cats22

Hey guys, having massive problems getting my QFT SS 750 with mechanical secondaries re-dialed in on my '69 427/390.

Bought the carb new last year and after numerous rounds of adjustments, finally got it dialed in just right last fall. Put the car up for the winter, and from the very first drive this spring, I'm back to square one ... maybe even worse than square one.

Everything runs perfectly - no hesitations, no bogging, no nothing - for the first 20 minutes or so until the engine gets nice and hot. Then the show starts. Begins with hesitations and bogging off the line and in low gears unless I gently press the throttle. About this time the idle RPM drops down to 750-800 (originally set at 900). And then she'll fully cut out whether going down the road at 20, 30, 40 mph, whatever. She'll immediately restart - but with the same hesitations and bogging - and then cut out again a few minutes later.

Crazy thing is that it only happens when the engine is nice and hot and it's under load. Never happens for the first 20 minutes or so from a cold start or when she's sitting in the driveway and has had a few minutes to recover after these episodes.

We've seen fuel spill out of the vents like geysers so we backed off both floats thinking she was flooding out and checked both needles and seats thinking they were getting stuck. That seemed to take care of the cutting out in the middle of the drive problem, but the hesitations and bogging are still there.

Fuel pump (mechanical) and inline filter checked out. Fuel pressure at a steady 6 psi so no issues there. Fuel lines clear. No positive pressure in gas tank. And I highly doubt it's related to a vacuum leak since it doesnt happen until the engine is warm. Happens with air cleaner on and off, so it's not a vent clearance issue as one of the QFT techs tried to tell us.

It's driving me crazy. The carb is perfectly tuned when engine is cold and/or when sitting in the driveway and not under load. The moment the engine gets hot and it's under load, it all goes to the dogs. Also, I have the mechanical secondaries set to 40%. They open up as they should when the engine is cold, but after each hot-engine-under-load driving episode, the secondaries barely open if at all and only get back to normal 40% opening after the car has had a good 5 minutes to just sit (engine still running) in the driveway.

Any thoughts before we throw an old Holley on there for testing purposes and someone who knows a lot more about this than I do gets the QFT on a bench and rips into it?

cooper9811

My guess is that your fuel is boiling in either the bowls or in the lines feeding it. I had similar issues several years ago when I switched to a Holley 4150. I added a heat shield under the carb, and a return line, and all those issues went away.

Not saying there might not be other issues, but it does sound a lot like like what I experienced. My opinion is that modern fuel boils much easier than what we used 25 or 30 years ago.

L82Dude

I just dealt with similar issue, found best way to tune carb was after it was at temperature. Let it idle for awhile till car heats up then adjust floats ( when engine off, may have to do it a few times ) set floats to just under middle of site glass. A vacuum gauge hooked up to full manifold vacuum helped on idle screws.


also, if you running a K&N air filter, through it away, found it restricted the air too much and caused secondaries to keep flooding


The Holley website has some good videos to help also under carb tuning. most of them show quickfuel tuning


your problem most likely is your floats, was mine , pull the float needles right out and check them, I ended up with something in sec float and after cleaning all is good now, found engine must be hot to set right

stumpshot

Had a similar problem with my QFT after the car sat all winter. My problem was water condensed in the fuel. Fuel in the bowls floats and then it starts sucking water. Dumped 2 cans of Heet in the tank and drove it after putting as much fresh fuel in as it would hold.
​​​​​​​Hope you figure it out!

Grumpy 427

My buddy had a 750 quick fuel as well. After days of issues we sent it back for a holley 750 hp. I have a 670 Avenger on my vette works great. and an AED750 on my duster, that works great too. very old school simple carbs.

As for the K&N filter, on the dyno we made back to back pulls, with the filter on we lost 10 foot pounds of torque it made the carb very fat. leaned it out 2 jet sizes and gained 16 feet torque back on the next pull.

dmruschell

I had similar percolation issues (fuel shooting out of the front bent) with a Holley Ultra SA 770 on my 79 Vette. I already had minimal hood clearance, so no room for spacers. I installed an Edelbrock 800cfm AVS and the car was drivable again with no fuel geysers.

ronarndt

I'm having the same issues with the QT SS 750 on my 68 with NOM 454. I've tried all of the suggestions in the above posts without complete success. Worst problem is getting the float levels too high and having fuel spew out the vent tubes. I replaced needles and seats and both floats. Have a insulator under the carb and a fuel pressure regulator and gauge set at 5 psi. Most annoying is the car runs fine sitting in the driveway and on short test drives, but has left me stranded twice on longer (10 mile) trips. I'll watch this post for other suggestions. Fingers crossed.

cv67

if your coil is good agree with the fuel thing.

Todays gas is just plain garbage; this problem is widespread doubt its the brand carb itself in fact was on the news yesterday, talk about raising octane nationwide

One more excuse to build a high compresion engine and spend the $ on gas....or EFI

MelWff

when fully hot have you tested for a vacuum leak at the throttle body to intake and the fuel bowls/metering blocks?

cats22

Yup, that's the next test. Got my vacuum gauge and a long vacuum line that I'll hook up to full manifold vacuum port on the carb. The gauge will sit on the passenger seat next to me so I can see exactly what's going on as I drive and the engine heats up.

BTW, like most of us, I also have a hood clearance issue. Have a Torker II on there so I barely have any clearance at all above the vent tubes, and that's with running a 14x2 air filter, not the standard 14x3. But like I said before, this isn't causing the issue since the same thing happens when I run it without an air cleaner at all. Yeah, I have a top end performance issue, but I'll deal with that another day...

Anyway, thanks for everyone's input and I'll circle back after running this test.

OldCarBum

Could be a pin hole in the float allowing it to fill with fuel.
A coil that's getting ready to fail will not supply the proper voltage to the distributor when they get warm.
I'd throw on a new one just to eliminate it as a possible issue.
You could have more than one issue going on.

ddawson

My SS 830 has been great. I followed this


Carb Tuning with Wideband and Vacuum

This is not an all about carbs guide. I’m not going to show off knowledge of fluid dynamics and venturi effect, or talk about pressure differential. I’m not going to talk about annular vs downleg boosters, or explain all the emulsion functions, internal passages, or what they do. I’m also not going to show you where the idle speed or mixture screws are. In fact, I’m not going to show you where anything is.

This guide assumes you know a bit about carbs, and are serious about really tuning it in. If you weren’t, you wouldn’t have a wideband and vacuum gauge hooked up. I will explain how to change each circuit for its intended operating range, and how to avoid common tuning errors of fixing symptoms, with the wrong circuits. Following these steps will give you the best balance of economy, and full power potential.

Base timing, and advance curve must be set prior to tuning. Refinements can be made later on, but it must be close to start. To set base/initial timing, start the engine, run until warm, then set a quick idle mixture. Set the idle RPM to the lowest idle that won’t stall. Advance or retard the timing until the highest vacuum at the chosen rpm is achieved. The idle speed screw will need to be turned with timing adjustments to keep the rpm the same, so this could take a while.

After this initial timing is set, check the amount of advance with a timing light. Write this down in a tuning log. You will then need to limit the total advance either with bushings, or in some cases, the slot must be partially welded, and filed to allow the correct amount of advance. If your initial timing was set at something high, like over 20* you might want to pull the initial back until you know how much mechanical advance you’re getting. Once the total amount of advance is proper, you can return the initial to the setting you wrote down. Choose a set of weights and springs that gives full advance at an RPM that is typically recommended for your engine. Most like around 2500.

Make sure you have the float level and fuel pressure set for what is recommended by the carb manufacturer, not your buddy, or that guy who solved his lean condition by running 12psi at idle.

This will get you close enough to dive into the carb circuits. If you didn’t already do this when installing the carb, pull it off the intake (I know, how annoying) and see where the transfer slot exposure is on the primary side. It should look like a square. You can put a jam nut on and lock it down which will prevent you from screwing with it later on, pun intended. This is important to isolate the idle mixture from the off idle/cruise mixture. With the carb back on, readjust idle speed if needed using the secondary screw, or if equipped, “idle ease” adjustment. Note that carbs with the idle ease, or 4 corner idle will need the secondary transfer slot set as the primary.

Now, to begin some baseline settings. Because of differences in combustion efficiency, compression, overlap etc, engines will “like” different AFR’s (air to fuel ratios). Acceptance of this is critical to optimizing your engine’s manners. This is why we will be using the trusty vacuum gauge once again before using any wideband data.

Idle Mixture:

This is a basic one, but it will greatly impact how you go about tuning your carb’s off idle/cruise circuit. More on that later. Warm the engine up to typical operating range.

Bottom out the mixture screws, then back them out 1-1 ½ turns. Working in equal amounts for driver’s and passenger’s sides, turn them in ¼ turn at a time, maybe even less, until the engine wants to die, but still stumbles along without doing so. Note the AFR.

This is the absolute leanest it will run at, but poorly at that. Now do the opposite, but do it for the highest vacuum reading possible. IGNORE the wideband at this point! Be sure not to go above the highest vacuum reading. Only open them up enough to hold the highest steady vacuum reading. Do this like you did for base timing, and readjust idle speed so you are getting your vacuum readings at the same engine speed. Only use the secondary idle speed screw, or if equipped, idle ease for speed adjustments. Don’t change the primary setting. Okay, now you can look at your wideband, and note the number.

This is where it “likes” to be, but will quite possibly be a very rich figure. Mine for example idles buttery smooth at a 12.0, but will tend to foul plugs quicker. It also idles as lean as 16 but sounds like it has a huge cam. At this point, lean it out until the vacuum, or rpm drops just a little bit, but doesn’t produce the choppy effects seen at the absolute lean mixture. This is going to be your best compromise of smooth idle, and clean plugs. Note this AFR.

Main Jets:

“But what about IFR (idle feed restrictors) and IAB’s (idle air bleeds)” you shout!? We will get back to them later. The selection of the main jet is a tough one to get your head around, but here is the real basic concept. The main jet needs only to be large enough to support cruise and light acceleration at engine speeds of approximately 2000-4000rpm. 4000 is really pushing it too. Who cruises at 4 grand!?

Warmed up to normal temp, drive the car on a flat level road at 2500rpm. Hold the throttle steady, and observe the wideband. You should be able to get away with 14’s for AFR at this range, even with a fairly radical engine because the combustion process is better than at idle, the advance is likely”all in” and there is very little load. If the engine is too rich, jet it down attempting to get it as lean as possible. This is where it doesn’t matter what your buddy’s engine likes, or some Honda tuner. Go lean until it starts surging or develops a steady miss. Go back to the leanest setting that doesn’t surge or miss, and note the AFR. Keep in mind that a steady miss could also be an indicator of a vacuum leak, ignition problems, or poor distribution. The vacuum gauge would be really jumpy if you had a leak though.

Going to a best lean cruise will likely create some drivability symptoms. This is where most guys say, “it needs bigger mains”. WRONG! Those problems will be cured with other circuits. Now we’re going to go back and flatten the fuel curve from idle through the 2500 main jet selection.

Idle Circuit Refinement:

To get a nice smooth fuel injection quality fuel curve, you’re likely going to have to modify some passages in the carb. Make a chart of idle, 1000rpm, 1200, 1500, 1800, 2000, 2200, 2500. With the engine warm, run the car at those speeds in neutral, just sitting in the driveway. At each of those speeds, carefully hold the throttle steady, and watch the wideband. Throttle movement will cause the readings to jump, so take your time, and let it find a steady number. Don’t just jot down the first steady number you see. Wall wetting from accelerator pump, leanout from slowly throttling down, etc can take a good 20 seconds or more to “clear out”.

There are 2 things we’re really interested in. Remember that good balanced idle AFR we achieved earlier? Is the number at 1200 richer or leaner? Is the number at 1800 richer or leaner than that? If the 1200 number is richer, the IFR (idle feed restrictor) needs to be smaller. If leaner, the opposite. If the value at 1800 is richer than at 1200, the IAB (idle air bleed) needs to be bigger. If leaner, the opposite. This is the area where most in town cruising occurs. This is where you can either get max economy, and clean plugs, or a swilling fuel drunk neanderthal that needs the plugs removed and cleaned every week.

This is also an area that requires a lot of time. Since the air bleed can affect the entire curve, you should change it with the restrictor. It should be a good match from the factory, but if the feed restrictor needs to go down, the bleed will seem to have a much more pronounced effect on the curve. As a general rule, if the restrictor goes smaller, the bleed will likely need to go smaller with it to maintain a flat curve. Change one, then the other, a little at a time until your fuel curve is smooth. Also, keep in mind that most engines will want a slightly richer AFR at, and just off idle. If that is your case, the curve should smoothly lean out from idle by around 1500rpm to what the mains tolerated, then stay there into the main circuit.

Carbs with screw in bleeds have it easy. If yours doesn’t, you will either need to experiment with wire to reduce the diameter, or best bet is to drill and tap the stock ones for 8-32 brass set screws. You’ll use the set screws later on too, so it is worth pulling the carb apart for this, and the ones mentioned later. If you read this whole thing first, you’ll know which ones to do. Same is the case for the IFR’s. If you don’t have screw in ones, drill and tap, but the screws will likely need to be smaller.

Another thing to consider is that air bleed changes in the idle circuit will tend to have the opposite effect on the signal to the mains. This is because by reducing the air going through the idle air bleed, more of it then pulls through the main. There could be some changes needed in the main jet after refining the idle circuit, but should be minimal.

Time for a test drive! Drive the car at varied speed, and terrains. With the idle circuit and mains smooth and lean, it could result in poor throttle response, and falling on its face under moderate part throttle acceleration. Why would we tune it like this then!? We will now begin curing drivability symptoms with the proper circuits!

Get the car up to a steady speed, say 25mph, then smoothly push down to accelerate. Don’t romp on it, but don’t baby pedal it. Think of trying to go up a slight hill, or smoothly pass someone. If the car’s AFR plunges lean, watch the vacuum gauge. Look for the lowest vacuum reading that it runs nice at, then the point that it stumbles badly at. Note those vacuum readings.

Accelerator Pump:

Next, romp on the throttle from a dead stop. Do it both to ½ throttle, and full throttle. It needs to be an immediate stab of the gas, not a roll into the throttle. Watch the AFR, and behavior of the car. If it goes lean, and recovers, but doesn’t stumble during that lean spike, you’re probably okay. More likely, is that it will pop through the intake, or stall, or come close then recover. Occasionally, it will go really rich and bog, then clear out.

If the first situation occurs, enlarge the primary squirter until the ½ throttle stabs are responsive, with no stumble or stutters. If the second situation occurs, go smaller until it smooths out. Do the same on the secondary to cover the full throttle stabs. I have found that with the double pumpers, equal squirters front and rear works best, but you might experiment with a smaller one on the primary for economy, and a bigger one on the secondary for full throttle hits.

Power Valve:

Once you can romp on it from a dead stop, go for another test drive. Most of the drivability issues should be gone now. The next step is to optimize the power valve, and PVCR’s (power valve channel restrictions). If you roll into the throttle, you will see the vacuum gauge drop. You want to watch the AFR for a lean out above light throttle, or a rich bog. If it goes rich too soon, a lower power valve needs to be used. To get close, use the lowest steady cruise vacuum reading you got from the first test drive. Go1-2 numbers lower for the PV. For example, if you have a big cam, and only pull 8” of vacuum at cruise, a #6 power valve should be used.

In most mild street and strip combos, the car will starve out for gas before the power valve opens because of lots of vacuum, and the lean main jets. This is where most folks put in bigger mains to cover it, but the best bet is to put the right power valve in, and “jet” the power valve circuit via the PVCR’s. Like with the low vacuum combo, pick a PV that opens just below the lowest part throttle vacuum reading. In some cases it may require a really early opening valve. The most aggressive one Holley makes is a 10.5.

After changing the PV watch the AFR’s as it comes in. You’ll know when it opens by watching the vacuum gauge. When you are under a part throttle, but not full throttle load, and the vacuum is below your PV opening point, look at the AFR. If it is lean, the PVCR’s need to be bigger. If it goes rich, they need to be smaller. If lean, use a pin vise set, or if you have steady hands, a cordless drill, to enlarge the passages a few thousandths at a time until the AFR under load is around a 12.5-13. For max economy, favor the 13. For max power, it will probably pull harder around 12.5. Experimentation will be required to find what your combo likes best. If it goes too rich, you have to reduce the size of the hole. Some folks have gotten away with JB weld, and redrilling, but the best bet is to drill and tap the holes to accept 8-32 brass set screws. This will allow you to keep a few sets to go back and forth with.

Secondary Jets:

Now for the fun part, full throttle blasts! Take the car somewhere you can run it out for a couple gears, while watching the AFR. Obviously a track is best, but do what you can and be realistic about the fact you will be distracted by gauges and need nobody else driving near you. I suggest running it out moderate throttle in first gear, opening it to full as you hit second, staying in it through redline, and a little bit into third. If the car goes lean, or pops or does ANYTHING weird, don’t stay on the throttle. As with the power valve, tune it for max pull. It is probably going to be around 12.5 that it pulls the hardest, but some are stronger a little lean, some like a fatter mix.

The last bit is to watch for a difference in AFR as it approaches your shift point. If it starts to lean out, the high speed air bleeds need to be a little smaller. If it gets richer, they need to be bigger. Again, carbs with screw in bleeds have this easy. If not, drill and tap for the same 8-32’s you used on the IAB’s and PVCR’s. This can take a lot of time and effort, but having an awesome running car is worth it!

The wideband can be a great tuning tool, but it can also become a sort of car guy illness. You can get real finicky and watch the wideband more than the road. Remember a carb isn’t fuel injection, but can be tuned dang close! At some point you have to decide when the car runs well enough, and just enjoy it. Also note that the aggressive initial timing, and lean mixtures can cause some pinging at light throttle. An adjustable vacuum advance can be used to cure that. If the pinging cannot be cured with timing, you might need to slightly enrich that portion of the fuel curve and compromise some economy.

Adjustable power valve:

Sometimes when tuning for economy, and your engine pulls a lot of vacuum it can create a lean spot. If your main jets are tuned small for cruise, you need something to cover the gap between primary and secondary fuel. That’s the power valve’s job. Holley only makes up to a 10″ vacuum valve as far as I know. I had a car that pulled 22″ of vacuum at light throttle cruise. Off idle acceleration was nice and crisp, but above that when I would go for more throttle it would nose over until the vacuum drop was enough to open the PVCR’s, even with the 10″ valve.

I decided to watch my AFR and vacuum to see when it was dropping lean. The car would surge at around 14″ of vacuum. I took a 10″ power valve and very carefully took it apart. I turned a new center shaft on the lathe, with extra threaded length. Once assembled, I was able to tighten the 2 knurled nuts for an earlier opening valve. After a few runs, turning the adjuster **** a 1/4 turn each drive, I was able to clear up the lean spot, without giving up the economical main jetting. The one in the picture used an aluminum shaft. I later made one with a brass shaft, and the high flow type valve. If there is interest I may look into production for these.

The Money Pit

Had the fuel boiling issue myself, and a 1/4" thick carb gasket solved that. There is no way you will ever tune the carb if you don't fix that problem first.

I spent a year putzing with mine, but my issue was more of an idle with my huge solid cam.....Anybody else would have just turned up the idle speed, and been done.

One other thought. How is your cooling system? You mentioned it runs fine while cold, so when the issue happens I assume the engine temps are getting up there. I run 160 degree T stat, and a hipo aluminum radiator, so my temps stay down.

cats22

At this point, there's absolutely nothing left to try but that.

Engine temp fine. Never really gets above 160 (ambient temp today is 90 and was running it hard), and only hits that when I'm sitting in heavy traffic.

Also had a vacuum gauge with me in the passenger seat. Everything there checked out fine. 12" and steady at warm idle before running it. Got up to 13.5-14" and steady at hot idle after about 15 minutes of running it hard and stayed around there and steady at hot idle for the rest of the run.

I got rid of the cut-out problem while driving (floats too high - was definitely flooding). No pin holes in primary float; pulled it and checked it. No stuck needles. Coil is basically brand new (bought it last year), and if that would be the trouble I'd have all sort of inconsistent idle and cut-outs.

The main problem is big time hesitation at the top of the throttle off the line while driving. If I roll the throttle slowly it doesn't happen, but the moment I hit it hard it bogs at the very top before recovering. Get tons of blue smoke coming out once it clears so something is causing too much fuel to hit the primaries at the top. Again, this is a brand new problem that I didnt have last year.

Crazy thing is that none of this happens when I have it in the driveway. And, yes, while the engine is hot after running it. It seems to tune perfectly - no hesitations, no bogs, no nothing, clean crisp throttle. I think I have it all figured out, and the moment I get it back on road... same darn thing.

ronarndt

I finally found the fuel pressure gauge I was using was defective and read way too low, causing my actual pressure to be set much too high and contributing to a problem getting the carb floats set correctly and flooding. Installed a different gauge, set pressure correctly and other problems went away.

fishslayer143

I ve had an SS750 QFT for 9 years.. I found out in the first day that any more than 6 PSI fuel pressure results in flooding at higher speed. turned it down to 5.5 psi and never had another issue since..

cats22

OK, so my journey continues. I swapped in a buddy's Holley carb and, guess what, it did the same exact thing.

Moved on to a compression test - and aside from badly fouled plugs across the board - I didnt like what I saw.

#s 1, 4, 5, 6 and 8 showed well at 155. #7 dropped to 146. #3 even lower to 132. And #2 came in at 105; plug was most fouled of all and completely wet with oil and build-up; not to mention smoke coming out of the hole when I removed the gauge after the test. Highly doubt I was getting a consistent spark on #2.

So I definitely have to deal with the rings and possibly a few worn cylinders.

dmruschell

Could it be possible that the geysers of fuel that the QFT (and I'm assuming Holley, since you said it was doing the same thing) were shooting out were flooding the car and caused the rings to wear or not seal properly?

In that line of thinking, could it be possible that the cylinders (since they had fouled plugs) still had some residual liquid gasoline in them that could affect the compression test?

I'm just hoping that there's a solution that's easier than tearing down the engine.

v2racing

Just look at it as a chance to upgrade the cam and do a performance valve job. Maybe a little bowl work. Alway a silver lining.

Mike

cats22

Sorry, to clarify, the "same thing" that the Holley is doing like the QFT is the initial hesitation/bogging down at the top of the accelerator when I get after it hard. The hesitation doesnt happen with either carb when I gently roll the accelerator. The geysers of fuel pouring out of the vents was only in the QFT and only b/c the floats were too high.

Anyway, the only option suggested in this thread that I havent yet tested is a "heat shield" of sorts for possibly percolating fuel. But again, I've been using the same fuel, same carb, same set-up, etc., for the last year and this whole mess only started this spring after being put away for the winter, so I'd be surprised if that's the issue.

Plus, the significantly inconsistent compression readings (ran the test 3 different times on 3 different days with exactly the same #s) and heavily fouled plugs now have me going in a different direction. I'll swap in new plugs this weekend and see how she reacts. BTW, here's a pic of the existing #2 plug...