Been dealing with a problem for a while and now it's finally time to get it fixed. Currently, my '86 F-150 has a stock 351w in it. 8.30:1 compression ratio, stock cam, manifolds, Weiand Stealth intake, Edelbrock 600 (1406) carb. I'm currently working on a new 351 for it with 9.03:1 comp., better heads, cam, etc..

The problem is gas boiling in the carb (or getting heated enough to make it evaporate fast). If I start it from cold when it has sat for a couple of days, I have to crank on it for a while before the carb gets gas. I have verified no gas in carb today as it sat for a week and only got three pumps before the accelerator pump emptied the bowl. Once it's running, it's fine. No signs of starvation at all. If I shut it off for just a couple of minutes, it'll start right back up with just bumping the starter. If it sits for ten or so minutes, I have to crank on it for a little bit before it'll catch. Once running again, it's fine.

Right now, it's got a 1/2" phenolic spacer with another 1/4" paper gasket under the carb. If I just plainly drive it, the carb body gets hot enough you can't hold your hand on it. The engine temp never gets over 195-200 degrees, verified with an Autometer temp gauge.

These intakes have an exhaust crossover that runs directly under the carb. I have heard if you plug that passage completely off, the intake can get ice built up in it in colder temps. True or not? I'm thinking of blocking it off anyway, or maybe making plates to cover the holes in the heads with a small hole in them to just let a small amount of exhaust gasses through.

The reason I'm thinking of blocking it off completely is there is no valley pan, so I would think heat from the valley would be enough to keep things warm enough to prevent ice (if that's really the case).

I have no emissions testing here and have nothing emission related, so no worries about not passing anything. I am also not concerned if it takes a couple of minutes longer to heat up because when it's that cold, I let it run for 5-10 minutes before driving.

The last 351 I had in my old '83 never had any problems like this at all, but it had a cast iron intake.

Any ideas or thoughts?

 

How much time do you have?

This is a subject near and dear to me. I've had the same heat-related issues. I can bore you to tears describing the mods I did, but it's late right now.

In the meantime, to give you something to ponder, I think the big variable has to do with the chemical makeup of modern fuel. This is strictly a theory of mine, but I'd really like to hear from an industry insider in the know.

When these trucks were designed, the vast majority of vehicles were carbureted. Protection from boiling, vapor lock, etc., was a HUGE consideration, so the refining process and additives were optimized in that regard.

30+ years later, and virtually all vehicles on the road are fuel injected. You just don't see too many carbureted dinosaurs like our trucks in daily use. Emissions are the primary factor in fuel chemistry now. Protection from heat-related issues just isn't that much of a concern anymore with fuel injection. I'm sure there are some minimum standard that must still be met, but no refiner is going to spend extra to beat that if so few consumers would be affected.

That's why I think our old trucks are so susceptible to heat-related fuel issues today. No doubt there are other factors, but I bet this is the biggest one. Once again, I'd really love to hear from an industry insider in the know.

Humbly submitted for your consideration.

 

Matthew fitted a heat shield beneath his new carburetor.

Whether it is reflecting engine heat or acting as a radiator (shedding heat before it gets up into the carburetor) he says it has helped.

 

I had this problem on my 400, a wooden spacer (made by Edelbrock IIRC, got it from Summit) seems to have cured it.

 

I too believe some of it has to do with todays fuel.


You did not say what you have for exh system. If using the stock exh manifolds does it still use the heat riser valve? If so does it work and if not is it stuck open?
It should be open when motor is up to temp so exh is not heating the intake.


I have seen heat shields that fit below carb, a pan type thing, may help.
I have also heard of people making spacers from wood to keep heat from the carb.
How hard is it to pull the intake? Some use tin foil stuffed into the exh cross over ports to keep heat off the intake. You could give that a try and if you find you need the heat come winter you can fish it out.


Only motor I have the cross overs blocked on at the heads/intake is a drag only motor AMC so cant say how it runs in winter.
Dave ----

 

Alrighty, had a good night's sleep and am on my second cup of coffee. You have been warned...

I took many of the same approaches as you did, noticed some incremental improvement, and then moved in SeriousOverkill™ territory. First I added an electric boost pump to reduce vapor lock issues in the line, and that was a big help. Then I took multiple steps to reduce heat reaching the carb, which helped even more. Finally, I went overboard and invented a system to introduce cooled air into a an insulated shroud around the carb. More rambling details on all of that to follow.

My truck is an '84 with the 351W in near stock trim. For most of the summer, the truck works hard hauling our cabover camper while towing a '48 Jeep. I had been having some bucking/chugging issues at high altitudes on hot days. I added an electric boost pump just forward of the tank selector valve. This pressurized the line feeding the stock mechanical fuel pump, and pretty much took care of that issue. With the poor mechanical pump having to suck fuel all the way from the back, that was an invitation to vapor lock. I also added a momentary toggle switch so I could run the pump before starting the engine. As my truck often sits for days, this makes quick work of refilling the carb and it would always start right up like that, saving wear and tear on the starter and battery.

If you do add an electric pump, make sure you include some sort of safety switch to shut off the pump in case of an accident. I added an oil pressure safety switch, based on the idea if the engine was still running and making oil pressure, it was safe to feed it fuel. You could also retrofit an intertia switch from later models. Don't count on your ability to shut off the pump, as you could easily be knocked out in an accident, and a broken fuel line could quickly lead to a fire.

After some heat-soak issues worsened several years later (heat soak at the carb, not vapor lock in the lines), I decided to eliminate the stock mechanical fuel pump completely. I removed the pump and installed a blocking plate over the opening on the timing cover. My thoughts were the mechanical pump sat in a very hot area and was contributing heat to the fuel passing through. Even if not much heat was transferred via the pump, I was working a game of adding up minor improvements. I removed the previously added boost pump and switched to a Carter P4600HP and 18-14U isolator mount kit, at the same location just downstream of the tank selector valve.

I've been running this for several years now with no problems. I also added a braided insulated cover (Heatshield Products #204012) over the metal fuel lines under the hood.

That took care of any vapor lock issues, but I have always had heat-soak issues at the carb as long as I've owned the truck (since 1992). On warm days, hot restarts were tricky until I learned to hold the throttle part way open while cranking. I'd always get a puff of black smoke from the exhaust. Based on the symptoms, most likely I had fuel boiling over and spilling into the intake manifold. Hot restarts after about 20 to 30 minutes were the worst.

I've never been able to fully cure the hot restart heat-soak, but it was manageable. However, several years ago I started having issues of the truck bucking and chugging after working hard. In each instance, it was several minutes AFTER the workout, while driving slowly but with the underhood temperatures still high. In two instances, I had to coast to the side of the road, open the hood, remove the air cleaner, and let everything cool off for about 30 minutes. Then the truck fired right back up as if nothing had happened. It was time to address underhood heat reaching the carb.

The factory setup was a phenolic spacer under the carb. That was it. I remember an RX-7 of mine (now that was a fun car) had a big phenolic spacer/shield combo, extending horizontally several inches around the carb. I decided to try something like that. I made mine from aluminum, with Thermo-Tec 13575 reflective heat barrier on the bottom. I used two phenolic spacers, with the aluminum sheet sandwiched in the middle. I did not put any heat barrier on the portion of the aluminum between the two spacers.

I also blocked off the exhaust crossover passage inside the intake manifold. The primary purpose of this passage is for faster warmup of the intake manifold, for improved atomization of the fuel/air mix until the engine is fully warm. It might help a teensy bit with carb icing, but since it's downstream of the icing point, not a whole lot. More on icing protection later. I have not noticed any cold start-up driveability issues.

I was hesitant to block off the exhaust crossover, as this is where exhaust gas is tapped for the EGR system. I'm not one of those "rip out all that pollution crap" guys. In fact, the EGR system basically slows down combustion under partial load conditions, allowing more timing advance. It doesn't affect max power or idle, as it only works at partial throttle. However, I found the EGR passages totally blocked with carbon, so I knew it hadn't been working for a long time. (My truck still passed Oregon's emissions testing with the EGR inop.)

I blocked off the exhaust crossover by modifying the intake manifold gasket. I'm using the big turkey pan one-piece style gasket, hoping it would act as a heat baffle under the manifold. I was able to find the multi-piece gaskets with the exhaust crossover ports blocked, but not on the turkey pan. I fitted a piece of .020" stainless to block each passage opening, held in place by the gasket. I secured it with a bit of RTV during assembly. It was tricky to install like this. If I were to do it again, I think I'd simply stuff the passage full of crumpled tin foil as previously suggested.

Blocking off the exhaust crossover also killed the heat source for the original thermostatic choke, so I had to convert that to electric.

Back to carb icing, even though I'm not sure if the exhaust crossover passage had much effect on that, I wanted to make sure that didn't become a problem. I made sure the stock heated air intake system was in good working order. This is the shroud around the passenger side exhaust manifold, with the corrugated tube feeding up to the bottom of the air cleaner snorkel. A thermostat downstream of the air filter ports vacuum to control the flapper valve in the snorkel tube, adding warm air as needed. I found the actuator leaking so replaced that. I've had no icing issues, even though Oregon winters are prime for that, with temps 35 to 50 and high humidity.

So far, so good. I've pretty much eliminated vapor lock issues in the lines. I've blocked off much of the radiant heat sources reaching the carb. Still, after being stranded twice due to heat soak at the carb, I didn't want to take any chances. We travel to some pretty remote areas on our camping trips, I didn't want to have that awkward conversation about which camping companion would become dinner after the rest of the food runs out. That's why I invented a system to actively cool the carb exterior.

The system is an aluminum shroud that loosely fits around the front of the carb and is open on the back. (I have a two-barrel Motorcraft 2150, with the float chamber on the front.) The outside of the shroud is covered in the same Thermo-Tec heat shield I used on the big shield under the carb. A shroud like that might actually trap heat, so don't add one unless you include cooling air like I did.

I removed the AC evaporator case and drilled a hole for a 1/2" cooling line. On the inside of the case housing, I fabricated a little scoop (maybe 1" deep x 3" wide) that diverts a fraction of the cold air leaving the evaporator. I haven't noticed any decrease in AC performance inside the cab, even with some of the airflow diverted like that. This diverted cold air runs through an insulated steel line (with a rubber flex section in the middle to allow for engine movement) and feeds the front of the new shroud around the carb.

All I can tell you is this cooling system works great and is slicker than Bill Clinton testifying under oath. Geek that I am, I rigged up a cheap remote thermostat, with the sensor on the exterior of the float chamber. It's not the true temperature of the fuel inside the chamber, of course, but I think it's a good relative indication. The hottest I've seen while driving has been around 130F, which didn't seem excessive to me. Normally I see 90 to 110. If I shut off the fan, the reading rises about 15 to 20 degrees, so I know it's working.

I wish I had taken temp readings before doing all these mods for comparison purposes, but the thermometer was an afterthought. After shutting down a hot engine, the temp usually climbs to about 150, so I know there's still a lot of radiant heat at play when everything is at rest. After a hot start, I'll usually wait until the temp drops to 130 or so, which happens very quickly with cold air blowing inside the shroud.

I would also suggest using only top quality name brand fuel. I am very partial to Chevron, with Shell as my second choice. Definitely stay away from grocery store fuel, but that's just my opinion.

I think the fuel used is very important. I have a '63 ****** wagon with a Chevy 350 transplant, from a circa 73 pickup. That engine is VERY sensitive to heat-soak issues. I am planning to make the same mods to that engine, although I'll have to use ambient air for cooling the carb, as it doesn't have AC. It's got so bad I won't drive it when the weather is above 85 degrees. I was driving home one afternoon last summer on an 80 degree day with no issues and passed my local Chevron station, forgetting I needed gas. Not wanting to turn around, I stopped at a 76 station instead. Within minutes, heat soak suddenly reared its ugly head. To make the ten miles home, I had to pull over twice, open the hood and remove the air cleaner to let everything cool.

So I think this solving these heat soak issues is a matter of adding up a series of minor improvements. There's no one magic fix. That has been my experience at least. I'll climb down from my soapbox now...

 

I'm going to mention a few things here first.

Here's the spacer I'm running: Edelbrock Carburetor Spacers 8723 - Free Shipping on Orders Over $99 at Summit Racing

It's labeled as a "Wood Fiber Laminate". I call it phenolic for short.

FuzzFace2 - Good catch on the exhaust. When I first got this truck and went through everything (there used to be an old thread on the rebuild of it), I cut the flap out of the heat riser, so nothing there. It's more of an adapter to use the stock manifolds and Y-pipe.

For the Y-pipe, I cut out where the two 2" tubes came together and remade that part. Both tubes now dump directly into 3" stainless tube, all the way back. Also running a 3" Magnaflow muffler, so it's pretty free-flowing after the Y-pipe.

As for taking things apart to try, no problem. Right now, I'm trying to drive the truck as little as possible (due to the engine itself that I can't fully trust). My other block is at the machine shop getting bored. When I get everything back, it'll be all apart so that's the time I want to do any modifications.

On the not trusting the current engine, I had a thread a little while back about double-checking work others do. https://www.ford-trucks.com/forums/1...with-pics.html After that problem was resolved, I put the old cam and lifters in. When I primed the oil system, I noticed a lot of oil coming out between the lifters and lifter bores. The block is shot and I want to drive it as little as possible to prevent a possible major problem. The next time I drive it is going to be to pick up my freshly bored block, then it's getting a heart transplant.

I would run a bigger (thicker) spacer, but that Stealth intake is already pretty tall and I have to run a bit smaller air cleaner (from a '81 F-150 with a 400, or maybe a 351M) to clear the cowl.

kr98664 - Awesome write-up. Thank you for that! Sounds like I'm having identical problems. Great thinking on the shroud for the carb too. If things just don't work out, I may be copying that to an extent. I don't have A/C anymore, so not sure where to pull clean, cold air.

I can understand the need for companies to change the fuel blends for todays cars. They also screwed those of us with older engines by taking the lead out years ago. They never did run right after that, even with the additive. Gasoline these days doesn't even smell like the gas I remember years ago growing up. Years ago gas was a cheap and an awesome parts cleaner, so I got many hours hovering over a pan cleaning parts, inhaling the fumes. Maybe that's part of my p p p problem. When I fill up the trucks, they only get premium, and from major companies like BP, Mobil, Citgo. I won't put anything with Ethanol in them. We have no Chevron stations around here, plenty of worthless Kwik Trips though. I wouldn't even use their gas as parts cleaner, it's that bad.

I do have an aluminum factory Ford heat shield here (I'm borrowing it for now) to use as a pattern to make a new one. I'm going to get something drawn up in CAD and cut it on the lasers at work.

For right now, I'm thinking:
*Valley pan, that will keep the hot oil off the lower side of the intake.
*Plug up the exhaust cross-over ports. I'll plug the heads as to try to keep the hot gasses off the edge of the intake.
*Aluminum heat shield to keep radiant heat off the carb.
*Hook up the heat riser on the air cleaner snorkel for cold starts.
*Heat wrap around fuel lines in engine bay.

I'm going to put a bunch of thought into all of this. That will take up my usually boring Sunday. Thanks everyone for your input so far. It is helping hearing I'm not alone and you were able to solve these problems, with different methods.

 

If you're building a new engine just get intake gaskets with the crossovers blocked.

Icing is due to pressure drop at the venturis and ambient humidity.
This is why intake air temperature is regulated with a stock air cleaner assembly.
(Obviously some heat in the carb and fuel itself helps)

Fuel blends winter/summer these days are largely dictated by market prices for the component hydrocarbons.
Fractional distillation is FAR in the past.
With catalytic reforming of feedstock molecules can be made larger or smaller at will.

This (vapor lock and boil off) is not an issue in a FI system that is kept sealed and under 30-40 psi all the way to the injector pintle.

Yes, I'm old enough to remember what gasoline and asphalt paving used to smell like when it was a fairly random blend of aromatics.

 

I had boiling fuel on the 454 in my boat, with a Holley double pumper. Most noticeable when starting after a brief time engine off, but not restarting immediately. 1/2" or so phenolic spacer cured it. I did have an intake gasket leak at some point and forget if I blocked the exhaust crossover or not. Suspect I did. Coldest I've run it in is around 40* water temp with air temp slightly warmer. Wetsuit weather for sure! In any event, I forget the timeline but a phenolic spacer cured my hot fuel issues.

 

One tidbit I forgot to mention involves region-specific and seasonal fuel blends. Even though you live in an area with no emissions testing, you are still very likely to be affected. In fact, areas without emissions testing often get the most troublesome fuel blends.

It goes like this: Region A has a bad pollution problem, due to geography, population density, etc. One part of the fix is to mandate certain fuel blends that help reduce whatever exhaust chemical is causing the most trouble in that region. In addition, the refiners may be required by law to make seasonal adjustments in the fuel. Region A might easily have a special winter blend and a special summer blend.

Meanwhile, you live in Region B and think you don't have to worry about getting those funky blends. But in reality, it's difficult to precisely forecast demand, and the refineries might end up with too much Region A winter blend at the cutover time for summer blend. So where do they sell it? They can't legally sell it in Region A, but there are no such restrictions in Region B. So guess who gets the funky blend that was not even intended for the current season?

Stuff like this happens all the time. I think rural areas near big cities probably get the worst of this horse trading. Just one more thing to keep in mind.

 

Great read guys. Good info. My big block has been giving me sort of the same problem. Except on mine the issue is the carb starts loading up really bad. Only after its gotten hot though.


Could this issue be worse with some carbs than others? I'm running a 850 double pumper as well.

 

Another good point I forgot all about. Our gas around here is HORRIBLE!! No matter where I fill up my '07 car, I get 27-30mpg MAX (in summer, easy on throttle, all highway). I had to drive to southern Illinois one time and had under 1/4 tank when I filled up down there. The car ran 35-36 mpg on that whole tank of gas. Once I filled up again from gas around here, the mileage dropped right off again. We must get the sludge off the bottom of the tanks.


What I've been working on so far is making "plugs" to press into the cross overs in the heads. My thinking is the further I can keep the hot gasses from the intake, the better. Yes, I could buy gaskets with the ports blocked, but I have a gasket set....and I'm a cheap bastage. More time than anything. Plus, I like making stuff. Maybe next week I'll start to work on a heat shield for the carb., and get the heat sleeve for the fuel lines. I'll keep updating as things get done. My block just got bored today, so I got to order pistons and stuff. In other words, got some time to kill until I get everything back.

 

What size engine, and what's been done to it?

I have a just over stock 460 with an Edelbrock 800 on it and it's waaaay too much. I need to go down to a 650 or 700 max. Also have a better radiator than stock and temps creep up a bit. When I had a 650 on it a few years back, it never ran warm.

 

My engine has been stroked out to 514 inches . Pretty good sized cam but nothing radical. Port and polished 429 heads. I don't remember all the details I built it about twenty years ago.

Crazy thing about my motor is that when I built it it never got over 160 degrees. That was the t stat I had in it . And it ran like a screaming ape! One day I had a brain fart and decided to take the truck on a 7 hour rd trip. Ever since then the temp gets up around 200, not beyond, and I've had this loading up problem. Everything else seems to check out.

That's interesting , I've never heard of carb size affecting engine temp. My engine has always had the same carb.

 

What a great idea? How are you making them?

Are the plugs removable? One concern I had was I might end up with driveability issues if the intake manifold stayed too cold. Wasn't an issue after all, but I wanted to keep any mods reversible just in case.