The Stovebolt Site Forums The Shop Area Engine Shop Drill hole in carburetor top to relieve vapor lock?

Yesterday I drove my 57 pickup about 15 miles downhill to a meeting at highway speed and it drove fine. The engine sat and heat soaked for about 45 minutes. Coming back uphill was a vapor lock nightmare. I had to pull off the road 6-8 times to allow the percolating fuel in the carb to cool enough to continue. By "uphill" and "downhill" I mean an elevation change from about 7600' to 5400' and back again. This is on a stock 235 with the one-barrel Rochester carb and mechanical fuel pump. The temperature was in the 60's. I could see the fuel vapor bubbles backing up into a glass-bowl fuel filter just ahead of the carburetor so I know that the percolation was in the carb bowl. Engine temperature with a 160 degree thermostat was below the halfway hash mark on the temperature gauge so the engine isn't running particularly hot. To combat air pollution, Colorado mandates a "winter blend" of gasoline that vaporizes at a lower temperature and that is what is in the tank. My fuel pump is working at 6 psi. A new rebuild kit was installed in the carb a couple hundred miles ago.

Before I go through the effort to re-plumb the fuel line between pump and carb to relocate the glass bowl fuel filter and avoid as much engine heat as possible, or add an electric fuel pump, I'm wondering about an easier fix. I have a spare carb (or two) that has a small hole drilled in the top to vent vapor done by some unknown frustrated driver some time in the past. I could see the possibility of introducing contaminants or the likelihood of the fuel in the carburetor bowl evaporating more quickly in a parked vehicle. What else am I missing?

My other trucks all have 235's in them and the hassle of vapor lock is always possible at my high elevation and during summer heat. But with Fall temps in the 60's and the winter fuel blend this is a bigger problem than usual. Is there anybody out there running a 235 with a vented carb successfully? I'm open to suggestions. Thanks!

I'm no Rochester expert, but 2 things stand out.

6psi is more than the service manual calls for.

Rochester one barrel carbs already have vented float bowls.

Is the plastic thermal isolator block present between the carburetor and the intake manifold? Try running a temporary rubber line between the fuel pump and the carburetor to see if the metal fuel line is part of the problem. 6 PSI is WAY too much fuel pressure for a Rochester carb.
Jerry

When I first started my test drives, I would get home and open the hood. I could hear the gas bubbling in my Rochester carburetor. I found out that my heat riser on the exhaust manifold was not opening. I'm still trying to figure out why my spring is not working. I bought a new one and I'm having the same result. So, I set my heat riser to be open all the time. As the weather gets colder it might take longer to warm up the carburetor (I have a Carter installed now). Not sure if your exhaust has a heat riser, but if it does I would check to see if it is working properly.

Bill- I have also wondered about the 6 psi pressure and that's why I mentioned it. As for the factory venting on the Rochester carbs, I'll check the shop manual exploded drawing and look for it. My other Rochester carburetors on my other trucks also show signs of percolation flowing back from the bowl into the glass fuel filter after use, but primarily during hot summer weather and not to this severe extent. Probably "summer blend" fuel in use during some of these other trips. This vapor lock condition is usually behind me for the season by late October.

Jerry- The factory thermal insulator is present. I like your idea of bypassing the metal fuel line temporarily as a test. This fuel pump was on the truck when I purchased it 2 years ago and appears fairly new.

Utah York- The manifold heat riser was not working when I bought the truck and I replaced the exhaust manifold with another that I bought on Ebay. I also thought that a stuck heat riser might be overheating the carb. That issue has been fixed now.

The truck idles and runs well with the 6 psi fuel pump when cold. It does not seem to be overwhelming the float needle/seat and overfilling the bowl. These roadside breakdowns on the ride home were a result of fuel starvation rather than flooding. This 57 has been a constant repair project reflecting many decades of neglect by previous owners. I'm still working the bugs out of it so that I can drive it some distance without fear of breakdown. I'm not quite there yet.

Thank you guys!
Dave

I like to put a Facet fuel pump as close to the fuel tank as possible. This has always been a good fix for me. They put out about 4.5 PSI. My main problem was fuel expanding and flooding the carburetor when I let a hot engine sit. The Facet pump allows fuel to flow back into the tank. And when you turn the key on, it pressurizes the fuel like at about 4.5 PSI.

Vapor lock is MUCH more likely to occur at idle rather than speed.

If you are having stalling from fuel starvation at highway speeds, check the fuel tank to make sure nothing is bouncing up and temporarily blocking the fuel pickup.

Jon

A Tennessee grasshopper is just exactly the right size to clog up the pickup tube in a 47 Mercury fuel tank. That car would run almost exactly 2 miles before the engine quit from fuel starvation- - - -repeatedly. It only took me three carburetors, two fuel pumps, and a flexible line between the firewall and the fuel pump to figure that one out back in 1962!

Crenweige- Is Facet a brand name or a type of pump? I'm not familiar with it.

Jon and Hotrod- I know that there is sediment in the bottom of my tank because I saw it when I replaced a broken sending unit last year. One of my projects for this coming winter is to completely clean the tank and seal coat the interior. I was hoping to use up the fuel in the tank by driving it rather than siphoning it. That could be the issue. I drove the truck a week ago to visit a friend. About 5 miles there and 5 miles back at 30-40 mph and without much elevation change. There was some sign of percolation when I got home, but the truck ran OK otherwise. So the truck tends to run well when I'm not demanding much of it but falters when I'm in a "pedal to the metal" situation and fully warmed up.

The float bowl on every carburetor has to be vented to atmosphere, otherwise the theory of basic carburation wouldn't work due to positive or negative pressure occurring in the float bowl. You may not see a hole, but a port or stand pipe somewhere in the carburetor is allowing atmospheric pressure to act on the float bowl. carbking, did I word that correctly?

Very commendable, grasshopper.

Jon

Pinetree, I think you'd be better off buying a new tank.

You can buy a brand new, made in Canada fuel tank from Rock Auto for $200. You can't beat that. No trying to get all loose debris and rust out of the original. No sloshing compound to swirl around inside the old one. No risk that you do it wrong and the coating doesn't adhere properly. Also, those coatings only last about 20 years before they start flaking off. 25 years ago I didn't worry about it. 20 years was so far away it didn't matter. Well, I replaced the fuel tank in the Galaxie and the Cadillac a few years ago because the 20 year mark came and went and little pieces of the coating started clogging up my carburetor! Whoda thunk it?

The Spectra tank is very high quality. In fact, I found it to be better quality than the original tank.

I bought mine from NPD because they had free shipping on orders over $200. I think the limit is $300 now. Check all the usual places to get the best deal. They all sell the Spectra tank.

Let me see if I can boil this down into a new narrative. Carburetor percolation, which I have been mislabeling as "vapor lock" is not the main problem. The carb doesn't require any additional holes as it's already vented. While today's crappy gas and my high altitude location are not ideal, my other trucks, all with 235's and stock fuel systems, run fine. I've already installed another exhaust manifold, carb rebuild kit, glass bowl fuel filter, gas tank sending unit, and basic tune-up parts.

The main problem is lack of fuel at peak engine demand. The previous owner was probably trying to address this with a new fuel pump. The 57 runs fine when I'm not requiring much from it. Before I spend more time and money on new parts, re-plumbing fuel lines, or swapping around parts from other trucks, I need to start with a known-to-be-clean fuel tank. (Thank you, Otto, for your new vs. refurbished gas tank recommendation). I am planning on taking care of the dirty tank issue this winter. I might as well just drain the tank as drive the truck around the neighborhood for another 100 miles at low speeds to use up fuel.

Are there any other considerations that I've missed? Thanks for all of the feedback so far!

When was the last time you had the fuel tank sending unit out of the tank. There should be a sock on the fuel pickup tube.

I replaced the original defective sending unit with the new one maybe 2 years ago. There is a sock on it that is larger than the stock filter (more surface area). I should mention that I've only put less than 300 miles on this truck since I bought it in March of 2024. Checking for excess crud on the sock or at the fuel pump inlet will be the first thing to check when I drain the tank. A couple of days ago, when all of this stalled truck by the side of the road stuff was occurring, I stopped by a buddy's house when the truck first started running poorly and we pulled the carburetor off the truck. I cranked the engine and observed what appeared to be a good amount of gas coming from the fuel line into a coffee can. The glass bowl fuel filter just in line before the carb has never shown any sediment. We opened up the carb to look for any obvious contaminants that might be blocking an orifice and saw nothing. We blew out all of the passages with compressed air and reassembled everything. During that disassembly/inspection/reassembly time, the engine cooled off considerably. Then came the long highway-speed uphill climb with all of the unplanned breaks by the side of the road. So if the fuel line sock is clogged, it took 300 miles traveled over 2 years to do it.

That's way more information than you asked for, Carl!

Debris in a fuel sock can come and go. When the pump is working it sucks and holds debris in the filter. When it's off, the debris can settle back into the fuel tank. It will be interesting to see if you find debris in the tank or sock. If the sock filter gets partly clogged, it could explain the difficulty on the hill when the most power (and thus, gas) is needed.

The gas in the bottom of the tank looked pretty cruddy when I was doing the sending unit replacement. I had so many more pressing repairs to do at the time that I put gas tank cleaning on the "to do later" list. That chore has now moved to the top of the list.

Could the amount of suction while the inlet from tank is blocked decrease the pressure and increase the likelihood of negative pressure resulting in fuel boiling leading to cavitation of the pump? Not just the obvious lack of fuel getting by but creating a secondary problem even if some fuel is getting out of the tank. Similar to a vacuum pump on an A/C line

I have a reasonably cheap Harbor Freight inspection camera, it really comes in handy this issues like yours. If you have a local Radiator Shop they should be able to boil your tank out at a minimal cost. If the tank proves to be fairly clean and free of rust you may want to check any rubber lines you may have, it could be one is collapsing.

... or letting in air.

When it acts up the next time, remove the gas cap. It may not be venting properly, causing a vacuum in the tank.

I'll try to supply some additional information that might be useful. All of the fuel lines are metal with flared fittings. There aren't any rubber hoses with metal clamps involved. I haven't noticed any leaking gasoline. The boiling (what I've been calling percolating) fuel vapor bubble moves from the carb bowl back into the glass bowl filter when the engine is at operating temperature and then gradually diminishes as the engine cools and the filter bowl refills with fuel. I haven't ever noticed a sign of air entering the filter from the inlet (pump) side. That 6 psi fuel pump pressure that I measured a couple of years ago was with a cold engine.

This high-altitude, winter-blend gasoline percolation is consistent with my other trucks with 235 engines and Rochester carbs.

As far as boiling out the tank goes, I do have a good radiator shop that I use. That would be part of the cost equation in a comparison of refurbishing the stock tank vs. buying a new repro.

If I left some needed information out of my answer, please ask again. I appreciate the interest and advice from all of you guys!

One more thing...

Carl, the existing gas cap is so old that I'm sure that there's no vacuum in the tank.

Check it to be sure.

Thank you.

https://facet-purolator.com It is a brand that I have been using for almost 70 years.

Which style of those pumps do you use Cren?

I see the Posi-Flo unit has several 2.5-4 psi options in 12v. and the Cube has a a couple 2-3.5psi options. Either is suitable for a Stovebolt.

No 6v options, though.

Ding, ding, ding! The winners of this month's "Stump the Stovebolters Prize" go to 52Carl for telling me to remove the gas cap and check for a vacuum, and to Otto for telling me to check it for sure. I reserve the "Senior Moment Booby Prize" for myself for forgetting that I had replaced the ancient gas cap with a new, "improved" one sometime recently. I should know better than to trust my memory when sitting in front of the computer past my bedtime!

In all seriousness, I had replaced the cracked rubber gasket in another gas cap with a new one that I cut out of some thicker scrap rubber and it really was snug. Too snug! This is the smoking gun of a recent change to the fuel system that I had forgotten about. I have found this "vapor lock" thread to be very instructive. The awareness that Rochester carbs are already vented, that the engine quitting at peak demand is a result of fuel starvation rather than gas boiling in the carb, and that there is a high-quality new alternative to the stock gas tank are also important insights for me to remember. I will follow through with cleaning or replacing the gas tank this winter. I know that this 57 can run well at this altitude on this fuel.

Thank you everybody for your good advice!

I still have some Facet 6 volt, but they are no longer being produced. I think if he is on 6 volt I would recommend a Delphi FE0552 Electric Fuel Pump. Delphi makes OEM as well as aftermarket. I would avoid Chinese knockoffs like are on Amazon and eBay.

Something that I learned around 70 years ago is that when a problem suddenly appears, the first thing I do is revisit the last thing I tried to fix.

The new gas caps have a valve in them that is supposed to open to allow fresh air into the tank to replace the fuel that's being sucked out by the fuel pump. Like most garbage new parts, they don't work like they should or fail soon after installation.

If yours is like the one below, disassemble it and remove the valve and spring.

Unfold those two metal tabs that hold the metal washer and all the plastic stuff in place. Remove the washer, plastic vent, and valve assembly. Under that vent with the five holes is a rubber valve and spring.

Throw out the valve and spring and reassemble the rest of the parts in the order you removed them. Fold those metal ears back into place and install the cap on your truck.

Sometimes the KISS answers are the best answers.

I want to let you guys know how it turned out. I went for a test drive with the old, original gas cap on the tank and that wasn't THE problem. Still stalling out when under full demand. It was a cool day and there wasn't any percolation in the carb so I could rule that out as well. I swapped out another known good running Rochester carb for the one on the 57 and that made no difference. I pulled the sending unit out of the tank and the filter was 80-85% blocked with rust. I could barely see any light shining through when I put a LED light right next to it. The entire inside of the tank that I could see is coated with rust. So, in the end, fuel percolation, altitude, and winter fuel blend were all blind alleys and the simplest answer was the right one. Thanks everyone for all your help!

Then you better get on the computer and order that new gas tank before the Christmas rush!

Ain't this place great?!

Buy yourself a Spectra Premium GM55B The best price I have seen is at Zoro.com $282.99. Use promo code VEHICLEP10926 for 20%off and free shipping. That brings it down to $226.39.

That's a good deal. Probably the best.

Check NPD as well because they also have a free shipping option.

RockAuto has the same tank for $208. You can get 5% off of that with a discount code. I don't know what the charge for shipping would be.

Pinetree - If your going for the best price, including free shipping, check out Autoplicty Spectra Premium GM55B tank.

At $204.51 (and again noted "free shipping" ), this is the best value currently available that I could find on the web this morning.

Says they have 6 in stock and usually delivers in 2 days, so maybe time to pull the trigger?

I'm detecting a strong lobbying campaign for tank replacement. I was just about to start researching pricing but you guys are way ahead of me! Let me look into this tank and I'll let you guys know how it turns out. Any one else in addition to Otto have this in their truck?

I just bought one from the Autoplicity listing that Dan found. They also gave me another $10 off for, I guess, being a new customer. Rock Auto wants $41 to ship it FedEx. Total delivered price including Colorado sales tax came to $217.87. I'm used to doing things the hard way. This was too easy! Thanks you guys!