carburator tuning question

[mf68chevelle]

I have a Holley 650 double pumper on my 383 in the Chevelle. It is running rich so I would like to re-jet it. I have a Autometer wideband air fuel gauge in the car. At wide open throttle it stays at about 10.0 I believe I would like it to be about 12.5 or so correct? My thought is to disconnect my secondary linkage and jet the front of the carb then reconnect the linkage and re-jet the secondaries. Any thoughts on this? Or how do you normally determine how to re-jet the primaries and secondaries. I also have a 770 street avenger (vacuum secondary) that I would like to re-jet for this motor also after I put a 50cc accelerator pump into it to try to get rid of the bog it has. Which carb do you think would be better for this motor the 650 or 770. 383 with Brodix IK200 heads 11:1 compression Hyd. roller cam with about .560 lift and 236 duration. I don't really know enough about carbs but I am trying to learn so any help on getting it set up would be appreciated.

[TC]

Re: carburator tuning question

I have a Holley 650 double pumper on my 383 in the Chevelle.

You could have stop right there. ;) A 650??, Dude I wouldn't run anything less than a 750 on a 350, that carb is way to small for a 383. The 770 would be better, probably need to put a different spring in the actuator for the secondaries, so they open slower, that should get rid of the bog. Or is it a "Tip In" bog, meaning does it do it when you barely hit the throttle, then it would be an accelerator pump issue. But if it bogs when you nail it you need to slow down the rate that the secondaries open and they make a spring kit for that.

And do not disconnect the secondary linkage, there will be nothing stopping it from opening, well unless you wire it shut somehow.

Also try this, turn the idle mixture screws all the way in(on both sides), does the engine stall??, if it doesn't then most likely you have a blown Power Valve, which could be the reason your running so rich.

[yellomalibu]

Re: carburetor tuning question

Nothing smaller than a 750 on a 350? I suppose if it's a high performance motor that sees lots of time around 7000 rpms, that's fine, but on a stock type 350 a 750 is way too much carb.
I have a 600 vac sec on my 350, even jetted down a size for fuel economy; runs fine.

That said, given the performance heads and cam he's running, I'd probably suggest getting the 770 tuned for it.

Just googled-
Looking at the Holley FAQ page http://www.holley.com/TechService/FA...ory=Carburetor It says to take the engine size multiply it by max rpm then divide it by 3456. For example: 383 x 6500 / 3456 = 720cfm carb. My stock type 350: 350 x 5000 / 3456 = 506 cfm carb... I guess my 600 vac sec is too big for the 350; hence why it runs fine with downsized jets.
There are several other tuning tips on that page as well.

[Barry Donovan]

Re: carburetor tuning question

Nothing smaller than a 750 on a 350? I suppose if it's a high performance motor that sees lots of time around 7000 rpms, that's fine, but on a stock type 350 a 750 is way too much carb.
I have a 600 vac sec on my 350, even jetted down a size for fuel economy; runs fine.

That said, given the performance heads and cam he's running, I'd probably suggest getting the 770 tuned for it.

Just googled-
Looking at the Holley FAQ page http://www.holley.com/TechService/FA...ory=Carburetor It says to take the engine size multiply it by max rpm then divide it by 3456. For example: 383 x 6500 / 3456 = 720cfm carb. My stock type 350: 350 x 5000 / 3456 = 506 cfm carb... I guess my 600 vac sec is too big for the 350; hence why it runs fine with downsized jets.
There are several other tuning tips on that page as well.

that cfm stuff is for lack of venturi'd young minds called fuel injection. kids these days.

I too had a 600 on a 350. the greatest engine ever.. it froze in the summer, and started all winter... no choke.

the 770 bogging is choking on not enough speed, there is all kinds of flow. You may find there is no contentment no matter what one does with the jets... and then you could boost it to humor the big fat slow venturi.


the special heads, and build... someone must have a nice combo written for you.

[Bamfster]

Re: carburator tuning question

I'd tune the accelerator pump and circuit before you put a 50cc pump on the vac sec. 650 or the 770 will be fine for the street but economy (I know it's a performance car) and driveability may be better with the 770. If you're running a tight converter and numerically low gears ... vacuum secondaries hands down.

Don't disconnect the secondary linkage to start jetting the DP. Personally when I tune jetting, I lean or richen ALL jets the same amount, and check MPH at the track ... that's the way I do it.

Double pumpers are set up rich in the idle circuit from the factory. They're designed for engines with a lot of duration and valve overlap. The idle circuit can be leaned out a bit fairly easily.

[mf68chevelle]

Re: carburator tuning question

I forgot to mention a couple of other helpful details about the car. I have a 2004r trans with a 3000 stall convertor and 3.73 gears. I drive the car quite a bit and have done 5 complete power tours with it. I normally take it to the track once a year. Last time I was at the track I ran a 12.44 @ 109 with a 1.88 60 foot. That was with the 770 on it, but it had a bad bog on the line and every time I start getting on it on the street it goes lean for a second and bogs. I increased the squirter size and it helped, but it needs a bigger pump. I had some ignition problems and at the time thought it was the carb. That is when I switched to the 650 double pumper, which got rid of my bog problem. It runs pretty good with the 650 on it, but it is rich,according to my air fuel meter it sits at about 10.2 when I put it in 3rd at 60 and run it to 120 at wide open throttle. I will probaly jet it down 2 jet sizes in the primarys and the secondarys and see where it goes to. My other thought is over the winter to buy a Holley 750 ultra double pumper which seems more like what I should be runing.

[IRONHEAD]

Re: carburator tuning question

I'd run the 650 DP or a 650 vac secondary carb.
750DP is way to much unless your spinning it to 6900rpm all the time..
the 770 vac sec will work but your throttle response will suffer..
check for blown powervalve.. the re jet the primary side if needed..
also. before you do anything, how do you know the autometer gauge is reading correctly?

[Bamfster]

Re: carburator tuning question

On the 770, what squirter and pump cam are you running? With the car idling, and in gear, what is your vacuum reading after adjusting idle mix for highest vacuum?> And what power valve is in it? I would agree that the DP is the way to go if you're taking it to the track regularly, but seeing how it's mainly street driven, I'd still go with the 770 vac sec carb. Cure the off idle stumble, get it jetted properly and then tune the secondaries, you'll be suprised at how the driveability will be with that carb.

Comparing cfm on a street car between a mechanical and vacuum secondary carb is kind of misleading. The nice thing about the vacuum sec carb is that it more or less only use what it needs in the cfm department (this is where tuning the secondaries comes in). A 750 DP on a street car with a small block is a bit overkill IMO.

[joe_rocket45]

Re: carburator tuning question

I forgot to mention a couple of other helpful details about the car. I have a 2004r trans with a 3000 stall convertor and 3.73 gears. I drive the car quite a bit and have done 5 complete power tours with it. I normally take it to the track once a year. Last time I was at the track I ran a 12.44 @ 109 with a 1.88 60 foot. That was with the 770 on it, but it had a bad bog on the line and every time I start getting on it on the street it goes lean for a second and bogs. I increased the squirter size and it helped, but it needs a bigger pump. I had some ignition problems and at the time thought it was the carb. That is when I switched to the 650 double pumper, which got rid of my bog problem. It runs pretty good with the 650 on it, but it is rich,according to my air fuel meter it sits at about 10.2 when I put it in 3rd at 60 and run it to 120 at wide open throttle. I will probaly jet it down 2 jet sizes in the primarys and the secondarys and see where it goes to. My other thought is over the winter to buy a Holley 750 ultra double pumper which seems more like what I should be runing.

I too think a 750 DP is what you need. My friend had a not so fast 302 mustang and we went all the way to a 800 DP, granted i had to modify it internally but it produced the best et of the 3 carbs he tried.
Keep in mind the carb calculators are assuming that that the engine will see 1.5" vacuum at WOT shift RPM, I have been told that closer to .5" produces the bast power.

the bog on your 770 (which being a street car is probably the best carb in reality) could possibly need the seconday springs one or two steps stiffer. in the past I have cured a bog in a "manual secondary single pumper" carb by drilling the squirter out to .055"... if you try this start increasing it's size slowly with a pin drill, test, enlarge, test ..

as far as your idea about unhooking the secondaries to tune a DP carb, in the past I have done this then tuned the secondaries after the primaries were right, but I was concerned that that metering on the primary side would change b/c it's seeing a different vacuum/airflow with the secondaries hooked back up.

[Ron Ward]

Re: carburator tuning question

Time for a Thermoquad?

;D

[Scott Liggett]

Re: carburator tuning question

Going lean right after tip in means that your accel pump circuit is a bit lean. If you have a 31 or 37 squirter, and it's still lean, then you need to go up on the pump cam. That's the little colored plastic thing on the linkage that the pump arm rides on. The white one is the smallest. Go up to a red or blue one you can buy a kit with several. Also, you want to adjust the pump lever arm so it's squirts fuel with the slightest touch of the throttle.

http://www.summitracing.com/parts/HLY-20-12/

Don't buy a 50 cc accel pump set up for primary side of the carb. Waste of money.

If it's bogging when you mash the gas, the secondary opening spring is too light. The heavier the car, the heavier the spring. I'd start with the mid range spring, then maybe go one step heavier on the springs.

http://www.summitracing.com/parts/HLY-20-13/

[DanStokes]

Re: carburator tuning question

1. Take it the dyno shop.
2. Do want Wayne (in my case) tells you.

I'm a crappy tuner and Wayne is great with 'em. So I spend the $$ and listen to him. I need to take it back and chase the off-idle bog out of mine - or maybe it doesn't matter as it only goes down the track at Maxton and I can drive past it.

Dan

[CDMBill]

Re: carburator tuning question

The advice with the 770 vac secondary seems on target in terms of changing secondary springs, I think that carb comes with the kit that makes those changes easy without removing the entire vacuum pod. I usually start with the purple form the typical selection package but you'll want to compare with what's in it first.

Is the 770 out of the box stock or has it been 'pre-tuned"?

The accel pump cam has two positions, often the throttle position you have it set to for idle purposes has used up much of the pump travel before you stab it. Try the adjustment suggested by Rebel and shoot for that tip in at the actual throttle angle you would use for a hot start, You may also find that a cam that has a longer shot is better and a slightly smaller shooter to compliment it. If you have short tires you probably don't need to chnage cams as you have plenty fo converter.

[mf68chevelle]

Re: carburator tuning question

I just took a look at the 770 carb. It is bone stock out of the box except i put a 35 squirter in it (stock was 25 I believe) and I put smaller jets in it. I believe I went down two jets in the front and one in the back. The squirter cam is a light greenish color in the #1 screw hole and the spring I don't see any color on, but I do have a new spring kit, and the carb came with 2 extra springs, black and yellow I believe. I also found an extra black squirter cam laying on my coffee table (don't ask me how it got there)
My tires are 28" tall L60-15 or 275/60r15.
I will probably get the 770 dialed in and put it back on the car. Thank you all for your advice!!

[TC]

Re: carburator tuning question

Put the stock jets back in it and see if the bogging goes away.