Percolating?

[SmithKid]

Recently (since warmer weather) I've noticed a strong smell of gasoline in the garage after shutting it down. I strongly suspect the carburetor is percolating as I have no spacer under the carburetor (Edelbrock Performer 600). Has anyone got experience with the best type spacer to use? My quick web-searching reveals there are several configurations and at least two materials. Even Edelbrock shows several different spacers on their web-site.

[redfalken]

I've got phenolic on mine. I've never had heat problems with the carb but installed it as insurance. When I researched at the time, phenolic had the best insulating properties and is very durable.

[SmithKid]

I also find there is a spacer/insulator made from a wood laminate. I had never heard of that, and was wondering if this is something new and wonderful. Summit has these. Some of the configurations I find are:
(A) One large hole for all 4 venturis. (B) Two large holes, one for each side. (C) Four holes; one for each venturi. However, they don't seem to be manufactured with the 4 holes sized to match the venturi size.

[ew1usnr]

You might want to check your gas cap gasket. If it isn't sealing, you might be smelling gas from the tank rather than the carburetor.

[redfalken]

Here's a nice little write-up I found on eBay:

How to Choose a Carburetor Spacer

It seems like there are hundreds of kinds of carburetor spacers on the market today. But no matter what it looks like or what material it's made from, a carb spacer has two tasks: insulate the carburetor from engine heat to keep the incoming air and fuel cooler, and act as a tuning aid to produce more power.


Spacer Materials


Let's start with a spacer's insulation properties. These are largely determined by the material a spacer is made from. There are four types of spacer material: wood, phenolic resin, polymer (plastic), and aluminum. Each has a different thermal conductivity (heat transfer) rating that tells you how well the material insulates against heat. The lower the rating number, the better the material insulates. Let's look at each type:
Aluminum is the worst insulating material of the bunch, with a thermal conductivity rating of 1,665.1. It does offer three advantages-it's very durable, are easily altered to help tune the intake charge, and is the only spacer material allowed by many racing sanctioning bodies.
Wood is the best insulating material, with a thermal conductivity rating of 1.02. It's also cheap and easy to shape. However, wood is the least resistant to gasoline and chemicals. It soaks up liquid easily, causing the spacer to warp, become porous (causing vacuum leaks), and break down faster. If you use wood spacers, inspect them regularly and replace if necessary.


Phenolic Resin has a thermal conductivity rating of 2.01. While not as good an insulator as wood, it is far more resistant to fuel and chemicals, making a resin spacer more durable.


Polymer is less expensive than phenolic resin. But with a thermal conductivity rating of 3.90, it does not insulate as well. Some polymers have been developed that insulate nearly as well as phenolic resin, so it pays to do your homework before you buy.

Spacers as Tuning Devices

Spacers fall into one of two categories. Open plenum spacers increase the total area of the intake manifold, which helps build mid-range and high-rpm horsepower. That makes them ideal for racing. Wood and aluminum are the better material choices for racers who reshape or stack spacers. Phenolic resin and polymer are good choices for spacers that won't be altered. Sanctioning body rules often dictate the type of spacer material you can use.


Four-hole plenum spacers are designed to increase the velocity of the air/fuel charge, which helps build low and mid-range torque. Spacers made from phenolic or polymer are ideal for street cars and trucks, providing the best balance of insulation and durability.
Spacer designers and racers often play with spacer configuration to get specific tuning effects at certain points in the engine's rpm range. Aluminum and wood spacers are ideal for this as their openings are easy to reshape with a die grinder.


Another tuning trick is to stack open plenum spacers. This increases the spacers' insulation factor and creates a mini-tunnel ram effect by giving the incoming air and fuel a taller, straighter shot into the manifold. Aluminum, wood, and phenolic resin spacers can be stacked.


Now that you know some of the basics concerning carburetor spacers, you should have a better idea of which type fits your needs.

[SmithKid]

Wow! Great info, Kenny. Thanks very much. I never thought to check eBay for info.

[Nathan289]

You will also find carb company making plastic carbs now too..


Wonder if it has anything to do with ethenol?

[BillP 98201]

It never ceases to amaze me how much info our senior members have, or find. Great job again!

[Luva65wagon]

You will also find carb company making plastic carbs now too..

Wonder if it has anything to do with ethenol?

Absolutely does. We deal with fuel monitoring systems at work http://www.floscan.com and the issue with ethanol has been plaguing us for a long time now. Car makers of course have shifted to all plastic fuel tanks and lines (where practical) and so it doesn't surprise me to see plastic carbs too. Interestingly the items we had the biggest issue with were diaphragms - like those used in accelerator pumps of carbs. Lot's of tweaking and testing compositions of materials to get these to last. I doubt that most carb-kit makers address this in their kits. Just a reason to stay in business for them.

Of late we've been dealing with ABS (the governing body, not the type of plastic) that is mandating components with fuel in them on boats withstand something like 1800 degrees for 5 minutes without melting (or some such number). So we are now redesigning sensors to be all stainless steel. Plastic carburetors would not pass ABS regulations.

From the sounds of it, talking with Gene last night, he's picked out a spacer. Gosh, now that I think of it, Gene, I hope all the linkages still reach...

[SmithKid]

I hope the linkage work, too. From what I read, I decided I want a 1" spacer, and that's what I ordered. My fingers will remain crossed until I find out if it'll work with my custom linkage.

[ew1usnr]

"Aluminum is the worst insulating material of the bunch, with a thermal conductivity rating of 1,665.1."

This is not the complete story, though. The aluminum is separated from the engine and carburetor by paper gaskets top and bottom. The conductivity of the paper gaskets should be similar to wood because they are made from wood fibers. The gaskets can be stacked if more insulation is required.

Heat would also be carried to the carburetor by the steel mounting studs regardless of whether the spacer were aluminum or phenolic.

[falcon cobra]

Try insulating the fuel line also, that foil covered bubble stuff really works good. I put a street demon carb on my falcon with a polymer center [ float bowl ] and it work great. no stink in the garage after a hot day anymore, I love it....jh

[ew1usnr]

I followed the suggestions in this thread and 1) added four more paper gaskets beneath my carburetor to make a 1/4" stack, 2) added four nylon washers under the nuts that screw onto the mounting studs, and 3) covered the fuel line with insulation at the point where it is near the exhaust manifold.

See:http://www.designengineering.com/category/catalog/design-engineering-inc/line-hose-protection/cool-tube
Technical notes: Nylon 6 is the most common commercial grade of nylon and the melting point of nylon 6 is 428ºF and it has a thermal conductivity of 0.25 where 1 W/(m.K) = 1 W/(m.oC) = 0.85984 kcal/(hr.m.oC) = 0.5779 Btu/(ft.hr.oF). By comparison the conductivity of paper = 0.05, aluminum = 205, and steel = 43.

Then I thought that even if the carburetor is thermally isolated from the engine, it would still have to heat up to whatever the engine compartment temperature is, and I wondered what that might be. I strapped a 0° - 220°F kitchen thermometer next to the air cleaner to measure the under-hood temperature and tried it out this evening.

1-DCP_4082.jpg

The air temperature was 85° and the thermometer read 160° after driving at 45 mph for a half hour. After the engine was turned off it read 135° after an hour and ten minutes with the hood down. After a short drive the temperature rose back up to 140°. I left the hood up after turning off the motor and after an hour the temperature had dropped to 120°. The volatilization temperature of summer-time gasoline is supposed to be 140°, so below that there should not be much gas smell coming off the carburetor and it should be safe to park in the gargage.