Q-jet tuning. Where's the starting line? 5.7L

[rude50]

I'm looking for empirical data / advise on Q-jet tuning for a 5.7L vortec alpha on a 19' searay. Would anyone like to share their Q-jet data on jets, metering rods, air bleed size, mixture / rmp performance?

I recently cupped two intake valves on a "FRUNKENSTEEN" 5.7l (see my post on cupping valves) so i'm looking to get educated and have some fun.

I've got a Q-jet that is a reman from Power basic PN: cpimc30 listed for mcm 5.7l bravo, mei260 ect. that is calling me to the subculture of Q-jet tuning geeks. I was thinking about putting it on my reincarnated 5.7l vortec alpha.

One big question I would like to start with is: "Air bleeds, whats up with them?" The brass insert air bleeds in the main veturies of the reman Q-jet look larger (0.084") than several other Q-jets I've seen on boats (< 0.050"). One far off example is a mcm205 carb with 0.030" bleeds.

 

[Bondo]

Re: Q-jet tuning. Where's the starting line? 5.7L

that is calling me to the subculture of Q-jet tuning geeks.

Ayuh,........

Your going to find that it's a Tiny Little subculture,.................
Maybe Non-existent around Here..........

See My last post on your Other Thread for My responce..........

 

[rodbolt]

Re: Q-jet tuning. Where's the starting line? 5.7L

nothing wrong with Q-jets.
they worked very well on almost every engine GM made.
my 70 w-30 olds came with one and in factory trim could spin the 14" redlines to a mid 13 in the standing quarter.
you can play with primary metering rods and jetting, power valve piston spring tension as well as secondary metering rods and cam.
the secondary air valve tenion can be adjusted as well.
at idle engine vacum holds the power valve down placing the tapered primary rods farther in the jet leaning out the fuel avalible at the venturi and thus making the engine run off the air/fuel passing the idle mixture screws and primary throttle blades. as throttles are opened engine vacum drops and the spring raises the piston allowing more fuel through the primary jets.
as the seconday throttles open and air starts flowing it causes the secondary air valve to open, the cam and rod holder pulls the tapered secondary rods out of the secondary jets allowing fuel to flow on the secondary side.
the Qjet has been about since around 1965 and is probably the widesest used 4bbl carb in history.
its infinatly adjustable but somewhat touchy if you dont understand air pressure and why carbs work to start with.
even then as you start modifying them they can cause stress lines in your face

 

[Bondo]

Re: Q-jet tuning. Where's the starting line? 5.7L

Ayuh,........

I guess the Biggest Advantage to the AFB is the fact that there are No Gaskets below the fuel level to leak on the motor..........

Tuning the Primaries involves 2 screws,.......
2ndaries are abit tougher to get at.......

This is My post from the Tuliped Valve thread.....

Ayuh,............

I guess you could keep throwing Good money after Bad with that Obsolete Rottenchester,........

Or,.......
You could click here,+ see that the AFB is a Much Easier,+ More Tuneable Carb.........

The Tuning Kit is only about $50.,+ is Packed Full of needles, springs,+ jets......

 

[John_S]

Re: Q-jet tuning. Where's the starting line? 5.7L

Here is a post with the info on Merc qjet rods and jets: http://forums.iboats.com/showthread.php?t=189135&highlight=jets+rods

A tuning guide I found very helpful for qjet understanding: http://www.florida4x4.com/tech/quadrajet/QJet_Tuning_Paper.doc

You will need to convert the carb to elec choke set-up, because vortec does not have exhaust crosover and mechanical heated choke provision.

My Q-jet set-up:

Primary jet: 0.069
Primary Rod: 0.042
Sec Rod: CH

PS: I have a bog when the secondaries open up, and will be trying the CL rods and adjustments to spring wind-up.

 

[rude50]

Re: Q-jet tuning. Where's the starting line? 5.7L

Inported john s: Great info. What engine did you don with the Q-jet? If I read the metering rod contour charts right, going from a CH to a CL will decrease the secoundary gas flow starting at about 40 deg. opening thru WOT. Is that what you want for a bog condition? So you have a "rich" bog not a lean hesitation?

 

[John_S]

Re: Q-jet tuning. Where's the starting line? 5.7L

I have a '97 5.7L, that I converted to vortec heads, Edelbrock Performer, and Merc marine roller cam.

The qjet I have was a reman purchased through I5 auto.

The secondaries (on mine) do not start to mechanically open until 3/4 throttle. They do fully open at full throttle.

Yes, I do believe it is either a rich condition or the air door spring adj. It is easy to change the sec rods, so will try that first.

I consider the bog minor. All other aspects of running have been fine.

 

[rude50]

Re: Q-jet tuning. Where's the starting line? 5.7L

Inport :

What size are the brass calibrated air bleeds on your 5.7 Qjet. The reman carb I bought has 0.084 primaries and 0.047 secondaries. Also what boat are you push'n with the 5.7

 

[John_S]

Re: Q-jet tuning. Where's the starting line? 5.7L

I'll have to look on the air bleeds. Probably won't be able to get to it, until this weekend. The boat is a 1997 2050 Bayliner CL, ie a 20' bowrider. It is somewhat narrower and has more deadrise than what is typical of the "euro" style bowriders. Pics: http://www.canadalake.org/OurBoatGallery.html

I believe it is key to which intake manifold you will going with. If you stay with the lower profile performer, something close to what I have should work. If going with a RPM or RPM airgap w/adapter, something more like the 350 mag specs would be in order.

 

[rude50]

Re: Q-jet tuning. Where's the starting line? 5.7L

Inport: Nice boat. Thanks for your time. Your aplication looks very close to mine in size, hull shape and style. I've got a 19' searay sorrento. No pics at this time.

 

[John_S]

Re: Q-jet tuning. Where's the starting line? 5.7L

Well, the weather is not cooperating. Yesterday, was very hot and humid followed by traveling thunderstorms. So far, today, on and off raining.

After checking a parts list, I am not sure of what part you are looking for. I am thinking you want a number off of the venturies. But, if it is something else, that does not require carb disassembly, will try to read it.

http://www.mercruiserparts.com/show_image.asp?ivar=images/CRUISER/816596/13.png