77403

I rebuilt a 403 for my 77 cutlass ….bored .20 and ported 7A heads 64cc combustion chamber scorpion 1.6 ratio roller rockers Howard hydraulic Roller roller lifters Lunati voodoo roller cam 2042071.. race tec flat tops for a final 10.5 to 1. I think I went to much cam for this torque monster. Their voodoo line has one better for lower rpm’s that’s another conversation. Edelbrock dual plain high rise. And eldelbrock 650 avs2. And that’s where my issue I have question on. I can get a good idle and runs good on primaries when I get on her and secondaries open she falls flat on her face and won’t recover. I have tried metering rods and jets of different sizes. And still just Stalls under load. Sitting still I can open them and no issue. So carb to small? According to calculations 650 should do it. But I just can’t get the stall out to see what this thing is really capable of. I am thinking of going with a sniper fire TBI but then I have to decide if worth changing distributor to allow sniper to control timing. Very frustrated at the moment. Any ideas/suggestions? Thanks in advance

Fun71

Well, the early QuadraJet carbs were 750 cfm and the later ones such as original on your 403 were 800 cfm. So with a much larger cam I would think you need more carb than that 650 cfm.

You left off the last "1" on that cam part number but I found it on Lunati's web site. Here are the specs so folks can give you some advice without havng to search for the cam specs:

https://www.lunatipower.com/voodoo-r...8-272-280.html

Advertised Duration (Int/Exh): 272/280
Duration @ .050 (Int/Exh): 221/229
Gross Valve Lift (Int/Exh): .549/.565
LSA/ICL: 112/106

edited to show later QJets are 800 cfm

77403

77403

thanks for the help on attachments I am new here. I am thinking I agree with the carb is to small and I am getting an air imbalance when the secondaries open. Yes the stock Quadra jet was a 750 on the 350 olds that was in it. Should of tried the 800 eldelbrock. Or found an older quadra jet that didn’t have the manifold choke heater and other accessories not needed.

oldcutlass

What is your timing settings? There is an old saying that most carb problems are timing related.

77403

it’s advanced 6 at idle seems to give me the best idle. I haven’t checked at other rpm’s. Do you have a recommendation for timing settings? Thank you

oldcutlass

I'm assuming an HEI, 6* is not enough especially with your cam. You need closer to 16/18 with vac advance disconnected at 750 RPM. The limit your vac advance to 10* with a mechanical stop and reconnect.

77403

yes that six is with advance disconnected. I will Try more advance. Dumb question but how to I tell where lock off my advance weights

oldcutlass

[QUOTE=77403;1462141Dumb question but how to I tell where lock off my advance weights[/QUOTE]
Your not locking off the advance weights, you're limiting the travel of the vacuum advance pin with a stop.

joe_padavano

Yes the carb is too small. No, that is not the cause of your problem. A properly tuned 650 won't fall flat on it's face like that, it will just slowly run out of steam at high RPMs. That isn't what you described.

Fun71

I agree with Joe, a too small carb would just restrict power and not fall off as described. Something else is amiss.

And as oldcutlass noted, with the GM HEI the initial timing should be set to 16-20 degrees, depending upon the distributor.

The reason for this is the total timing (initial + mechanical advance) should be 32-36 degrees, and most GM HEI distributors have around 14-18 degrees mechanical advance, so the initial needs to be set to a value that gives the appropriate total advance.


I have an HEI from a 77-79 403 and its vacuum advance canister is 24 degrees so I made a stop similar to what oldcutlass posted to change it to 10 degrees.

cutlassefi

Make sure your fuel pump can keep up.
Scratch the idea of using a Qjet. There are more modern, easier to tune carbs available today. Buy a wideband O2 and tune it right.

olds 307 and 403

As said, check your fuel pump output. You probably need more timing as well, especially if you are running a HEI. The later Qjets are 800 cfm and better than early carbs but need idle circuit mods, even the early carbs will need passages enlarged. Unless you are willing to drill passages or send it out, get a Holley style carb in at least 750 cfm. My Qjet is nearly spot on with the AEM wideband. I have a 800 cfm Qjet set up for a very aggressive cam I would part with. Fully rebuilt, never seen gas with Cliff Ruggles premium parts and rebused primary throttle shafts and Cliff's high performance choke pull off.

77403

[QUOTE=oldcutlass;1462147]Your not locking off the advance weights, you're limiting the travel of the vacuum advance pin with a stop.


Thanks for the tips….Can I achieve the same thing with an adjustable vacuum advance that adjusts with an Allen wrench in the canister? Or in your experience you can’t adjust down far enough?

77403

thanks for the good advice I just got the timing tape and adjustable vacuum advance canister hoping I can adjust that down to 10 percent max. In case I ever want to try another cam I can adjust hopefully. Is there a specific rpm to see the max advance with the GM/HEI?

gs72

The allen screw adjustment just adjust at what vacuum it starts coming in. It does not limit how much advance.

olds 307 and 403

Are you sure? My adjustable vacuum advance, the Allen screw adjusts the amount of advance. You want your mechanical advance in by 3000 rpm.

77403

Thanks I will give it a whirl

77403

Thanks I will keep that in mind on the Qjet. My buddy has a new AVS2 800cfm in the box he doesn’t need for his build and said I can try it to see. After I get timing finished up and new dual exhaust on.