1968 quadrajet question

[Roald P.]

Hello

I have been trying to find out why my new "remanufactured" quadrajet causes my engine to 'knock' (68' 327 cui, auto)

I think its lean, a buddy of mine tore it down and found the following:

prim. jets .068
rods .033
idle tubes .038
down ch. .052
lower air bl. .060
upper air bl. .090

Sec. rods CH/.055
POE well restr. .030
POE restr. .040
tube restr. .023
M hanger

the lower idle air bleeds: .062
the upper idle air bleeds: .090

main air bleeds in airhorn: .052
main air bleeds in body: .114

No APT system

What shoud it be for a correct 1968 qjet specified for a 300 HP engine with auto and air?

Thanks,

Roald

[Edward J.]

Re: 1968 quadrajet question

Roald, possiably a timing issue, advanced to far, maybe the check the vaccum advance operation,and the mechanical advance operation ,engine temp.- seem okay? a vaccum leak will lean out the mixture, but there should be a tell-tell sign, like the idle maybe rough.

How does the car run and idle? other than the pinging.

[Roald P.]

Re: 1968 quadrajet question

Roald, possiably a timing issue, advanced to far, maybe the check the vaccum advance operation,and the mechanical advance operation ,engine temp.- seem okay? a vaccum leak will lean out the mixture, but there should be a tell-tell sign, like the idle maybe rough.

How does the car run and idle? other than the pinging.

it runs fine, idles good, no vacuum leaks, timing set very conservative at max total 30 degree's and vacuum advance disabled, still knock... I changed the carb to a Holley that was setup for a 350 cui and all pinging/knocks problems went away, but I would rather use the qjet for the original look

[Clem Z.]

Re: 1968 quadrajet question

check to see if the EGR valve is working because with out the exhaust gas to cool the intake charge it will ping..

[Terry M.]

Re: 1968 quadrajet question

Clem, Unless somkeone has made some changes there is no EGR on a 1968 -- thank goodness.

[Clem Z.]

Re: 1968 quadrajet question

Hello

I have been trying to find out why my new "remanufactured" quadrajet causes my engine to 'knock' (68' 327 cui, auto)

I think its lean, a buddy of mine tore it down and found the following:

prim. jets .068
rods .033
idle tubes .038
down ch. .052
lower air bl. .060
upper air bl. .090

Sec. rods CH/.055
POE well restr. .030
POE restr. .040
tube restr. .023
M hanger

the lower idle air bleeds: .062
the upper idle air bleeds: .090

main air bleeds in airhorn: .052
main air bleeds in body: .114

No APT system

What shoud it be for a correct 1968 qjet specified for a 300 HP engine with auto and air?

Thanks,

Roald

the 68 327 corvette carb #7028208 should have #71 jet,#46 pri rod and a sec rod stamped "AN" with a .070 tip

[Edward J.]

Re: 1968 quadrajet question

Roald, I find that the holley carbs are jetted quite differently, than the Q/jet, depending on the cfm.s of your holley, and the Q/jet may indeed make the difference of a slightly richer mixture with the holley.is the Q-jet carb. a factory carb, for you 68/300 engine?

I dont have any spec.s on the metering of you carb. but there is one member that posts quite often. with spec. on the carbs.as I have rebuild many carbs in my day, from dealership experience. if all the internal spec.s are on such as the float level, metering rods, jetting is the only other thing.

[Roald P.]

Re: 1968 quadrajet question

Roald, I find that the holley carbs are jetted quite differently, than the Q/jet, depending on the cfm.s of your holley, and the Q/jet may indeed make the difference of a slightly richer mixture with the holley.is the Q-jet carb. a factory carb, for you 68/300 engine?

the holley was a 670 street avenger and an old 600 cfm spread bore holley. I tested two holley carbs both worked without knocking and the engine had more power with them.

no, the qjet is a rebuilt from zip corvette - the original is unfortunately long gone

[Jim T.]

Re: 1968 quadrajet question

the holley was a 670 street avenger and an old 600 cfm spread bore holley. I tested two holley carbs both worked without knocking and the engine had more power with them.

no, the qjet is a rebuilt from zip corvette - the original is unfortunately long gone

Roald since 1974 I have been using a Holley 6210 650 spreadbore double pumper on my original owner 68. Just recently put a Holley renew kit in it. I like the positve control of the secondarys with this carb. Best change I have ever made to the car. Great performance on the original L79 327/350HP with M21 and 3:70 rear.

[Edward J.]

Re: 1968 quadrajet question

Roald, sometimes carb.s out of the box are a hit or miss. one carb. number may fit a dozen or more applications. as Clem posted the metering rod sizes to be different than what you posted. there are many good quality rebuilders in the driveline or maybe some of the TSB members know. and in general, they know all the tricks of the metering needed for todays gasoline formulas.

[Roald P.]

Re: 1968 quadrajet question

the 68 327 corvette carb #7028208 should have #71 jet,#46 pri rod and a sec rod stamped "AN" with a .070 tip

Thanks

[Paul H.]

Re: 1968 quadrajet question

Roald since 1974 I have been using a Holley 6210 650 spreadbore double pumper on my original owner 68. Just recently put a Holley renew kit in it. I like the positve control of the secondarys with this carb. Best change I have ever made to the car. Great performance on the original L79 327/350HP with M21 and 3:70 rear.

Did the same with my otherwise stock 68 427/390. Felt like I bolted on 100 horsepower. Great throttle response and great low and mid-range torque. I know many swear by the Q-Jet but there are very few people out there who rebuild them properly.