I'm having some issues with the Holley 4160 I recently installed. If i try to rev the engine much beyond 1500 rpm it will backfire through the carburetor. I've checked the power valve (was my first thought) and it is still intact. Floats are set correctly, and I've got the idle mixture screws set to highest vacuum possible (21 inches). In drive the engine idles at around 17" of vacuum.

The engine is brand new with around 300 miles on it. Everything in the ignition system has been replaced, and the timing has not been touched since set originally.

I'm wondering if I need to swap out the accelerator pump cam for another one, or not? Currently it has the white cam installed, set on the 1st hole. I'm also wondering if I need to change the fuel discharge nozzle out as well, but I have not checked to see what is currently installed. I'm also thinking it might be the accel pump itself but haven't investigated any further.

Any suggestions guys?

 

Eliminate other potential causes before touching the carb...especially the ignition system including the timing. Which 4160 do you have?...there is a bunch of them.

 

I'll take a look and get the list number off the air horn tomorrow so I can tell ya' specifically which one I've got. Everything in the ignition is brand new, as it was replaced when the engine was bought and installed. Everything under the hood has about 300mi on it just like the engine, I wanted a fresh start for everything!

I know new does not always equal good, but I would think that everything should be in working order.

 

Harte3, I got the list # off the air horn of the carb this morning. It is a 80457-2. Hope that helps in troubleshooting whatever is wrong with this thing.

 

I have an 80457 on one of mine. (600cfm) It has some minor differences from the generic 1850, but not much.

The acceleration circuit is most likely not the culprit. You could disconnect the accelerator pump arm entirely and you'd still be able to rev it up.
The accelerator pump covers the lean hole when you throttle it quickly and the engine vacuum changes too fast for the normal jets of the carb to keep up. If you accelerate slowly, it doesn't factor in.
If you have the wrong size, and throttle it quickly, the engine'll simply struggle to keep up, or stall from lack of fuel. I've never had it backfire from it.

In all honesty, it doesn't sound like the carb at all. Carb issues are often bogs, hesitations, stumbles, rich exhaust, that kinda thing.

However, did you have another carb you were using before that didn't have this issue? On the same rebuild, ignition, everything?

 

To be honest, no this is the only carb I've used for this build and don't have access to another one.

The reason why I'm suspecting there might be something with the carb (I really should of said this at first...) is that when the build was complete and I was driving the truck around if you were rolling along at around 40mph and stomped it, it would fall on its face until you let up on the go pedal. Now I am getting the same behavior, in park if i stab the throttle from an idle.

If I ease the pedal down, I can slowly build RPM but not all at once.

I'm basically trying to figure out what would be the best method to tune this thing as I don't have much experience with 4bbls. I'm just as stumped as you guys as to what is causing it to backfire if you stab the throttle suddenly.

 

If you stab the throttle and it falls on its face, that does sound like an accelerator pump issue.
What size squirter do you have in there? If you look under the choke plate, you can see the number embossed on the front of the squirter. If it's stock, it's most likely a 25, which in almost all cases I've come across, is too small. (If you want carb tuning parts, try AllCarbs. They have WAY better deals than most anywhere else.)

I could possibly see it being a combination of the lean spike with the pump and timing. If you have too much timing, and it's running really lean when you step on it, maybe that could cause a backfire?


Another thing you can try (I had to do this with my 600cfm 4bbl on the 300), is restrict the idle air bleeds on the top of the carburetor with some small gauge wire. I think it was around 26 - 30 gauge. Pretty small.
If you look down the throat of the carb, these are the air bleeds:



#1 is the air used to meter the idle. #2 is the air used to meter the main jets. You can also see the squirter in the center.

At cruise and low RPM acceleration, you'll be using the idle (idle isn't just for when you're sitting still!) If you put the wire in and it surges during cruise, you know it's too big.
This really cured some stumbles for me that my accelerator pump wasn't taking care of.



Either way, no matter how many times I've worked and tuned lean spots with all the 4bbls I've worked on, I've never once had a backfire from it.

 

Also, is the carb new or used/rebuilt?

 

The carb is supposed to be recently rebuilt, but was given to me by my F-I-L. I haven't a clue as to what it was originally on and tuned for, but I will be asking tonight to see if he knows.

I'm going back outside in a few minutes to pull the squirter, and check the clearance on the accel pump linkage.

 

Do you have heat running to the intake? If not, that's the first move. Next would be a larger squirter. I think most end up with a #37 squirter. Next, a different pump cam. I run a blue cam in the first hole. Make all changes one-at-a-time. Do not make them all at once. Keep notes on the changes so if you have to back up to square one you will know where that is.

 

The squirter is a #31, I have heat to the intake, and I will have to go buy a set of pump cams from the parts store.

I did find out when I removed the squirter that it was partially stopped up so I cleaned that up. It didn't make much of an improvement, but it did help some. Next, I advanced the timing a little bit and got a bunch of the stumble cleared up!

I'm off to the parts store soon to grab some pump cams and check on getting some squirters.

 

I'd highly recommend getting your squirters from AllCarbs.com.

The very few parts stores that sell squirters usually charge the Holley price, which is around $20 for ONE set (they always come in a pack of two.)
Allcarbs sells them for $5.50 ea, or you can get a tuning set for around $22 that comes with 5. Big savings if you need to get a few to play around with.
That is, if money is an issue.



The other adjustment to be sure of is your pump arm. The accelerator pump squirter needs to be shooting fuel into the venturis the moment you even tap the throttle. A gentle tap of the finger should make at least some gas come out. If there's a momentary laps, this'll cause bogs.


As for the timing, I have a feeling that's where your backfire's coming from. Are you timing with a light? Or with a vacuum gauge? Double checking to be sure the vacuum advance is disconnected?

 

I've been timing it with a vacuum gauge, with the vacuum advance disconnected. At idle it pulls a strong 21in of vacuum, and the idle is dead smooth.

I checked and re-checked the pump arm and it is where all the Holley tuning videos I've watched say it should be, which is .015. Looking down the throat of the carb with the the engine off, the very instant the throttle is touched I start getting a steady squirt of fuel. So i think that the rest of the accelerator circuit is working properly, but I still need to change out that pump cam.

Thanks for the tip on AllCarbs, I'll def be going there instead to pick up my squirter's!

 

Sounds like your pump arm is as it should be. If a larger squirter and a more aggressive cam don't fix your bog, I highly recommend restricting the idle air bleeds. It's what I had to do on mine. I believe it's simply due to it being a 600cfm carb.

Is it still backfiring?

 

On a rough stab of the throttle it will spit a mist of gas back out the carb, but no it is not backfiring like it was before. What did you use to restrict the air bleeds on yours?