shield over that since this pic was taken, no difference.

http://s785.photobucket.com/user/67dart ... n.jpg.html

I now have double heat shields, the one you see here (and I put additional ceramic insulation on it) and I put another one over the exhaust manifolds. See below pics:

http://s785.photobucket.com/user/67dart ... .html?o=35

http://s785.photobucket.com/user/67dart ... .html?o=30

http://s785.photobucket.com/user/67dart ... m.jpg.html


The heat valve in the manifold isn't the cause. Though I have it fixed in the "on" position now, it was doing this prior when the valve was working properly as well. I do have a short period of rough idle when it's below 40 degrees, but it's only about 60 seconds.

Good point on the gas fumes/sparking though.

I think ultimately it would be better to have fuel injection so I could use higher fuel pressure like today's fuel is designed for. But for now, it's fun to play with things to keep the carbs cooler.

Brian

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One observation on a possible condition you've not noted specifically.

Your heat shield appears to be aluminum, and rather a large surface. Is it possible that it's soaking up general engine bay temp, and becoming essentially a "hot plate" under the bowls, thus contributing as much to the problem as it is correcting it?

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Have you considered adding a fuel return line so the carbs are always receiving cool fuel without vapour bubbles in it?
Not seein' the filter in the pic.
That's going to greatly aggravate the exact situation you describe.
Well, yeah.
You'd be having the same issue, or worse, with yesterday's gasoline.

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Dan, the filter is below that pressure gauge, down by the frame where it's pretty cool. Recirculating the fuel with a return line at the filter wouldn't make it any cooler, maybe a few degrees cooler, maybe. Perhaps if I could recirculate at the carb it might help more.

My heat shields over the manifolds are SS, and my shield at the carbs is aluminum, yes. I don't think it is heating the carbs, specifically acting as a big heater. The Al heat shield reduced heat at the carbs significantly and when I added the SS shields between that and the exhaust manifolds it went down another 15 degrees at the carbs.

Dan, if that heat riser is always open during operation (once the engine warms up) then how could it make the issue worse by being in the position it always is during operation anyway? I can tell you that the carb issue is no different before or after I tacked that flapper in place.

I believe the issue is bad circulation of air in the engine bay. When in traffic, it builds up pretty quickly. If I idle in the drive way with the hood open, I can idle for a long time with no heat build up on the carbs, but with the hood closed it's a different matter. I've done some trials in the garage with the hood closed and when it starts to boil over, I open the hood and take temperatures and the air cleaners are really hot, while down by the heat shield and phenolic spacer it is much cooler - hence my theory on super heated air intake being the problem. My air cleaners suck air from directly under the hood surface and at the highest spot under the hood; while even factory snouts were pointed away from that and lower. I think I've just not factored for the geometry and location of my air cleaners, they're in the worst possible spot there is as far as sucking in hot air.

Also, for some reason this engine bay simply has bad air circulation geometry. The engine bay temps rise dramatically when sitting still with the hood closed; when driving, even 10-15 mph, enough air is forced in there to circulate the hot air out.

I'm hoping that cold air intake will keep the carbs cool from the inside; I've read other's have done this and it helps/solves their issue.

Brian

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Yup, that would be the idea.
Slant-6 heat risers don't have an "open" or a "closed" like V8 ones do. Slant-6 heat risers have heat-on and heat-off. You said "I have it fixed in the 'on' position".

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...and, regarding the engine itself, the radiator I have keeps the engine temperature steady as a rock...when I'm in traffic it's not the engine that gets hotter, just the carbs. My temp gauge moves very little during these episodes, the cooling system is overkill for my application, small block V8 radiator with triple core. I think that's rated for like a 700 hp engine while I'm nearer the 200 hp mark...

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How much fuel pressure are you running?

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