Hi guys I'm new but have had a couple of blow through setups in the past ill start out with. My setup

This is all in my 74 ford maverick car has a th350 with a 3100 stall and a 3.27 gear ratio in a 8.8



5.3 gm ls
220/220-.575/.575 on a 114
Matching springs
Victor jr intake
Msd 6010 25* initial to 1,200 then drops to 13*
Stock plugs
Cast manifolds 2.25 y pipe into a 70mm with a 1.0 exhaust a.r
46mmm wastegate set to 10psi
Spectre hat 3.5" charge pipe and no intercooler

Carb
850 main body on a 750 base plate (passages in main modded for base) (yes it was ran previously by me)
Downleg boosters
4781 metering blocks (850)
Hi speed air bleeds deleted
.093 pcvr in primary
70 primary jets
88 secondary jets with extensions
6.5" power valve (blowout protector removed)
Nitrophyl floats
1:1 linkage
50cc front pump with brown cam
30cc secondary pump with pink cam
45 primary squirter
35 secondary
Hollow srews
And vent tube extensions on both bowl (flared)
7psi fuel pressure rises 1:1 with boost (a1000 pump)

I've tried every squirter and pump cam combo known to man swapped 30 and 50cc pumps raised and lowered fuel pressure and bowl levels swapped power valves and can't get rid of this stumble it happens at 1500 or so rpm and follows through till if is in full boost (2800) it free revs clean and cruises decent above the stumble rpm. Wot pulls clean with a afr of 11.7 that is rock steady and one in the stumble goes to 18.1+ and will cough through the carb. I'm about at my witts end here with it. That why I came to you guys I feel its a transition issue and. Wanted some opinions.


I'm ready to put the hose back on itGRRRR

 

Does it do this when you stab the throttle?
Does it do this when you slowly roll into the throttle and when you reach part throttle it goes dead lean?
What do the plugs look like?
At 1200 rpm your pulling timing down to 13 deg. even before boost?

 

Yes it' will make a quick pop through the carb with some light hesitation then the a/f goes rock solid with boost I've never brought it over the hesitation point and hit it.

Yes it is horrible when you slowly roll in the throttle like you would drive your daily

Plugs are a dark tan as id expect with the in boost a/f ratio the heat range is a little hot as shown by the strap.

You can feel it drop power in the stumble

And yes the timing tapers off probly a little fast



















stumblde

 

for the life of me i can't understand why guys try to run timing like this. i highly doubt a factory injection system kills timing like that for driveability. that 70 jet is also way to small when used with a down leg booster. try a 78-80..

 

I just copied someone elses curve. I ran it previously at 30* when it was n/a
I'm still learning these ls based motors and what they like


Jetting up or down doesn't change the pop or stumble but changes my wot a/f dramaticly
N/a I ran a 72 with the stock pcvr channel

 

I'm thinking timing as well .. up it a bit and you will probably need the extra jet to bring the afr back in check.
My thinking is that is way low at that rpm.

 

Yep , im agreeing on the timing too. I wouldnt pull timing until your well into boost. Even then you shouldnt need less than 17-20 at 10 psi.

Your running this on 93 oct. correct?

 

I'll try changing it tonight

And yes on garbage 10% ethanol 93

 

ok guys i changed the curve some with a good improvement not 100% yet still leans out and coughs but the rpm moved up this time

25* to 2,500 then steps down to 16* by 3,000
adding 15* till it hits about 12 " of vac then it tapers to 10* then straight 0* added with boost
it idles at 38.2* seems to be happy there as it revs super crisp (but getting some rattle at cruise probly going to bring this down to around 34*)


now when it stumbles i am showing it around 2,400 with 1-2 psi in the hat (no gauge on intake side) and if i stay part throttle (20-50%) opening it will taper lean then pop through the carb immediate flooring and it has no stumble i can build the full 10 psi on the foot brake in 2nd and 3rd with no stutter (just a temporary lean from 2,400-2,800) then it goes 11.3

it is cruising at 13.2 from 1,400-till the stumble idle is 13.8-14.2 swing car seems happy at the afr's still 11.7 in boost to the rev limiter

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I did a turbo setup that kept doing what yours is doing in regards to the part throttle lean issue.
Annular booster fixed the problem. No other changes were made to the carb. Has something to do with the throttle bladesl being partially open so to much air flows on the outside of the booster and it starts to loose signal based on how much air is actually going in. With the larger booster more air goes thru the booster and it has a better signal at part throttle and full throttle. I think i had to jet down a tad for WOT but my main jet stayed the same.

 

If it doenst snow tonight im ging to bring the curve up to 3k then taper it off and see if it goes away its a night and day differance right now

i wonder if maybe opening the idle jet in the metering block would richen this transtion if raising the timing doesnt help

i may have to go to a annular booster sure hate to change them lol

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I've drove it all day and revised the curve better but still leans out, do you guys think changing the pv will help? It has a 6.5 in it and makes 16hg of vaccume

 

I think the problem is the 1-9/16 venturi with skinny little downlegs on a 5.3L... JMHO

 

Put a 1:1 linkage on my carb , and it lowered the fuel curve dramatically . Especially transition into boost .

 

Ill swap the boosters then will report back tommorow

 

Good advice there ...

 

I've already have a 1:1 on mine

 

Swapped the boosters to some straight legs I had and drilled the idle jets on both blocks to .051

Even though the gauge still shows a slight lean out it drives great now I am going to have to lean out my wot some it moved to the high 10's

 

Nice ...

 

Hopefully dropping the secondary jet a coupe won't effect it