750 Edelbrock carb tuning issue

mrmustangman357 · Sep 8, 2007

Hello all:
Here is the setup:
351W
9.8:1 compression
Trick flow TW heads
.530 lift 230 dur @.050 roller cam
performer rpm air gap manifold
edelbrock 1407 750 vacuum secondary carb
36 degrees total timing (~20) with vacuum advance plugged into manifold vacuum to idle better (no pinging or signs of knocking)

I can get the cruise and WOT tuning correct, but what is wierd is that my wideband O2 shows the motor going dead lean (>15:1) around 1500 rpm at light cruise, just off idle. It happens regardless of how I wanna get in the throttle, so I think I ruled out the accelerator pump. I can creep into the throttle and it still happens around 1500 rpm. After reading the edelbrock manual, it suggested using a stiffer step-up spring. I used the stiffest one and the lean spot is still there. The carb runs like a top everywhere else (way better than my holley 4150), but I would like to attack this one bad spot. What do you think?

rbohm · Sep 8, 2007

change the primary metering rods that have a smaller low speed diameter, and maintains the same high speed diameter. if you look at the metering rod, the area you want to change is the thickest part. it sounds like you want to make that part 3 sizes smaller to start. this just takes a little looking through the edelbrock parts book.

10secgoal · Sep 8, 2007

You are going the right route. But it seems you are out of options as far as edelbrock is concerned. I don't think changing to amount of gas will help, as it looks like it;s just a timing issue on the transition. Only thing you can do now is raise the float level to get the mains to flow a little sooner. A holley has numerous power valves for this, and unfortunately edel only has 5.

mrmustangman357 · Sep 8, 2007

I should also mention that the lean spot occurs around 1500 with high idle vacuum (15 inches of Hg), but everywhere else it is fine. I put in the richest rod from the calibration kit I can go without swappping a jet and the lean spot is still there, but then it reads ~12.5 at all other cruising modes. 10sec, how does the process of tuning with the stepup springs go? is there anything else I can do?

mrmustangman357 · Sep 8, 2007

well, now i decided to attack my idle issue: I made some videos to show you guys exactly what is going on while I try to tune it. I went through the entire carb last night to make sure there wasn't any debris in it. The carb has less than 1000 miles mind you and I have had this issue since the motor was put in so I am still in uncharted territory.
The car almost acts like it has a vacuum leak, but i replaced the intake gaskets! the pcv valve isn't leaking either: it holds the same vacuum regardless if it is hooked up or disconnected from the system (and the carb nipple plugged).
Here are the videos I made today to demonstrate. Apologies for the bad audio, it was taken on a camera:
Walkaround:
at Idle:

Idle in gear (someone else in car):
You can find all the other vids of the car (and others) at my youtube account: take a gander at my signature

jikelly · Sep 9, 2007

I had a similar situation on my 625 carter carb. I finally got it out by playing with the choke setting. I think I wound up with it at 2 notches lean, two marks to the left of the center mark.

I would drive the car to see how it felt, adjust it, drive, adjust, etc till I got it feeling right.

mrmustangman357 · Sep 9, 2007

the choke? Mine is a manual choke, and I wired it open.

I checked today with the car in gear and the step up springs aren't letting it bounce around. The rods stay down the entire time.

oh yeah, despite the fact I have HEI, I checked the dwell and it reads 35 at idle

iapexl8r · Sep 9, 2007

Just watched your vids car sounds great. Does it stumble when driving?. What I noticed is your air fuel ratio meter is not a "wide band" unit. (http://store.summitracing.com/egnsearch.asp?N=400065+302787+115&autoview=sku). I would suggest visiting a dyno facility with a wideband a/f meter to verify the "lean" condition on the dyno. You might be able to verify the condition by driving the car @ the 1500 rpm mark for about block then pull a plug and read it. If it is white you have a problem, not white dont worry about it and cruise on. chris

BlwnGazkit · Sep 9, 2007

It still seems like you may have a vacuum leak. Can you plug ALL vacuum lines and hook the gage up and see what it reads? I think you said you had a C4 tranny, is your vacuum modulator leaking maybe?

mrmustangman357 · Sep 9, 2007

when i take off initially, with little throttle input, when it gets in that 1500-1800 rpm window with high vacuum, it stumbles like it is dying, then when i give more throttle input, the gauge comes back on. I plugged the vacuum lines and nothing. i even tried using the olf mechanic trick of propane around the intake snd still nothing. I am starting to think it may just be because of the cam. The vacuum gauge reads rock steady at cruise (15-16" or so) which tells me the valves aren't sticking open or running crazy.

When I do the rear axle this winter, I will be going to a dyno next year. Bring on the bragging rights!

10secgoal · Sep 9, 2007

The propane trick is for the inside of the motor. Fill the motor with propane, and it will tell you in the underside of the intake is leaking. Just start filling the valve cover. But disconnect the pcv if you have one.

jikelly · Sep 10, 2007

You should get an electric choke.

BlwnGazkit · Sep 10, 2007

The choke is only used to enrich the starting mixture of a cold engine. it serves no purpose other than that. Having said that, using a choke will allow you to run a slightly leaner mixture than would otherwise start easily but it doesn't seem to be that he's too rich.

mrmustangman357 · Sep 10, 2007

called the edelbrock tech line and they told me exactly what you said 10sec, they said set the float higher and try different metering rods for the lean spot. I may also try toying with the static idle advance and see if it wants more. Perhaps I can adjust in more vacuum advance in my canister if it wants more. I read another article online that long duration cams can do fine with the timing around 24 degrees, but i have 35 and still lagging? This is really frustrating

jikelly · Sep 11, 2007

Did you solve your issue yet??

I'm having a similar problem with my carter 750 on my rebuilt 351w. I haven't played with it too much, just acknowledged the issue and moved on to setting the clutch. I might get to move my car tonight, get it off the stands and do the base/idle tuning. But probably I won't get too because I'm supposed to cook dinner. We're having fish.

Honest though, you should get an electric choke, it makes life much easier, and take my word for it the choke setting affects part throttle response. You'll get it figured out though. I really like this carb design because of the ease of tuning. I have played with them a lot over the years. remember to try and only make one change at a time when tuning.

I'm going to be moving to EFI though. I'm going to do the megasquirt thing like bottlefed where you use a holley throttle body. That I bet is going to be a tuning challenge. It's gonna be a hell of a lot of fun rigging up the EFI system.

Best of luck!!

sixtyfive · Oct 4, 2007

mrmustangman357 said:
called the edelbrock tech line and they told me exactly what you said 10sec, they said set the float higher and try different metering rods for the lean spot. I may also try toying with the static idle advance and see if it wants more. Perhaps I can adjust in more vacuum advance in my canister if it wants more. I read another article online that long duration cams can do fine with the timing around 24 degrees, but i have 35 and still lagging? This is really frustrating

Did you end up getting yours straightened out?

I know you have an automatic, but is the problem aparent at 1500 in all gears?

I am running the same series carburetor in the 600 cfm form with an electric choke, and I am noticing a dead spot when I punch on on for a second or two and then it goes away and the car runs like a raped ape. It happens right after shifting gears of course.

I also hear a very similar whooshing sound which sounds like a vacuum leak

phast69 · Oct 4, 2007

I'm the same as you guys...getting that dead...no response sometimes when I punch it. But once it kicks it's good to go. As I was telling 10sec in an IM earlier....I've considered throwing this edelbrock carb in the trash so many times it isn't funny. You can get it to run correctly for a lil bit and then it's back to the way it was...PITA. But that should be changing w/ in the next year as I plan on buying one of 10sec's turbo kits for my car and I'm going w/ a Holley.

Good luck guys. I've had tech's look at it, I've messed w/ it, etc....but we're all the same in which we're more familiar w/ Holley's. Edelbrock carbs have a reputation and I've noticed why over the past 3 or so years....

sixtyfive · Oct 4, 2007

I had a mopar guy who has a pair of challengers which got about 800 horsepower each adjust mine, he mentioned that the 600cfm may not be large enough and I should step up to the 750cfm. Edelbrock says otherwise, they suggest the 750 cfm for 408CI engines and higher.

I got him to set my cruise / WOT ( this is what the front screws are, right?) the car ran ALOT better then before, but this was in damn near 90 degree weather, which he said it should be set different since id say the average temp is 70 when I am driving, and now it is back to the same way it was before.

phast69 do you also hear a whooshing vacuum sound??

Do either of you run the edelbrock air/fuel monitor?
http://www.edelbrock.com/automotive_new/mc/carbs_acc/access_thun_perf.shtml

Do I need to change my metering rods and springs??

MRmustangman357 did you change yours yet?? did this help?

phast69 · Oct 4, 2007

65: I don't get the whooshing sound AFAIK...then again I have a sever hearing loss and therefore I may not be hearing it. I do not use the monitor.

mrmustangman357 · Oct 5, 2007

no, I finally surmised there has to be a casting issue in the idle slot, it starts to lean surge at cruise, despite the fact that I have the fattest rod combo I can go for cruise. I think my cruise is still on the idle slot, so that would also support the idea my idle stuff is bad. I'm senfing it to eddybrok at the end of the season

750 Edelbrock carb tuning issue

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