Holley problem: leaking power valve? - Chevelle Tech
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post #1 of 37 (permalink) Old Nov 10th, 19, 6:08 PM Thread Starter
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Holley problem: leaking power valve?

I had the L79 out for probably the last time of the season. I changed the oil and took it for a decent drive and just enjoyed it for probably the last time of 2019. Anyway, once it warmed up I noticed a new issue. When idling it would surge a bit, up and down. Idle would drop to about 500 (it can barely run at 500, normal idle is 800-900), stumble a bit, recover and then stumble again, up and down. It would never die.

I also noticed that when cruising through town at 20-25 miles an hour in second or third with the throttle steady but barely open it would surge and buck. When the choke was on or I was into the throttle or I was cruising on the highway it ran just fine. It ran fine while accelerating so it doesn't appear to be a clogged fuel filter.

One thought crossed my mind that it might be a vacuum leak, but a vacuum leak would cause a steady problem, but a leaking power valve would cause a surge as it dripped, dripped, dripped. Could be a fuel drip from some other source but I don't know from where.

I haven't checked ANYTHING yet. First things to easily check will be the fuel bowl levels and the o-ring and diaphragm in the vacuum secondaries can and check that the vacuum advance can holds a vacuum too.

The L79 carb is a weird little 4150 with two metering blocks but only one power valve. The rear metering block only contains jets. I haven't really touched the carb probably since the 80's when I rebuilt it completely during the car's first restoration. Once or twice in the past I've had issue with needle valves sticking open but that simply results in lots of fuel dumping down the intake and causing it to die. This issue is completely different.

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post #2 of 37 (permalink) Old Nov 10th, 19, 6:16 PM
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Re: Holley problem: leaking power valve?

i would replace the needle and seat assy.
see if you have positive results ...

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post #3 of 37 (permalink) Old Nov 10th, 19, 7:43 PM Thread Starter
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Re: Holley problem: leaking power valve?

Well, that was easy. I went up to the shop to assess what spare gaskets I have lying around and found I have tons of them. I even have a few spare power valves and lots of extra parts. So, I thought I'd look at the carb and see if I ever added those blue gaskets that don't stick to things. When I looked at the carb I found that the whole front of the carb was wet and the primary bores were wet. Looks like I have a sticky needle valve. Maybe just some junk on the seat since I never got a full bore flooding of the engine.

Anyway, I pulled the carb. I'll play with it more next weekend when it warms up again. The weekdays coming look pretty chilly. I should have enough parts on hand to do whatever repairs I need to make as long as I don't have to pull the vacuum secondaries can off the body. I have no idea why those little gaskets that seal that to the body are so hard to find and so expensive when you do find them. I have tons of bowl and metering block gaskets, o-rings for the crossover tube and spare needle valves and gaskets. Speedway is just a few miles away though should I need anything more.

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post #4 of 37 (permalink) Old Nov 10th, 19, 8:18 PM
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Re: Holley problem: leaking power valve?

Rich, considering history of the car it's likely the 0-ring on the primary N&S has given up the ghost. If you have or can get a #8 O-ring you can fix it without even pulling the front bowl. Or, since I have a couple hundred I could send you some. If you want send me your mailing address via PM and I'll get some out to you.

Is that Q on your pickup still happy? That's been like 10 years ago now.

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post #5 of 37 (permalink) Old Nov 10th, 19, 10:29 PM Thread Starter
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Re: Holley problem: leaking power valve?

Quote:
Originally Posted by Tom Mobley View Post
Rich, considering history of the car it's likely the 0-ring on the primary N&S has given up the ghost. If you have or can get a #8 O-ring you can fix it without even pulling the front bowl. Or, since I have a couple hundred I could send you some. If you want send me your mailing address via PM and I'll get some out to you.

Is that Q on your pickup still happy? That's been like 10 years ago now.
The only problem with the Q-Jet I've had since you laid hands on it was when the fuel filter got clogged. Discovered that one day when I went to pass on a busy two-lane and it ran out of poop halfway through the pass! Made it home alive and got a new filter in it right away.

I think I have a bunch of spare o-rings but I'll check and let you know if I can't find any. Could be a float with a hole in it or dirt in the bowl so I think I'd better pull the bowl off and check it all out. That would also allow me to check the power valve just to be sure about it too. I recall seeing a few needle valves still sealed in their baggie. I never use any fuel with alcohol in it but nothing lasts forever.
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post #6 of 37 (permalink) Old Nov 11th, 19, 11:57 AM
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Re: Holley problem: leaking power valve?

Sounds like you're on the right track but I would like to remind everyone that a power valve cannot effect idle unless it is completely ruptured and leaking fuel into the vacuum chamber behind it. The power valve supplies fuel to the main circuit only and the main circuit is not active at idle.

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post #7 of 37 (permalink) Old Nov 12th, 19, 10:45 PM Thread Starter
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Re: Holley problem: leaking power valve?

I got the carb home and just couldn't wait for warmer weather so I pulled the front bowl off. No dirt, absolutely clean as a whistle. The needle valve moves freely. The power valve moves freely. I haven't pulled either out to inspect closer but it is obvious fuel had been at least oozing out of the vent on the top of the bowl and out the vents in the throats of the front barrels. So SOMETHING was causing the front bowl to overfill. Float level hadn't been touched in years.

I was kind of hoping it would be something obvious. I guess since I have it off the engine and at least partially apart I might as well regasket it and generally rebuild it. I guess I'll have to go buy a new kit just to make sure I have all the right bits and pieces. Oh well, a Trick Kit is only about $32.
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post #8 of 37 (permalink) Old Nov 13th, 19, 9:34 AM Thread Starter
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Re: Holley problem: leaking power valve?

I suppose I should also install the power valve protection kit while I'm at it. 125-500

Although, this is an all original number matching vintage carb so maybe I shouldn't mess with it that far. Never actually had an issue with needing power valve protection anyway.

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post #9 of 37 (permalink) Old Nov 13th, 19, 10:02 AM
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Re: Holley problem: leaking power valve?

What do they say Rich?
It's only original once. I'd leave it.

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post #10 of 37 (permalink) Old Nov 13th, 19, 10:11 AM
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Re: Holley problem: leaking power valve?

Put a good kit in it, replace both floats and forget about the power valve blowout protector. If you need help, feel free to give me a shout.
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post #11 of 37 (permalink) Old Nov 13th, 19, 10:43 AM Thread Starter
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Re: Holley problem: leaking power valve?

Quote:
Originally Posted by Vintage Musclecar View Post
Put a good kit in it, replace both floats and forget about the power valve blowout protector. If you need help, feel free to give me a shout.
Which are better, brass or hard foam? They are side hung 4150 floats, looks like brass is 116-4 and foam is 216-9.

I only use Holley parts on my Holley. I plan to use the Holley 37-119 Trick Kit to do the rebuild. But maybe 37-1544 would be better? The 1544 has a lot of extra parts I wouldn't use though. There is also the 37-1542 called the fast kit, not sure how it differs from the 119 though.

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post #12 of 37 (permalink) Old Nov 13th, 19, 12:46 PM
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Re: Holley problem: leaking power valve?

Brass floats.

The 37-119 kit should be fine.

While it's apart, replace the secondary diaphragm. P/N 135-4.

Use grease on the transfer tube O-rings when you install the bowls. Put the O-ring at the very end of the transfer tube, lightly smear the ring with grease and slip & twist the tube into the bowl making sure the O-ring doesn't slip past the flare on the tube.

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post #13 of 37 (permalink) Old Nov 13th, 19, 1:07 PM Thread Starter
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Re: Holley problem: leaking power valve?

Quote:
Originally Posted by Vintage Musclecar View Post
Brass floats.

The 37-119 kit should be fine.

While it's apart, replace the secondary diaphragm. P/N 135-4.

Use grease on the transfer tube O-rings when you install the bowls. Put the O-ring at the very end of the transfer tube, lightly smear the ring with grease and slip & twist the tube into the bowl making sure the O-ring doesn't slip past the flare on the tube.
Yeah, that diaphragm used to be part of the kit. I'll be sure to get one of those.

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post #14 of 37 (permalink) Old Nov 13th, 19, 4:16 PM Thread Starter
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Re: Holley problem: leaking power valve?

I got all the parts I needed. It sure it great to live so close to Speedway Motors so I can get parts like these the same day I need/want them. I did skip the power valve protection kit. I got a Trick Kit, secondaries diaphragm and new floats.

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post #15 of 37 (permalink) Old Nov 13th, 19, 5:11 PM
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Re: Holley problem: leaking power valve?

Haha, I always "suck" my PVs to ensure they're kosher! Good luck Rich, I too am in the needle/seat camp, as they'll often fail "just because".

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