LT1 stroker tuning issue - help

[aaronkl84corvett]

I'd love some help here - Normally, i'm a whiz at getting Holleys dialed in but this one is perplexing me.

Basically the engine is a stroked 1970 LT1 - with a 3.875 crank - sorta -
its got dart iron eagle heads that were CNCed to look like 186s -

has the stock LT1 camshaft (i know, alot of cubes and heads for .435 lift)
z28 intake and a 4053 DZ Holley - which is the same carb for both the odl DZ302 and the L78 engines. So other than dialing in the jetting which according to my Innovate O2, i'm on the spot with that -

The engine runs great, clean, plenty strong throughout the entire powerband - with only one problem... off idle stumble....its the classic case of needing more fuel when you open the blades - BUT i've fooled with timing, pump cams, etc... and the closest i get is with a HUGE #40 squirter -

i have a hard time believing it needs more... ignition is locked out at 38 advance - car fires right up, idles clean, etc.

should i keep enlarging the squirter? seems like overkill but when i go down - it gets worse

[earntaz]

The latest issue of Hot Rod (June) has an article with exactly the same issues with a 383 ... check it out ... alot of good info. TAZ

[VintageMusclecar]

Assuming everything else is right, I have never needed more than a .028" squirter and the #218 pump cam (which is stock on the 4053) on a 780.

Make sure your pump arm isn't bent/tweaked which limits travel. The end that rides on the cam sometimes gets bent upwards. If everything's set right, the pump arm should move immediately upon throttle opening. FORGET that old wife's tale about .015" clearance at WOT, that's a myth that needs to go away (along with that crap about picking a power valve that's 1/2 of idle vacuum, but that's a rant for another time).

If that checks out, make sure the ball isn't stuck in the pump well on the bottom of the bowl, and that it doesn't have too much clearance--.~.015" is sufficient.

*EDIT* get rid of that locked out distributor set-up. 16°-18° initial and ~36° by 3000 rpm should work nicely.

[earntaz]

[aaronkl84corvett]

Originally Posted by VintageMusclecar

Assuming everything else is right, I have never needed more than a .028" squirter and the #218 pump cam (which is stock on the 4053) on a 780.

Make sure your pump arm isn't bent/tweaked which limits travel. The end that rides on the cam sometimes gets bent upwards. If everything's set right, the pump arm should move immediately upon throttle opening. FORGET that old wife's tale about .015" clearance at WOT, that's a myth that needs to go away (along with that crap about picking a power valve that's 1/2 of idle vacuum, but that's a rant for another time).

If that checks out, make sure the ball isn't stuck in the pump well on the bottom of the bowl, and that it doesn't have too much clearance--.~.015" is sufficient.

*EDIT* get rid of that locked out distributor set-up. 16°-18° initial and ~36° by 3000 rpm should work nicely.

all of the checks out - and I find the same issue with 2 different carbs -

as for the timing - i've had it everywhere from the stock Z28 curve to locked out at 36- it does very little to solve this issue - with or without vacuum advance.

[seventieshow]

Originally Posted by aaronkl84corvett

all of the checks out - and I find the same issue with 2 different carbs -

as for the timing - i've had it everywhere from the stock Z28 curve to locked out at 36- it does very little to solve this issue - with or without vacuum advance.

If you are sure it's not a carb or timing issue you could spend some time looking for an air leak elsewhere, maybe the intake. I think a .031" is the largest squirter I've ever used. Not crazy about locked timing on a street car with a relatively mild camshaft.

[aaronkl84corvett]

i will say that the idle mixtures on a stock 4053 do little to nothing.

i will look for leaks and see if I can find what gives.