Chasing lean condition at WOT

[baxternick]

So my quadrajet has been great for normal every day driving around town. The engine is a healthy Cadillac 500 9-1. I installed a wideband gauge to monitor A/F ratio a few weeks back.  General driving around town is between 12.8-13.2.  Even moderate acceleration never is much above 13.3.  The issue I have is when I open up the throttle the ratio quickly climbs as I approach my shift point of 5200 rpm.  It is above 14.2 at times.  I have been trying to tune it out with different seconday rod and hanger combos.  All with crap results. 

The current setup is 75 Primary Jets, 50M primary rods, M hanger, and AX Rods. I tried a different hanger (G) and the car would have a huge bog even when secondary doors were tight.  I then tried the G hanger and some DA rods and all that did was cause a smaller bog and a VERY LEAN ratio.  I had to stop the pull at 4000 rpm. Am I wrong working on the secondary side?  Should I try some larger jets first on the primary side?  I was going to try a 76 jet with my 50m rods, but I didn't want to hurt current fuel MPG at cruise if I can help it.   

Thanks in advance for any help you provide!

[Frank400]

What is the number on your Quadrajet and what was done to it ?  What mods have you done to the 500 (camshaft, intake, etc) .

    The numbers you are stating regarding around town cruising seem kind of rich to me.

 

[Marx3]

Sounds rich to me too.
75 jets and 50 rods sound like the calibration of a large MAB's Chevy unit, but the AFR you see at city driving suggests otherwise. 
If I were you, I would get that sorted first.
 Know a 500 engine will need a lot of fuel at wot.
Did my buddy's Eldo once, and I couldn't believe it kept pinging, no matter how high I bent the hanger arms. It had gotten DA's too.

[77cruiser]

It's probably running out of fuel at higher rpm. What size fuel inlet seat? How much fuel pressure?

[baxternick]

The Carb Number is 17059206.  It is a larger MAB unit built by Jet Performance.  The engine is a 71 Model 500 30 over.  The cam is a MTS 15 Cam with Duration @ .050” range: 234, and Advertised duration range: 286-295.  I have a Edlebrock performer intake, and MTS shaft roller tip rockers. The car is a 54 Chevy that comes in at 3400lbs without me in it. The compression is 9-1.

 The car was having horrible bog issues over the summer which ended up being my fuel system starving for fuel with stock pump.  I installed a ROBBMC pump, and 8an line all the way to the tank.  Tank has 1/2" pickup as well.  That cured the problem for the most part.  I then decided to install a Innovate A/F gauge just to help with tuning.  I was alarmed to see the A/F climbing above 14 when the rpm climbed.  I actually kept a close eye on it today while driving to work and I was wrong in my cruise A/F ratios.  They are actually 13.2-13.5ish.  Once I crack it open a little to accelerate mildly they drop to 12.8-13.2.  Again these are 50M rods, so they have the larger tips.

  Fuel pressure seems to hover 4.5-4.8lbs around 4500RPM.  My needle and seat are .132" according to Jet when I asked them. The car starts and idles beautifully.  It has been a pleasure to drive on my daily commute.  Just alarming to see it leaning out when I do the occasional full throttle pull.

I currently have DA, CE, and AX Secondary Rods.  I also have B, G, U, and M hangers.  The factory carb that was cracked on my 500 when I purchased it was a  U Hanger, CE Rods, primary jet was a 74, and primary rod was 47B.

[Marx3]

.118 MAB's, 75 jets and 50M rods sounds exactly like any smog SBC 350 would be calibrated... Your 500 deserves better! Nice setup btw. I remember your thread on cadillacpower.

If I were you, I would at least get a hold of some of Cliff's 44 primary rods. 
Dont think you wil ever get full control over the afr's, with that large mab calibration.
Are you able to drill and tap the MAB's, so you can install screw ( or press-in ) air bleeds, to re-size the MAB's?


I rebuilt the Qjet on my buddy's stock ( pure stock ) 1976 500. I bumped up the timing some and at WOT, the engine would ping like crazy, even with DA rods, untill I got them high enough ( bent the hanger upward ).
Cliff later confirmed, that these huge Caddy engines need ALOT of fuel.
If I recall correctly, I ended up with .070/.070 MAB's and 72 or 73 jets with Cliff's 44 rods plus DA's. With the advance curve made longer ( stock HEI ) and total timing around 30, that big old Eldorado does 0-60 in about 8 seconds. With just at carb and timing tune. ( wouldnt even spin the tires with the stock setup )

[77cruiser]

You might have to open up the seat to .140. Can you up your fuel pressure a bit?

[Frank400]

M rods have the large .036 tip for single main air bleed carbs.  If you have dual .118 air bleeds, you need different primary metering rods, the ones with .026" tip.  That's why you're running lean while under heavy load. 

    Frank.

[Cliff Ruggles]

Correct, "M" series metering rods are a poor choice for those carburetors, and for sure it needs more fuel everyplace.

The "stock" set-up for many of the later large MAB carburetors used 77 main jets and 48 or larger "K" metering rods, and they were pretty lean at part throttle on the pathetically anemic smog engines they were used on.

If I were building one of those carburetors here, it would get smaller MAB's and a completely different calibration, to effectively feed the big 500cid Caddy engine......FWIW.....Cliff

[Jeffs68]

Just to satisfy my curiosity, did you have any drivability issues, or was the A/F ratio the reason you started making changes.

I wouldn't be too concerned with the motor going lean at WOT, how long do you stay at 5K + Revs?

[baxternick]

The engine always had a small bog when I floored it and would go stupid rich no matter what hanger I used when under 3k rpm. Then once I broke the 4K rpm the engine started to lean out bad.  I would see no change in performance once I got through the initial bog.  The engine would pull strong even when that lean at WOT. I was just concerned about engine damage with it that lean.  I just installed 77 jets with 53M rods.  This is close to the same cruise area I had before, but a good amount more at wot on primaries.  This allowed me to tone back secondaries to a M hanger that stopped the bog.  First drive today was good.  Air fuel at cruise is 13.3-13.5.  WOT is now 13.2 at most. 

[Jeffs68]

Nice! Looks like you got the right combination!

[Cliff Ruggles]

Large MAB carburetors never used "M" series metering rods, and stock 350 SBC low compression smog engines with crappy 882 heads on them used 76 or 77 jets and "K" series metering rods with .026" tips......FWIW.......Cliff

[Marx3]

So if this is indeed a large mab unit, then the calibration is just even worse for a big Cadillac 500 !