Please help with carb recipe - 455 GTO

[lrust]

Hi Cliff,

Just bought your book and went through my carb.  Carb info and car setup is below.

Carb number: 17057504  (0357 AYO)

Idle screw hole:  .094 - idle screws were not bottoming out
secondary hangar - K
secondary rods: DA
primary jet: 75
primary rods - tapered:  .025, .040 and .062
fuel seat: .135
idle tube: .0595
idle down channel: .0465
lower idle air bleed: .0785
upper idle air bleed in air horn: .0935
upper idle air bleeds main casting: .0465
accelerator pump discharge holes: .073
main air bleed (main body): .115
main air bleed (air horn): .115
secondary brass tube restriction: .043
holes in throttle plates: .11

Carb has APT
Carb does appear to have idle bypass

I bought it second hand and used it for a year.  It was refurbed by Jet Products as a "stage 3" unit.
My car is 67 GTO, 3800 lbs, with 455 and 670 heads (stock), cast intake with headers, TH400 and ATI 10 inch converter with 3.55s.  Car would not idle below ~ 1000 RPMs with MVA (I know you are not a fan of MVA) and a bunch of initial timing.  Car pulls hard at WOT.  I was not able to get AFR meter working reliably so I can't comment on mixture.

Heads are currently out for porting and should flow ~ 260 ish.

When reading your book I thought I should go with recipe 3 until I found that I had a "large main air bleed" carb, like on page 112.

Need recipe recommendations.....

Thanks in advance for the response as well as the help you have provided on the PY forums.

Lowell

[lrust]

forgot to mention:  SD OF cam with 1.5 rockers.

[Cliff Ruggles]

If your numbers are correct it is a MESS!

I can't believe it could have been used for a year with any level of success, it needs a LOT of changes to get it where it needs to be for that application.

Did you at least put one of our kits in it?  I would have also made recommendations and sent tuning parts to get it straightened out......Cliff

[lrust]

Gulp....I was afraid you were going to say that.

Please give me your recommendations on parts to buy and kits from you and I will start working on it (part numbers if easily available).  I would send to you but I understand how busy you are.

Carb is still apart - it is clean and in good shape - just needs proper tuning. 

Thanks,
Lowell

[Cliff Ruggles]

Lowell, when you get time call the shop and we can go over a few things and get you on the right path to getting the carb up to par for what you are doing.

We're available 8am-4pm, M-F EST, and most Saturdays 8am-noon......Cliff

[lrust]

Sent in the carb.  Have you had a chance to look at it yet?

Thanks,
Lowell

[lrust]

Cliff,

Attached is the chassis dyno sheet from my engine with specs above as well as the QJet that you reworked.  I haven't changed any carb settings.  I has 69 secondary jets.

Engine specs are above except with 1.65 rocker arms.  Initial timing at 10 plus 20 of mechanical advance.  Ambient temp was about 90 degrees and car was at normal operating temperature.

Part throttle driving, my LM1 AFR meter reads about 14.5.  The attached sheet shows AFR of around 15 then transitioning into 13.xx as the power gets steeper.

Overall it seems lean (WOT and normal driving). 

I am interested in your comments on the transition at about 4,500 RPMs - it seems that the opening rate of the secondary choke pull off is too slow (I have not measured yet).  If so, it seems that my WOT jetting is ok if not a little lean, just needs to come in faster? 

I would be interested in your comments on this transition and other jetting recommendations.

Thanks,
Lowell

[Cliff Ruggles]

I wouldn't worry about part throttle A/F as 14.5 is well within range.  It really depends on engine load and how the engine runs.  When an engine isn't happy at light part throttle they surge, lack power, and let you know about it.   I've seen some go well into the 15's at part throttle cruise and still be happy.  The engine combo and distributor tuning play a big role there as well.

The pull-off shouldn't be holding back the secondary airflaps till 4500rpm's.  Compress the pull-off and make sure it is fully open in about 2 seconds or slightly less.  Also make sure the airflaps and the linkage aren't binding anyplace and they open smoothly.

If all that looks fine, I'd try lightening up the spring tension slightly to bring them in a little sooner......Cliff

[lrust]

Cliff,

I checked the diaphram and it closed in about a second and the linkages were all operated freely so I chose not to change anything.  Went to track yesterday for first time.

Launched cleanly in ~70F temps with good track prep.  Car ran best ever of:

60 - 1.615
1/8 7.24 / 95 mph
1/4 11.41 / 117

Second pass - all 5 OEM style lug nuts, trim ring and center cap exploded off the rim towards the wall on the driver side.  Fortunately no body or rim damage.  I replaced the studs and nuts and kept racing.  Got hotter during the day and track prep was worse.  Could only manage two more 11.6 ETs with tire spin.

Thanks again for the support.  I think my combo is doing well so far, need to work on suspension.

Lowell

[Cliff Ruggles]

Yikes, good news that nothing was damaged and that you are running well, very respectable ET and MPH.

Traction is the key for good ET's for sure and you have no control over track prep, I run into that sort of thing all the time.....Cliff