Carb/power valve settings for a 4.3L V6!

loco

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Hi all!
So my accellerator pump was shot, so I spoke to real steel in the UK, who found a compatible rebuild kit for my carb (Holley 75007-2).
The carb's obviously a marine model, and the rebuild kit was for a car. Everything went in OK, but the power valve that came out of the carb was unmarked. The power valve that came with the rebuild kit was a 6.5, and felt like it had a lot more resistance to the one that came out.

I just bolted the rebuilt carb on last night, and got it fired up. I fiddled about with the idle screws a bit to get it idling a bit better, and was getting around 12-13in/Hg at idle. That said, the idle is still lumpy, so I think there is more work to be done getting it right, but it was dark and raining, so I left it last night.

Anyway, my question is: do I need a lighter power valve for marine use? Does anyone know what these carbs would have had in them from stock?

I'm obviously looking for the best performance, and hopefully some efficiency. My fuel flow meter goes up to 65litres/hour when I have WOT, and it's only a 23ft boat, pushing 220HP!

Thanks in advance.
 

Scott Danforth

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6.5 power valve is for a car NOT a marine application

you should be looking at a 2.5 or 3.5 for marine use. most likely the 2.5

available at any Holley retailer.
 

loco

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Thanks!
Is it because of the type of load? I'd like to understand the mechanics behind it.

How do I find whether I need the 2.5 or 3.5?
I.e. On the Holley site, it says to use a PV of half the vacuum pressure. Presumably different for marine use.

Interestingly, I messsged Holley, and they said the carb originally came with a 6.5, but it didn't seem right to me.
 

Scott Danforth

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The value is the opening vacuum. Since marine motors are always under load (very low manifold vacuum), you need a lower power valve setting than you would in a car.

Most of my Holley marine carbs have been #25 power valves. My truck and hotrod ran #65
 

jimmbo

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All I could find for all the 4.3 engines was a 2.5" power valve. Closest I could find was R75006A, but as I said all the 4.3s showed 2.5". A 6" would just open sooner. At WOT a 4.3 is gonna be hard pressed to have more than an in or so pressure drop on a 500 cfm 2bbl

Just for others info, On the 5 liter and 5.7 engine the Volvo service manuals shows power valves in the 8.5 and 9.5 ranges. I think that's a bit high, the 2bbl Holley on my 5.7 has a power valve of 5 or 6"(been 13 yrs since I looked at it). At WOT, my engine pulls 4" of vacuum. A 2.5" would be closed at WOT. Shows the carb 2bbl 500( when flow tested like a 4bbl, 370cfm) is a wee bit tiny for the 5.7, but, it is the perfect size for a 5 liter running 5000rpm.
 
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loco

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Ok, thanks, all. I'll go 2.5. Corroborates with the feel of the PV I replaced.
Rather disappointing that Holley themselves gave poor advice.
 

jimmbo

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You yourself said it was a rebuild kit for an auto. Holley does make kits for marine carbs too, whether or not they would come with the correct power valve is unknown as the kit would cover several carb numbers. Power Valves are not necessarily a mandatory replace item. Most are needlessly replaced.
 

loco

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Right, I got the thing fitted, with the 2.5 power valve. I lowered the idle so that the butterflies were totally closed, and if I screwed the idle mixture screws all the way in, the engine stopped very quickly. I tuned the idle settings as best I could, and it seems OK.
I find the vacuum and RPM gradually die as it idles though - check out the video and see what you think... does that vacuum look a bit unstable, too? (sorry about the squealy belt)

Next, I took the boat out for a test. Conditions were choppy, and the hull and props are filthy so I couldn't really run flat-out, but low-to mid range pickup was a hundred times better thanks to the fixed idle pump.

Getting towards the higher RPMs - 4000 and above, the engine felt a little flat. Presumably, that could either be the PV, or the vacuum secondaries.

So, to test the vacuum secondaries are opening, should they be opening when I rev the engine out of gear in the harbour, or does the engine need to be under load, so they would only open when I'm going full pelt at sea, and would need an assistant to hold the engine bay open and check?
any other thoughts welcome!!
 

Scott Danforth

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At 4000 rpm the power valve isnt in the picture. You should be into the secondaries by then

What spring did you use on the vacuum diaphragm?
 

loco

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I reused the spring that was in the diaphragm. I went down today to see if I could block the tubes in the primary and secondary barrels that feed the diaphragm, so I could compress it and check for leaks, but I couldn't fit my fat fingers past the primary venturi, so I'll have to take it apart, or just test it at sea.

Before I put the 2.5 power valve into the carb, I pushed it against the PV that came out, and the 2.5 had more resistance, so I think it's a harder spring. That said, the one that came out could be up to 30 years old, so could have weakened.
 

Scott Danforth

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Put a paper clip on the diaphragm rod, if the diaphragm moves, the paper clip gets moved.

Learned that trick from an old hot rodder about 35 years ago
 

loco

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Ok, I took the boat out today, and ran with the air cleaner off. As I suspected, the secondaries weren't opening. Going WOT, she'd get around 3K, then would bog. I went in with a screwdriver and manually operated the secondaries, and the boat instantly had more power and went all the way to top speed (top speed with dirty props and hull, at least). When the engine was working hard, I was able to let go from manually operating the secondaries, so the vacuum at this point was enough to hold itself open.

So, the carb came back with me to the workshop. I blew air past the hole in the primaries, and after getting the positioning right, I could get the vacuum secondaries to open,so the nozzle wasn't blocked.

I took the vacuum unit off, squished it in, and held a finger over the hole. It stayed in the compressed position, so I'm certain the diaphragm is ok.
I blasted air to clear the nozzles in the carb body, but it all looked fine.

so, I'm a bit confused here. There's a red spring in it, but the same spring has been in there since forever, and worked in the past.
Here are my thoughts:

- The 2.5 power valve is still too much for this engine. This is the only thing that's really changed since the rebuild (other than gaskets), and pushing it against the unmarked PV that came out of the carb, the original had a softer spring rate.
- There was a leak between the vacuum unit and the carb body, and the engine wouldn't be able to get enough vacuum.
- With filthy props and hull, there's too much drag on the boat (seems unlikely, as I'm sure she's been dirtier, and still got up to speed).

I think my next play will be to put the original PV in there, and replace the seal between the carb body and vacuum unit. If it was running too rich and bogging in the mid-range, then maybe it was just shy of getting the right vacuum, etc.

Any other thoughts or ideas very much welcome, though!!
 

Scott Danforth

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Should be a yellow, orange or white spring in the vac diaphragm. Most marine applications are white

The purple spring is too high a spring rate.

Holley doesnt have a red spring
 
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loco

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No Title

This is the spring that came out. It seems Holley don't offer a red spring, as you say, so I'm at a loss right now, as it looks distinctly red. I can't help but feel that this carb maybe had some non-standard parts in it for the volvo marine application - the unmarked power valve that's softer than a 2.5, a red spring, etc.

Before blowing cash on a quick-change cap and set of springs, I think it would be best to see if I can get it going with the spring that presumably worked ok before. Looking for a replacement cork seal, they're crazy expensive and come in blocks of 20. Could I not use a rubber o-ring instead, and achieve the same seal?

i.e instead of this:
https://www.holley.com/products/fue...onents/vacuum_secondary_parts/parts/108-67-20

Use something like this:
http://www.ebay.co.uk/itm/50-Pcs-In...hash=item43bbce5053:m:mD3WfOa0YAz7hBr8pRnDq1A
 

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Scott Danforth

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Thats more faded purple. Get a holley spring kit, put in the white spring
 

loco

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Just an update on this one, I've picked up a Holley spring set, and have been trying to compare the spring that came out with the ones in the kit, but it's so dfficult to tell. It's certainly on the softer end, and comparable to the yellow or white.

I found this Holley document that lists many carbs as coming with a red spring which is interesting (search for the 75007, for example). I mailed Holley tech support to ask how the red compares to the other springs, but they haven't got back to me yet:
http://documents.holley.com/techlibrary_carb_numerical_listing.pdf

If the red is indeed softer than the white, then It would make sense to put it in, but I'll give the boat a run with the white spring this weekend and see how it goes with the fresh cork seal, etc.
 

Scott Danforth

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I personally like the blue non-stick gaskets over the cork gaskets. they are reusable and dont stick to the bowls or metering plates.
 

Lou C

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I don't know Holley carbs at all but thinking back to tuning my old school American V8s with carbs, they all pulled a higher vaccum reading at idle, then you see in that vid. More like 17-20 inches of mercury. Low readings like that can be due to vaccum leaks, late ignition timing, or low compression...
 

Scott Danforth

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Boat motors are always under load, and have almost no manifold vacuum
 

jimmbo

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Moat marine engines do best with the black spring(slowest opening). The last thing you want are the Secondaries opening too soon or too quickly
 
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