As a heads up, it's very poor forum etiquette to hijack a thread, especially if it isn't even the same topic as what you're asking about. If you want help on a subject, please start a new thread.

 

Hi all,

Did an interesting test that I thought was worth sharing.

I disconnected the arm from the secondaries so that they couldn't open and then went for a test drive.

The first thing I noticed was that when I got on it, it was still dropping down to a super rich 10.0 AFR. I've always thought it was the secondaries causing the rich condition, but now I can see that it isn't.
My guess is that it's the power valve, and my PVCRs (Power Valve Channel Restrictions) are way too big:



When the powervalve opens all the way, it's just dumping gas in. I'll play around with restricting these with wires, as I have everything else.


The second thing I noticed, and the one that really surprised me, is that when I did a full RPM pull, the primaries took me all the way up to 4500 RPMs without leaning out.
I'm not totally sure what to think of this since that would mean that there is really no need for the secondaries, and that those tiny primaries flow a LOT of cfm. Mathematically, a 300ci engine @ 4500 RPMs = 390cfm. Accounting for volumetric efficiency, that's still around 330cfm.
Does that mean on a 650cfm spreadbore carb that the tiny primaries are flowing over 50% of that? I can't see that.

This leaves me a little puzzled. Thoughts?

 

Hmm that's very interesting. I'm not sure what to think about that. Except to try it myself once mine it up and running

 

So I gave this a try yesterday. As you can see, that's QUITE a reduction in the fuel opening for the power valve. However, the AFR was still around 11.5 when it opened fully. Ideally, it should be around 12.0 - 12.5, maybe 13 or so, so I think I need to reduce it some more.

Also, I'm also still baffled that at WOT, it'll stay at 11 AFR all the way up to 4500 RPMs. What good are my secondaries doing me?

 

The extra fuel through the secondaries is accompanying the extra air through the secondaries and thus maintaining the AFR?

 

Must've missed it. In my post from Saturday, I said I disconnected the arm to the secondaries. They're currently not opening. So those little primaries are providing a rich AFR all the way through the entire RPM range. I would have thought they would have run out of breath around 2000 or so.

 

The small primaries probably increase the velocity of the air flow and thus a bit stronger signal to the fuel metering systems and you are getting more fuel because of that? I'm just guessing on some of this......

 

Harte3, I was thinking along the same lines. Kind of like when you place your hand over the carburetor throat of a lawn mower... the vacuum is strong enough to literally flood the carburetor.

 

So what happens to the AFR with the secondaries? And how much, if at all, do they open at WOT? Maybe get some Prussian Blue or something so you can see if and how much they do open.

Get one of those PV plugs and see how the AFR looks without one. Do they sell low-flow PVs? I wonder if they sell roll pins wee enough to fit those passages.

 

What's the fuel inlet pressure and how do the floats/fuel bowls look?

 

Even though the secondaries are disconnected, could it be that the secondary idle system (if that carb has one) is still having fuel drawn through it?

 

Hi all,
Thanks for the ideas and sorry for the late reply.

Getting a power valve plug seems like a really good idea to help me really isolate each of the fuel systems and seeing what they're doing individually. Sometimes it can be tricky seeing what's contributing where.

As for the AFR all the way up the RPM range, I did a little more looking into it. And yes, the primaries are supplying enough fuel flow to pull all the way up to 4500 RPMs.

However, the engine vacuum keeps climbing during that pull. If it's 0.5 hg at WOT at 1000 - 1500 RPMs, it'll climb up to around 3 - 5hg at WOT when it's up to around 4500. So, even though it can do it, it's a ristriction, and is where the secondaries come in. They'll just allow MORE air/fuel to get in, and allow the engine to get up to speed faster.


I don't have a fuel pressure gauge (something I still need to get).

I'll have to do some more looking into things here soon, but it dropped from about 60° to around 5° in a week, so I've been hiding indoors.... where it's safe.

 

"...it dropped from about 60° to around 5° in a week, so I've been hiding indoors.... where it's safe."

Me too! Time to plug in the tank heater!

 

I had just bought two of them and planned on putting them in this weekend. Now, I'm worried my hand would freeze to the block.

 

A little update, and possibly a final verdict.

I did some long distance driving for Christmas this last week. The trip is about 150 miles each way of easy, straight interstate driving, so it's a good judge of gas mileage.

On the way there, I had two issues. One, I had a terrible headwind the whole way. Two, this was combined with the fact that I forgot I was toying with some settings and had a 10.5 powervalve in it. Needless to say, it was in the powervalve the entire time. Kind of a sucky thing to be driving and seeing your wideband read 10.5 - 11.0 AFR for two hours straight. In the few off chances it wasn't, it was reading a nice 16.0, which is about where I wanted it. I ended up getting 11.5 mpg. My average speed was 60 - 70mph. Not really a very valid test, but it's good to note that that's probably the worst gas mileage I can get.

While at my parents, I swapped out the 10.5 pv for a 6.5 pv.

On the way back, it was a clear, calm day (perfect for driving), and I wanted to see what it could do. I cruised the 150 miles back at 75mph the entire way. RPMs were around 2500 and my AFR read a nice, healthy 15 - 15.5 AFR 95% of the way (aside from dipping down into the powervalve on steeper hills, as expected).
I ended up getting 13.2mpg on the way back.


As for the carburetor, I don't think I can get the cruise tuned any better than it is. 35 - 65mph is around 16:1 AFR and 75 is around 15 - 15.5. That's just about perfect.
However, I really think it should have done better than 13.2mpg. With the 600cfm generi-Holley I had on there before, I was getting 15 - 16mpg at that speed and 17 - 19 at 65mph.

Unless there's another issue going on, like an ignition issue, timing, dragging brakes, that sorta thing, I think I'm finally going to surmise that this carb just loves to guzzle gas. I can't do any better than 11.5 - 12mpg in town (used to get 15 with the Holley) and on the open road, 15 is about the best I've done. It drives really nice, and has been one of the better mannered carbs I've had, but it just loves its fuel!

Another oddity is that I've had the secondaries disconnected for over a month now and have never noticed their absence. Those tiny primaries supply TONS of power all the way up to 75 mph. It's strange pulling up a steep grade at 75mph and being able to maintain speed on those tiny primaries. If that's the case, what do I need secondaries for? I have all the power I want.

On that note, if I have all the power I want with the primaries, then that means that small primaries aren't saving me the slightest bit of fuel (which has been obvious).

Considering that the stock jetting is 64, I'm starting to think that Holley didn't really design a spreadbore with this carb. I remember when I asked them a few questions about it once, they simply disregarded them all and said that it's only intention is a Quadrajet replacement, not a universal carburetor. I'm thinking they took a squarebore and modified the baseplate to fit a Quadrajet, and then tuned it to run properly. It truly does not seem to be metered as a spreadbore.

Not sure what direction to go next. I kinda wish I had a fresh 390 cfm to play with, now that I have the wideband.