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Q jet swap info

greggome

Active member
Joined
Jul 25, 2014
Messages
38
Location
florida
Corvette
1968 corvette bronze coupe
I am switching out my Holley spreadbore carb for a Q jet on my 68 327/350 and want to be ready to install it without surprises. Not being familiar with the Q jets attachments I have included an attached photo for questions on the pictured rear of the carb.
1. What does the threaded hole at the base of the carb attach to and if not applicable what size plug will fit to plug it?
2. just above that hole is a rectangle square showing 2 screw holes, what attaches to this?
3. there is the brass tube at the top, what attaches here?

Lastly at the fuel inlet on the holley its a male fitting that my rubber fuel line attaches to. The q jet has a threaded female inlet. are there male threaded inlets that can be screwed in the Q jet so i can keep the rubber fuel line set up I have? What size do I need? 830912641_o.jpg
 

Tom Bryant

Well-known member
Administrator
Joined
Nov 9, 2000
Messages
7,238
Location
Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
Is that the correct number carb for your 1968 L79?

That big vacuum port in the rear of the base plate is usually used for power brakes. It is a pipe thread. If you don't have power brakes you can get a brass plug at any auto parts store and probably any decent hardware store. That small diameter vacuum tube coming out the rear of the top of the carb is used to supply vacuum to emissions controls. If you don't need it you can get a rubber vacuum cap at an auto parts store to cap it off.

The rectangular opening in the back of the carb body is for the idle compensator assembly and the holes are for the screws that hold the cover on. That looks to be a re-manufactured carb. I would think that the compensator would be on it.

As for the fuel line, if you are going this far why not buy a correct reproduction line kit from one of the parts houses and get rid of the rubber hose? Paragon and others will have the pre bent line kit plus correct filter, clamps, and fittings to make it look original again.

Tom
 

greggome

Active member
Joined
Jul 25, 2014
Messages
38
Location
florida
Corvette
1968 corvette bronze coupe
Thanks for the info on the Q Jet info. That cleared up some areas of concern before I do this swap. Would not having the cover on the idle compensator be a problem?

At this point I plan on keeping the rubber hose set up to allow me to be able to swap back to the Holley after trying out the Q Jet to see which I like best. Any idea if they sell adapters to screw into the Q jets female fuel inlet that would convert it to a male fitting so a rubber fuel hose can be attached?
 

Tom Bryant

Well-known member
Administrator
Joined
Nov 9, 2000
Messages
7,238
Location
Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
You can get a hose nipple for your size of hose that will thread into the inverted flare fuel inlet nut on the carb. It's a standard part and should be available at the auto parts store.

From enlarging your carb picture it appears that there are parts missing in the idle compensator well. You must have the compensator and cover in place for the carb to idle properly.

Here's something I found to help you understand it's function. This carb should be nearly the same as yours. The compensator is described on the third page.

Quadrajet


Tom
 

SVO

Well-known member
Joined
Jan 11, 2011
Messages
332
Location
Texas Gulf Coast
Corvette
69 L68 Vert, 72 LS-5 Cpe, 72 LT-1 Air 'Vert
Tom,
I don't think that all models of the Q-jet used a hot idle compensator. The HIC was a bi-metal piece that slipped into the well on the backside of the bowl casting. There was a circular area in the well which was drilled with a opening, which required a small circular cork gasket to seal with the IHC. If there isn't a drilled opening in the IHC well, then the carb doesn't require the IHC. I have seen O-jet breakout diagrams that notes the "IHC assembly and cover, if required".
 

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