Hello,

I've got excessive fuel consumption and needed to change the main jets a lot on my 950cfm Holley 4150 carb so that it didn't get lean on WOT. Currently 99 in the secondaries and 92 on the first circuit. Holley Carb Tech line claims I've got insufficient booster signal for my combo and carb would be to big for my application.

They suggested to switch to a 870 Street Avenger with vacuum secondaries but I'd first like to try to get the job done with my 950cfm. So pls school me choosing the right Booster.

Summit offers AED Boosters in this size:
0.136 0.140 0.160 0.180

Thats my combo:
500cui RB, 11,5:1 with OOTB 440source heads, Hughes flat tapped hydraulic cam with 256/260 duration and .600 lift with 1,6 Roller Rockers. 950cfm Holley carb and Edelbrock Performer RPM Intake manifold, Headers and 3' TTI Exhaust. 2,76 Rear Axle and 1800RPM Converter. I know the heads are limiting me currently so I might upgrade those in the future thats why I wanna keep the 950 carb as well. The intention of the car is high speed highway driving in germany. I don't care about 60foot times etc. It needs to hook from 60 up to 150mph on the highway!

Many thanx in advance

This one could be fuel delivery. I don't run 99's in my 1150 dominator. Another thought, what is the alcohol content in German fuel?

That carb is definitely not too big. Jets will change the overall fuel curve. You might have to correct the emulsion circuits and high speed air bleeds. Plus, you're not going to change boosters in a 4150 carb on your own too fast. Special alignment and installer tools are needed. Get with Thumper. He'll get you squared away.

get with Thumper here, I imagine he will chime in here eventually.

seems the tendency of carb tunes lately is the a bunch of emulsion area in the metering blocks and large main air bleeds. this seems to require large jets and also a tendency to be rich right as the mains start and gradually get leaner as the main air bleed start to have more influence (ie. as rpm's increase)

If it has 1 3/8 boosters then it's a bit small BUT the hwy gears and tight vert are important and the opposite happens in my world. A smaller carb on a bigger engine signals stronger/better so they go rich up top. You NEED a wide band and I can fix that carb for ya no problem if u'd like......Read below and check out my FB page cos there's LOTS of pics, videos and happy customers including on here as well.............Over emulsified carbs are the norm these days and simple to correct.........

There’s way too many “950” carbs out there to know what you have with that as a description.

If it’s a genuine Holley, what’s the part number and how old is it?

Holley is wrong. Again.

This carb purchased 2/2017:
https://www.summitracing.com/int/parts/hly-0-82951sa

For sure you have something going on if that carb is still lean with it jetted up that far.

Frankly, I doubt it’s the carb itself.

How are you determining it’s lean at wot?

If you’re sure that’s what actually happening, IMO the best course of action would be to swap the carb with a “known good” carb and see if any unusual jetting is required.

That street hp950 should be super rich at part throttle with that jetting.
As for the wot, I’m assuming you haven’t removed/plugged the PV’s.

With the current main jets AFR is fine!
We test drove it with the digital FAST Wideband Digital Air/Fuel Meters.
And its super rich at part throttle yes I experienced this driving to sweden in that setup 2500 kilometers.
I have a 750cfm Mighty Demon soon available from my sweptline but this should be too small.

Sounds like you’ll need to increase the pvcr some, so you can run smaller jets.
I’d try decreasing the hsab a bit too.

If it was me I'd go back to stock jets and then try to find the real problem. The stock jet size should be fine for that carb so if you're going lean at WOT then you either have a problem with your power valve circuit, or you have fuel pressure problem at WOT, or you have some other problem.

Sounds like you fixed some unknown problem by putting big jets in the carb and now you have multiple problems. Best step is to go back to box stock jetting and try again. Get it running smooth at idle, then make sure it is working for low speed and cruise. Don't worry about WOT until you have everything else working just fine. Then you can focus on one thing at a time.

On a street car you shouldn't adjust AFR at wide open throttle with the main jets. Drag racers do that as a short cut but for a street car you need to adjust WOT by using the power valve circuit. That is what it is there for.

for a street car you need to adjust WOT by using the power valve circuit

This^^^

Partially true but the same crap happens to many and JETS are the least of the solution it's the EMULSION and hi speeds bleeds that fix em every time for me and those in the know.............

I don’t necessarily agree that the 82951 carburetor would work satisfactorily at wot in its ootb configuration........ since I’ve never tried one myself.
I’ve tested enough carbs on the dyno over the years to know that sometimes they don’t.

I know someone who recently dyno tested a 505 with a new XP950 which had all kinds of issues(and required jets well over 100 to get the wot a/f ratio in line).

Recently, on Brads motor, several of the carbs we ran were still on the lean side at wot, and horribly rich at part throttle...... in large part to the giant jets.
These were far enough off that pvcr tuning wasn’t going to cure them.

There were obviously no “other issues” with fuel delivery or the test equipment, as not all the carbs tested showed these tendencies.
Sometimes it’s simply a poorly executed carb configuration.

As an example, the old 4500 3 circuit carbs(pre-HP design) had a tough time flowing enough fuel through the metering blocks and boosters in large ci high HP applications.
The area of the main well, minus the area taken up by the idle tube was about the size of a 97-98 jet.
Trying to increase the jetting beyond that showed essentially no difference.
If you happened to have a combo that would work with jets that were 97-100, the part throttle operation was often in the 7’s or 8’s a/f ratio.

This was an inherent design problem with those carbs.

Remove the idle tubes, ream out the main well, increase the diameter of the crossover leg, increase the booster feed hole size...... and you could have the a/f ratio at wot be richer by a couple of points, which would allow you to take a bunch of jet out..... which would clean up the part throttle operation.

There are several areas along the path the fuel travels within the carb that can greatly impact the ability of the jet to be the true determining factor in how much fuel can get through the circuit...... especially when the demand is high.

The OP is obviously not an experienced carb tuner so my advice is for his situation. Street cars are driven primarily at low to moderate throttle openings so it would be wise for him to get the carb correct there rather than mess up the part throttle by using a jet size that is way too big.

He should get the carb working correctly on the main circuit at cruise and then try to figure out the WOT issue. It is possible that the power valve circuit in that carb can't provide enough fuel for WOT. In that case he'll need to work with a carb expert to modify the circuit and/or get a different carb. Crutching it with a big jet is a bad solution for a street car although it does work okay on the dyno and at the drag strip.

My point was that with the factory sized 84 jets, you may not be able to get another 8 and 15 sizes worth of jet out of pvcr sizing.
A properly configured 1.56x1.75 carb on that motor combo should be plenty rich at wot with 84/84 jetting and pv’s on both ends.

The fact that it required adding 8 and 15 jet sizes to get the wot a/f ratio right means either the fuel supply into the bowls is lacking, the carb is defective, or the configuration is wrong.

Looking at a couple of dyno runs from Brads motor...... at the top end of the pull, two carbs showing fuel flow in the 3-teens lbs/hr.
One had 75pv/84 jets, the other 88pv/98.
The 88/98 carb was way rich at part throttle, and had a basic configuration(1.52/1.75 downleg)that shouldn’t require anywhere near that much jet to achieve proper wot a/f ratios.
That carb didn’t need “tuning”, it needed the relationship between the booster response curve, booster feed hole size, and possibly main well sizing changed....... if you were looking to have a better balance between part throttle vs wot operation.

Since the OP bought the carb new, I have to assume it was tried with the ootb calibration...... and ended up being too lean at wot.
Which lead him to chase it until he ended up where he is now.

I’d say he is already at the point where he could use some advice from a carb specialist, preferably one who has some experience with a holley 82951.

As to the original question from the OP about the boosters, I would think for that basic configuration, 1.56x1.75 downleg, you’d want the .180 feed hole..... especially if you’re looking for more flow from less jet.
Frankly, I’d kinda expect that to be what it came with...... and if it didn’t, it probably should have.

Be worth a call to Holley and see what size they come with.

Looking at an old BG catalog, they used .184 feed holes in their downleg carbs with 1.500 and 1.562 Venturi sizeing.

Thanx for all the input so far. I learned a lot in the last days.
Fuel supply is by a mecanical fuel pump above 100gph.

I will stock up power valves and High Speed blades and will restart with stock jetting, adjusting in idle and first gear to get the most vacuum and choose the power valve accordingly. Than will try to enrich by the air bleedes.

Need to find out what is the stock bleed size and which steps I should do.
Main jets two sizes at least but Air Bleeds?

The fuel pump “rating” means almost nothing.

If you haven’t done it yet, you need to plumb a fuel pressure gauge into the line up near the carb and see what the pressure is at wot.
If it’s less than 5psi, that’s the first thing that should be addressed.

I would be sure I had at least a 3/8” supply line from end to end, including the pick up.

Since you’re talking about doing prolonged wot operation, I would be upsizing the tank vent as well........ I’d do a 3/8” vent.

You need to determine for sure that the fuel supply is keeping up with demand before trying to fix the carb.

One more thing, where do you have the floats adjusted?

The power valve rating should have no affect on your problem, at wot they should all be open.
The pvcr is the feed hole in the metering block supplied by the PV.
It’s likely they will need to be enlarged from what they are now so that they contribute more enrichment when the PV opens.

A couple of follow up questions......
-What valve springs are on the heads?
-does the a/f meter show lean fairly quickly after you go to wot, or does it take a little time for that to happen?
-Does it seem to get suddenly worse when you reach a certain rpm? If so, what rpm is that?

3/8 sized Fuel sending unit and fuel lines all way. Not tested the fuel pressure as it hooks now with the large main jets and it doesn't seem to lack fuel in the bowls but I can check.

Hughes Cam HUG SEH5660BL-8 with corresponding HUG 1107 Valve Springs.
It went fairly quick lean and pinged until we raised the main jets.

If I dont want to end up boring the metering blocks for increased power valve feeding I gues i can just enrich thru the Air bleeds.
At least I know the sizes from Holley:
Idle Air: 74 & High Speed Air: 25