Fuel loading when hot

[67HEAVEN]

Any ideas?

After driving until the engine is good and warm, if I leave it shut down for a half-hour or so, it seems to load up with fuel.

Holding the accelerator to the floor usually gets me going after a few cranks, but the smell of fuel and the puff of blue out of the sidepipes is nasty.

I've also noticed a more pronounced smell of gas in the garage after a cruise.

I've checked for leaks, and there appear to be none. Fuelie was over yesterday, and had a look at a few things (two heads better than one). Nothing obvious springs to mind. There's no purcolating down the butterflies after shutdown either.

The float levels are right too. :confused

I found a temporary preventative cure, however. If I turn off the electric fuel pump about 30 seconds prior to shut down, the problem goes away at re-start. Since this pretty much empties the bowls, it would seem to suggest that the fuel is cooking in the bowls after shut down.

One more thing. Once the engine is good and warm, I occasionally get a notceable but very brief engine-stumble when turning a hard corner in either direction.

502 c.i. / Holley 850 with electric choke

Any ideas?

 

[IH2LOSE]

Bob what pump do you have in the car and what is the psi?

Could be the pump is high pressure and the pressure pushes into the carb after setting a while.

As we all know high pressure has to go to low pressure even if the floats are satisfied and closed

 

[67HEAVEN]

Larry,

It's the Holley #12-802-1 fuel pump:

Flows 110 GPH (free flow)
Flows 88 GPH at 9 PSI
Maximum pressure is 14 PSI

 

[IH2LOSE]

Bob I thought on a standard carburated motor we were only supposed to run 6 to 8 psi

the simple answer is it sounds like heat soak/perculation

 

[67HEAVEN]

Larry,

It came bundled with the Holley 12-803 Flow Regulator, in the same package.

http://www.holley.com/12-803.asp

 

[67HEAVEN]

the simple answer is it sounds like heat soak/perculation

Yeah, it does. It got lots worse on the trip to Bowling Green last year. There was some excitement in the convoy, if I remember correctly.

Does anyone know where to source the aluminum heat shield (flat plate) that fits under the Holley carb? I had one years ago, but it grew legs.

 

[Paranoid]

They show up on eBay occassionally. 3969835

 

[gmjunkie]

Any ideas?

After driving until the engine is good and warm, if I leave it shut down for a half-hour or so, it seems to load up with fuel.

Holding the accelerator to the floor usually gets me going after a few cranks, but the smell of fuel and the puff of blue out of the sidepipes is nasty.

I've also noticed a more pronounced smell of gas in the garage after a cruise.

I've checked for leaks, and there appear to be none. Fuelie was over yesterday, and had a look at a few things (two heads better than one). Nothing obvious springs to mind. There's no purcolating down the butterflies after shutdown either.

The float levels are right too. :confused

I found a temporary preventative cure, however. If I turn off the electric fuel pump about 30 seconds prior to shut down, the problem goes away at re-start. Since this pretty much empties the bowls, it would seem to suggest that the fuel is cooking in the bowls after shut down.

One more thing. Once the engine is good and warm, I occasionally get a notceable but very brief engine-stumble when turning a hard corner in either direction.

502 c.i. / Holley 850 with electric choke

Any ideas?

67 have you thought about the Power valve in carb? It can only take 1 little backfire(usely more,but 1 is all it takes sometimes) to pop a small hole in diaphram, and that's all it takes for fuel run in through valve rubber membrane down the vacuum ports behind the metering block out of float bowl under throttle plates into intake!! You'll never see it LOOKING down through the Top!!
67, Did I ever tell ya how much I LOVE Holley's??

"Holley" Trusted by Auto Arsonist World Wide!!

 

[gmjunkie]

They show up on eBay occassionally. 3969835

67 check with my son on E-bay 97s10ss4.3

He coughs up Old GM stuff all the time!! He's a BIG BLOCK BOY!! If it's not in his Items for sale,just e-mail him through E-Bay (He WILL get Back to Ya)!! Here's his 66 SS 396 Chevelle he's been Restoring for the Last 10 years!! Took him 6yr's to gather all the part's he wanted to Replace!! It will be finished this summer after Dad gets there to Blow the RED stain on it!! I don't know why he wants me to Stain it ,He's as good a Painter as I am!!

 

[paul67]

The heat shield can be ordered from your local speedshop or even the speciality catalogue at Canadian Tire.

I tend to agree with the 5-6psi for the fuel pump.

http://www.holley.com/108-70.asp

 

[67HEAVEN]

The heat shield can be ordered from your local speedshop or even the speciality catalogue at Canadian Tire.

I tend to agree with the 5-6psi for the fuel pump.

http://www.holley.com/108-70.asp

Ahhhhh, thanks Paul. I'll get one of the Holley shields. As for the psi, the Flow Regulator is adjusted to 7psi, and I have an inline pressure gauge that confirms it.

I'd hesitate to drop it any lower, 'cause when I put my foot in it, it takes an oil-tanker to keep that monster satisfied.

 

[67HEAVEN]

67 have you thought about the Power valve in carb? It can only take 1 little backfire(usely more,but 1 is all it takes sometimes) to pop a small hole in diaphram, and that's all it takes for fuel run in through valve rubber membrane down the vacuum ports behind the metering block out of float bowl under throttle plates into intake!! You'll never see it LOOKING down through the Top!!
67, Did I ever tell ya how much I LOVE Holley's??

"Holley" Trusted by Auto Arsonist World Wide!!

Junk,

If cooling things down a bit doesn't solve it, I'll pull the Holley apart and check your theory.

 

[Tom Bryant]

A couple of questions:

What is the pressure regulator set at? I bought the same one for my system and also bought a liqued filled pressure guage so I can actually see what the real pressure is at rpm. Mount it in line between the regulator and the carb. You should be able to get the same reading mounting it in the second outlet port of the regulator.

A follow up to that question would be do the fuel bowls continue to fill after the engine is shut down for awhile due to too high of residual fuel line pressure and drain down into the the intake ? I'm not that sharp on Holleys so I'm not sure this can even happen but if pressure causes fuel to continue to seep past the needle and seats as it sits it has to go someplace.

It seems like the Holley ads claim most new carbs have power valve blowout protection and that a device can be retrofit to carbs not having it. Does your carb have it?

I would add the heat shield. Also does your intake have exhaust gas crossover and is the passage blocked off to keep the intake cooler?

Tom

Summit 800115

 

[motorman]

do you have the heat riser crossover in the intake blocked if not do it to remove the exhaust heat from the carb and intake manifold. even when gasoline was good back in the 60s i always blocked the crossover in any corvette i owned that had a aluminum intake to prevent the fuel from boiling in the bowls

 

[67HEAVEN]

A couple of questions:

What is the pressure regulator set at? I bought the same one for my system and also bought a liqued filled pressure guage so I can actually see what the real pressure is at rpm. Mount it in line between the regulator and the carb. You should be able to get the same reading mounting it in the second outlet port of the regulator.

A follow up to that question would be do the fuel bowls continue to fill after the engine is shut down for awhile due to too high of residual fuel line pressure and drain down into the the intake ? I'm not that sharp on Holleys so I'm not sure this can even happen but if pressure causes fuel to continue to seep past the needle and seats as it sits it has to go someplace.

It seems like the Holley ads claim most new carbs have power valve blowout protection and that a device can be retrofit to carbs not having it. Does your carb have it?

I would add the heat shield. Also does your intake have exhaust gas crossover and is the passage blocked off to keep the intake cooler?

Tom

Summit 800115

Tom,

Yes, I have an oil-filled pressure gauge. The regulator is set for 7psi. I'll record the readings at various rpms. The 850cfm carb came with the 502, so it's a couple of years old.

The 502 intake does not have an exhaust gas crossover port.

 

[motorman]

i have seen as low as 4/5 PSI fuel pressure on a 600 HP race engine with no problems as long as the fuel flow is there. i also would lower the float level to 1/16" below the opening in the float bowls. you could also onstall a bypass between the input and output of the fuel pump with a .015 diameter hole restrictor to allow the fuel pressure to drop to 0 when you shut off the engine. the pump you have has enought capacity to overcome this small loss when running

 

[gmjunkie]

i have seen as low as 4/5 PSI fuel pressure on a 600 HP race engine with no problems as long as the fuel flow is there. i also would lower the float level to 1/16" below the opening in the float bowls.

I will agree with that All the way!!:cool

 

[paul67]

Second thoughts for your consideration. That heatshield was, I believe, developed for the 1970-71 LT-1 with Holley that had boilover problems. That plagued me also two summers ago when the original 4160 was on my car. I bought the heatshield from a local speedshop but unfortunately it did not fit. There was interference in several areas such as the rear manifold vacuum fitting and if I remember correctly, the t-stat housing. I tried some mods but basically fouled up on cutting. And because of other Holley issues, I just gave up and converted to Edelbrock. I am not familiar with your engine but fitment might come into play.

With the Edelbrock I added some insurance in the form of a 3/8" phenolic spacer (see pic). I did not run the car without it but I can confirm that with it, I have not experienced any fuel delivery problems. That might be an option for you as well.

 

[67ragtop]

Bob,

I had the exact same symptoms on my L-79 with a new 600 cfm Holley model O-80457S. The problem was with the accelerator pump. The newer Holley’s have a rubber umbrella valve under the pump diaphragm. This is basically a one way valve which remains closed unless the pump is drawing in fuel. Since the pump is sitting right on top of the hot intake, the fuel inside the pump heats up and expands. The closed valve prevents the fuel from returning into the bowl so it is forced out the nozzle (this is not an issue with the older style carbs that have the ball bearing type valve which remains open). In my case this started 10 – 15 minutes after parking. It also didn’t start until several months after I installed the carburetor. That has me thinking the rubber valve might be designed to stay open like the ball bearing type but is deforming after use.

It sounds like you definitely have some type of percolation problem. If you watch the carb after a hot run you should see where the fuel is leaking from. It’s either going to be the accelerator pump or the boosters. Since there is a half hour delay before the leaks starts, I don’t think it is pressure related.

Mike

 

[Gerry]

Hi Bob,

I know some real hots--t race guys who are carb specialists and will ask them. Does the stumble you get under hard cornering always happen, possible fuel pickup problem? Might be a clue. My car also gives off gas smell in the garage after I shut down but I run the edelbrock mechanical fuel delivery system and a mighty demon 950. Don't have the other problems you are experiencing so perhaps the smell doesn't have relationship with them. I'll get back after I ask experts.

Best regards,
Gerry