slickkery

My Holley 3247 (4150 series) has a long history of running very rich (especially at WOT) on my L-72. I finally bought an air/fuel meter using dual wide band O2 sensors to tune it, but was surprised that switching to much leaner main jets didn't have more of an impact. The carb had #72 primary and #78 secondary, and 8.5"Hg power valves which are only a year old. Before the switch, the AFR was 12.5 to 13 at cruise, and 9.5 to 10 at WOT. After switching to #68 primary and #74 secondary, the AFR at cruise improved to 14 to 14.5, but the WOT remained 9.5 to 10!

Assuming I don't want to buy new metering blocks that have replaceable PVCRs, what else can I do to get my WOT AFR closer to 13? Will switching to #70 secondary help get me there?

Thanks in advance!

Sam

P.S. The 3247 Holley for the L-72 engine is spec'd with #70 primary and #76 secondary jets, and 8.5"Hg power valves. Mine has a crisp 850 rpm at 11"Hg manifold vacuum. I live at 300 feet elevation and it was a humid 85 degrees today. I only run ethanol-free 110 octane leaded gas.

jim lockwood

I'm not a Holley expert. However I am in the process of helping a neighbor dial in the Holley on his F*rd 351-C. Point being, I'm going down the same road as you.

One thing we've found is that his power valve was opening at inappropriate times.... in particular, during light, part throttle acceleration. That was making his engine's "cruise" mixture appear too rich. We fixed this by installing a power valve with a lower opening threshold. Point being that your power valve threshold is very close to your engine's idle vacuum and you might be well served by also switching to a lower-opening valve.

Your WOT mixture is so rich that the accuracy of your reading could be off. My suggestion is to swap in even smaller secondary jets. The mixture will eventually lean out and you'll see it on your gauge.

If after you replace your power valve, the cruise mixture is still too rich, then you should consider leaner primary jets. But not before.

belgiumbarry

spec for the power valve is half the idle vacuum.So you would need a 5.5 power valve.
is the bowl fuel level secondary not to high ? could try it a bit lower ?

slickkery

Thanks Jim! I should add the the pig rich WOT reading was noticed on the engine dyno after the 2017 rebuild (.030 over, 11.6:1 static CR, stock grind cam specs, 3247 service carb purchased in 2014 with correct specs for L-72), confirmed at my local shop using their AFR meter a few weeks ago, and confirmed again using my AFR meter.

If my PV was opening too soon, I'd be seeing way rich AFR at cruise, but I'm not, so in my case, I am thinking the problem lies elsewhere.

DansYellow66

Yes, a 5.5 power valve - should be 1/2 of your best idle vacuum. But that won’t resolve the rich WOT condition. You might try aggressively cleaning your primary and secondary air bleeds out with cleaner an even a small piece of wire as a restricted air bleed will cause a rich condition. Sometimes ethanol (and maybe any fuel) will leave deposits in them that harden, even though normally no fuel runs through them. The atomized fuel fumes I guess during shut down, heat soak.

slickkery

Thanks Barry. You bring up a good point. On its website, Holley recommends using the "manifold vacuum divided by 2" method to determine PV size for an engine. However, I think most would agree this is overly simplified, very conservative, and not optimized for a high-performance engine such as the L-72. The L-72 was spec'd with the Holley 3247 carb which was outfitted with the 8.5"Hg PV in both primary and secondary metering blocks.

slickkery

Thanks Dan. I will try cleaning out the high speed air bleeds on both primary and secondary.

I'm wondering now what if the vacuum secondary is not working properly? Could that cause this pig rich WOT condition?

jim lockwood

Um, maybe.

Keep in mind that the power valve isn't an on-off switch. As manifold vacuum reaches the threshold of the valve, the valve will begin to open, providing a little additional fuel which will make AFR go richer. As manifold vacuum continues to drop, the valve will open more. Given the very narrow spread between your idle vacuum and your PV threshold, it's possible and even probable that your PV is opening a little during cruise.

This isn't idle speculation. This exact phenomenon bit us with the F*rd this week. In the case of the F*rd, there was a much greater spread between idle vacuum and PV threshold than with your engine. Switch to a PV with a lower vacuum threshold cleared up the issue.

It'd be a simple experiment to temporarily install a 5.5 PV, as Barry suggests, and see if there is any effect on your cruise mixture. I bet there will be.

And, BTW, at what vehicle speed are you measuring your cruise AFR? You want to be going fast enough that there is no contribution by the idle circuit. For my neighbor's F*rd, that's around 70 MPH.

slickkery

Hmmm, that's news to me. From everything I've read, a properly functioning power valve IS essentially an "on-off switch" that will not open at all until the manifold vacuum threshold is met, and therefore does not open "as manifold reaches the threshold."

Cruise AFRs were obtained at 2,000, 2500 and 3000 rpm, which in my car with its 4.11 gearing, equates to 40, 50 and 60 mph.

jim lockwood

At 2000 and 2500, it's probable you are getting some mixture richening from the idle circuit. With 4.11 gears, even at 3000, the engine is so lightly loaded that it's possible the idle circuit is still partially active. I'd either go faster or find a stretch of road with a modest upgrade so the throttle plates open far enough that the idle circuit is positively off.

DansYellow66

In addition to the above you might try sending a PM to member Tbarb if he doesn’t chime in on your thread and see what he can add. He’s helped me on a lot of carb issues.

To kind of add to Jim’s comment on the idle circuit, I think what he’s telling you is that the transition circuit is shared with the curb idle circuit and it (actually both) can stay active with the throttle lightly opened up.

slickkery

Very interesting! I'll take it out now and report back with the results. It's only 65 now and less humid, so imagine the AFRs will also read a bit leaner than yesterday evening's hot and muggy conditions. Thanks!

slickkery

Good to know, thank you, and hope that some Holley gurus are able to chime in too.

1Hotrodz

Like others have said; going to a 5.5" power valve is a good move. But the WOT AFR is still low which could mean an air side blockage in the metering block of the secondaries, since you reduced the jet size of the secondaries without reducing the WOT AFR.

The air bleeds in the metering block are there to reduce the vacuum signal to the fuel jets, hence the word bleed. If the air bleeds are restricted or clogged it will increase the signal to the fuel jets thereby causing a richer mixture. It might be time to rebuild the carb. or at a minimum inspect the metering blocks.

slickkery

Thank you for the tips. I will remove the carb and clean out the air bleeds with wire and carb cleaner. Regarding rebuilding the carb: although the carb was purchased 9 years ago, it only has about 4,000 miles on it and has only had ethanol-free leaded 110 octane race gas, so is a rebuild warranted?

slickkery

Just got back from another outing and here are the results: idle AFR was 14, cruise AFR was between 13.5 to 14.5 in 4th gear and rpms between 2500 and 4000, and cruise AFR dropped to 12.6 to 13 when in 3rd gear at a constant 4,000 rpms (I don't have the right road for 4,000 rpms in 4th gear); and WOT AFR was between 9.7 to 10 across 3 different "floorings." As expected, the results are slightly leaner than yesterday due to the mild temps and lower humidity, but still way too rich at WOT especially.

jim lockwood

Your primary power valve is opening.

larrywalk

WHOA!... Let's look at what is really happening. At cruise, especially with high numeric gearing and the light loads on the engine, the intake manifold vacuum will be very high and the throttle barely open at all. In this case, the idle fuel restrictor (IFR) restricts fuel that passes through to the idle and transition circuits. When you're cruising, your intake manifold vacuum is probably 18 to 20" Hg and the power valve (PV) will be held strongly closed. Although it may seem that reducing the main jet size will help, restricting the IFR will be the solution.

On the other hand, to address the very rich WOT problem, one needs to first get the AFR correct in the mid-range of 15" Hg to 10" Hg which keeps the PV closed and the transition circuit mainly ineffective. I prefer 13.5 to 14.5 AFR in this range. This is the range most affected by the main jets.

WOT AFR is most affected by the PVCRs. After the PV opens (vacuum less than the PV opening point), the PVCRs are MAJOR determinants of the WOT AFR. To put it in context, the PCVR metering area is roughly the same as the metering area of the main jets, essentially doubling the total metering area after the PV opens. So, reducing the main jet size to affect the WOT is largely ineffective. Restrict the PVCR holes beneath the PV after the mid-range is set.

For a better understanding, consider WOT as manifold vacuum less than 5" Hg. OK, now that the WOT range is set, what about the range of 5" to 10" of manifold vacuum? This is the range in which PV selection becomes important and testing is required. While driving AND having a vacuum gauge and AFR gauge visible, and in the range of 3,000 rpm, note how the AFR changes as you slowly add throttle passing through the 5-10" range at a rate of about 1 second per inch of vacuum (it helps to have a long hill to help hold speed relatively constant). This transition between mid-range and WOT is determined by the PV selection. If the AFR goes too lean at 10", a 10.5 PV is needed. If the AFR only goes too lean at 6", a 6.5 PV is needed. Similarly, if AFR goes lean at 8", use an 8.5 PV. (This is the best method to determine which PV is needed, not the flawed vacuum at idle fable. When one studies how the Holley is engineered, the PVCR fuel enters the bottom of the main well joining the fuel from the main jets. Both sources then travel upwards past the emulsion holes to the boosters. Since the boosters aren't flowing at idle, it doesn't matter whether the PV is open or not - it only matters when the main circuit is flowing through the boosters.)

I just recently did this with a Holley 3310-1 vacuum secondary carburetor. To cut the richness of the IFRs, I added restrictors using fine-diameter wire bent into a V with legs about 1/4" long, one leg into the IFR hole, the other leg into the adjacent hole at the bottom of the main well. I determined the diameter and then the area of the IFRs and subtracted the area of the wire's cross-section. I achieved about a 10% area reduction which worked well. To cut the richness of the PVCRs, I used the same method and used .031" diameter wire bent into an "L" with legs about 3/8" long. One leg went into the PVCR hole, the other was laid along the outside of the threads and was trapped after the PV was screwed into place. Good results also. By the way, if one wanted to use screw-in jets, one could drill and tap for replaceable PVCR and IFR jets, but at least this quick method found the tune that I needed.

By the way, I have used vacuum gauges in every vehicle I've owned since 1963 and have learned a lot from them. One of the big takeaways is that with cruising rpm above 3,000, the vacuum is very high; with overdrive and rpm below 2,000, the vacuum is a lot lower - both need to be considered when tuning a carburetor.

larrywalk

Looking at your post #16, a PVCR restriction is warranted.

slickkery

Interesting! I neglected to mention that after the shop tightened the valve lash on the solid lifters, the manifold vacuum at idle dropped from 12.5"Hg to 11"Hg. But even at 11"Hg, it passes the 2" rule advocated by Duke, Hinkley, Lars, etc. Of course, if my vacuum gauge is reading high by 0.5"Hg, then suddenly I'm right at the edge of passing the 2" rule. Hmmm.

I'll order a pair of 7.5"Hg power valves from Holley just in case, but has anybody heard about WOT richening due to a problem with the vacuum secondary not functioning properly? Something like, if the vacuum diaphragm doesn't open the secondary throttle blades all the way, a pig rich condition occurs?? Anybody here heard of that?