[82ltdqs]

Did you resolve this? I’m having the same issue. Holley 750 HP body Dp. Quick lean spike while at a 40 mph cruise at 2000 rpm or so, on light acceleration only. Not sustained. Only when I slightly speed up to keep up with traffic. AFR gauge shows a quick lean spike, as quick stumble, recovers rich quickly, then stabilizes. Happens in about a second. All other conditions are perfect. Wot takeoff from idle is perfect. I have tried everything. This is very annoying. Any help?

[kyle_blake]

Gee , i know i never changed the 50cc or power valve. i did resolve it. i think i jetted up 2 sizes or returned to 72. Richen up idle screws a 1/4 turn. Then i hooked up my vac advance that i limited ....i used 8-15 degrees limited connected to ported. i think the ported connection helped solve it

[chiphead]

What does idle and 2000 RPM cruise AFR look like? Is the primary accel pump set correctly? This sounds like my issue with a QF750 DP. That's about when the transition to the mains starts and the idle circuit tapers off. If your low speed AFRs are trending lean, decrease the LSAB 4-6 sizes and see how it responds. That will richen up the idle and the transition slot, and keep the transition working longer, ideally until the primaries achieve pullover.

IF that doesn't help, go up one size on the IFRs. It should fix it, or at least show richer AFRs in the same RPM range. Then maybe open up the LSABs again, or install transition restrictions if the transition slot is then too rich but the AFR before pullover is about right.

[82ltdqs]

Originally Posted by chiphead

What does idle and 2000 RPM cruise AFR look like? Is the primary accel pump set correctly? This sounds like my issue with a QF750 DP. That's about when the transition to the mains starts and the idle circuit tapers off. If your low speed AFRs are trending lean, decrease the LSAB 4-6 sizes and see how it responds. That will richen up the idle and the transition slot, and keep the transition working longer, ideally until the primaries achieve pullover.

IF that doesn't help, go up one size on the IFRs. It should fix it, or at least show richer AFRs in the same RPM range. Then maybe open up the LSABs again, or install transition restrictions if the transition slot is then too rich but the AFR before pullover is about right.

Ok, here goes. Long version. The carb came with .075 IAB in all four corners and .036 IFR in all four corners. It was pig rich. I mean stinky watery eyes and immediate black plugs on idle and cruise. I did not have an AFR back then so I do t know how rich. Also, I barely drove it enough to notice if it had this 2000 rpm stumble. Funny thing is that there is no stumble during those hard carburator testing drives. Driving hard to see if it stumbles. You know those hard throttle launches. . It only does the stumble when driving regular and not trying to induce the stumble. Never got a change fire to how Smokey and rich it was. Four corner mixture screws were at like 3/4 turns.


Dropped to .033 IFR and the same .075 IAB and it was better but super rich. Like in the 12’s at low cruise. That’s when I noticed the stumble. I figured it was a rich stumble, so I dropped all the way down to .029 IFR and .070 IAB. It ran so nice and clean. Mid 14’s at cruise. Idle screws at 1-1/8. Still had the stumble. Car felt a little weak.

Figuring it was now too lean, I went to .031 IFR and .073 IAB. Not bad. Cruise in mid 13’s. Idle mixture screws at 1 turn. Feels faster here.

Idle AFR you say? 13.5. Regardless the above settings, this engine gets the highest vacuum and rpm at idle in gear at 13.5. Once I put it from neutral to drive over and over and readjust for the lowest rpm drop when going into gear. That means a hair richer. Where it bounces between 13.2-13.5.

Anyway, regardless of the settings, tats stumble remained.

Should I try .065 IAB? I’m worried I’ll have a pig rich transition again. My pump arm is set just right. I even rebuilt the entire accelerator pump circuit. From the umbrella check valve up.


The only thing is I noticed my front shooter streams are more like a cone. I tried other nozzles and it only did that on the primaries. I noticed fuel seeps from the top and bottom shooter gasket. I noticed the nozzle screw is loose and the threads are stripping. Could that be it? I already have hollow screws. I’m going to helicoil it. I already have the kit.

[chiphead]

Yeah, fix the accel pump circuit next. At least you'll know its not a root cause if it keeps doing it.

What I've found in that throttle plate position at curb idle is super important, like crazy important. My 750 QF was the same. When I started working with it, it was pig rich, blacken the plugs, didnt want to idle, etc. I squared the primaries on the transition slot. Set the idle with the secondaries. Leaned out the idle mix screws once the engine was fully heat soaked. The end result is it leaned out the idle like you said, 13.2-14.0 at curb idle. Then it would lean out at light cruise and surge. Had the flat spot yours had too. Then pullover was late and it ran kinda lean-ish once it was on the primaries. I had to change to a proform metering block to see a major improvement.

Right now I'm .031 IFR, .066 IAB, white cam, 31/33 shooters, 71 primary, 80 secondary, 6.5PV, .054 PVCR. 4-bleed emulsion with .026" holes. 9.7 CR Poncho 400 with 2802 cam. 14" at 850 RPM.

Honestly, most of the issue with tuning these carbs is the high IFR location. All the new carbs have the high IFR. The curb idle tends to be pig rich but the low speed cruise is lean, without a bunch of work. Relocating the IFR to the low position tends to lean out much less on the transition. This lets you reduce the IFR for decent curb idle and reasonable AFR on the transition into pullover.

[82ltdqs]

Originally Posted by chiphead

Yeah, fix the accel pump circuit next. At least you'll know its not a root cause if it keeps doing it.

What I've found in that throttle plate position at curb idle is super important, like crazy important. My 750 QF was the same. When I started working with it, it was pig rich, blacken the plugs, didnt want to idle, etc. I squared the primaries on the transition slot. Set the idle with the secondaries. Leaned out the idle mix screws once the engine was fully heat soaked. The end result is it leaned out the idle like you said, 13.2-14.0 at curb idle. Then it would lean out at light cruise and surge. Had the flat spot yours had too. Then pullover was late and it ran kinda lean-ish once it was on the primaries. I had to change to a proform metering block to see a major improvement.

Right now I'm .031 IFR, .066 IAB, white cam, 31/33 shooters, 71 primary, 80 secondary, 6.5PV, .054 PVCR. 4-bleed emulsion with .026" holes. 9.7 CR Poncho 400 with 2802 cam. 14" at 850 RPM.

Honestly, most of the issue with tuning these carbs is the high IFR location. All the new carbs have the high IFR. The curb idle tends to be pig rich but the low speed cruise is lean, without a bunch of work. Relocating the IFR to the low position tends to lean out much less on the transition. This lets you reduce the IFR for decent curb idle and reasonable AFR on the transition into pullover.

Thank you for the real life data and actual sizes and numbers. I can’t find and info on this particular symptom other than theory.

My curb idle is at 3/4 turn. Exactly .030 of the primary transition slot showing. Rear transitions are higher and not showing. Rear curb idle is closed. This carb hates the secondaries cracked at all. I get an actual stumble on all acceleration conditions if even cracked 1/8. My initial timing is how I set my idle to 900 in N.

I will fix the nozzle threads then move to what you did.

Should I close the transition and fatten the idle?
Say to .020?

Should I open the transition and lean the idle?
Say to .040?

Or should I leave it alone and just drop to like .065 IAB?

Awesome info man. Thank you. Been dealing with this a while. Many threads on this topic but are never followed up on with resolutions.

Should I close

[82ltdqs]

Also…. Yeah emulsions. AED has 5 emulsions, not including a kill bleed, for a total of 6, per corner. All .027. I did fill in a few holes as an experiment. Believing that too much emulsion was delaying my mains I tried:

Open
Closed
Open
Closed
Open

Then I tried

Closed
Open
Closed
Open
Closed

No difference. Bought a new metering block with all of them open and it ran better. Tried HSAB brown .028 to .035. No change. Also numerous jettings.

It’s definitely my transition and or accelerator circuit(s).

I’m just not sure which circuit to make more dominant. Idle or cruise.

[chiphead]

My carb is a 4-corner idle, and the mixture screws are about 5/8 turns out. The primaries have maybe .025" of the transfer slot exposed. I use the secondaries to add some bypass air, I'm guessing about 1/2 turn on the secondary screw from closed. They're about as open as the primaries on curb idle. Secondary transfer slot not exposed as the t slot on the secondaries is higher in the bore than the primaries.

I read on your other post the timing is locked out? A lot of carb problems are really ignition problems. I dont have any experience with locked out timing on a street car. Seems like it would buck at low RPM? Like cliff said, having to add a buttload of low RPM timing is a crutch for static compression/camshaft combos that are mismatched. I expect the engine will want a lot of fuel enrichment at low port speeds, so that's part of why it's complaining.

What's your cranking compression PSI? If under ~155 PSI, you could advance the cam 4* to get some dynamic compression back.

[82ltdqs]

I have not run a compression check yet. I spoke to my engine builder and he said this engine recipe he builds with a high stall converter is made for locked timing.

I’m going to fix my pump nozzle threads then crack my secondaries and fatten my idle.

Thank you

[Cliff R]

Locked timing.....on a street engine?

Very poor advice you are getting someplace. If you didn't have a fancy whiz-bang distributor you could tune with a cell phone you'd knock the nose off the starter the first time you tried to crank the engine fully warmed up and heat soaked.

I build these engines N/A to and past 700hp and they end up in vehicles running pretty solidly into the the 9's in the 1/4 mile weighing nearly 2 tons and even on those engines with upwards pf 14 to 1 compression I put a very short curve in the distributor vs locking it out just so we don' t have to use a toggle switch to kill the ignition until the engine is spinning over pretty fast.......FWIW.......

[Cliff R]

Just some notes for the folks on here trying to fine tune a carb at very light engine load and low throttle openings. This is and can be a difficult area to tune, especially on aftermarket "high performance" carburetors with large diameter venturi area and small booster in the middle of them. The lack of sensitivity from the boosters requires a LOT of fuel to be brought in from the idle system and transfer slots.

The PV is there to add fuel with quick/heavy throttle movements when vacuum falls off quickly, VA timing falls out, and MORE fuel is needed for the additional engine load, so it is NOT a player at very light engine load.

Same with the accl pump. When you are driving very gently with very slow and light throttle apply the accl pump does very little. It is there to cover up a potential lean condition with heavy and quick throttle movements to provide a nice shot of fuel before the main system catches up.

In most cases the cure to solving transition issues, lean steady cruising or coming back off idle to light cruising you are looking to put fuel in via the transfer slots then from the boosters once they come on line.

Timing in that same range is also a big player. Not adding any or enough requires even more fuel to make things happy. For Holley and Holley clones look into the metering blocks and air bleeds for the cure vs pump cams, squirter size, more pump capacity, and PV's with higher vacuum ratings......FWIW.....

[Formulajones]

Another point worth adding is that many people get too fixated on a specific air fuel ratio. Mostly because it's constantly regurgitated on the internet.
There are way too many opinions on what is lean or rich.

Truth is, any particular engine is going to like what it likes and trying to force a lean afr on it sometimes ends in frustration.

A fact to keep in mind, the current fuel available has at least 10 percent ethanol. Stoich is 14.0 for that fuel and many times you'll find more than 10 percent ethanol mixed in.
Food for thought.

Also, on top of that, an engine that may lack some efficiency either with not enough compression or cam timing events that don't agree with the rest of the combo isn't going to run very well with super lean afr's.

Lastly, everyone's opinion on what is lean or rich is different. That really needs to be put aside and just give the engine the afr it's asking for.
If you're shooting for a 14 afr at idle and light tip in and the engine just doesn't respond favorably to it, then you just have to give it more fuel and get over the fact that it's just not happy being that lean.
Idling at a 13.5 or even a 13.0 afr, if that's what an engine wants, is not overly rich, it doesn't blacken plugs and doesn't wash cylinders. Honestly it really doesn't even affect mpg enough to matter either.

Tuning with a wideband is a neat tool and fun to really dive into carb circuits but at the end of the day how well the drivability is, that's what matters and should be the main focus. Sometimes with a particular engine combo you just have to live with a slightly richer idle and pump circuit afr to make it happy, and focus your leaner efforts at steady state cruise, while making sure wot afr is in a safe zone. Let the rest fall where it may.

[steve25]

For all of you Q-jet haters out there, this type of carb issue is where it’s small primary’s and 3 booster rings in each bore allow the carb to shine in the hands of a tuner like Cliff, or under his guidance.

[82ltdqs]

Originally Posted by Cliff R

Locked timing.....on a street engine?

Very poor advice you are getting someplace. If you didn't have a fancy whiz-bang distributor you could tune with a cell phone you'd knock the nose off the starter the first time you tried to crank the engine fully warmed up and heat soaked.

I build these engines N/A to and past 700hp and they end up in vehicles running pretty solidly into the the 9's in the 1/4 mile weighing nearly 2 tons and even on those engines with upwards pf 14 to 1 compression I put a very short curve in the distributor vs locking it out just so we don' t have to use a toggle switch to kill the ignition until the engine is spinning over pretty fast.......FWIW.......

I was looking at my old distributor. Non electronic. I had it set to 22 initial and all in by 3000 rpm at 36 degrees. That’s a 14 degree mechanical sweep. The vacuum canister was hooked to manifold for and it has a 12 degrees, or so, vacuum advance with a sensitive canister from performance distributors. So at idle and slow cruise, under 3000 rpm, it was at 34 degrees. With my 3200 stall, I was basically locked out already. Most everyone is.

I read threads about too much timing from people. They claim ONLY to be running 16 initial and 36 total and say how efficient their set ups are because they don’t need much timing, only to later say how they run 14 inches of vacuum advance at manifold vacuum and say how much better their engines idles with the vacuum advance plugged in. So in reality they are actually idling and low speed cruising at 30 degrees of timing and high speed cruising at around 50 degrees.

So if I’m locked out at 36, with a high stall, it seems the same. Plus this distributor can add vacuum advance whenever I want. And can be reprogrammed to a traditional timing table in 10 seconds.

Right now I have it locked out at 36 and the distributor simulates being plugged into ported vacuum, leaving the idle out of the vacuum advance equation. Plus it has start retard.

[82ltdqs]

Originally Posted by steve25

For all of you Q-jet haters out there, this type of carb issue is where it’s small primary’s and 3 booster rings in each bore allow the carb to shine in the hands of a tuner like Cliff, or under his guidance.

AGREED! I just don’t have the skills to tune a whet and can’t find a qjet builder for my set up.

[82ltdqs]

Originally Posted by Cliff R

Just some notes for the folks on here trying to fine tune a carb at very light engine load and low throttle openings. This is and can be a difficult area to tune, especially on aftermarket "high performance" carburetors with large diameter venturi area and small booster in the middle of them. The lack of sensitivity from the boosters requires a LOT of fuel to be brought in from the idle system and transfer slots.

The PV is there to add fuel with quick/heavy throttle movements when vacuum falls off quickly, VA timing falls out, and MORE fuel is needed for the additional engine load, so it is NOT a player at very light engine load.

Same with the accl pump. When you are driving very gently with very slow and light throttle apply the accl pump does very little. It is there to cover up a potential lean condition with heavy and quick throttle movements to provide a nice shot of fuel before the main system catches up.

In most cases the cure to solving transition issues, lean steady cruising or coming back off idle to light cruising you are looking to put fuel in via the transfer slots then from the boosters once they come on line.

Timing in that same range is also a big player. Not adding any or enough requires even more fuel to make things happy. For Holley and Holley clones look into the metering blocks and air bleeds for the cure vs pump cams, squirter size, more pump capacity, and PV's with higher vacuum ratings......FWIW.....

Yeah, this seems to be the case. More timing does help the symptom.

[82ltdqs]

What do you think would help here?

Reducing the IAB’s will just make the idle and transition richer.

More IFR and more IAB, in theory, should have extended the transition range while keeping me from going too rich, but that did not work.

Richer transition with more transition showing with a leaner idle?

Vs

Richer idle with less transition and leaner transition?

[82ltdqs]

Originally Posted by Formulajones

Another point worth adding is that many people get too fixated on a specific air fuel ratio. Mostly because it's constantly regurgitated on the internet.
There are way too many opinions on what is lean or rich.

Truth is, any particular engine is going to like what it likes and trying to force a lean afr on it sometimes ends in frustration.

A fact to keep in mind, the current fuel available has at least 10 percent ethanol. Stoich is 14.0 for that fuel and many times you'll find more than 10 percent ethanol mixed in.
Food for thought.

Also, on top of that, an engine that may lack some efficiency either with not enough compression or cam timing events that don't agree with the rest of the combo isn't going to run very well with super lean afr's.

Lastly, everyone's opinion on what is lean or rich is different. That really needs to be put aside and just give the engine the afr it's asking for.
If you're shooting for a 14 afr at idle and light tip in and the engine just doesn't respond favorably to it, then you just have to give it more fuel and get over the fact that it's just not happy being that lean.
Idling at a 13.5 or even a 13.0 afr, if that's what an engine wants, is not overly rich, it doesn't blacken plugs and doesn't wash cylinders. Honestly it really doesn't even affect mpg enough to matter either.

Tuning with a wideband is a neat tool and fun to really dive into carb circuits but at the end of the day how well the drivability is, that's what matters and should be the main focus. Sometimes with a particular engine combo you just have to live with a slightly richer idle and pump circuit afr to make it happy, and focus your leaner efforts at steady state cruise, while making sure wot afr is in a safe zone. Let the rest fall where it may.

You are right brother. I send down that rabbit hole. My engine idles the best at 13.2 or so AFR. Just need to find that happy AFR at my point of stumble.