[wr17]

Originally Posted by Z Code 400

In my post I mentioned weak/damaged PP spring. What I meant was too strong a PP spring that is 'fluttering' up and down at idle. You should check this right away by removing the APT plug in the top of the Q-Jet and checking to see where the APT is.

i've tested that and the primary rods are down at idle.
i select the spring by testing it in a core carb hooked to the car, so i'm sure it's right.

Originally Posted by Z Code 400

I also agree that much more data is needed with regards to yur carburetor. I recently had a Q-Jet that was built precisely to one of Cliff's recipe's and it never ran right. I could never find any problems with the carburetor; everything was sized right, but it idled poorly. I replaced the carb with a core, performed the exact same modifications and it ran perfectly.

i've try 3 different carbs (800 and 750 cfm), that works great on other engines, with the same bad iddle

Originally Posted by Z Code 400

Most importantly, you need to adjust the rate of your vacuum advance to be certain that it does not 'flutter' at your idle vacuum reading!

i've test that too with an HEI that had an adjustable vaccum advance

Originally Posted by Z Code 400

I have found that the tight LSA camshafts, in lower to moderate compression engines, respond very well to more timing at idle. I know that many people feel the MVA (manifold vacuum advance) is a waste of time, but I use it most successfully in lower SCR street applications. When you experimented with MVA and had poor results, I believe that this is a direct result of your low idle vaccum. I have seen this condition cause the vacuum advance to 'flutter' at idle and cause eratic, 'rolling' idle issues.
I hope you find this information helpful. If you need jets or metering rods, I have plenty to share.
Vous êtes bienvenu mon ami...Robert

my 462 had 10,3:1 CR, which is not high but can not be considered as low for a street engine
i've test everything from 10° to 20° advance at iddle and past 15° there's no major change.

many thanks for your help Robert

[wr17]

Originally Posted by 67cruiser

WR17, Did you pull the idle tubes and resize them???I believe you never stated jet size and rod size, along with idle tube size, Cliff is exactly right, could use some more details on the carb. idle circuit is probably ware your trouble are.

the carb is a 800cfm that came from my 301 turbo, rebuilt it with Cliff R book and i've follow the third recipe.
idle tube : 0.40
idle down : 0.063
main jet : 0.073
primary rods : 0.044

[Z Code 400]

WR17,

10.30:1 SCR is much to high for that UD profile. That profile is designed for low to medium compression engines. I have used that profile in a few 455's but never with over 9.50:1 SCR.

I think your engine would be much happier with a 744 or something close to that in specification...Robert

[Cliff R]

The biggest problems we see with carbs sent here that will not idle well after rebuilding and recalibrations are:

Distributor springs too light and the advance mechanism too quick, allowing some timing at idle speed to come in, move around, then fall off when a load is placed on the engine.

Manifold vacuum to the distributor without verifying that the canister is fully applied at the vacuum the engine sees at idle speed.

Improper indexing/alignment of the secondary throttle plates.

No clearance between the link that applies the secondaries and the arm that actuates them, keeping them or allowing them to come slightly open at idle speed.

Twisted primary throttle shafts from failing to hold the fast idle parts to remove the screw at the end of the shaft.

Excessive clearance between the primary throttle shaft and the throttle body.

Installation of bronze bushings for the secondary throttle shaft. Yes, we do NOT recomend doing this. The secondary shaft requires some clearance so that the throttle plates can fully seat each time they close.

Twisted secondary throttle shafts, rare, but I see about 1 in 25 or so twisted enough to cause a leak on one side more than the other.

Excessively warped airhorns/poor sealing at the gasket between the airhorn and main body.......Cliff

[Z Code 400]

Cliff,

In general, at what vacuum reading will the PP springs in your kits actuate????

[Z Code 400]

Updates????

[wr17]

just came back from the drag race.
before running, i've set the initial at 14°, ported vaccum (with ported vaccum, the iddle is stronger)
the engine make good power, too much for my 69 firebird conv body and too much for the 10 bolt 8.2 axle : the limited slip were "game over" after the first day with street tires on very bad asphalt (we don't have permanent dragstrip here, we use small aiports that are closed for the week-end). i can burn the dire all the 1/8 long without opening the secondary.!!! impossible to drive !!!!
i've set my governor at 5400 rpm, but before the axle were out of order, the trans doesn't not want to shift automaticaly and the engine saw 6500 rpm with no problem and seem to want more.
next day with the axle out, the trans shifted at 5400 like it should.
i don't know why but i'm sure that someone here will tell me why.

One thing is sure, this 462 will go in my 80 Pace car trans-am for the next year so i can put drag tires and my 69 bird conv will be reconditionned to factory spec with a moderate 400 and will see the sea, sun, beach and cool cruising

[Z Code 400]

Sounds like the engine is definitely making power. I have also noticed that the UD cam specs you are using do not seem to fall off in the upper rpm ranges as some have reported.

Now, what are your plans for a 400???

[rohrt]

When I had my 455 with this HD cam in my 77 GP is was kinda scary how quickly it would climb past 6000rpm. Scary since I still have stock rods. I broke loose the P275/60s power breaking from a dead start and it hit 6500 for a second or two. Jim Buttle talked me into the cam and the 2800 rpm stall it has ran great. I do agree with Cliff that the 110* lobes suck bad for a good idle.

[wr17]

you can hear my engine here :
http://www.youtube.com/watch?v=f-VtuMmt0d8