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Old 05-06-13, 04:07PM   #1
jeffb
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Default vacuum break assembly/Q-jet?

Hello, this may be an interesting topic. I have an old modified Q-jet from Brad Urban's carburetor shop. It has all of the non ported vacuum ports (ports above throttle plate) plugged and epoxied. I bought a new Mallory distributor with vacuum advance and a Summit CD ignition box for street/strip driving purposes. The only place I can tee into a non ported vacuum source is at the vacuum brake assembly (which connects to the rear butterfly), which controls the premature opening of the rear butterlfy. At idle, with throttle closed, there is a good amount of vacuum here, i didn't measure with vacuum gauge yet. I set my initial timing at 10 degrees. The car will barely idle, the idle is so low. Then I connect the vacuum advance to the vacuum port, it picks the idle right up. But isn't this defeating the purpose of the vacuum advance, since it is already advancing the timing while the car isn't even moving? The car starts good hot, runs like hell at full throttle, and doesn't ping, but the thing just doesn't sound good idling, especially in gear. I have a rebuilt 4160 Holley I want to put on, because that has the vacuum port for vacuum advance. Does this Holley port have vacuum at idle? I want to prove my new ignition, which has a slight miss while cruising 35-40 mph ( I believe may be plug wires ~ only 2000 miles on them, or the MSD used coil). I want to correct this before I throw another tuning problem in the mix with the rebuilt carb. Any ideas??
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68 Firebird, 455, forged rods/pistons, 1971 #96 unported but polished heads, Holley Street Dominator intake, Q-jet for now, 1 7/8" headers, Ultradyne hydraulic cam, .231-.239@.050, .538/.558 lift with 1.65 Comp Cams roller rockers, TH350 trans, 2800 stall converter, 3.42 gears.
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Old 05-06-13, 08:36PM   #2
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Sounds like that 10 degrees Initial isn't enough. And you need more timing. So yeah when you plug in the vacum for the disb it advances and the idle picks up. Sounds like it just need more timing at idle. Nothing wrong with that. Leave the vacum line plug in and see if it pings. What is the total timing set at.
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Old 05-07-13, 09:24AM   #3
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Gach- Hello, I have to check total timing, I was going to do that and the wife ran out of the house screaming to stop revving the car, I will do it when she goes somewhere, she gets very "excited" when I leave the car idling in the driveway. So I guess it is normal to have vacuum there and for the idle to pick up. Thanks, Jeff
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68 Firebird, 455, forged rods/pistons, 1971 #96 unported but polished heads, Holley Street Dominator intake, Q-jet for now, 1 7/8" headers, Ultradyne hydraulic cam, .231-.239@.050, .538/.558 lift with 1.65 Comp Cams roller rockers, TH350 trans, 2800 stall converter, 3.42 gears.
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Old 05-08-13, 11:08PM   #4
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It depends on what type of vacuum advance can is on the distributor too.
Does Mallory say to put it on full time manifold vacuum ( taken from below the carb throttle blades )or to put it on a ported source of vacuum (taken from above the throttle blades)?
Ported vacuum spark advance will react gradually as the throttle opening is increased until the centrifugal advance takes over.
Sometimes engines came with ported thermal vacuum swtches that would put the distributor on full manifold vac if the coolan temps got too hot.
Usually on AC equipped cars in stop and go traffic traffic.
That and a Hot idle compensator valve on some stock carbs.
It helps the engine cooling in those situations.
The vacuum advance can on the Mallory may be adjustable too.
Check the tech lit that came with the unit.
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Old 05-09-13, 09:24AM   #5
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Sprint- It says to put it on ported source, I am not sure if it is adjustable or not, thanks for the reply.
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Old 05-10-13, 04:00AM   #6
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You will have to re-establish one of the "ported " vacuum ports on the carburetor.

That will be a port that has it's source opening drilled above the throttle blades.It will have little or no vacuum on it at idle.
That's the connection/source for the vacuum advance on your distributor.
Compare your carb to a stock Q jet to see where the port would be.
Once yo clear the epoxy out of that port you may have to install a new hose nipple in it.
A piece of small dia brake line will work .make sure it's the right OD for the vacuum line to fit snugly over it.
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Old 04-23-15, 11:08AM   #7
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Just asking, so the vacum ports are all epoxied up so drill another right next to the area. Just looked at my q-jet i have at least 3 spots i can drill holes in the base plate. All you need to do is get a old brake line 1/4 o.d. and install it.
Just make sure its below the throttle blads. Better yet just drill out the epoxy
real easy. Hope it helps you out a little. Are you running metering rods & a power piston??? What size idle ????? you running???? Main air bleeds??
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