Quickfuel problem
 

Put a quickfuel slayer on my 395 stroker. At idle and accelerating, it runs fine. Lined out on the highway, it constantly stumbles. Any ideas? Carb is fairly new.

First check your ignition. Then, make sure your idle screws are adjusted correctly. If it still stumbles at that point, you probably need more primary jet.

Also keep in mind that your most critical carb adjustment is WOT because that is where you can do damage.

How do your plugs look?

What rpm is it stumbling at? Partial throttle?

Stumble = Lean

You need more fuel. At highway cruise you are likely not into the main metering, so changing jets wouldn't help.

You are likely in the transition zone from the idle circuit to main metering.

Tell us what rpm it occurs.

John

I found that my light-throttle cruise mixture was affected by both the transition circuit and the primary jet, but you have a good point.

You should go through the basics first: ignition, idle screws, throttle plate position (with carb off) so transfer slot is properly exposed, and measure vacuum so you have an idea what power valve to use. You need to make sure all those things are nailed down and if you still have the problem then add some main primary jet.

It would be good to have a little more information about your motor. A lean continuous miss at highway cruise is sometimes brought on by too much total advance with a motor with vacuum advance. Some motor, because of their aggressive initial and mechanical advance curves and high compression, can only tolerate a minor amount of additional vacuum advance at cruise speed. The continuous miss is because the engine is firing far enough ahead of TDC that the flame front is spreading sufficiently to cause a slight kick-back which feels like a light miss.

If you have vacuum advance then it's simple to plug it temporarily and run the car. If the miss is gone then that's the issue and you need a vacuum can with less vacuum advance.

An engine idles on the idle circuit but runs at light load on the transition circuit. As load increases the primary jets start coming into play. Most carbs are considered to be too rich in the transition circuit - thus the common need to install transition circuit bleed restrictors. Usually a carb that is a bit large for an engine (like most enthusiasts will install on their hot rod engines) will run rich. If that carb is put on an engine at the upper end of the scale that the manufacture recommends it for - it may run a little lean. Most carbs don't have provision for easy adjustment of the transition system - possibly some of the upper scale units like Quick Fuel do - I don't know.

But, it would have to be pretty lean to actually be stumbling and missing at light cruise throttle. That's what makes me wonder if you have vacuum advance and have an over-advanced timing situation.

Rough at cruising speed, no matter what rpm. this carb has acted the same on two engines. 1st was a 325hp sbc and now on my 400hp 395 sbf. I pulled a 750dp holley off that needed a rebuild and tried the Quickfuel. Ran good with the holley, just leaked down a bit.

In that case it sounds like timing can be ruled out. Just have to pull the Quick Fuel apart and see what's in it and what you can see. Maybe worn throttle shafts or some sort internal vacuum leak. Seems I remember Quick Fuel might have replaceable transition circuit bleeds - possibly they need to be richened up.

Picked up a kit for the Holley today. Carb is in chemtool bath right now. Gonna go that route.

First off that is a great carburetor for the money. If you want to do it right buy a wide band O2 sensor that goes in the dash, but you can just use it as a tool and not put it in the dash. It's a lot cheaper that way. I started tuning with a wide band three years ago and would never, never go back. I use an Innovate #3844 MTX-L that I can also hook my laptop to it and use the charting feature. When you get done just unplug it and put it back in the box and use as a tool for the next car. This last summer I used it to tune a friends show car with an Edelbrock ProFlo 2 Fuel Injection System that he gave up on. Install the bung between the collector and the muffler and tape the wiring on the side of the car and over the door. With a wide band and a vacuum gauge you can tell exactly what circuit you are in and no more guessing.

Do the following one step at a time and the adjust before you go to the next step. If you skip around you can mess up what you just fixed. The circuits are: idle, idle transfer, primary main metering, power circuit, and secondary main metering. The accelerator pump circuit is a whole other issue not addressed here.

1. Idle/ Idle Transfer
Start with your idle circuit, then from about 1300-1800 rpms, depending on the engine, you can check the idle transfer circuit. This carb has small restrictor jets to adjust the transfer circuit. On the Slayer I believe stock restrictors are #32s but I don't think the restrictors are marked like main jets. This is where you get most of you gas mileage from. You are looking for 14-14.7 AFR. Now change restrictors if needed.

2. Primary Main Metering
From about 1,800-2,100 rpms you will be starting the primary main metering. In third gear very slowly open the throttle and run the engine up from 1,800- 3,000 rpms. Take about 30 seconds to do this and watch O2 sensor from 2500-3000 rpms for your primary main metering AFR. Do this with your secondary linkage disconnected. Opening slowly keeps the power valve out of the picture and stabilizes the AFR to get a good reading. You are looking for 13.75-15 AFR about. Change jets if needed.

3. Power Valve Circuit
With a vacuum gauge attached, (I run a long clear plastic tube from Home Depot out the front scoop taped on the hood and over the passenger door). In third gear put your foot in it and when you get below the vacuum rating on your power valve, usually stock is 6.5 in., read what your AFR is. The power valve has two restrictor jets. They are under the power valve and I believe they are not marked, and are #55. You are looking for about 12.75-13.25 AFR. Most are way rich. Change restrictors if needed.

4. Secondary Main Metering
Hook up your secondaries and do some high rpms runs, but make sure your vacuum is above the power valve rating. You are shooting for around 13.5-14.25 AFR.

5. Wide Open Throttle
Lastly do some wide open throttle runs which will open the power valve and the secondaries. Everything will be all in. You are looking for 12.25-12.5 AFR. To keep your speed down you can do this is third gear, but it will still be pretty fast. Don't over rev your engine.

Now the problem is how do you know what restrictors and jets to buy. I go through each of these steps first and see where they are and estimate what I will need doing the math based on percentages. I order the jets and restrictors from Quick Fuel. They seem to be cheapest. Do the testing with a passenger to watch the O2 and vacuum gauge, and do testing on a open road with no traffic. It could take up to a tank of gas. You may have to do this a few times and order more jets.

One hint: Have the passenger use a cell phone and record with video the readings of each step. Say which step you are doing in the recording so you don't get confused. Because these cars are so fast the runs are usually really short. Write down every jet and restrictor change and don't lose track. When you get done you should have a great running carb. I really like the process and just think it's fun to get these dialed in.

Good luck.

Sorry you changed carbs. That Quick Fuel is a great carb and totally adjustable. You most likely will not be able to adjust the idle transfer or the power valve circuits on the Holley.

This is a really good post. Two additional comments:

(1) If your local gas has 10% ethanol, then stoich is about 4% lower (14.1 vs 14.7). So adjust your targets down across the board.

(2) If you have a big cam with a lot of overlap, then the readings on a wideband are inaccurate below about 3k RPM, which is when a big cam typically gets more efficient. If you have a big cam and you don't know this, your readings won't all make sense. Ask me how I know...

Try this easy solution first.

Open up the idle screws about a quarter turn more making the idle/transition circuit a bit richer. Go for a drive and cruise at a steady 2000 rpm. See if that doesn't cure the problem. Not all cars run best at the highest idle vacuum (leanest idle mixture). Mine doesn't.

Thanks for all the good info. I threw a kit in the Holley and I'm back in business. Wbulk, when I get some time, I will put the QFT back on and go through the steps.

Sounds like a squirter problem to me. Call quickfuel. They are awesome to deal with and will figure this out for you.

Sounds like a squirter problem to me. Call quickfuel. They are awesome to deal with and will figure this out for you.

Thanks, stevofiveo.
After looking at your avatar, I think we're all gonna have a squirter problem.

Same problem happen to me at Mid Ohio with my 392 stroker. Just needed more fuel in the bowels at WOT. holley 770, not more pressure, just an adjustment. It was never driving on the street.
rlt3214

This is good stuff! I am about to re-do my exhaust and will plan on putting in the bungs as well! Thanks!:3DSMILE:

When I tuned mine I used an Innovate meter and their clunky bu adequate graphing software. That way I had a graph to go with my noted for every set of changes. This stuff is tricky but you'll get it right.