Blown power valve?
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=50263
Printed Date: November-22-2024 at 8:19pm
Topic: Blown power valve?
Posted By: uncle-buck
Subject: Blown power valve?
Date Posted: September-28-2021 at 5:52pm
| Took the boat out on Sunday and had several backfires. Later discovered (with Jonny Quest's generous assistance) that I had over-tightened the rotor screws on my new D.U.I. distributor - so it was not advancing. Now fixed, but am wondering if the power valve in the Holley carb is blown. Ran the engine this afternoon and closed both mixture screws all the way. The engine stumbled, but continued to run. That means the power valve is toast, doesn't it? |
Replies:
Posted By: Gary S
Date Posted: September-28-2021 at 6:46pm
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What year? 1992 Holley started installing power valve backfire check valves. They make a kit to add one if you have an earlier one ------------- http://www.correctcraftfan.com/diaries/details.asp?ID=1711&sort=&pagenum=1&yrstart=1966&yrend=1970" rel="nofollow - 69 Mustang HM SS 95 Nautique Super Sport |
Posted By: uncle-buck
Date Posted: September-28-2021 at 6:55pm
| The carb was purchased from SKIDIM in 1995. |
Posted By: KENO
Date Posted: September-28-2021 at 8:28pm
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There are a variety of things that can let it keep running with the idle mixture screws shut, blown power valve being one of them. How's it start and run right now?
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Posted By: uncle-buck
Date Posted: September-28-2021 at 8:54pm
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Starts and runs well in the driveway. Will probably pull the primary fuel bowl and inspect the power valve before getting back on the water. Thanks |
Posted By: uncle-buck
Date Posted: September-29-2021 at 2:04pm
| Checked manifold vacuum at idle while tuning the mixture screws and got a max. of 14 in-Hg. So I replaced the power valve with a 6.5. |
Posted By: MrMcD
Date Posted: September-29-2021 at 8:04pm
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Does it run better? ------------- |
Posted By: uncle-buck
Date Posted: September-29-2021 at 8:06pm
| Haven't taken it out yet. It runs well in the driveway. |
Posted By: MrMcD
Date Posted: September-29-2021 at 9:39pm
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There have been notes on this forum to help choose the correct power valve. Might take a look to be sure a 6.5 is correct. If my memory was better I would remember. Glad you have it running better. ------------- |
Posted By: Jonny Quest
Date Posted: September-30-2021 at 12:40am
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I use the old school rule-of-thumb for power valve selection is the formula of the inches of Hg at idle, divided in half (round down). So, 10” Hg at idle calls for a number 4.5 power valve. So, if Uncle Buck had 14” Hg, a 6.5 PV sounds about right. However...I've seen both 6.5 power valves and 2.5 power valves in 4160 carbs that appear to be OEM factory configuration Go figure. When I retro-fit my old 1994 with a new carb, I installed a QuickFuel M-600. That carb came from the factory with a 6.5 power valve. The M-600 is more-or-less an upgraded version of the Holley 4160. JQ
------------- Current 2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
Posted By: KENO
Date Posted: September-30-2021 at 7:38am
Kinda makes you wonder what those Holley engineers could possibly be thinking when they started putting 2.5 power valves in marine 4160s maybe 30 years ago. 
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Posted By: Jonny Quest
Date Posted: September-30-2021 at 3:08pm
Yup. A bit of a head scratcher... I looked on the Holley website and found this: [Quoting liberally from the Holley.com website] "The purpose of the power valve is to improve part-throttle drivability, air-fuel ratio, and fuel mileage. With high manifold vacuum in a part-throttle situation, the low pressure “pulls” on the power valve keeping it closed. As the throttle opens and the load increases, the manifold vacuum will drop. At a calibrated point, a spring inside the power valve will overcome the low manifold vacuum and open the valve. When the power valve opens, it directs fuel from the float bowl into the main well in the metering block. Why is an additional circuit is necessary? Why not just increase the jet size and eliminate this circuit? The reason is that there are times when the engine will operate at a light throttle. The power valve circuit delivers a rather substantial amount of additional fuel when at near wide-open-throttle (WOT). By reducing the primary jetting at part-throttle, the engine operates at a leaner air-fuel ratio which prevents fouling plugs and washing the cylinder walls with excess fuel, which can damage piston rings. Generally, the power valve circuit will introduce the rough equivalent of 6 to 8 jet sizes worth of additional fuel. This reduces the primary jet size by that same amount. Each power valve is rated to open at a given manifold vacuum level (generally expressed in inches of mercury, or “ Hg). For example, the most commonly used Holley power valve is rated at 6.5“ Hg, which means the valve will open when manifold vacuum drops to 6.5“ Hg or lower. Basic tuning recommendations: Use your vacuum gauge to read the idle vacuum of a fully warm engine at idle in gear. Let’s put this idle vacuum at 13 “Hg. If we divide the idle vacuum in half, this results in a 6.5“ Hg power valve as a great starting place for tuning. This will open the power valve when the manifold vacuum reaches 6.5“ Hg or lower. Some tuners prefer to open the power valve a little earlier to prevent the engine from running lean at heavier part-throttle that could cause detonation problems. In this case, the tuner might raise the power valve closer to 8.5 or perhaps 9.0 “Hg." ******** So...based on what Holley says, it seems that the 2.5 power valve would allow for the engine to run a bit leaner throughout a larger range of "part throttle", thusly providing a bit better fuel economy. The improved fuel economy from a 2.5 PV may come by preventing the PV opening too soon before WOT. By contrast, the 6.5 PV would open much sooner in a part-throttle scenario. It would be a fun experiment to run an "A vs B" scenario on the lake. JQ
------------- Current 2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
KENO wrote:Posted By: MrMcD
Date Posted: September-30-2021 at 4:54pm
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Interesting discussion, makes me wonder how the Quadrajet or Carter AFB carbs accomplish the same thing without power valves? The Holley and the other designs all seem to work when tuned proper so they must have hidden secrets. ------------- |
Posted By: KENO
Date Posted: September-30-2021 at 5:12pm
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Just for you JQ, here's a quote from an old post. That boat had a 2.5 originally in it's carburetor, used a 6.5 for a while and went back to a 2.5 a year later. Gas use was marginally lower with the 2.5. Nothing scientific, but my wallet was a little heavier when the 2.5 was being used. Check those idle vacuum numbers compared to Buck's idle in his driveway
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Posted By: KENO
Date Posted: September-30-2021 at 5:19pm
Midrange tuning needles /springs along with main jet sizing in Carter/Weber/Edelbrock and the Q jet too.
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Posted By: MrMcD
Date Posted: September-30-2021 at 10:31pm
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There you go, a good answer. I have rebuilt several carbs but I have to say I don't think I ever understood them. Switched my old Nautique from a AFB that was on it when I bought it back to a Holley. Played with the tune on that Holley for half a summer before it was dialed in for my engine. Then it worked the next 10 years. An experienced guy would have tuned it in a couple hours. It was a used Holley that I picked up with problems but I was young and broke at the time so we made it work. The AFB was just worn out. ------------- |
Posted By: Jonny Quest
Date Posted: October-01-2021 at 2:08am
Good info, KENO. For those operating constantly in the 3,000 to 3,500 RPM range, the 6.5 PV may be a good option. JQ ------------- Current 2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
Posted By: MrMcD
Date Posted: October-01-2021 at 2:27am
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I think the numbers shown would drop with a load, pulling a good skier at 36 MPH, usually 3,600 RPM the engine would have less vacuum, maybe that is why Nautique or PCM chose a lower number power valve for some engines. The heavier the load the more you open the throttle plates which drop your vacuum number. ------------- |
Posted By: uncle-buck
Date Posted: October-01-2021 at 8:41am
| Just noticed that the new Holley carb which is calibrated for the Ford 351W engine come with a 2.5 power valve and the 703-29 rebuild kit for the 1995 carb I have on my boat comes with a 5.0 power valve. |
Posted By: KENO
Date Posted: October-01-2021 at 9:00am
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So.......what's your list number? You probably didn't read the part that says the kit comes with a 5.0 power valve, your application may vary. Read the tech resources section in the link below. Kinda funny though that probably every carburetor listed for this kit came with a 2.5 power valve. They must have had some 5.0 valves to get rid of or something http://www.holley.com/products/fuel_systems/carburetor_components/rebuild_kits/marine_renew_kit/parts/703-29" rel="nofollow - link
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Posted By: uncle-buck
Date Posted: October-01-2021 at 9:11am
| The list no. is 50419-1. Holley's website says to use the 703-29 kit. It comes with the 5.0 power valve. |
Posted By: KENO
Date Posted: October-01-2021 at 9:13am
| Did you read the tech resources section of the link above? |
Posted By: uncle-buck
Date Posted: October-01-2021 at 9:22am
| Yes. I simply thought it was interesting that carbs which were supposedly calibrated for the same engine come with different PVs. |
Posted By: KENO
Date Posted: October-01-2021 at 9:32am
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Those carburetors listed aren't all for marine 351 engines Holley information in lots of cases is confusing at best.
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Posted By: Jonny Quest
Date Posted: October-01-2021 at 11:20am
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The more learnin’ I get, the more confused I become. 🤪 JQ ------------- Current 2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
Posted By: uncle-buck
Date Posted: October-01-2021 at 11:48pm
| FWIW, the 5.0 power valve (pulled from the 1995 carb which was subjected to a couple of backfires last weekend) was not blown. Apparently, Holley's backfire protection feature works. |
Posted By: KENO
Date Posted: October-02-2021 at 5:42am
Imagine that............. the little ball and spring work like advertised
Well just to add to your confusion, the original power valve listed for your 50419-1 is a 2.5, so where'd the 5.0 come from? A previous rebuild or something? What's a guy to do? I guess I'd launch it and see how it runs |
Posted By: uncle-buck
Date Posted: October-02-2021 at 6:04am
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Right. The 5.0 PV came from the rebuild kit installed last year. Planning to launch on Sunday... |
Posted By: Jonny Quest
Date Posted: October-02-2021 at 8:04pm
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I'll bet a ski run behind my little Nautique that 9 out of 10 people couldn't distinguish between a 2.5, 5.0 or 6.5 PV on back-to-back tests in the same boat. JQ
------------- Current 2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
Posted By: TucsonMike
Date Posted: March-27-2022 at 8:47pm
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I realize this is a bit late to the party - I don't recall which mine is (maybe 6.5?), but on my 1994 closed bow nautique (19.5ft) it seems calibrated for more power when opening the throttle from a 36mph cruise - like you'd want when the skier is pulling hard after rounding the buoy. ...I found this thread when searching for 'float bowl'. I'll post about that next... ------------- SkierMike |
Posted By: TucsonMike
Date Posted: March-27-2022 at 8:52pm
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Almost forgot - mine always (since I bought it in 1998) ran rich at idle. Got gradually worse until it would die frequently. Went down the power valve rabbit hole after the metering screws test. turns out the screws holding the secondary metering block were very loose, allowing fuel bast the block. Runs great after fixing that. ------------- SkierMike |
Posted By: TucsonMike
Date Posted: April-17-2022 at 7:56pm
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I finished the repair. Mine (1994 closed bow 351 pro boss) always did have a 2.5 in it. ------------- SkierMike |