Technical Help with lean spot on Holley carb

Hello all! I am new here and new to carburetors, and was hopping this awesome community could help me out with a problem I am having. I am having an issue when I am driving 20mph or slower the cars air fuel is fine about 13.3:1 and if I go to get on the throttle at all the afr shoots up to 20+ and the car stutters, and doesn't want to go anywhere. The few times I have gotten past this point that car takes off like a rocket. So I believe the issue is in my transition circuit,but once again I am not that skilled in carbs just yet so I could be completely wrong.

I can tell you the carb is probibly to big for the car it's a 402 bb with a mild cam, heads, and air gap intake.

But thus far I have put 10.5 power valves in it and messed with the jets and changed the squirters to 35s (I tried 40s and it didnt help anything ). I also installed different pump cams( it seems to like the brown The most. ) i have triple checked the accelerator pump circuit to make sure it's set up correctly. fuel levels look great as well. Timing also looks good 20 at idle and 35 all in. I am just stumped at this point. The car idles great around 650 -700. And in park I can rev it to 6000 without issue. It's only under load in that one specific speed/ rpm range. I thought about changing air bleeds, but I wanted to reach out and see if you guys had any better ideas. Thanks in advance.

 

How much vacuum a idle. Which holley? What gear? How tall tires?

20 degrees of lead is a ton by the way.

 

Sounds like you need a REO pump and a hollow accelerator pump screw

 

Do NOT do this!

 

Take a vacuum measurement at idle, which should be around 10 in. Your power valve needs to be 1/2 of that reading.

 

I agree with 57 , sounds like the power valve.

 

I can double check later today, but I want to say last I checked the car pulled around 17 in HG vac at idle and I have tried a 6.5, 8.5 and a 10.5 pv thus far.

 

It has hallow screws in it already

 

It's a Holley 850 ultra xp, it has around 17 in HG vac at idle I can double check later today though.

 

How far are the throttle blades open at idle. They MAY be open PAST the transition circuit.

 

Without a photo it is speculation on my part, but the most common mistake on a Holley is people tighten the pump adjustment nut instead of loosening it. The tighter ( the more you compress the spring ) the less travel less pump shot you have.

 

Which size jets you running? What do the plugs look like? I have almost the exact same set up and I’m running a 700 DP. Bigger isn’t always better. Transition slot should be almost squared off when looking from under the carb. I fought a lean stumble for months. Jets was to small. I also tried different cams on the carb and squirters also. Mine was off idle stumble though. Not after moving.

 

Loudbang's suggestion about the transition circuit is a good place to start.

 

l

 

Do yourself a favor.
https://www.holley.com/support/carburetor/

 

What ignition are you using? Most Delco distributors the mechanical or vacuum advance is worn out. Maybe you just need to learn how to drive a car with a hot cam. Lol try more advance and see what happens.let me know

 

This is a very common problem. People get a screw driver and turn on the curb idle screw with no clue as to what that is doing to the idle circuit. There is a slot in each barrel of the primary located just where the ****erfly hits the barrel wall. This slot should be between .020" and .040" in height exposed below the throttle blades looking at the bottom of the carb. The best way to set idle on a Holley is to remove carb and set the curb idle such that there is a square hole about .020" of the transition slots exposed. Turn the curb idle screw until there is about .040" of the slot is exposed. Now you know the range that the curb idle screw can be adjusted. Install the carb with .020" of TS exposed. Start the engine. If the idle speed needs adjusted, adjust it with the secondary throttle stop on the bottom of the carb. The idle speed can be adjusted up slightly from there with the curb idle screw as long as the trans slot exposure does not exceed .040"
This could very well be your problem. When the TS is overexposed at idle there is not enough TS left to cover the lean spot at throttle hit.

 

With 17" of vacuum you're going the wrong direction on power valves. You should be less than half the vacuum reading, so no more than a #8 power valve.
It sounds like you're too much timing advance also, and I can't help but wonder how the engine cranks over when warmed up? I'd be down around 14 degrees and check the timing as you bring rpm's up to ensure the advance is working properly.
If all else fails, I'd tear the carb apart and give it a thorough cleaning.

 

According to your diagnostic description you're going lean from low speed to stomp it. Showing my age we'd say you need to "cover the hole" in your accel circuits. I know power valves are counter intuitive with regard to numbers. I won't say which way is up, been a minute for me. Pump circuit needs to dump immediately. Look inside, engine off, just touch the throttle from a wee bit off idle. It should still shoot. You need pump shot duration and volume increased. With a bigger carb than needed you need the biggest shooters and longest duration. Distributor; is the vacuum ported or manifold? Could play a role. If manifold it drops back at the crack of the throttle, ported it pulls in more as engine speed increases. Raw thoughts...

 

It doesn’t sound like it’s not getting a pump shot…..or not enough of one.

When you hit the throttle it should go richer for a second or two or until it clears the pump shot. At light cruise 13/1 is still a bit rich.

Yes that carb is big for that engine.

 

I don't think that carb is big for that engine at all. But I still don't understand why everyone needs a DP four corner idle carb for the street. JMO Don't get me wrong they LOVE selling these high dollar ****ers. LOL Lippy

 

Okay update time.

Yesterday I checked the vacuum (it was 16in HG). I moved the timing down as per the suggestions and I also went back to the 40 pump nozzles. Reset all the transfer slots to .030. and the problem still persisted. So I called Holley they told me to put a smaller power valve in it. They suggested a 6.5. lucky I had those. So I put them in. And it did the same thing.

So I slept on it and today I moved the linkage for progressive to 1:1 and the issue is gone. It's probably not the best way to fix it but it worked.

 

Did you ever check/set the front pump adjustment ? going with the 1.1 linkage brings the rear pump on all the time so it can help with a lean condition as you have found out.

 

You have to understand a double pumper with four corner idle is what? Now you have a four barrell with accelerator pumps all the time. Going one to one it's on all four barrels all the time. Idle and all the way through the rev range. It was not designed to work that way. So all your doing is covering up a problem with another problem. It's very hard to explain. TA DAD had the correct thoughts on this. Now we are getting into emulsion tube sizes and it gets way over my head. Cause I'm not there. Holley knows, tell them your problem, And I don't think it has anything to do with power valves. And if you think that carb is too big, I have run a dominator on a small block. Not on the street though LOL Lippy

 

Yes, that just reaffirmed what your AFR gauge was telling you. As said before, now you have a full time 4 barrel. MPG's will suffer a lot.
What vehicle is this in? A big heavy vehicle will exacerbate a lean spot with a DP.
A bigger power valve can help get fuel in quicker. Bigger power valve channel restrictors can get more in. Different pump cams can get more duration out of the squirter.
Remember that the AFR gauge is not the last word. If you have one O2 sensor you are only monitoring the average of one bank.

 

It's a 67 Impala. And I'm sure it will chug gas now, but I'll take that trade off till I can solve whatever was wrong. As to your point about the power valve that's why I was surprised when Holley suggested I put a 6.5 in it.

 

Yeah lol. It's definitely not lean anymore. I did check the pump arm adjustment again it looked correct but I went ahead and completely reset it and check tolerances. If I had to theorize on a way to actually fix the issue. it would be to move the linkage arm back to progressive and then adjust it so that the secondaries opened a little sooner.

 

I would go through the front pump circuit, make sure it is all clean from the metering block on. The pump itself can get hard if it has some age on it and not allow a full shot. By bringing the rear squirter on early it sounds like the front one is causing your lean condition.

 

Holley 850 Ultra XP on a mild 402 wouldn’t have been my first choice for a street carb. Those are setup with an engine that uses long duration camshafts and lots of timing. In the premium Holleys like yours you have the green accelerator pump gaskets which are better or more impervious to todays fuel but over time can still harden up resulting in a poor pump shot. As said above I’d start looking there.

 

That's not a bad idea I might go through it. But It's a brand new carb and I put a 50cc pump on it because I thought the duration was causing the lean spot.