Hot Restarting

[George C.]

I am having problems with my 1970 LT-1 starting when hot. I am being told this problem is due to modern fuel percolating in the carburetor and I should install an insulated carburetor base gasket, spacer or shield.

I have checked Doc Rebuild and he does not have any of these items in stock. The heat raiser is wired open.

I want to keep the car original, but I still want to drive it.

Has anyone had this problem and how did you solve it?

[John D.]

Re: Hot Restarting

I had a similar hot start problem with my 67 L79 and it got much better when I wired the heat riser in the open position.

[Keith W.]

Re: Hot Restarting

George
Years ago I had the same issue - vapour lock fuel in carb (what ever that was) and header pipe running too close to the solenoid - always embarrassing hanging around a petrol station or trying to fill your car at the furthest pump - best of luck

[Joe L.]

Re: Hot Restarting

I am having problems with my 1970 LT-1 starting when hot. I am being told this problem is due to modern fuel percolating in the carburetor and I should install an insulated carburetor base gasket, spacer or shield.

I have checked Doc Rebuild and he does not have any of these items in stock. The heat raiser is wired open.

I want to keep the car original, but I still want to drive it.

Has anyone had this problem and how did you solve it?

George-------


First of all, what sort of hot restart problem are you having? Does the engine crank excessively but not start or does it not crank or crank very weakly?

As far as base gaskets go, I'm very surprised that Dr. Rebuild does not have any of the insulating type base gaskets in stock. He's usually a very reliable source for these kind of parts. In fact, I'd say he's the "King of Carburetor Base Gaskets". In any event, Corvette Central has the 1970 w/EEC style insulation for your application under their number 353197. This is an aluminum plate heat shield combined with base gaskets. The 1971-72 style 1/4" thick insulating gaskets for Holley carbs seem to be a lot more difficult to find these days. Originally, they were GM #3999259 but that's long-since GM-discontinued. Holley #108-12 might work.

Keep in mind that using a thicker-than-stock base gasket will also require a longer auto choke rod.

[Duke W.]

Re: Hot Restarting

Your emission controlled engine has severely retarded spark advance at idle and low load that throws a lot more heat into the engine. Setting up the spark advance map similar to the 365/375 HP 327s will go a long way toward cooling things down and making hot starts easier, but the lower mid range boiling point of E10 is definitely a culprit. Insulating all engine compartment fuel pipes and hoses also helps with documented temperature drops of 20-30 degrees. There is at least one long thread on this topic with photos that you should be able to find by searching.

[Frank D.]

Re: Hot Restarting

"Heat Soak" is a common problem with today's gas with carbureator cars. After 20-30 minutes of sitting after a hot run the heat can migrate into the carb raising fuel bowl temps. You can actually hear gas percolating very often when this happens, another indication is fuel dribbling out of the carb's nozzles. The raw fuel sits on the intake runners and when you go to start the car you are basically starting a flooded car. Which is why using starting techniques for flooding work.

To wit, holding the gas pedal on the floor while cranking. A less stressful starting technique is to crank the engine with your foot OFF the gas for 3-5 seconds; then slowly depress the gas pedal about 1/3 of the way down so as not to activate the accelerator pump and then crank the engine. It should light off immediately and run rough for a few seconds but then straighten out.

Phenolic or wood carb spacers are often used to mitigate the problem....as noted this can cause hood clearance issues, cause re-jiggering of carb/throttle linkage and often requires longer carb-to-manifold studs..

An infrared temp gun aimed at the fuel bowls after a hot run and letting the car set for 25 minutes can confirm the issue -- anything north of say 140* is getting into the trouble zone..

Vapor lock is generally described in different terms and it can occur as you are running down the road. Fuel entering the fuel pump is vaporized and the result is fuel starvation at the carb. And, no, clothespins on the fuel lines do not help.

[Steve G.]

Re: Hot Restarting

There is a lot of misinformation around about this "percolating fuel". What is now being blamed on the fuel was correctly diagnosed back in the day as just plain old carb/choke problems, too high a float setting, float damage or fuel leaking past the needle and seat, choke not coming off properly. As we found ourselves with fewer mechanics that know about carbureted engines we seem to have more percing fuel incidents.

According to the people that actually make the fuel, fuel perc and vapour lock peaked with fuels made in 1985 and have constantly been dropping to now being non existent.

Fuel that doesn't vapourize doesn't burn. In order to get the fuel to vapourize properly with a carburetor you had to add heat, lots of it.That's the primary reason for the exhaust crossover and heat riser. Fuels were formulated to vapourize easily to aid the process. It was a balancing act to get full vaporization and not have vapour lock.

Modern fuel injection atomizes the fuel by forcing it through a nozzle under pressure. This vastly improved atomization eliminated the need for the heat riser/exhaust heat and allowed the fuel manufacturers to make the fuel less volatile. This also helped reduce evaporative emissions.

Things to consider:

You will have a hard time keeping under hood temps below 140 with the air that fills the engine compartment blasting through a rad filled with 180 degree coolant.

If you reduce the temp at the base of the carb, what's going to vaporize your fuel properly? Poorly vaporized fuel results in incomplete burning of the fuel, higher fuel consumption, fouled plugs, cool temp stumbles, poor to non existent choke function...

Why do only a handfull of cars seem to have this "percing" problem with everyone using essentially the same fuel formulation.

When we encountered this problem back in the days of more volatile fuels, we just fixed them. Find someone that knows the systems, not just how to restore the finish on the carb. I've got pics of absolute junk that came from at least one of the high profile "carb specialists" .
Steve g

[Larry E.]

Re: Hot Restarting

FWIW: Picture below is the GM plate/gasket. It came two ways;one for Holley's and one for the Rochester Carb. I
bought several of each back then(Not for sale) and they worked for me.
Also if the car is not used on a regular time schedule>Av Gas is your answer. Expensive but it will cure your problem.
Larry

[David M.]

Re: Hot Restarting

I'll toss my 2 cent hat into the ring. I own a nut n bolt restored 68 442 (we like 68 GMs). I could not get rid of the hot start problem for years. Tried everything. Part of the problem is the fuel. I witnessed it percolating out of the pull over ports on the Quadra-Jet. So I installed a laminated base gasket (stacks of aluminum and gasket material about 1.5" thick) and fine-tuned the timing curve and lowered the float a 32nd or two. It improved but not by much. Next I tried nothing but ethanol free gas. We're getting better its not burping over. I recommend ethanol free in anything carbureted. Ethanol rots carbs nicely. Next, I removed the stove flap and thermal spring from the driver's side exhaust manifold and welded up the holes. Still improving but not cured. The cure...I removed the behemoth intake and welded in cast iron block off plugs. No more hot soak. The divorced choke needed to be tweaked so it would pull off fast. An electric choke is an option here. No more burnt paint on the intake. The car is cold blooded and needs to warm up a lot in winter storage. Don't notice it on summer days. 100% improvement. This car like all others loves cool dense air. There's enough heat under the hood of this Olds big block with its 50lb 14" across intake, that atomization inst a problem. I know this won't help the guys trying to stay 100% performance correct, but with me, it was function over form. Only I know this modification was performed. Only a trained eye will spot the stove flap shaft and spring delete. It's a condition 2-3 car that does see road time. I wanted it to behave. It runs & starts flawlessly now.

[Mark E.]

Re: Hot Restarting

Fuel that doesn't vapourize doesn't burn. In order to get the fuel to vapourize properly with a carburetor you had to add heat, lots of it.That's the primary reason for the exhaust crossover and heat riser.
Steve g

Here's an interesting Hemming's article about the role of a carburetor. https://www.hemmings.com/magazine/hc...n/3715001.html

A quote: "For gasoline to be burned in an engine, three things need to occur. It must be atomized (broken into small particles); emulsified (mixed with air); and vaporized (changed into a rarefied form)."

Maybe the experts will chime in, but there are two forms of vaporization: evaporation and boiling. Evaporation can happen at low temperatures, especially if the liquid is atomized (broken up). Gasoline vaporizes at a fairly low temperature; that's why you can smell such a small quantity when it's spilled. Carburetors vaporize gasoline by releasing it in an atomized state in a low pressure area, which increases the rate of vaporization even further.

So the low vaporization temperature of gasoline combined with the carburetor's role of atomizing it in a low pressure state enables our cars to run with a cool charge of air. Many cars run better with a cool intake manifold and cold air, which effectively increases the density of the air/fuel charge in the combustion chamber. Ever see bags of ice on an intake in preparation for a drag race?

[Gene M.]

Re: Hot Restarting

Thread and plug the heat passages under the carburetor. This has been covered in the past postings. Also avoid E10 gas if possible. I have my cars intakes plugged off and no hot/warm/cold start issues. And yes remove flap in heat riser. All assuming this is a fairly warm weather car. No winter exposure............

[Duke W.]

Re: Hot Restarting

Maybe the experts will chime in, but there are two forms of vaporization: evaporation and boiling.

No, that's gobbledygook. You can't rely on typical "consumer" publications to accurately discuss science. (You can include the NYT, WSJ, and WaPo in that group!) All liquids have a vapor pressure that is a function of temperature and the higher the temperature, the higher the vapor pressure. The definition of "boiling" is when the vapor pressure reaches the same value as the surrounding ambient pressure. The classic case is water. At sea level, the vapor pressure of water is the same as standard sea level pressure, 29.92" Hg, at 212F, which is the "boiling point" of water. It evaporates at lower temperatures, albeit slower. Otherwise puddles would never dry up after a rain. But gasoline is not a homogeneous (all the same molecule) substance. Gasoline is made up of hundreds of different hydrocarbon species from dissolved butane that boils at about 30F (at standard sea level pressure) to longer chain, heavier HC molecules, a very small percentage of which boil at 350F or more. The attached gasoline "distillation curves" will be the subject of an article I am co-authoring for The Corvette Restorer, and it will also be distributed to chapter newsletter editors for publication if they wish to do so.

[Mark E.]

Re: Hot Restarting

I understand what you're saying, but part of it is inconsistent with the Hemming's article?