Holley 4150 Fuel Inlet Leakage - Team Camaro Tech
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  #1  
Old Sep 14th, 09, 01:09 PM
Bigblox Bigblox is offline

Grant
 
Join Date: Mar 2007
Location: Oregon
Posts: 19
Default Holley 4150 Fuel Inlet Leakage

I'm hoping someone here may have experienced the same thing I've started seeing in the past couple of weeks with the rear fuel inlet fitting leaking on my 1970 Holley 4150 Dual Feed Carb.

This carb is the only thing non-original to my `68 L72 427 (M21), and I'm using all stock configuration and diameter fuel lines with the factory style brass Tee fitting that splits the fuel lines to each inlet from the fuel pump. I'm also using the ceramic Inlet Filters and springs.

My problem started with the rear inlet fitting leaking slightly between the gasket and the rear Fuel Bowl that it threads into. It's hard to tell, but I believe there is also some leakage at the Fuel Line fitting as well. I've since replaced the Inlet Fitting Gasket several times, and replaced the Inlet Fitting itself with a new Holley fitting with no change. The threads in the rear Fuel Bowl seem to be in good shape, as I can thread the new Inlet Fitting into the bowl finger tight up to the gasket. I have always been careful not to over tighten the fitting or the rear Fuel Line.

The front bowl fitting, Tee lines, and Fuel Pump line are completely dry, and don't leak a drop. I noticed after pulling the rear line off again (for about the forth time in as many days) just after running the engine for about 15 minutes, there was a considerable amount of back-pressure in the Rear Fuel Bowl as I loosened the Inlet Line. It's been a while since I've delt with Double Pumper problems like this, but if I remeber correctly, the fuel in the bowl will just leak out when the line is loosened, and not actually have a whole lot of pressure behind it.

My questions for the knowledgeable Holley guys out there would be:

> Is it possible there might be some kind of restriction in the rear Fuel Bowl circuit that's causing back-pressure and blowing this Inlet seal out? As I mentioned, I've replaced several gaskets and the Inlet Fitting now, and they all still seem to be in good shape when I pull them out.

> Could the threads in the rear Fuel Bowl be damaged enough to cause this kind of fuel leakage? They don't appear to be, and I would think the gasket itself would still do its job. (There don't appear to be any nicks or damage to the sealing surfaces either).

Sorry to ramble on, but this is becoming frustrating. Any input or advice would be greatly appreciated.

Thanks,

Bigblox
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  #2  
Old Sep 14th, 09, 01:44 PM
alanrw alanrw is offline
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Alan
 
Join Date: Oct 2007
Location: SoCal
Posts: 3,910
Default Re: Holley 4150 Fuel Inlet Leakage

The fuel pump is only putting out like 7 psi which is nothing. If you get a fuel gush when you loosen the fitting, all you are doing is releasing the residual fuel pressure that stays between the fuel pump and the fuel bowl as the needle and seat are closed.

Have you looked real, real close at the fitting and the bowl thread surface to make sure all is in a good state? Possible tiny crack?

There was a post last week which turned out to be a microfracture in the metal fuel line.

You could always temporarily rig up a rubber fuel line from the junction block to the rear bowl with new fittings to see if that solves/isolates the problem.

alan
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  #3  
Old Sep 14th, 09, 03:12 PM
Bigblox Bigblox is offline

Grant
 
Join Date: Mar 2007
Location: Oregon
Posts: 19
Default Re: Holley 4150 Fuel Inlet Leakage

Never thought about a "micro-crack" under the Inlet on the rear bowl. That seems very likely given the fact that no amount of throwing new fittings and gaskets at it changes anything. I'll see if I can get a small mirror under there to check before I pull the carb off.

Pretty sure it's not the Fuel Line itself. The wettest area does seem to be directly underneath or behind the Inlet.

Thanks for the input. Greatly appreciated.

Bigblox
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  #4  
Old Sep 14th, 09, 06:01 PM
rj68RS rj68RS is offline
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Russ
 
Join Date: Jul 2009
Location: Gilroy,CA
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Default Re: Holley 4150 Fuel Inlet Leakage

I know you said you are careful not to over tighten the fittings but I've seen a lot of folks wrench the heck out of the fittings and then they leak. If it's not the line fittings, could be that the female threads in the bowl have been worn enough from removal and installs that fuel is seeping past the seal and down the threads. You said you can thread the fitting up by hand till it hits the gasket - how many threads are still showing on the fitting ?

You could try putting fuel safe thread seal on the fittings just to test if that is the problem but you need to be really careful not to get any of it into the ceramic filter. Clean the parts well, apply small amount of sealer, install, let sit a minimum of 2 hours - over night is better. If this works, it is not a suggested long term fix. Get new a new bowl and fitting to properly fix the leak.
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  #5  
Old Sep 14th, 09, 07:28 PM
SY1 SY1 is offline
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Dave
 
Join Date: May 2001
Location: Michigan
Posts: 3,235
Default Re: Holley 4150 Fuel Inlet Leakage

Grant there are only a couple of things that can cause this. Sounds like you already know this, but just to set the record straight...........The inlet uses an inverted flare fitting which is metal to metal seal at the flare on the line to the inverted flare inlet. The threads on the tube nut to inlet provide no sealing, nor do the threads on the inverted flare inlet fitting to the fuel bowl. The later get all it's sealing from the gasket under the inverted flare inlet when threaded into the bowl housing.

That said the only sealing surfaces are the gasket surface on the fuel bowl housing and the flare and inverted flare on the inlet fitting. Inspect these very closely. Especially the inverted flare. If the inverted flare is scored or scratched replace it. If the double flare line is cracked or scored or scratched on the sealing surface replace it. I'd try swapping the inlet fitting from front to rear fuel bowl housings and see if the leak follows. If it does all you need is an inexpensive inverted flare inlet fitting. Make sure you get the right size as they are available in 5/16 and 3/8" for the 4150.

Make sure your fuel lines are double flared, as single flare tends to crack much more easily. Take a close look at the flare under the tube nut. Make sure is is smooth and evenly distributed around it's entire circumference with regards to how it sits in the nut. Good luck let us know what you find.
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  #6  
Old Sep 15th, 09, 10:07 AM
BPOS BPOS is offline
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Al
 
Join Date: Nov 2002
Location: Vancouver, WA
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Default Re: Holley 4150 Fuel Inlet Leakage

You aren't by chance using a reproduction fuel line are you? If you are, it's my guess that your leak is at the flare fitting that seals the tubing to the fuel inlet nut. I used a repro Z/28 fuel line and and could not get the flare to seal properly because the tubing they use is super hard. I replaced that section with some steel tubing from the parts store - double flared it myself and installed it - leak was gone. The tubing is not as hard and conforms to the flare easier.

It might LOOK like the leak is coming from the thin metal gasket between the inlet nut and the bowl, but my guess is that it's leaking at the flare.
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  #7  
Old Sep 15th, 09, 07:36 PM
SY1 SY1 is offline
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Dave
 
Join Date: May 2001
Location: Michigan
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Wink Re: Holley 4150 Fuel Inlet Leakage

Al good point! A lot of after market lines are now available in stainless steel and that stuff is quite hard and difficult to work with. Could very well be his problem.

I also got thinking that it's possible the flares are the wrong angle, could be AN instead of SAE and if that's the case AND it's stainless it may never seal well on the inverted flare SAE. No matter how tight you torque the tube nut AN 37 degree stainless is not going to want to draw up and seal on the 45 degree inverted flare.
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  #8  
Old Sep 15th, 09, 08:33 PM
BPOS BPOS is offline
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Al
 
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Default Re: Holley 4150 Fuel Inlet Leakage

Dave

My repro line set wasn't stainless, but just really hard plated steel. The steel brake/fuel line I got at NAPA was considerably softer and sealed up no problem. I've heard SS can be a real bugger not only to flare, but to seal as well. That is why I have steered clear of it.
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  #9  
Old Sep 16th, 09, 05:57 PM
SY1 SY1 is offline
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Dave
 
Join Date: May 2001
Location: Michigan
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Default Re: Holley 4150 Fuel Inlet Leakage

Al I hadn't thought of that, but you're right there probably are different hardnesses of steel line. Maybe even different wall thicknesses. I usually do like you and buy the soft steel stock, some extra tube nuts and double flare and bend my own. It's easy and kind of fun.
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