Need Tuning after dyno run

[leeklm]

Got the Pontiac 455 on the dyno today, and first run with my new 17059272.  I built this carb to specs under "recipe 2" in the book, including the drilling of idle bypass holes, which were not present, and only tested it in the garage on a stock low compression 400 just to be sure nothing major was wrong.  The carb seemed to run well during this minimal quick test.

On the dyno, the idle was a little rough, which could be smoothed out by applying a some choke.  During the pull, the engine was plenty lean and was not smooth.  I attached the dyno sheet if that helps regarding AF ratio, etc.  For comparrison, we picked up about 30hp & 30tq after bolting on a carb the dyno guy had.  Unfortunetly, we were running short on time to do much for adjustments.  Also attached is the dyno run with another qjet for comparisson.

One adjustment I made was to back the APT screw out to the max, which helped some at cruise rpm, but minimal.  Also, seemed like I really had to loosen the secondary flap, otherwise would not open.

Here are my carb & engine specs (all parts bought from Cliff)... Any thoughts or suggestions would be greatly appreciated!

Main Jet = 73
Main Rods = .021 - .039
Sec Rods = CE

462ci w/20" of vacuum
468/489 lift cam w/219 & 227 duration @.050   112 lobe

[Shark Racer]

It looks like the secondary function is the culprit for the leanness; what have you set your air-valve spring tension to?

Going with off the shelf-stuff, it's a big leap to CC secondary rods, but that will certainly richen you up. You might hunt down a couple pairs of rods in the middle.

[Cliff Ruggles]

We don't sell CE metering rods.

What primary metering rods are you using?

Did you modify the stop on the secondary airflap shaft linkage so the airflaps can open further?

I would also install solid secondary fuel nozzles if the factory ones are cross-drilled?

Did the idle mixture screws have any effect?

What size idle tubes, down channel and idle bypass air were used?.....Cliff

[leeklm]

Thank you in advance Cliff for the help!  Here is additional info...

Secondary Rods are a set of Edelbrock CE that I had...  I did open the secondary flap restriction, added a stop screw, and adjusted opening according to book.

Primary rods = 44 (bought from Cliff   Measurements above are what I took from the fat & skinny part of rods

Idle Tubes = .036 (I think I installed these w/o changing when purchased in my kit)

Down Channel = .05

Idle bypass air, which I drilled into the baseplate passages = .098

I was not in the engine room when the tech made an initial adjustment of the idle screws.  It seemed to smooth the idle some, but still needed a little choke to smooth it out.  Unfortunetly, we did not have time to go back and make more adjustments.  At $70/hr, we had to keep things moving   I am tempted to spend another $300 for more fine tuning & experimenting at the dyno, but that also requires another day off of work, which is not always bad... Besides, I would probably get more results spending the $300 with Cliff

The secondary nozzles are cross-drilled.  Can these usually be extracted from another carb w/o damaging, or do you sell new?

Lee

[Cliff Ruggles]

We sell new ones, insert a punch that just fits into them and work them in a circle, they will fall right out....Cliff

[leeklm]

Cliff, I will be calling you for another parts order.  In preparation for that call, what are your thoughts on the above specs?  It feels like i must have something wrong here, as have never had these issues on my previous qjet builds.  (All 2 of them

[73 Z28]

Primary rods = 44 (bought from Cliff   Measurements above are what I took from the fat & skinny part of rods

Idle Tubes = .036 (I think I installed these w/o changing when purchased in my kit)

Down Channel = .05

Idle bypass air, which I drilled into the baseplate passages = .098

Lee


Lee,

From your description of the problem and your calibration info.  It looks like you need to increase the main jet size to 74's and open the idle down channel to .062 and increase the idle tube to .037-.038 Tipping in the choke and getting it to run better indicates your too lean on the primary side. This is quite common in big block engines with mild cams.

Paul

[73 Z28]

Two other things. What size did you drill out the idle mixture screw holes to?  They should be minimum .095 and then you can adjust the idle mixture screws at about 4 1/2 turns out from fully seated. Your apt should be set 3.5 turns up from fully seated to start with. This will give you a good starting point for dialing in the qjet to your big block.

Paul

[leeklm]

Thanks Paul.  I did drill my idle screws out according to the book.  If my memory is correct, .095 sounds about right.

Since I am running lean and down at least 30hp at full throttle, it seems that my primary & seondary flow needs help as well.  At full open throttle, how much of the CFM is coming from the primaries as compared to the secondaries?

As I prepare to do more testing, what would reccomendations be for extra primary and secondary rods to have on hand?

Thanks again!

[73 Z28]

Thanks Paul.  I did drill my idle screws out according to the book.  If my memory is correct, .095 sounds about right.

Since I am running lean and down at least 30hp at full throttle, it seems that my primary & seondary flow needs help as well.  At full open throttle, how much of the CFM is coming from the primaries as compared to the secondaries?

As I prepare to do more testing, what would reccomendations be for extra primary and secondary rods to have on hand?

Thanks again!

Lee,

The big block q-jets I've calibrated, I typically use 74 or 75 main jets and 44k or 45k metering rods for the primary side.
On the secondary side, I prefer CV, AU or CK rods. Typically the qjet flows 250cfm through the primarys and 500cfm through the secondarys, but as GM made 2 (well technically 3) sizes of q-jets. The primarys on the 800cfm q-jets are closer to 300cfm. When calibrating a q-jet for a big block,  the carb is trying to deliver fuel to approx. 50 to 100 more cubic inches of motor so more fuel on the primary side will be needed.

Paul

[leeklm]

Getting closer, i think...  I opened idle down channel from .05 to .06, and confirmed the idle tubes at .036.  Running the car up on jack stands (we still have a foot of snow...) and I am no longer lean.  If anything, it seems to idle a little rich, and not quite as smooth as I would like.  Not bad, just not great.  With the 73 jets and 44 rods, I need to have the APT out only about 1 turn, which then seems to run good at part throttle using the tip-in procedure.  Turning the mixture screws in will shut the motor down, so they seem to be working as they should.  With the holes drilled out to .095, I am running the needles about 4 turns out.

My next jet sizes in the shop were 70, so I tried those, and was able to back the APT screw out about 4 turns for good part throttle running.  It "felt" leaner, but hard to say with no load.

I then proceeded to try another (cliff recipe) qjet that was running like a champ on a stock 400, along with a Holley 750.  Funny thing is, they all idled and ran very similar at part throttle! 

I will wait for spring before I do much more so I can road test to get a better feel on how things are running under load.  Maybe it is closer to right than I think it is?

[Cliff Ruggles]

70 main jets aren't enough for that carburetor number, it will be lean at heavy part throttle....Cliff

[leeklm]

Thanks, I did assume it would be too lean under load with the 70s, but wanted to make a big change to see how it impacted my idle and part throttle testing.

I will go back to the 73s you sold me with the kit for street testing. 

By the way, what float setting do you like for this carb?  My earlier Qjet was about .25, but the build sheet on this one calls for something closer to 17/32 if I recall correctly.

LM

[Cliff Ruggles]

9/32 to 1/4" is best, depending on the actual fuel pressure. 

Many late model carburetors used a lower setting, when the factory used higher pressure fuel pumps and return systems to keep the fuel circulating for improved cooling....Cliff

[leeklm]

While reading the idle circuit chapter again in Cliff's book, I understand the challenge of getting the proper air/fuel mixture and amounts to the engine. Obviously my 455 is a big change from the 301 my donor carb originated...

Although I did add .098 idle bypass air holes in the baseplate, I still seem to struggle with an rich idle condition.  My engine builder used a typical Holley 500 2bbl while running on the test stand, and we were pulling a solid 20" of vacuum.  My current vacuum with either of my qjets or holley 750 is about 16", and not as smooth of an idle (again, not "bad", but not perfect).  Part idle runs excellent, and can be easily adjusted with the APT.

At idle, I can pull the 5/16" PCV line and the idle speed picks up considerably, and idles better!  A vacuum leak this large tells me I need more air at idle, correct?  While closing down the idle mixture screws, I can restrict fuel to the point of killing the engine, but my vacuum reading never picks up.  Again, more air is needed?

From reading the book, it appears that holes in the primary plates are rarely needed when you have bypass air, so I am hesitant to go down that path, although I do have an identical parts carb for new plates... Also, while this engine is at 468 cubes, it has a pretty mild cam and approx 9.3CR, so nothing even close to radical.

I am stuck, what do you guys suggest for a next step to lean out my idle circuit?  Or, am I out in left field here?