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Old 06-07-2007, 05:46 PM
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Carb jetting theory?

I'm trying to get my Holley 4150 750cfm DP carb jetted right on my motor. I did a complete rebuild kit on it, but i still have to get it tuned in. I also bought a jet set so I can swap out whatever sizes I need. As you will read I am a newbie when it comes to carb tuning...

I just need a little bit of background knowledge to do this properly. Since I do not have a wideband sensor on the motor I am told that the best way to get my jetting right is to read the plugs after driving it for a bit, is this true?

Also, I'm curious about the theory behind jet adjustments and CFM. I think the carb may be a little oversized for my current engine combo. I know that because the way that carbs works, usually an oversize carb will still work to a decent degree on a motor. My question is, will a carb a little oversized tend to run lean because CFM is airflow and not fuelflow, or does it maintain a consant ratio no matter the CFM size, or does it run rich from too much fuel?

Then the idiot basic question is... running rich = jets too big, and running lean = jets too small?

Is there a good way to adjust both sides of the carb? I was thinking perhaps disconnect throttle linkage to the secondaries to adjust the primaries (or just dont crack the throttle open too much). And for the secondaries I wasn't sure, but I thought I could either do it at the track after I make a pass down the 1/4 mile or I could just maintain the primary/secondary ratio that the carb comes with standard (70/76 i believe) and do the secondaries according to the primaries.

Any input/help here?

Thanks guys,
Kevin

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Old 06-07-2007, 11:54 PM
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The best way to do it is with an O2 sensor. I bought an Edelbrock sensor and use it to tune all my carbs.

Simply braze a proper size nut into the exhaust pope just past the header and screw in the sensor. Then watch the O2 readings at various speeds, accelerations, idle etc. Bigger jets run richer, smaller jets run leaner. Shoot for a 14:1 air/fuel ratio for economy (cruising), and a 12:1 for power/acceleration. Set idle and initial timing using a vacuum gauge - set both for max vacuum at best idle. Set to factory spec for stock engine, lowest stable idle speed for modified engine. Choose power valve for 12:1 ratio on acceleration. Play with primary jet size for 14:1 ratio on cruise.

Screw in a short bolt to plug the collar when done tuning.

Now for the size of that carb. The above jet sizing is based on using a properly sized VACUUM secondary carb. For a 350, that would be 600 - 650cfm. Running a 750 DP on a a 350 is WAY TOO MUCH CARB! You will have to over jet and over accelerator pump to compensate for low air velocity in the carb venturis especially during acceleration. Cram your foot on the accelerator, open those huge butterflies and the engine will bog due to the velocity dying in the venturis thus killing gas flow. Only way to compensate is to pump a bunch of gas w/ the accelerator pump = lots of $$$$3.50 gas. The DPs were and are intended for drag racers who run at WOT all the time and don't need all the jewelery associated with a vacuum secondary. DPs are not intended for street use. Period. You can tune them to work on the street but at the expense of mileage and usually poor bottom end performance.
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Old 06-08-2007, 01:41 AM
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Thanks for the reply Willys36,

I am still working with my exhaust setup so im not sure whether i want to get an o2 setup right off the bat. I may have to read plugs for a little while until I decide I want to buy that equipment and run my exhaust.

The 750DP I am using probably is overkill. I'm using it on a street/strip vehicle with a modified 350 (~425hp). I did have a holley 750 vaccum on it previously but the DP actually improved my ET and trap speeds over the vacuum carb even when not tuned right. I have been too stubborn to purchase a 650 because i have had plans of making more power with better heads and eventually a single turbo setup. I may scrounge a few junkyards to see if i can experiment with one for cheap.

Right now I want to get it in the ballpark range just to toy with. Is that edelbrock unit a wideband reader or just a regular o2 system? I have been told that widebands are the only worthwhile systems to use... not sure
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Old 06-08-2007, 06:11 PM
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It's just the cheapie O2 indicator. Of course the wide band instrument is superior but I have good result with my little indicator. My guess is that you didn't have the vacuum 750 tuned right with the proper diaphragm spring, jets, etc. Done right they should keep up with a DP, no sweat.
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Old 06-08-2007, 10:32 PM
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i dont have anything to add to the topic but i did want to ask you how you like the Edelbrock a/f setup ? im looking into getting one and aside from making my own setup, id eyeing the Edelbrock. just looking for opinions on it..

Thanks,
Dale

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Originally Posted by willys36@aol.com
The best way to do it is with an O2 sensor. I bought an Edelbrock sensor and use it to tune all my carbs.

Simply braze a proper size nut into the exhaust pope just past the header and screw in the sensor. Then watch the O2 readings at various speeds, accelerations, idle etc. Bigger jets run richer, smaller jets run leaner. Shoot for a 14:1 air/fuel ratio for economy (cruising), and a 12:1 for power/acceleration. Set idle and initial timing using a vacuum gauge - set both for max vacuum at best idle. Set to factory spec for stock engine, lowest stable idle speed for modified engine. Choose power valve for 12:1 ratio on acceleration. Play with primary jet size for 14:1 ratio on cruise.

Screw in a short bolt to plug the collar when done tuning.

Now for the size of that carb. The above jet sizing is based on using a properly sized VACUUM secondary carb. For a 350, that would be 600 - 650cfm. Running a 750 DP on a a 350 is WAY TOO MUCH CARB! You will have to over jet and over accelerator pump to compensate for low air velocity in the carb venturis especially during acceleration. Cram your foot on the accelerator, open those huge butterflies and the engine will bog due to the velocity dying in the venturis thus killing gas flow. Only way to compensate is to pump a bunch of gas w/ the accelerator pump = lots of $$$$3.50 gas. The DPs were and are intended for drag racers who run at WOT all the time and don't need all the jewelery associated with a vacuum secondary. DPs are not intended for street use. Period. You can tune them to work on the street but at the expense of mileage and usually poor bottom end performance.
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Old 06-08-2007, 11:05 PM
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I'm not a whiz at tuning Holleys, but I will add my take on it.

Running a carb that's too big won't hurt top end power. It will sacrifice the low range torque and drivability on the street, but 750 for a racer 350 isn't too big. If I were putting 425 hp on the street I might do a 600 or 650 just to help streetability, but I wouldn't worry about it.

The main thing is that too much carb provides more flow when you floor it than you need. The secret to peak power and strip tuning is max power. The secret to street tuning is peak velocity. The bigger flow path of the larger carb just means that the air flowing through the carb is slower for any given combo compared to a smaller carb. That means that you're asking the carb to accurately meter flow and mixture with less velocity and it can cause headaches when tuning.

I deal mostly with Qjets (very forgiving) and Carter/Edelbrock carbs. Typically if you put too much carb on it, it runs rich at idle and cruise and a bit lean at WOT. Dropping the primaries a jet size and adding a bit to the secondaries usually gets things going in the right direction. but like I said, I can't speak intelligently about Holleys.

Its all a little dance. More flow can get you a few more hp at the expense of drivability, but on a strip car that stalls at 3500 and only lives at WOT, who cares? A few HP is a few HP. On the street, you want a carb that doesn't restrict at the top but isn't too big on the bottom.

I'm a big fan of going on the conservative side of carb flow, but you have to ask yourself how much it really matters (that is to say; what percentage of the time your car spends on street/strip.)
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Old 06-09-2007, 12:17 AM
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Sensor works fine. Uses the same zirconium oxide O2 sensor that we use in the oil fields to control emissions in industrial steam generators. The analog colored light display that mine has is not high resolution of course but it still lets me tune the carb pretty well. I know for sure when my ratio is in the 12 -13 or 14 -16 range which is much better than trying to do it reading plugs. If you want more precise readout, you can go up scale to industrial digital readouts but that is $$$$$$$$$$$$$$$$$.
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Old 06-09-2007, 08:00 AM
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Here are some basics to get you going. I'll be very brief and concise to get you going.

Adjust the front and rear floats with the car running so that just a trickle of gas flows out the plug holes.

Balance the idle mix screws to achieve a stable idle. The engine should stumble if you turn one side in too far as that's leaning out that side of the circuit.

Remember what size Power Valve you have. That number will tell you at what point on vacuum the power valve will richen up the primary circuit.

Write down your primary and secondary jets, or your primary metering number.

Drive the car around on light throttle without any heavy throttle action. It should run smooth and crisp. If it surges a bit the idle mix screws might be too lean, If it's muddy you are too rich. Slight, very slight changes can be made with the idle mix screws but if it's too lean, surges or very poor throttle response it's lean and jetting needs to be increased. If it's muddy feeling on the throttle or you have a poor tone on the exhaust it's rich.

After you get nice response with light cruise and light throttle you can make some harder pulls but you need a vacuum gauge. You need to verify with a vacuum gauge taped to the windshield where the power valve is opening up and affecting the fuel mix. If it opens too soon you can go rich on the fuel mix, too late and it can lean it out. If your engine idles at 18 or so inches of vacuum a 10 PV or 8.5 should get you in the ballpark.

On wide open throttle pulls again, it should pull smooth with no surging or bucking, lean. If it's muddy and smokes black out the exhaust it's rich. The only way at this point to read the plugs is to chop the throttle and turn the engine off as soon as you let off the throttle. Not too safe on the street. Even then, without some experience you will have some trouble reading the banding at the base of the insulator to get a read on the fireband. You can make an easy visual looking at the base of the thread area that the ground strap attaches to. It should be clolored all the way around charcoal gray. The ground strap should show a heat line about 1/2 up the strap, giving you a feel for timing.

Bogging off the line or when hitting the throttle hard usually is not enough accel pump shot. Real muddy and soft off the line is usually too much pump shot.

These are some general, basic guidelines to get you going.
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