Racing Fuel Systems

Hi all, new member and first post.

Engine is:
10.25-1ish comp 396
Dual plane Weiand intake
Stock exh manifolds
Cam is 226/234@50 on 110 lobe centers.

I've been thinking about trying a Q-jet on my car versus the Holley's I've had on since forever so I asked a few pals if they had any collecting dust they didn't mind me ruining.

I got a 17085586 and took it apart, cleaned it up, had a pump cup to replace petrified one and tried carb on engine.

It ran badly as expected with main dribbling with throttle blades being way to open and bad stumble at low speed with mix screws not really responding.

Took it apart and made a few mods:

Idle tubes were small probably .033 so I drilled to .037
Down restriction was at .046 so I drilled to .052
Mixture screw holes were small probably .065 so I drilled to .093

Tried again and it is better and going the right way.

Float is at 5/16"
Mix screws are at 3 3/4 out
850rpm idle is smoother and vacuum is more steady and not hunting from the dripping mains.

Still pretty close to having main dribble because throttle blades are pretty far up trans slot so a slight opening of blades and it starts dribbling.

Still surging a bit in the lower rpms at cruise.

Hoping to not go too far with drilling but wanting advice on where to go from here.

Because it seems lean still I was thinking idle tubes to .039
down restriction to .055
Perhaps add some idle bypass also to move blades further down trans slots??
Thx,
D

What is your idle bypass measurement?
Have you modified the air lid for access to the APT feature? Use it to try and dial out the lean surge at cruise.

Did you happen to measure your idle air bleeds upper & lower?

1…I like to use the road test to dial in the APT setting.
2…the idle bypass is measured at the baseplate primary bores…it’s the two lowest holes.

The transfer slot position is THE most important adjustment, & the SECOND most important adjustment is ign timing at idle. Both adjustments are linked.
You do these first, & before drilling any holes.
Many do not realise that the IFRs are enlarged to supply more fuel for low speed cruise [ which supply the T slots ], NOT FOR MORE IDLE FUEL. There is always enough fuel for idle....it there wasn't the engine would simply stall!! Do not enlarge the IFRs any more at this stage.


Next step would be idle timing. Looking at the engine size & cam, you will need about 25-35* of timing at idle. Simple to check: engine idling, & starting at about 12* BTDC, turn dist CCW slowly to advance timing. When you get the highest rpm, stop & check what the timing is. That is the best idle timing for THAT combo. A couple of degrees either way is ok. One way of getting this extra timing is to use man vac adv [ MVA ], which GM used until 1968. If the test shows 34* & you had 12* initial, you can use MVA to add the extra 22* at idle. You MUST use an adjustable VA unit because of the lowered vacuum. Turn the Allen Key fully CW.

With more timing, the engine will idle faster [ it is making more HP ]. The t/blades can be closed. The T slot position can now be checked & if more than 0.040" is showing, bypass air should be added to get 040 or less. APT, main jets, rods come LAST.

Congrats on using a QJ, second only to a TQ.

More on MVA, scroll down to post #6.

www.hotrodders.com/forum/vacuum-advanvc ... 47495.html

GTO Geoff is right.

With that you have picked a hard carb to tune for your big block. That carb is a 1985 truck carb , I believe. I have no records for that model. I beleive it would be a single main bleed model ( first picture ).
Can you tell us what size jets , primary rods, and secondary rods are in it.
I believe you will end with enlarging the idle bypass air to around .080 to .100". You might as well measure them now.