The result is that the A/F ratio is quite rich (401 w/crossram and Holley 4160s,

My mechanical engineer friend says that the first thing to do is to enlarge

the main/high speed air bleeds to introduce more air into the system. Then,

he says, I can change the secondary metering plates to a smaller "jet" opening.

He feels that a valve might be burned in the process of dyno-ing the engine if I don't

modify the air bleeds first.

The dyno shop and another carb buddy say no, first thing to do is to change the plates

and that any main/high speed air bleed changes will be inconsequential compared

to the plate changes.

Which direction is "more correct?"  Is there a more effective direction to take here?

See graph below.

Full throttle began @ approx. 3k rpm.

It seems to be running good.

A little rich is preferred to detonation and

worse, back fires.

IMO don't waste your time unless you spend

most of your time at 4000 rpm +

A back fire with a plenum that big

is a scary thing.

the best way to fix this rich at WOT condition is to invest in a wide band o2 gauge and sensor, get a base line then start with the simplest adjustment and re road test and just move forward from there. or take a bunch of carb parts  to the dyno . its best to take baby steps with the adjustments. get a note book and write everything down so you can keep track of all changes.

-chris

But I've only worked on Chryslers way back when, so take that for what it's worth - maybe nothing!

The thing is big and the variables of air pressure, temperature, humidity ... all effect your A/F mixture. Mechanical secondary's may be good one day and terrible the next. Let the engine tell the carburetors what it needs.

There's no vacuum info given (although it was 11" @ idle) and no BSFC that 

I can see, although when the car was being dynoed I vaguely recall that acronym

on the screen.  Yes, there's room for secondary blocks and I've thought about 

going that route.  There is no surging going on and a stumble was fixed.

It's a pity you don't have the BSFC numbers.  That, IMHO, is one of the first things you want to look at as it is an indication of how efficient the engine is.  If it is relatively constant that is a good thing.  If it is all over the place, falling, or rising with rpm, then you have some kind of problem.  A/F ratio is not the entire answer.

The "blocking secondary's" is just stating there is TOO much fuel to burn. That's the O2 sensors connected for the run.

It's a nice curve for HP and Torque, but too low on the RPM scale for this setup to be at peak power.

It's basically very good at pumping enriched fuel.

What was done to stop the "stumble"? 

Guessing that the secondary's were put in early.

Boris is correct, your better off rich.

That stated, my first impression if keeping the manual secondary's, is to enrich the primaries and drive out the secondary's to open at a higher RPM. You want the secondary's to open just before the primary's start to lean out. This is definitely not the case looking at the dyno sheet.

To elaborate - that sharp "U" between 3000 and 3250 RPM, the goal should be to flatten out that "U". You can see the primary's leaning out very quickly at 3250 RPM. 

I'll inquire about the BSFC info.  Thanks for replying everyone.

You might find that quarter mile performance is better with a richer, maybe 11.8 AFR than typical 12.5 WOT target for max power.  but 10.0 is too rich

From what I've gathered, BSFC is measured on an engine dyno.  I'm doing a chassis dyno.

Hmm.  I guess it depends on how old the chassis dyno is and what features it has.  I just did a cursory search on Google and found that BSFC is available on chassis dynos that have a fuel flow input and are configured to calculate BSFC.

The primary jets have nothing to do with idle mixture.

The implication is that the throttle plates are being held open and you need more air for idle. This just adds to AFR complications.

Putting back the original primary jets and adjusting the idle mixture for idle quality would be the first step. If you need more air, cracking the secondary's slightly open achieves this without increasing AFR. They used to drill holes in the primary throttle plates, but cracking open the secondary's has the same effect and can be adjusted. Try this with just a screwdriver first and see if your idle quality improves.

If you can get your idle circuits working the way they should be, the rest is much easier.

As I understand it, it is typical to have to lean out the carburetors on dual applications.

 

https://www.hotrod.com/articles/ccrp-0807-holley-carburetors-basics-guide/" rel="nofollow - https://www.hotrod.com/articles/ccrp-0807-holley-carburetors-basics-guide/

This has the holes drilled into the Primary butterfly's that I did not recommend doing.

PHAT69AMX has more information on specific carburetors, but the basics of setting idle are in the article. Setting the Primary butterflies is crucial to have your idle circuit working properly. As the article also states you should be running your Primary circuit rich, especially on that massive intake. All the Mopar's were set quite rich on the primaries and idle circuits.

After reading the reply above, the newer carburetor may not be an option of cracking open the Secondary's without having fuel flow. Don't know about these.

I like the idea of running without the Secondary's to do an initial tuning, but the Primary butterflies and idle circuit have to be set up before adjusting your Primary jetting. At least on the older 4160's.

Interesting info with the "...very small secondary Main Air Bleeds..."  A buddy who's

Oh, and I just have to bring up... has any adjustment at all been done to the Secondary Idle Speed Screw adjustment ?... Not to my knowledge, as they didn't remove the carbs.

You'll need to drop the primary jets after, say 8 sizes and reinstall the power valve. Ideally the combo of the primary jet size and power valve channel restrictor will be equal to the AFR of the squared jets at WOT. That means more dyno testing, or buy an AFR gauge if you haven't already. Possible to maximize fuel efficiency while cruising and WOT power. After you know your max power with squared jets, if you don't have an AFR gauge, you could do timed runs with the stopwatch from say 3500 to 5500. Find the smallest primary jet with big enough PVCR to match the squared performance and it should consume less gas while cruising.

I think idle and especially transition tuning is way more tedious than WOT tuning, and not nearly as fun.

"465 CFM Universal Off-Road Carburetor".

  What model number Holleys are you using?

It's just like Phat's post above yours:  4160/9776

I still feel that unless you  are running at high

engine speeds all the time, just drive it and enjoy it.

I suggest the issue has to do with that huge plenum

and the poor intake "signal" that comes with it.

Very poor at low engine speeds.

Jetted to be correct at lower engine speeds as engine speeds

increase the signal becomes strong and it goes rich.

I do not have a clue how to tune that issue out.

But I can tell you that having seen hundreds of Harley

and Harley Davidson based engines on the dyno that

power does not really go away till you get to 10:1 fuel air.

They smoke a bit and 13:1~ typically makes the best power.

Pushing the compression up in the 14:1 range makes an engine

more sensitive to being overly rich.

I could say that I am a confirmed Holley hater and toss a pair of 600

Carters on it, but as I see it your rig is making EXCELLENT power

and the issue is well out of the normal driving engine speeds.

As a comparison in big radial airplane engines fuel air ratios

are in "auto rich" settings and are typically in the 10:1 range for take off.

Best max power is in the 12:1 range.

As I see it you have no problem here unless it is running like pooh. 

about the history of anything as "esoteric" as a carburetor, esp. one that one is

using!

last update on this saga:  went back to the dyno facility and i'd already installed

the .067 sec. metering plates. first dyno run showed a near flat curve of the a/f

ratio and this metering plate was the ticket!  a/f went from a 10:1 at its richest

point to an approx. 12.2:1 at highest rpm.  supposedly, i gained 20 hp!  i'm done!

the techs said to ignore the large "humps," as that's when there's supposedly a rush

of air into the sensor area when full throttle is applied.  the fairly "flat" curve was

not able to be duplicated by the same guy after we drilled the plates to .071 for just

a little bit more fuel.

Good to see the improved A/F Ratio results, Thanks !

Very nice!

For the $35, I'd tend to put a 134-9, or what was originally installed, back in without the drilling the from 0.067" to 0.071".

That leaning out to above 16:1 may cause problems with fuel changes.

Regardless, your AFR looks much better now.  Glad you got it sorted out

BB

I wonder what would happen with a little more carburetor also, see what the original design specifications resulted in. I should add I've never seen or worked on an STR11 and certainly could not afford it or the carburetor (paypal) setup. I do have an 1x1850 at home though!

The sec. metering plate was manufactured @ .067.  It was decided to drill to .071 to slightly

enrichen the flow.  Primaries jets were originally .062 and were replaced with .059 jets for a

slightly leaner flow.  Thanks for your feedback.