Tuning help with new carb on strong 408

[aengineguy]

Hi All,

Have always been happy with my Holley 750 HP, except when starting cold. There is no choke and would take about 4 to 5 times of starting to keep running, then good. I purchased a new Quickfuel 850 with mechanical secondaries and electric choke. Throttle response is markedly improved. As with the previous Holley, I had to go with a 2.5 PV as vacuum is low...engine is pretty radical. I had the Holley off and on about 10 times getting it dialed in; squaring the transition slots, secondary opening screw and ultimately stepping up on drilled holes in the primary butterflies. I thought I had read everything about doing all this, but maybe I am all wet. The new Quickfuel is really rich at idle, as was the Holley when I started. The four corner idle screws have no authority at present. Do I have to go about drilling the primary butterflies again on the new carb to get the transition slots squared up again? I am sure that I am probably running on that circuit at present at idle. It worked out well on the Holley. What about the idle jets. Will installing larger ones help me at all? I am planning on a wide band A\F gage, but not installed yet.

Thanks........!!

 

[70aarcuda]

1st where is the timing set? and how much initial timing is there in the engine?

 

[aengineguy]

1st where is the timing set? and how much initial timing is there in the engine?

I am at 24 initial with about 36 total. Here are most of my specs...

• Current CID 408
91 LA block .030 over, zero deck, line bored, Hughes Main girdle
Extensive oil system mods
Melling oil pump
• Kevco oil pan and pickup with large pickup tube
• Scat forged 4.00” crank
• Scat H beams con rods
• Keith Black forged pistons
• Edelbrock Magnum Heads fully CNC ported by Hughes, 2.08" intakes, milled back to 58cc chambers. Around 300 cfm @ .600 lift
• 10.65 static compression
• 1.6 stainless steel rocker arm
• ARP head and main studs 190 Ksi
• Hughes Camshaft
o Intake Lift .610
o Exhaust Lift .614
o Intake Duration @ .050 valve lift 252
o Exhaust Duration @ .050 valve lift 256
o 106 degrees Lobe separation angle
• Solid lifters with EDM holes
• Port matched Edelbrock Air gap Intake
• TTI Industries 1 7/8 Headers
• TTI Industries 3" dual exhaust with X pipe to the rear bumper
• SFI Certified Damper and Flex Plate
• Mallory Magnetic Breakerless #42 Distributor
• Mallory Hyfire 6AL Capacitive Discharge box with rev. limiter
• Edelbrock 160 GPH pump
• Dual Fram Fuel Suction filters
• -8 AN lines to regulator and -6 AN to carb

 

[russhal]

You need to change the air bleeds. In board are for idle curcuit. If it's rich, you need go larger I think. Qf has a great tech line. I had the same problem. My 408 is much milder. I have a 750. Don't go cutting the carb up. It's designed to be fully tunable.

 

[Woods74]

Wide-band time.

http://ls1tech.com/forums/carburete...t-level-holley-tuning-serious-ocd-tuners.html

 

[AJ/FormS]

I think the engine wants more air. Are you running a PCV? I dont see one.

 

[Hipo65]

Wide band O2!!!!!!!!!! How much of your transition slots are exposed? If too much it will idle rich & have hesitation when accelerating from off idle. Demons had a sweet air bleed right into the center of the carb to get the idle speed up & not expose the transition slots. Or the old school way is drill holes in the throttle blades.

 

[SRT_DSTRHOLC]

I agree on the wideband. Be suprised how far off aome things really are

 

[360duster]

Hi, i´m in the same boat with my AED 750. Even after completely removing the IAB´s (the outer ones in the carb body) it´s running pretty rich at idle. I will change the Idle feed restrictors in the metering blocks to smaller ones - i think that is possible with your carb, too.

This thing must pull like a freight train with the heads and cam combo.....cool, i can imagine this can run 10s in the quarter if tuned properly.

Michael

 

[Hipo65]

Again before you start doing mods pull carb off & make sure the transition slots are not over exposed 1st!

 

[russhal]

He's right. No more than .04 of the slot should be exposed. You may have to drill a small 1/8 hole in each butterfly if it won't idle. It wants more air at idle. A result of a large cam profile

 

[AJ/FormS]

I really hate to mention this, but what are the chances that PV is leaking?Right through the diaphragm.You wont see it in the mains.The intake floor might be wet.
I would put a plug in there until you get the idle sorted.
Jus saying

 

[aengineguy]

You need to change the air bleeds. In board are for idle curcuit. If it's rich, you need go larger I think. Qf has a great tech line. I had the same problem. My 408 is much milder. I have a 750. Don't go cutting the carb up. It's designed to be fully tunable.

Yep, did do that, went up 2 sizes at the recommendation of QF tech. Was able to get some adjustment of the idle screws, but now am running into a problem with it randomly popping loud in the exhaust, about every half second. The idle screws were at about 3/4 turn out (gave me the highest vac) and I took them out another 1/2 turn and the popping went away. I will have some time to goof with it this weekend.

Wide-band time.

Totally agree!

http://ls1tech.com/forums/carburete...t-level-holley-tuning-serious-ocd-tuners.html

Yep!

I think the engine wants more air. Are you running a PCV? I dont see one.

No, I am not running a PCV. Think I will install one this weekend. However, with the 750 HP Holley I had been running, everything was just great. Racked up about 1500 miles on it.

Wide band O2!!!!!!!!!! How much of your transition slots are exposed? If too much it will idle rich & have hesitation when accelerating from off idle. Demons had a sweet air bleed right into the center of the carb to get the idle speed up & not expose the transition slots. Or the old school way is drill holes in the throttle blades.

Have not had the time to take the new QF off and check the slots. I had to do this with the Holley several times before getting perfect. However, maybe in my stupidity, I did drill the primary blades out. I stepped up a few times before everything was right with 3/16" holes.

I agree on the wideband. Be suprised how far off aome things really are

Agree!

Hi, i´m in the same boat with my AED 750. Even after completely removing the IAB´s (the outer ones in the carb body) it´s running pretty rich at idle. I will change the Idle feed restrictors in the metering blocks to smaller ones - i think that is possible with your carb, too.

Could be....have to keep in mind.

This thing must pull like a freight train with the heads and cam combo.....cool, i can imagine this can run 10s in the quarter if tuned properly.

Michael

Yes, it is really strong. Agree, should run in the 10's, but I need to cage it and get more room in the back for larger tires.....have not tubbed it yet.

Again before you start doing mods pull carb off & make sure the transition slots are not over exposed 1st!

Yes, on my adgenda!

He's right. No more than .04 of the slot should be exposed. You may have to drill a small 1/8 hole in each butterfly if it won't idle. It wants more air at idle. A result of a large cam profile

Agree, may be a last option after trying different air bleeds, pcv, etc.

I really hate to mention this, but what are the chances that PV is leaking?Right through the diaphragm.You wont see it in the mains.The intake floor might be wet.
I would put a plug in there until you get the idle sorted.
Jus saying

It is possible, when I start goofing with everything this weekend, will check.

The Dart is my first money pit....am also trying to get some brake issues sorted out on my second money pit (Roadrunner)......as many of you know!

 

[russhal]

Wow! You answered everyone! You'll get it. Can be a very drawn out process. The high end carbs offer Alot of adjustment you don't get from the standard holley. I played with the air bleeds on my with only slight changes. Finally went old school and drilled the butterflies. Idle improved greatly and so did transition from idle to wide open thottle. Really made it pleasant to drive.

 

[AJ/FormS]

Two of 3/16 holes is a mighty big idle air bypass.That works out to .055 in sq. Thats the equivalent of a .265 round hole! You wont find that much bypass air in the pcv, unless you gut it. However, IMO, I still recommend you run one.
And Im still interested in plugging the PV circuit during the idle tuning.I hate having to tune twice.
I havent worked on anything near as nasty as your engine. I wonder if your combo might be one of those that could benefit from using manifold vacuum on the vacuum advance can. You could still have the same 24* initial, but as soon as the engine runs, it could pull in another 8* to12* or so.These degrees would drop out as manifold vacuum drops out with increasing throttle. And the 36* degrees of total idle timing, might just be the answer. I guess the hard part would be getting the v-can to activate at the relatively low idle vacuum, yet drop out when its supposed to.
I feel the Dcr on your combo is wanting more idle timing.
I think MSD makes a neat dizzy with selectable electronic curves,including a start-retard,and a vacuum curve, that might also be of interest.
With the higher idle timing, the engine might be much happier.I guess it would be easy enough to prove, before buying parts.Just crank some more timing in and see what happens.You may be able to reduce the curb idle and/or secondary cracking screws, to the point that the mixture screws begin to function correctly.
Or maybe you have already tried that?

 

[aengineguy]

Wow! You answered everyone! You'll get it. Can be a very drawn out process. The high end carbs offer Alot of adjustment you don't get from the standard holley. I played with the air bleeds on my with only slight changes. Finally went old school and drilled the butterflies. Idle improved greatly and so did transition from idle to wide open thottle. Really made it pleasant to drive.

russhal...yeah, I like to acknowledge the effort people take to help with my issues. I was super happy with the 750HP after getting it dialed in, no lag, no hesitation, just instant wwwaaaaa! I just did not like the hard starting when cold. If I cannot get the carb dialed in with idle jets, I will not hesitate to start boring holes, besides, throttle blades can be replaced, if at sometime I want to sell the carb, but not in the foreseeable future.

Two of 3/16 holes is a mighty big idle air bypass.That works out to .055 in sq. Thats the equivalent of a .265 round hole! You wont find that much bypass air in the pcv, unless you gut it. However, IMO, I still recommend you run one.
And Im still interested in plugging the PV circuit during the idle tuning.I hate having to tune twice.
I havent worked on anything near as nasty as your engine. I wonder if your combo might be one of those that could benefit from using manifold vacuum on the vacuum advance can. You could still have the same 24* initial, but as soon as the engine runs, it could pull in another 8* to12* or so.These degrees would drop out as manifold vacuum drops out with increasing throttle. And the 36* degrees of total idle timing, might just be the answer. I guess the hard part would be getting the v-can to activate at the relatively low idle vacuum, yet drop out when its supposed to.
I feel the Dcr on your combo is wanting more idle timing.
I think MSD makes a neat dizzy with selectable electronic curves,including a start-retard,and a vacuum curve, that might also be of interest.
With the higher idle timing, the engine might be much happier.I guess it would be easy enough to prove, before buying parts.Just crank some more timing in and see what happens.You may be able to reduce the curb idle and/or secondary cracking screws, to the point that the mixture screws begin to function correctly.
Or maybe you have already tried that?

AJ, your initial words sound like me, all the numbers, area and diameters. I was a tool maker and now an engineer, so I like getting down to the nats southside. Anyway, tomorrow morning I start goofing with the thing. I am going to do one thing at a time so I don't miss anything. My dizzy has no vacuum advance, if I understood you correctly. I would have liked to gone with a good programmable one, but I have spent my lunch money on this engine for the next 100 years. I will put your thoughts into my list of possible things to do. Thanks.....

 

[AJ/FormS]

I hear you on spending. But you have come too far to let a couple of hundred more stop you from getting a nice tune.
I suggest getting her running, warm it up nice, then put the timing lite on it and start giving her some more idle timing(after plugging the PV circuit).Then take it from there. Once she responds favorably, you can assess the benefit to dollars ratio, and make a decision on the new dizzy purchase.
All the best to you