91 camaro racer

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Trying to tune out a very rich light throttle cruise issue with my speed demon 650. Have set my curb idle to best vacuum, normal acceleration is good at low 14's, my only issue is very rich light throttle cruise. It is in the low 12's. I do not have screw in ifr's or iab's. It is also a 4 corner idle carb. I am wanting to use the wire in the ifr method to see if that helps before I modify the carb. Since it is 4 corner I will have to put the wire in all 4 ifr's right? Then readjust the idle mix screws. Correct?

Fast355

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Does this carb have a functioning power valve or is it plugged off?

I would suggest lean best idle rather than maximum vacuum. Lean the mixture screws down until the engine just starts to slow down. Do it on all 4 corners. Will run substantially leaner even at light throttle but not mess with the acceleration fuel curve much.

91 camaro racer

iTrader: (2)

functioning power valve, 6.5. for the mixture screws, do 1 at a time until the idle slows or turn each one a little at a time until the idle slows? i have made progress on the light cruise. used a small strand of wire in each IFR and the cruise AFR went from 12.2 to 13.0.

Fast355

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You can turn them one at a time an even amount or all a little same difference. Generally I go back in 1/2 to 1 full turn from maximum vacuum. I would try 1/2 turn each and then go 1 full turn, if it is still idling smoothly maybe go more, just depends on what it does. On a two screw carb leaning to lean best it drops the rpm about 25 per screw. Generally I start with my idle 50 rpm higher than desired and then lean out the screws to get it to drop. You want your idle around 15:1 anyway. If you remove the accelerator pump cam you want the tip-in with a very slight steady increase of throttle slightly lean, like 15.5-16:1. If it stays the same or worse richens, your primary jets are too big. The various systems of the carb overlap and you want the mixture as steady as possible if you want it to perform and transition well at all throttle openings. The idle mix needs to be right, the transition off-idle needs to be right and the throttle plates closed enough not to uncover the transition slots at idle. If the transition slots are uncovered at idle the transition from idle to off idle will be crappy as there will be no additional fuel to cover up the lean spot before the main metering comes in. The average person will increase the jet size. Making it run rich at cruise.

Does this carb have a rear secondary throttle plate idle speed screw? I have always had good luck evenly opening the front and rear idle screws and evenly adjusting the mixture screws. Helps prevent uncovering the primary transition slots and causing the main metering to come on too early.

Fast355

iTrader: (2)

Actually curiousity just bit me and I had to look. Your carb should have an adjustable idle air bypass system. I guarantee if you have opened the idle speed screw or screws rather than opening the idle eazy air bypass valve you have upset the transition ports. Open up the bypass some and close down the idle screws to compensate. The extra air bypassing the venturis will lean you out at light throttle and idle. You want the plates as closed as possible with only a tiny square of the transition slot exposed when viewed from the bottom of the carb. Bypass air is a good thing. Will lean out your whole idle, off-idle and cruise calibration. If it leans the idle mixture and off idle too much, can always increase the opening on the mixture screws to compensate. You do not want the transition slots supplying any of the idle fuel.

https://mooregoodink.com/demon-wants...rs-in-minutes/

sofakingdom

iTrader: (1)

6.5 is too low of a vacuum for Holley-style design carb on a motor anything remotely resembling "streetable" to operate at. Yours is not "the same" as a Holley of course, but it works largely the same way.

That PV is possibly perfect for a car with more cam, and no idle vacuum; not so much for something in which "cruise" is a concern, let alone a priority.

The jets are probably too large, to cover up the vast "gulf" between cruise vacuum and the point where you FINALLY open the throttle enough to lower the vacuum enough for the PV to open.

Might want to take a look at the Holley Tuning sticky on this forum, particularly the parts where it describes how to set up the throttle openings at idle so that the transition system isn't continuously sucked dry at idle, the cruise mixture is right, and the PV opens when the vacuum drops to a point where the engine actually begins to need it.

Fast355

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8.5 would probably be better for the power valve but IMO it is not the main issue. Main issue is probably not enough if any idle bypass air and the transition slots are probably exposed. Working correctly the engine should idle on the idle screw ports alone and the transition slots cover the gap in fueling between idle and the start of the main metering. Fix that relationship and you will not need huge jets to cover up the gap between.

On my Q-Jet that was an OEM 305 carb, going on a 383 I had to rework the idle feed, idle channel and idle bypass as well as the main metering jets and rods and power piston spring to make the transition smooth and seamless. Had to make the idle down (fuel feed tube restrictions) and idle channel restrictions (restrict the mixture of emulsified idle feed fuel and idle port bleed air) larger for more fuel delivery to the idle mixture screw ports and transition slots. Also had to open up the idle mixture screw ports to get enough fuel to idle the engine solely on. Then had to enlarge the idle air bypass ports to get the 383 enough air to idle without the throttle plates exposing the transition slots. Then the main jets had to be larger and the metering rods smaller to supply enough fuel for the mains. Finally used a 8 in/hg power piston spring to enrich the mixture more quickly. I adjusted the APT lean stop once the idle mixture was correctly set to lean the cruise afr out. Then had to increase the accelerator pump shot to match the larger engine, higher flowing heads and larger plenum intake. Even at high vacuum when you lightly accelerate on the primaries the transition slots are uncovered and then the vacuum signal increases at the boosters which pulls more fuel. First a light drizzle then more and more as the throttle plate opens and engine speed increases. Good rule of thumb is power valve or piston spring that works at half the engines idle vacuum. My 383 makes slightly more than 2x the idle vacuum the power piston enriches at and it works great.

sofakingdom

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Large jets and small air bleeds produce very similar results.

In this case, the carb most likely has too much jet, expecting that the vacuum at any sort of RPMs that it would ordinarily run at, will be fairly low; either at idle because it has a large cam, or because it's not untended to cruise AT ALL, but rather, RACE. (full power all the time more or less, at relatively high RPM) But, since if it has a small enough cam to be "streetable", and it's CRUISING (low RPM, low load, NOT racing), and d00d who owns the car is complaining about STREET behavior, then more likely, it has relatively high idle vacuum, and of course, it gets used for CRUISING, which is inherently a relatively high vacuum situation. When operated like that, it will cruise at let's say 14 - 18" of vacuum or maybe even more, but with a 6.5 PV, the vacuum has to drop ALL THE WAY from that high cruising vacuum to 6.5 before any power enrichment occurs. In that case, the jets HAVE TO BE rich enough to support whatever power level is demanded between cruise vacuum and PV vacuum, and that's how the aftermarket carb factories set up their carbs. The usual misbehavior that results with a Holley type of racing-oriented carb operated on the street is, terrible cruise gas mileage, fouled plugs, a LOOOOOOOOOOOOONNNNNNNNNNNNNNGGGGGGGGGGGG flat spot when giving it throttle, and a sudden SPIKE of power when the PV opening point is finally reached as the throttle is opened more and more.

Q-Jet design is COMPLETELY different. The things that work for one of those DON'T work for a Holley design, or vice-versa. Whatever worked for a Q-Jet on your van is IRRELEVANT to this guy's situation.

91, IGNORE "Fast" 's advice, except for his comment about the transition slots; and go to the Holley Tuning sticky on this forum. Which incidentally includes the proper method for setting up the transition slots.

Fast355

iTrader: (2)

OP do not ignore what I have to say if you actually want to fix your problem. I have fixed a dozen Holley carbs. It is not the power valve causing his issue. Power valve is for WOT enrichment not cruising enrichment. I doubt the main metering is even in effect until the car is running 60+ mph. The idle and transition slots feed the engine until the main metering take over. Main metering probably comes on around 1/4 throttle or so on the primaries, maybe a little less. Same principle that works on a Q-Jet, works on an Edelbrock or yes even a Holley. They still have the same systems in place. It needs the throttle plates closed below the transition slots, the idle mixture screws opened up to supply all the idle fuel and the idle bypass air valve opened enough to get the engine to idle properly in gear. Bypass air will lean it out at cruising speeds. It is the answer! Before the bypass valve was incorperated aka older 4150 and 4160 carbs, I have drilled throttle plates with 0.050" holes. Started on the secondary side and then moved to the primary side if secondaries could not do it alone. The secondary plates on that 1406 I built are drilled 0.070" per side just to keep the primaries out of the transition slots. I do the secondary side so that they do not pull excess fuel and bring the primary metering on earlier. Modern EFI cars and trucks do not go into power enrichment until 90% throttle and often after a moderate delay in time, yet they run fine. You do not need a super rich mixture under part throttle acceleration to get good throttle response and performance.

Fast355

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Just to further prove primary main metering has little to do with it, even at 2,000 rpm on a 383 this 600 cfm Edelbrock was only beginning to pull a very small amount of fuel out of the primary boosters. 600 cfm Edelbrock has smaller primaries than that 650 Holley does. ~90% of the fuel was being supplied by the idle and transition slots and a decent amount of bypass air was flowing through the secondary throttle plates and venturis without creating enough pressure drop in the venturi to pull fuel into it. It was not running excessively rich at all.

Fast355

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Just to further prove primary main metering has little to do with it, even at 2,000 rpm on a 383 this 600 cfm Edelbrock was only beginning to pull a very small amount of fuel out of the primary boosters. 600 cfm Edelbrock has smaller primaries than that 650 Holley does. ~90% of the fuel was being supplied by the idle and transition slots and a decent amount of bypass air was flowing through the secondary throttle plates and venturis without creating enough pressure drop in the venturi to pull fuel into it. It was not running excessively rich at all. Edelbrock ran 7 minutes and 24 seconds on 1 quart of gasoline at 2,000 rpm. At that rate of consumption that is nearly a full 30 minutes on a gallon. Granted with load it would have pulled more fuel, but the throttle is barely cracked at a steady cruising speed anyway. The basic science of the carburetor does not change regardless whose name is on the label. This carb has fatter economy tips, and thinner power tips than the normal 1406 and has a .101 main jet rather than a 0.95". The power piston springs are 8 in/hg. It is super resonsive even opening the throttle from 23 in/hg vacuum. The venturi effect pulls more and more fuel into the venturi as the vacuum in the venturi builds with increased throttle.

91 camaro racer

iTrader: (2)

ok i will respond to each after the quotes in bold to make it easier....

i do have a secondary throttle plate screw that is easily accessed from the top with an allen wrench. i have adjusted the primary throttle to show a square with the transition slot then used the secondary to control idle speed.

mine is an older Barry Grant version prior to the addition of the idle eze.

i agree that the 6.5 PV is too low of one. The original plan was to dive deep into the carb and tune it as best i could, plans changed and now i am waiting for a fuel injection set-up to arrive. Now i am just doing the basics to have it as a decent cruiser until that arrives to add up miles for my rear gear break in. my idle vacuum is right around 12" parked in gear. i actually just raised the primary jets from 65 to 68 to help with a lean (15.5-16) moderate acceleration. this change helped by lowering it to ~14. I have looked at the tuning sticky and like i said, planned on following it but then plans changed.

agreed it probably does need a higher PV. Transition slot is good.

Granted it is currently just used to drive around until miles get put on the rear gears, the overall purpose of the car is for drag racing. not that that really matters for this topic.

I understand that the PV is for very aggressive to WOT driving. i did a long drive today (24 miles) to see what the changes i completed yesterday did. so far it seems to be better and closer to where it needs to be. i am now cruising in the low to mid 13's (not perfect but better than the low 12's it was doing. also my moderate acceleration is better at mid 14's. i havent accelerated enough yet to even really get into the secondaries or to open the PV. even after getting nice and warmed up from the drive, mix of city and highway, the car behaved well and 95% of the time the AFR's looked ok. i do appreciate all the replies and suggestions. Even though i will be going fuel injected soon, i still want to learn as much as i can. Thank you both for the help!

sofakingdom

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I should mention also, people's familiarity with EFI at the same time as lack of carb experience, leads them to overestimate the need for high fuel pressure. Carbs don't work off of FP; they work off of VOLUME. As long as the fuel system can keep the carb's fuel bowls full, then it's supplying enough pressure. If it can do this at .001 psi, then .001 psi is all it needs. That's an exaggeration of course, but the point is the point. Generally, in a real-world installation and especially one that's been converted from EFI to a carb, FP falls off as delivery (engine demand) requirements increase; i.e., high RPM, high power. At the moment the engine needs more fuel, the fuel system falls on its face. To move x number of gallons per hour of fuel past the needle valve, requires more pressure than to move x/2 gallons or whatever. Therefore the ideal FP for a carb would be LOW at idle and cruise, but HIGH at high RPMs etc. But most fuel systems can't do that; most can't even keep it constant across the "dynamic range" of engine demand. The usual response is, crank up the FP, to try to "crutch" it, particularly if it leans out going through the traps or at the top of each gear; but that's not the right answer.

Resist the temptation to keep cranking up the FP to cover up a perceived fuel starvation at high RPMs. Instead, focus on DELIVERY: gallons per minute, not pounds per square inch. Look for restrictions to eliminate, especially on the "suction" side of the pump. (disabled in-tank pump, anyone?) Most carbs don't ever need more than 5 psi of FP, and that much only at high flow rates. If you have a regulator, set it NO HIGHER than that. Lack of fuel bowl level control can be a yuuuuujjjjjjje contributor to a rich condition, which of course is what happens when the needle valve is overwhelmed, or when the slightest drop of the float, allows MASSIVE amounts of fuel to come squirting in. This sort of thing can happen just from going over bumps as you drive. Don't let that trip you up. It's IMPOSSIBLE to "tune" a carb if it doesn't have unconditional control of the fuel bowl level.

Fast355

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x2 on the fuel delivery issue. That being said a factory in tank EFI pump easily fuels way more hp with a carb than it does with EFI. Just needs a return style regulator. GMs old ZZ3 conversion package actually had an electric pump and a bypass regulator to keep the carb fed for that HO 350 and it worked well. One of the first changes on my old G20 van when it was carb still was a TBi tank, TBI fuel lines and an in-tank Vortec pump. Vortec pump flows something like 190 LPH at 58 psi. At 5 psi it flows ALOT more. It cured the fuel starvation issue I was having in hot weather by continually pumping cool fuel from the tank to the bypass regulator mounted on the intake manifold of the engine itself. The regulator I had was also vacuum referenced. With the vacuum line hooked up it had about 2 psi at idle and about 5 psi without vacuum. Worked GREAT with both an Edelbrock and Q-Jet. At one point as well I had a Jaguar fuel cooler installed in the suction line between the a/c compressor and the accumulator. Used the cool refrigerant returning from the compressor to cool the fuel feeding the carb. Helped keep the fuel in the float bowl cool on a 110°F day.

91 camaro racer

iTrader: (2)

i am currently running a stock TBI fuel pump. this is routed to an adjustable regulator set at 5.5 psi.