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Super stock crossram jetting for dual AFB chokeless 750s

miller said:
Have to wonder just how much difference, flow and distribution, there is between the two?
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You would have to have an O2 sensor on each primary header pipe to figure that out. I guess we will never really know. I think the old timers did most of the homework for us. What we do with all this information is up to us now. Spread the joy! and the Mopar love.

Stan
 
coloradodave said:
FYI, I was referring the member to the manual to get an idea of the relationship between the jets and metering rods and how changing either one affects the mixture, which also applies to the Carter Super Comp carburetors of old.
I keep seeing references to staggered jetting. The specs given were for WOT for the Chrysler super stock cars. Is that what your ride is going to be used for? If so, the sizes and manifold modifications in the Direct Connection manual will be applicable; however, if the car is street driven, those numbers will have very little relevance. A wide band dual reading A/F ratio gauge with a sensor for each bank if running a crossram will help get the jetting spot on.
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I hope I didn't offend you. You are correct that the manuals and literature will help everyone understand the theory of this type of carb and is a great place to start for the inexperienced.
 
stage3 said:
in my shop..they start down..I crack open the throttle,,they go up..without any movement of the throttle blades..they go back down on there own..is this correct?.thanks..to lean it out some,,75-65 rods?without changing jets....
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That's what we did to save time as long as the new rods are giving or taking away fuel in the right RPM range where you might be having issues. If it stumbles right off the bat you can move the accerator pump rod towards the plunger. This will give more fuel on initial throttle. If you move the rod down a hole, away from the pump it gives less initial fuel when accelerating from a dead stop. The rods, jets, and accelerator pump all affect the total outcome.

Stan
 
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prostockstan said:
I like to get to the point where it pops lean and go up larger with the primary jet or smaller with the rod. The larger diameter of the rod is the low rpm diameter of .071 the small diameter of the rod is the high rpm end of the rod. The high and low numbers of the rods switch around so see what rpm is the issue and tune the rod according. Smaller diameter rods flow more fuel around the rod. The larger the rod end the leaner . Remember the rod goes down into the primary jet and fuel is flowing around the rod ends.
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Unless im missing something, I don't agree with your rod theroy. At low vacuum (wot) the rod is up with the small diameter blocking the jet. At lighter throttle (depending on engine vacuum and spring selection) will determine what portion of the rod blocks the jet. Rpm does not control rod movement. Rod movement does not control fuel curve based on rpm.
Doug
 
prostockstan said:
I hope I didn't offend you. You are correct that the manuals and literature will help everyone understand the theory of this type of carb and is a great place to start for the inexperienced.
Click to expand...

Not offended at all, Stan. We're all here to give advice when we have the knowledge and to receive advice when we need it. BTW, all my experience is with a port matched factory crossram and ported factory max wedge heads. I ran 750 AFBs first and then Edelbrock 500 cfm carbs later. I now run two FiTech throttle bodies and do my tuning on a handheld device...
 
dvw said:
Unless im missing something, I don't agree with your rod theroy. At low vacuum (wot) the rod is up with the small diameter blocking the jet. At lighter throttle (depending on engine vacuum and spring selection) will determine what portion of the rod blocks the jet. Rpm does not control rod movement. Rod movement does not control fuel curve based on rpm.
Doug
Click to expand...
dvw said:
Unless im missing something, I don't agree with your rod theroy. At low vacuum (wot) the rod is up with the small diameter blocking the jet. At lighter throttle (depending on engine vacuum and spring selection) will determine what portion of the rod blocks the jet. Rpm does not control rod movement. Rod movement does not control fuel curve based on rpm.
Doug
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Vacuum is the only thing that controls the rods. I only meant that we we had a stumble at 4000-4500 we went with a different rod which gave us more fuel at that rpm range.
Stan
 
I am really interested in all that I have read here.One question came up.I was just thinking of the Altitude of where you are, and the difference that might be as far as tuning in reference to density altitude in your location ? The altitude of Lancaster Pa is around 360ft.Where I'm at in Nevada it's 2500ft. So would the size of the jets and metering rods be different in each location according to atmospheric pressure ?
 
Darter6 said:
I am really interested in all that I have read here.One question came up.I was just thinking of the Altitude of where you are, and the difference that might be as far as tuning in reference to density altitude in your location ? The altitude of Lancaster Pa is around 360ft.Where I'm at in Nevada it's 2500ft. So would the size of the jets and metering rods be different in each location according to atmospheric pressure ?
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2200 ft is barely enough to worry about, maybe one step up which is about 4%. The distribution in these manifolds is nothing to write home about anyway. (look at the jet spread to start, wow). Make the seat of the pants feel good and enjoy. The max wedge I alluded to previously is based in Phoenix. Stock factory jetted 3705's. Drives excellent and gets 14.5 mph highway.
Doug
 
Good , my car came from Pittsburgh Pa, altitude around 800.It came with paperwork of all the stuff done to it.Jetting was one of the papers.
 
set it up per stage 2 specs,,.104 primarys...089-.071 secondarys..supposed to be .069's but my jet assortment was missing that 1...65-52 rods with orange springs.original 3705 carters..runs good with the new porcelain plug wires..finally running on all 8...also..the pinging/detonation I had with the .101's and the wrong metering rod springs is gone....edelbrock says in there guideline that jetting motors up and spring tuning can eliminate a lot of detonation...have to keep the idle high though..cam does not like idling and to put in gear below 1500 rpm's.will slow and die/foul the motor...duration is 320-330....
 
have to keep the idle high though..cam does not like idling and to put in gear below 1500 rpm's.will slow and die/foul the motor...duration is 320-330....[/QUOTE]
Check for a vacuum leak, it should idle. When warm my racecar motor will idle at 1000, 2 Eddy 750's. Cam is 285/[email protected]/.800"
Doug
 
dvw said:
have to keep the idle high though..cam does not like idling and to put in gear below 1500 rpm's.will slow and die/foul the motor...duration is 320-330....
Click to expand...
Check for a vacuum leak, it should idle. When warm my racecar motor will idle at 1000, 2 Eddy 750's. Cam is 285/[email protected]/.800"
Doug[/QUOTE]
that's what I thought also..pro stock cars idle,,there r so many points for a potential vacuum leak..bolts,bolt covers.etc..
 
Time to take the unlit propane torch and go over everything, carb mountings, around perimeter of casting etc just to double check. I chased a weird idle prob for several months. The intake casting was really porous looking all over but especially underneath. So i made block off plates for carbs and intake openings and pressurized the intake sure enough, I had some air bubbles on the valley pan side...it was a bad casting. I had that area tig welded. Once I got it back, I painted the entire bottom with glyptal paint. Reinstalled it and low and behold it ran sooooo much better, idled better etc. Not saying this is your problem, but it was something that happened to me.
 
^^^^^^I agree. I have heard about that more than once.
 
100_0417.jpg
i put together this "cheat" sheet for jet and metering rod changes. i figured the area of all jets and rods and add or subtract the areas to figure the percentage of change. i believe figuring percentages is more accurate than just pulling parts out of a box. i know the photo isn't great but this way i always know what i'm doing.
 
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