4.8 carb tuning help
 

Hoping someone can help me out. I got a 4.8 with a Cam Motion Billet Core 224/228 112+2 , holley dual plane intake, msd box timing set at 0 rpm 15° at 1800 34° down to 29° by 4400 on out. Rev set at 6500. Summit 600 carb with 66 primary 68 secondary. With 700r with 2400 stall. Phew. Thanks that's everything ,now issue. I have got the cruise down pretty good. Usually with hang around 14.0 to 14.6 but anytike you go full throttle it goes really rich and jet size hasn't made much different sluggish to. I have set the floats per instructaions little lower then half way in glass. At a lost do I jet the secondary down more? Any help would be greatly appreciated.

Re: 4.8 carb tuning help
 

What's your idle vacuum?
What power valve do you have?

Re: 4.8 carb tuning help
 

I'm not familiar with the Summit carbs, but I'll help the best I can.

A few questions and suggestions first.
1: Your timing seems kind of low. My cam is slightly more aggressive, but most carbed applications I've seen use more timing. I had my timing set a 38* all in by 1200 rpm and held flat all the way past the rev limit. Is there any particular reason to back the timing back off after 4400rpm?

2: What's your thermostat temperature, and what's the typical operating coolant temp?

3: Vacuum or mechanical secondaries?

4: As geezer mentioned, power valve and idle vacuum numbers are important.

We can move forward once we have this info.

Re: 4.8 carb tuning help
 

When I did vacuum gauge on it it's around 16 but that in neutral as I didn't have anybody to hold foot on break in gear. The power valve is a 6.5 that comes factory with summit carb. From my understanding it's the holley 4010 style carb. Timing was just taking little info from Web and making my own base . I'm open to suggestions. Thanks guys for helping

Re: 4.8 carb tuning help
 

Power valve is ok.
Usually you want it to be about 1/2 what your in gear vac is.

What's your idle rpm?
Can you adjust either mixture screw in all the way without the engine quitting?

Re: 4.8 carb tuning help
 

it idles about 700 to 800 in neutral and 600 in gear.
if i turn idle screws in all the way it will die. they are about a 1 1/4 turn out.

Re: 4.8 carb tuning help
 

Vacuum secondaries. Thermostat 180 and truck stays right around there.

Re: 4.8 carb tuning help
 

Sounds like all is normal.
Should be rich at full throttle when the power valve opens.
Only thing you might need to fine tune is the spring in the vac pot.

Re: 4.8 carb tuning help
 

The afr gauge goes to 10.1 and even lower sometimes and the truck feels like it just falls flat. The old 350 had more get up and go.

Re: 4.8 carb tuning help
 

LIke I said, try a different spring in the vac pot.
What exactly is your timing curve like?
What's initial timing?
How much in the mechanical?
All in by when?

Re: 4.8 carb tuning help
 

At 0 it's 15° at 1000 36° up to 3500 then it ramps down to 32 all the way to 6500. Which way you think I should go for vacuum spring?

Re: 4.8 carb tuning help
 

You'll need to translate your timing into 'gearhead english'!
Or give me a link to how you set it.

As for a spring you'll need to experiment. Go stronger first.

Re: 4.8 carb tuning help
 

Keep the timing at 36* all the way through the rpm range. There's no need to step it down. However, the reduced timing shouldn't be causing a fat A/F ratio. According to the info you've posted above, everything seems to be in good working order.

What's your rear gear ratio?

The secondary springs are color coded. Typically the dark to light colors indicate heavy to light. As fat as the fuel curve is, I'd start with the heaviest(black) spring and see what happens.

I actually find it odd, that a vacuum secondary carb is dumping so much raw fuel in at WOT.

Re: 4.8 carb tuning help
 

The carbed LS engines use a programmable timing box that uses a graph table to set total timing instead of initial and advance settings that adjusted with springs and weights like distributors. That means typical "gearhead English" doesn't apply. lol When I first started learning to set and adjust timing curves on mine, I felt like a....well....you know the old term about a monkey and football.

A little info on it here.--> https://ls1tech.com/forums/carburete...iming-set.html

Re: 4.8 carb tuning help
 

Sweetc10, did you get this sorted out?

Re: 4.8 carb tuning help
 

Sorry for the late response. I have been in class for work last couple weeks haven't had time to mess with it. I did raise the timing and it runs better but still at wot it's a sluggish and goes fat. I will try the soring and see what happens

Re: 4.8 carb tuning help
 

Just to put things in prespective . My 750 on 388 ci sbc is 13 at idle and all in at 33 at 2600 . It is jetted 69Front 69 rear no power valve. The 4.8 is like 284ci I always aim for 12.5 to 12.8 WOT.

Re: 4.8 carb tuning help
 

The afr gauge shows rich but plugs are white

Re: 4.8 carb tuning help
 

I got my vacuum number is 14 at idle in gear. I'm still getting a lean spot just off idle

Re: 4.8 carb tuning help
 

For the off idle try a little bigger squirter and may want to try a accellerator pump cam change.
Jimmy

Re: 4.8 carb tuning help
 

Sounds like to me every thing is working fairly good except wot . It sounds like that engine is not liking the jet selection . You said you tried different jets but no mention what they were and the results .

First thing I would do is disconnect the secondary and go for a ride and see what the af is at wot . You need to tune the primary first . If rich drop 2 sizes each time until you reach your goal . Hook back up the secondary and go for a ride again at wot and see what af is again . If still rich drop 2 sizes until you reach your goal . ( or go up if lean )

As for the stumble off idle , as mentioned above tuning the squirter should cure that . It has nothing to do with the wot .

Power valve sounds like it is fine . If not it would be running real rich at idle .

Also you timing seems fine . It its a common practice to set the timing a little lower in the peak torque range where the extra timing is not needed . The LS msd timing boxes allows for custom tuning .

Re: 4.8 carb tuning help
 

Thanks for advice guys, I will give it a shot and report how it goes. As for jets I have changed to is one size down on primary and two sizes down on secondary.

Re: 4.8 carb tuning help
 

DO let us know how it goes . ALso it take at least 2 jet size difference to see much difference . Just keep moving them until you get your af where you want it .

Re: 4.8 carb tuning help
 

Well I changed the primary jets to 68 and secondary to 66 and so far runs pretty good. Still got a bit of a lean spot In light throttle position. Haven't had much time to do anything else to it.