Q-jet to 1409, lost 200 rpms???

[John_S]

I put on the new 1409 Sunday AM. It runs smoother than ailing Q-jet, but lost 200 rpms on wot. The secondaries are opening all the way, but have not found someone to watch the air door at 50mph. While I haven't tried tuning yet, surprised by the loss. Given this is basicly a stock 350 vortec, surprised it would be this far off. Even though this is a 600cfm carb, and merc used 750cfm on their 350 Mag and LX, the VE formulas say it should be more than enough.

Anyone had a similiar experience?

BTW, the q-jet air door bog is gone, but also the wail. I was kinda liking that sound.

 

[MikDee]

Re: Q-jet to 1409, lost 200 rpms???

Sorry, didn't know the 1409 was a 600CFM carb? It just shows to go ya, a 750CFM Qjet is sometimes better, even though the book says No! Maybe some jetting would help?
Shoot, many years ago, I put a random Qjet on a 75' Chevy Monza 262V8, and it really woke it up, Damn, I guess no one told it the carb was too big ?! I've even had them on V6's without any issues. At one time I had a 780CFM Holley double pumper on a lightly modified 350, (74' Nova SS 4spd) that my cousin (a certified Chevy mechanic) put on before I bought the car for my son, with no issues (but that's supposed to be a Big no, no also, according to the books )

 

[freak007]

Re: Q-jet to 1409, lost 200 rpms???

IIAC the 1409 is the marine version of the AVS... If so then the secondary should be vacuum operated... they will only open as far as the engine requires...

As for what your engine requires... the formula that most calculators use is:

CID x RPM x V.E. / 3456 = CFM

This is an old formula and it works (fairly) well... inputing your information (guessing high on volumetric efficiency) we come up with:

350 x 4800 (?) x .9 / 3456 = 437.5 CFM being sufficient...

Due to upgrades in engine and carb technology since the original formulas inception, we (drag racers and carb tuners) have changed the formula to:

CID x RPM x V.E. / 2820 = CFM OR

350 x 4800 x .9 / 2820 = 536 CFM


Either way you look at it, as long as your carb flows what it is supposed to your carb size is fine... but does it really flow what it is supposed to?? Think about this for a minute:

Four bbl carbs flow is RATED at 1.5hg (20.4 in/h2o). Most flow benches in the motorsports industry measure dry air at low pressure drops, commonly 10 inches of water column (10 in/h2o or .73 in/hg), one bore or venturi at a time. This result is multiplied by a factor to arrive at what that bore would flow at a higher pressure (i.e. 1.5hg or 20.4 inches of water), then that figure is multiplied by four, (in the case of a 4 barrel carburetor) to get the C.F.M. rating. This rating is usually much higher than it actually is.... Besides previously mentioned airflow calculations, multiplying, reading and measuring errors, four bores will not flow four times what one bore will flow when they are close together, as in a 4 barrel carburetor. The bores are actually ?fighting? for the same air. Another factor is, no provision is made for fuel flow - the actual air flow (of a single bore) will be 8% to 16% less ?wet? than dry flow rating simply because fuel takes up space... space that air can no longer flow through... It is very possible (I will go so far as to say LIKELY) that your "600 CFM" carb really flows only ~470 CFM under "real world" conditions...

Confused yet???


The BEST way to size your carb is by using a vacuum guage... I do not know if your boat has a "manifold vacuum port" or not... but if it does you should hook up a vacuum gauge to it and find out what it reads at WOT (under your typical load)... For a heavy vehicle, or one that needs a wide usable RPM range, you should shoot for .6 - 1 in/hg... if you have more than that your carb is too small (or your secondaries aren't opening enough)... less than that and you're over carbed...


If your not confused then I did a better job explaining than I thought :tup:

If you are confused, ask and I will try to help

 

[wca_tim]

Re: Q-jet to 1409, lost 200 rpms???

great post above, and good info. I could follow...

John, while I'm no expert, I've been involved in setting up 5 or 6 of these in the past year and not one was right on out of the box. we could always do just a little better and most were lean by a metering rod number or two... None of them was a direct comparison on a vortec 350 with no other mods though. Good luck on jetting it. I'll be interested to see how the plugs look and hwere you wind up, etc... Best Regards

 

[mtnrat]

Re: Q-jet to 1409, lost 200 rpms???

To me the bottom line is the advise of Dennis Moore in Small Block Chevy Marine Performance, The Q jet is the best SBC Marine carburetor. Not an exact quote but the guru spoketh. Ok I will quote "The Quadrajet is by far the best carburetor of the three different brands for overall performance on stock and modified marine engines up to 475 horsepower." Page 159.

 

[freak007]

Re: Q-jet to 1409, lost 200 rpms???

To me the bottom line is the advise of Dennis Moore in Small Block Chevy Marine Performance, The Q jet is the best SBC Marine carburetor. Not an exact quote but the guru spoketh. Ok I will quote "The Quadrajet is by far the best carburetor of the three different brands for overall performance on stock and modified marine engines up to 475 horsepower." Page 159.

THANK YOU!!!

If you can power a 6000 RPM 496 that pushes a 3600 pound car deep into the 11's on a "Quadra-Junk" then these marine engines shouldn't be an issue at all


JMHO

 

[John_S]

Re: Q-jet to 1409, lost 200 rpms???

I will be headed back to lake within a couple weeks. I will be taking more equipment with me this time, including a vacuum gauge. I installed a port on the intake when I converted to vortec heads.

FWIW, Merc service manual lists a 5.7L 4brl, which uses a 600cfm weber @ 235hp, and the LX and 350 MAG use a 750cfm @ 250hp.

I will post further results, and might be able to swap the Q-jet back in.

 

[MikDee]

Re: Q-jet to 1409, lost 200 rpms???

Hey John, I was only bustin on ya I think once you run that carb a bit you'll gain your RPM back. The engine just has to get used to it, IMO
Either way, even if you need to jet it some, I think you'll be fine! Unless you have a special preference for the Qjet? I personally prefer the Edelbrook, but I wouldn't go out of my way to get one, unless my Qjet was totally shot.
Good Luck Man! Mike

 

[John_S]

Re: Q-jet to 1409, lost 200 rpms???

My first step will be to verify everything is still "good", and that some other variable didn't creep in to impact wot. Timing, vacuum, compression

2nd, check thet secondaries are physically opening all the way, and air door is not sticking. Can't believe I forgot to check the obvious!

3rd will be to fine tune the idle mixture and speed, which shouldn't effect the wot. Followed by runs with vacuum gauge attached. I will try 2 steps richer, followed by 2 steps leaner. If either doesn't get the rpms back and vacuum seems high for wot, will go back to a larger carb.

Hopefully, the parts for the Q-jet will be in hand such that I could retest that.

Other reading: http://www.carcraft.com/techarticles/selecting_right_carb/index.html

 

[freak007]

Re: Q-jet to 1409, lost 200 rpms???

My first step will be to verify everything is still "good", and that some other variable didn't creep in to impact wot. Timing, vacuum, compression

2nd, check thet secondaries are physically opening all the way, and air door is not sticking. Can't believe I forgot to check the obvious!

3rd will be to fine tune the idle mixture and speed, which shouldn't effect the wot. Followed by runs with vacuum gauge attached. I will try 2 steps richer, followed by 2 steps leaner. If either doesn't get the rpms back and vacuum seems high for wot, will go back to a larger carb.

Hopefully, the parts for the Q-jet will be in hand such that I could retest that.

Other reading: http://www.carcraft.com/techarticles/selecting_right_carb/index.html

absolutely... but keep in mind that 2 jet sizes might not be enough to get you "right"... but if your tach is sensitive enough you should notice a change of some sort... just keep going up (or down) until you no longer see any improvement...

I am not totally familiar with the AVS, but here is *my* technique for tuning... Keep in mind this is a "generic" technique...

Wire the secondaries shut so you are only making changes to one side at a time... many people believe that the secondaries make the biggest difference, it reality the secondaries are only functioning at ~75% or more throttle... the primaries are always working...

Set your float level - all other adjustments are pointless if your float level is wrong

Start "tuning" with the main system... change your jets (going bigger or smaller as needed) until your WOT RPM maxes out (or in the case of a car max trap speed)...

Then work on your cruise range... there are several ways to get this "right"... many people like to use plug readings... others use a wideband O2 sensor (if you have $3-500 for equipment this is easy)... I personally use wideband, but what works well for me in a pinch (and before my Innovate setup) is adjusting off the way the engine runs. When the carb set up with the correct metering for cruise & idle, there will be a slight hesitation upon acceleration if the accelerator pump is disconnected. I disconnect the pump this with the engine running so I don?t have any trouble starting the engine without it... With the pump disconnected and with the engine running in neutral, ?flick? the throttle just a little. If the engine actually feels more responsive with the pump disconnected, your cruise metering area is too rich If you get a severe stumble, or if the engine dies, you?re on the lean side. When the metering is correct you will get a slight hesitation when the pump is disconnected.


Now you are ready to adjust the secondary jets (or rods). Unlock your secondaries, and again adjust the jets (or rods) for max rpm (or in the case of a car max trap speed). Disregard any bogging you experience, all you are worried about is you max power once the engine stabilizes...

Adjust the idle mixture/pilot jet - Set your idle speed as low as you can without stalling (usually ~500 rpm, keep it low so that adjustments have an obvious effect) Hook your vacuum gauge to your manifold vacuum and adjust all your mixture screws evenly until you reach maximum vacuum. If turning the screws in (leaner) has little or no effect you probably need a smaller idle (or pilot) jet, if you have to turn the screws more than 3 turns out (from lightly seated) then you probably need a larger jet... some carbs use an "air jet" for idle instead of a "pilot jet", on "air jet" carb turning the mixture screw in RICHENS the mixture and out leans it... so the above jet size adjustments need to be reversed... as you make your idle mixture adjustments it is likely your idle speed will increase... make sure you idle it back down... once the mixture is perfect, set your idle speed to factory spec. At this time you will also set you timing (initial and confirm correct advance)


Now you can adjust your accelerator pump (if applicable)... make adjustment in both speed and volume to achieve the best throttle response when going from idle to ~1/2 throttle while under load... DO NOT open the throttle far enough to open the secondaries when you are tuning the accelerator pump... all adjustments here are done by "seat of the pants" and trial and error... more of an art than a science...


FINALLY it is time to adjust your secondary opening rate... adjust your secondaries to open as fast as possible without causing a "bog" when going quickly to WOT...


your done... enjoy your newly found power...