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Holley problem: leaking power valve?

12K views 36 replies 12 participants last post by  Vintage Musclecar 
#1 ·
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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#3 ·
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.
 
#4 ·
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.
 
#5 ·
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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#6 ·
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.
 
#7 ·
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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#8 ·
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.
 
#11 · (Edited)
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.
 
#12 ·
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.
 
#14 ·
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.
 
#16 ·
Brass, always brass. Nitrophyl tend to lose their coating, saturate, and sink, foam degrades faster than the Nitrophyl, especially with "gasahol" fuel mixes.

As far as the PV leaking, if that was happening, the carb would lose fuel into the manifold, and eventually into the pan when the engine was shut down, to the bottom level of the PV leak. If it were severe, one cylinder could fill with fuel, and when attempted to restart, bend a rod, kill a piston, etc.
 
#18 · (Edited)
Is there anything in the main body or the throttle plate that would be compromised if I submerse these parts in parts cleaner? The parts washer I have is filled with pretty mild cleaner but it does a good job of dissolving grease. It is not the super aggressive carb parts dip that gives you cancer if you look at it sideways.

The rear bowl had just a tiny bit of dirt in it. The power valve holds a vacuum and the needle valves both looked good but are still going into the dustbin.

I visited Eric's website and was interested in the page about how gasket mating surfaces are rarely as flat as they should be. I found this to be entirely true in my case. I gently sanded the metering block faces of the main body and the base of the main body to take down some of the high spots. The main body has a fairly rough surface in these areas and I'm not going to keep sanding to completely eliminate them (it would take days and days) but I got them much better. Nothing left that the gaskets can't easily take care of.

Concerning the choke plate screws, when I put them back in would blue loctite be sufficient to keep them in place? Same question on the throttle plate screws. I know the throttle plate screws all have lock washers on them but a little extra security never hurts as long as it doesn't damage the screws or main body screw holes.
 
#19 ·
I'd install the PV protector. Like anyone would ever know short of rebuilding the carb. Before it was a part you could buy we would drill the hole oversize cut the spring from a ballpoint pen to length put a small steel ball on top of the spring then reassemble the carb. If you don't install it you better hope it never backfires.
It's only original once, so I guess you better not use the blue gaskets either. BS

later, dozer


p.s. stake the choke plate and throttle plate screws
 
#20 ·
I use blue locktite on the screws and a file on the main bodies when needed. I never dip base plates because you can push dirt into the shaft bushing and you'll never get all the solvent out without removing the shafts but the only thing to worry about is the Teflon srips on the throttle shafts.

We've all heard about backfires rupturing power valves and Holley took steps to keep it from happening but I've never actually seen it happen. Keep your tune right, don't lean on it when it's cold and you shouldn't have to worry about a $5 power valve. I've never understood all the pearl-clutching about power valves- they're cheap and easy to replace. I must have 20 6.5s laying around that I'll never use.
 
#22 ·
Take this for whatever it's worth;

I've been rebuilding carbs for 40+ years, professionally for 20. I have literally built thousands of Holley carbs, and I haven't installed a power valve blowout protector in a single unit.

The number of come-backs I've had due to a blown power valve?

Zero.
 
#23 ·
I have a buddy who is one of those guys for whom every silver cloud has a dark lining. He was always lamenting "there goes the power valve" after the slightest spit / stumble of the engine. Seemed like he wanted a reason to take the carbs apart? I seriously doubt he actually had many failures (if any) and I am SURE he did not have any where the number of failures he fretted about.
 
#28 ·
All the parts are cleaned up so the rebuild can begin this weekend. In fact, I think the old 37-119 Trick Kit I have may have enough parts to do the rebuild. I could take the new one back or just put it on the shelf for future use. I must have bought the kit years ago, probably to get one or two little parts I needed at the time.
 
#29 ·
If you need any small parts to keep from having to open the new kit please let me know. I buy all the parts individually and in bulk so I have a small speedshop's level of supplies. I'd be happy to send you whatever you're lacking.
 
#30 ·
I only dress the threaded metering block boss ends of the body for flat, as the screw hole bosses bend easily when some Gorilla uses 57 foot pounds of screw torque to hold a leaky metering block in place after it was bent so far out of shape it won't work.

For the metering blocks, I lay them out with their bent curvature up in the center, and VERY LIGHTLY press them back flat. Way too easy to turn a metering block into instant junk with other methods.

Before I get plastered by everybody for what I am saying, please remember, I worked in the Pro-Stock carb dept of Holley Racing for 5 years, band sawing Dominator's apart, making them into "Split Duals", then helping tune them on race cars.

As far as the leaking PV, a few ways to get them to leak, frm the wrong gasket, to the over tightening in the metering block, but what aI was asking, and got the right answer, was, did the vacuum diaphragm get punctured, was there fuel leaking into the manifold? I have seen a very small tear in a PV diaphragm load the entire bowl volume down to the hole, into the intake manifold, cylinder, and oil pan. NONE of ahy of that is good at all. Glad that wasn't the issue.

The PV save device is simply to make sure the PV diaphragm doesn't have failure issues if the engine backfires. The reverse pressure can easily kill the PV diaphagm.
 
#32 ·
All buttoned up and ready to go back on. I did end up opening the new Trick Kit, there were just a number of parts I needed which I did not have on hand from the old kit.

So now the bowl screw gaskets are hard plastic? I suppose that will make them reusable, but I sure hope they seal okay and don't melt from fuel contact.

I also found that the springs that support the floats are different. Not a big deal, just never noticed before. One has about twice as many coils as the other. They both supply about the same amount of support, just look different. Hope it doesn't matter which one goes where.

And no parts left over! Yay!
 

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#33 ·
I don't prefer the plastic screws, but, they don't tear, nor separate, and they are reusable. I always coat those paper gaskets with anti-seize. BTW, bowl screw torque is 60 INCH POUNDS, (5 foot pounds), NO TIGHTER.

The different design springs help further dampen float operation when the carbs are mounted on off-road, and/or, high vibration environments.
 
#34 ·
Dave,
Not trying to hijack this thread but every time I see a post by you i wonder the following. Are you the same Dave Ray that used to be on Graveyard Carz?
 
#36 ·
I take it that is some sort of TV show, like Overhaulin' and other disasters. Nope, not me, but I do know a very small handful of them. I have always been too busy fixing stuff from those sort of people to spend time on shows like that.

Yes, that is the old board name I used to have.

Right now, I am watching highlights from earlier today, Valencia, Espana, FIM Moto E Race 1, no gasoline engines, only electric Grands Prix race bikes. Strange to hear varying whining of electric motors, instead of 4 stroke exhaust notes.
 
#37 ·
Whatever paper material they're using these days for bowl screw gaskets is garbage--same goes for power valve gaskets. You hold your lip wrong and the damned things shear in half when you tighten them down, and guarandamnteed they're gonn'a leak.

I used to soak both of them with a light oil (typically 3-in-1) and let them soak for a minute before installing them. This allowed the surfaces to slip when tightening them down and improved the odds of the gasket shearing in half to 50/50. :|

It also allowed the gaskets to compress far better than when dry--you'd be amazed at how many carbs I've had on my bench that came in with LOOSE power valves (and bowl screws). I'm sure they were tight when they were built, but the gaskets obviously took a set once the carb went into service and loosened everything up.

I've since switched to the black nylon bowl screw gaskets, and I had custom Teflon power valve gaskets made a while back. ZERO problems since, but you do have to sauce the bowl screw gaskets up before assembling. The material is pretty "sticky" when it's dry, and often the friction will stop the bowl screw from turning before the screw is actually tight and the gaskets are compressed. A little lube alleviates the problem.

BTW, Rich, I answered your PM.
 
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