Author Topic: Vapor lock with different carb?  (Read 1847 times)

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chilly460

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Vapor lock with different carb?
« on: April 13, 2021, 07:22:58 PM »
Fairlane I picked up a couple weeks back had a nice Pro Systems 850 on it but owner didn’t include with the deal, but let me use it to get the car home as carb that was on it needed a rebuild.  I had an AED 750 HO on the shelf, basically new, so threw that on it.  It runs great but I’ve discovered it vapor locks BAD.  Did it the other day rolling 50mph on a 80* day, several times bad enough to stall the engine while running along at 2500rpm.  I pulled the air cleaner figuring it may be acting as a heat sink, it seems to have boiled over so bad it shot gas out the vents and hit the underside of the hood.  I insulated the fuel line and drove it today, vapor locked again in 66* weather with engine at 180* just hitting two stoplights. 

Just put the 850 back on it, ran the same exact route, stopped and let it get to 200* and shut it down to heat soak, let it sit for five minutes and started it.  Ran absolutely fine on the way home, so I’m convinced it’s the carb and not general heat soak issue with the fuel line specifically 

Anyone experienced this kind of issue going from a Holley to another Holley?  I have a thick insulating carb pad to try but I’m not confident, it’s doing it on very mild days while moving so can’t imagine this’ll be enough but we will see.  Any ideas what else to check??  Already checked fuel pressure, it was actually down to 3psi as fuel was boiling, 7-8 normally
« Last Edit: April 13, 2021, 07:25:47 PM by chilly460 »

Jb427

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Re: Vapor lock with different carb?
« Reply #1 on: April 13, 2021, 08:30:06 PM »
Have you had a look at the entire fuel line/system that it is not getting unwanted heat soak and can you fit a carb spacer ? try a small plastic or plywood carb spacer to try and remove some of the unwanted heat soak.
« Last Edit: April 13, 2021, 08:32:11 PM by Jb427 »

475fetoploader

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Re: Vapor lock with different carb?
« Reply #2 on: April 13, 2021, 08:49:50 PM »
Any chance the floats are sunk, pushing fuel out the vent? Bad needle and seat maybe? Or just the float level had crept up too high.  Seems strange it’s heat soak, if carb A does it, Carb B doesn’t.
1967  Fairlane Tunnel Wedge on Proports.
1975 4x4 461 f.e. 4speed Dual Quads on 38’s
Love many, Trust few. Always paddle your own canoe.

chilly460

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Re: Vapor lock with different carb?
« Reply #3 on: April 14, 2021, 06:25:49 AM »
Have you had a look at the entire fuel line/system that it is not getting unwanted heat soak and can you fit a carb spacer ? try a small plastic or plywood carb spacer to try and remove some of the unwanted heat soak.

I insulated the fuel line in the engine bay, I used an infrared thermometer and traced the fuel line, it started to heat up where it was routed over the shock tower so I started there.  I have a phenolic spacer and a thick insulating gasket I will put on, but I was shooting for an A-B test as it seemed like it was the carb more than the fuel line. 

chilly460

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Re: Vapor lock with different carb?
« Reply #4 on: April 14, 2021, 06:28:28 AM »
Any chance the floats are sunk, pushing fuel out the vent? Bad needle and seat maybe? Or just the float level had crept up too high.  Seems strange it’s heat soak, if carb A does it, Carb B doesn’t.

I was able to hop out and check it the second time I did it, the fuel level looked completely normal.  It's been tough to really check it because it's usually dying on the road with traffic involved, but now that I know it's an issue I can be ready to check it when it starts to happen.  Fuel level is fine every time I've checked it, which has been often since I installed the carb and have been tuning idle/etc. 

cjshaker

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Re: Vapor lock with different carb?
« Reply #5 on: April 14, 2021, 06:55:37 AM »
I've had quite a few cars that vapor locked over the years, from flatheads and FE's to 6 cylinders and small blocks, but none of them vapor locked while cruising down the road, it was always from a take-off after sitting idling on a hot day or after being shut off for a short time (heat soak). The fuel is being used to quickly when cruising, and you typically have lots of air flowing around to keep the carb cool enough not to boil.

For the carb to boil at cruise, I'd think the engine heat must be off the charts, or exhaust was being directed at the carb. Neither seems likely though, especially at 66* ambient air temp, not to mention another Holley didn't have the issue. I'd probably be looking at another issue as the cause. Have you checked the plugs to see if they're sooty? Maybe the floats momentarily stick and flood the carb?
Doug Smith


'69 R-code Mach 1, 427 MR, 2x4, Jerico, 4.30 Locker
'70 F-350 390
'55 Ford Customline 2dr
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My427stang

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Re: Vapor lock with different carb?
« Reply #6 on: April 14, 2021, 07:08:20 AM »
Just a stab, but if one works, one doesn't.

Could be a different style needle/seat putting less pressure on the seat, could be a different volume of fuel at the same float level (bent float), could be slight differences in baseplate material (reaching for that one but brainstorming), enough difference in mixture that one is causing a cooling effect beneath it (also reaching with that one, I doubt it)

I'd give AED a call or stop by, see what they think, after they go through the basics that you already know,  they will likely have some ideas too LOL

I will tell you this, no idea why, but in the heat of Vegas I had some minor vapor lock pre-EFI, but in Hampton Roads, I was stranded at times it was so bad. SoVA apparently has lower atmospheric pressure or ancient alien influence or something LOL



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Ross
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- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, Erson SFT cam, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 461 cid FE, headers, Victor Pro-flo EFI, Comp Custom HFT cam, 3.50 9 inch

chilly460

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Re: Vapor lock with different carb?
« Reply #7 on: April 14, 2021, 08:31:41 AM »
I'm leaving the 850 on it and will drive it on a warmer day, once the weather turns, to confirm it's not doing it.  If the 850 doesn't act up, I'll put the plastic spacer and the insulated gasket on it and see if the AED improves. 

Doug, I just did plugs, the old ones looked fine.  Engine heat isn't an issue, usually 165* going down the road, will roll up to 180* but that's about it.  Underhood heat doesn't really seem that bad when I pop the hood, the shock towers are pretty warm as expected but most other surfaces aren't anything crazy IMO.  I do agree it's very strange that it's occurring at cruise, the car has a 427 hood on it so it's getting a ton of fresh air moving over the carb itself. 

I'll check the floats in the 750 for any issues, easy check since it's off the car right now. 

Ross, I will do a little more debugging and then give AED a call if still not solved (want to 100% confirm it's a carb issue). 

e philpott

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Re: Vapor lock with different carb?
« Reply #8 on: April 14, 2021, 09:56:15 AM »
I've had quite a few cars that vapor locked over the years, from flatheads and FE's to 6 cylinders and small blocks, but none of them vapor locked while cruising down the road, it was always from a take-off after sitting idling on a hot day or after being shut off for a short time (heat soak). The fuel is being used to quickly when cruising, and you typically have lots of air flowing around to keep the carb cool enough not to boil.

For the carb to boil at cruise, I'd think the engine heat must be off the charts, or exhaust was being directed at the carb. Neither seems likely though, especially at 66* ambient air temp, not to mention another Holley didn't have the issue. I'd probably be looking at another issue as the cause. Have you checked the plugs to see if they're sooty? Maybe the floats momentarily stick and flood the carb?

X2 ... among others the worse was my 1983 Plymouth Scamp with 2.2 4 cylinder , idle no problem but take off good luck , lol , had a farmer customer with a Dodge Rampage just like my Plymouth ( Horizon/Omni pickup truck ) and he had about 20 wooden cloth line pins on the fuel line leading up to the carb and it seemed to fix his , never tried it on mine

cleandan

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Re: Vapor lock with different carb?
« Reply #9 on: April 22, 2021, 09:56:38 AM »
Just a few mental ramblings to ponder.
What are the two carbs made from? Zamak? Billet Aluminum? A combination of each?

Do the carbs have differing fuel pressure requirements...For instance, a Carter AFB likes about 4psi where a universal Holley 1850 will take 7psi.
If you were set up to run 3-4 psi, but are using a carb that wants 6-8psi, it may boil easy.

Could you somehow be pulling more air over/across the carb that boils easy causing a bit of a low pressure area in the general air around the carb? A stretch I know, but it has happened.

Do you have a Go-Pro style camera...or something like this? (there are some REALLY cheap Chinese versions that work great for this work...Like $40) If so, set it up to video the carb with detail as you drive. Then drive around and do your best to make the issue happen...Look at video and maybe you will see something that points in a useful direction.

cammerfe

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Re: Vapor lock with different carb?
« Reply #10 on: April 22, 2021, 11:08:47 PM »
The 8BA engine with the fuel pump on the back of the intake manifold was well known for vapor lock.

When I went to a Sidewinder on my '67 Cougar XR7/427, I employed one of the stack of thin spacers/ aluminum heat shields combos to create more surface area for radiating under the Holley. That worked as well. I was running 13:1 compression---long story, don't ask---and I also went to a vapor injection system that used isopropyl alcohol. No more problems after I did the several suggested tricks.

KS
« Last Edit: April 22, 2021, 11:10:44 PM by cammerfe »

chilly460

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Re: Vapor lock with different carb?
« Reply #11 on: April 24, 2021, 09:55:36 AM »
Just a few mental ramblings to ponder.
What are the two carbs made from? Zamak? Billet Aluminum? A combination of each?

Do the carbs have differing fuel pressure requirements...For instance, a Carter AFB likes about 4psi where a universal Holley 1850 will take 7psi.
If you were set up to run 3-4 psi, but are using a carb that wants 6-8psi, it may boil easy.

Could you somehow be pulling more air over/across the carb that boils easy causing a bit of a low pressure area in the general air around the carb? A stretch I know, but it has happened.

Do you have a Go-Pro style camera...or something like this? (there are some REALLY cheap Chinese versions that work great for this work...Like $40) If so, set it up to video the carb with detail as you drive. Then drive around and do your best to make the issue happen...Look at video and maybe you will see something that points in a useful direction.

You may be on to something, the 750 is the traditional Holley pot metal, the 850 is aluminum.  Hard to believe heat conductivity is that different but it is one difference.  Both are Holley based needle and seat, and float, so you’d think pressure reqs would be same. 

I do have a GoPro I could run in there, not sure if it’d get enough light to be able to video but easy enough to mock up and check

cleandan

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Re: Vapor lock with different carb?
« Reply #12 on: April 25, 2021, 10:58:02 AM »
Just a few mental ramblings to ponder.
What are the two carbs made from? Zamak? Billet Aluminum? A combination of each?

Do the carbs have differing fuel pressure requirements...For instance, a Carter AFB likes about 4psi where a universal Holley 1850 will take 7psi.
If you were set up to run 3-4 psi, but are using a carb that wants 6-8psi, it may boil easy.

Could you somehow be pulling more air over/across the carb that boils easy causing a bit of a low pressure area in the general air around the carb? A stretch I know, but it has happened.

Do you have a Go-Pro style camera...or something like this? (there are some REALLY cheap Chinese versions that work great for this work...Like $40) If so, set it up to video the carb with detail as you drive. Then drive around and do your best to make the issue happen...Look at video and maybe you will see something that points in a useful direction.

You may be on to something, the 750 is the traditional Holley pot metal, the 850 is aluminum.  Hard to believe heat conductivity is that different but it is one difference.  Both are Holley based needle and seat, and float, so you’d think pressure reqs would be same. 

I do have a GoPro I could run in there, not sure if it’d get enough light to be able to video but easy enough to mock up and check

It may be time for you to get that clear plexiglass hood you have been thinking about since you first saw the movie GREASE.

Drew Pojedinec

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Re: Vapor lock with different carb?
« Reply #13 on: April 25, 2021, 03:21:29 PM »
You may be on to something, the 750 is the traditional Holley pot metal, the 850 is aluminum.  Hard to believe heat conductivity is that different but it is one difference.  Both are Holley based needle and seat, and float, so you’d think pressure reqs would be same. 

If one uses a larger seat, the sealing requirements change.
yes the thermal conductivity is different, never studied how much.
Also float type, float level, etc.
All the little details.