4150 Lacks Throttle Response When Hot, Help Please

[kryptik]

It has a manual choke. The choke, however, is pinned open at the moment.

[makoshark]

I would try 2 things. First, I would use a phenolic carb spacer to see if you are getting heat soak in the fuel bowls. Sounds to me you are. Also, a smaller carb would definitely give you more throttle response. A friends 454 was he built for his mud truck was doing something similar to you. Someone suggested a smaller carb and so he bolted on a Holley 670 Avenger and it cured his woes.

[kryptik]

Have tired a phenolic spacer (two actually) with negligible change. My old carb was a 680cfm, the problem was still present with that carb.

[makoshark]

You have a zz383, right? Are the heads on the motor the zz4 heads? If they are, then there's your problem. Those heads just cannot support the power your trying to achieve in the RPM's you mentioned.

[kryptik]

I agree. My dyno curve can attest to that. Power drops away at 5,400rpm...weak. However, 2500-5400rpm it runs very strong.

[makoshark]

If you have a specific power goal you want to reach, then you will probably have to change the heads and cam. If you just want to optimize what you have and make it run the best it can, then I suggest installing a 650dp and a different cam. What cam are you running now? Believe it or not, the GM Hotcam would run very well with your motor.

[kryptik]

Cam is stock zz383, which I believe is a bit more aggressive than the HOT cam. I'm not ready to start changing parts because I'm saving for an LS build. I simply want to get this combination running properly, which I feel is totally achievable --> When I have the idle set on the money, I can pull almost 20" of vacuum.

[MrQuick]

The issue that I'm having is that as my engine warms up to operating temp (180F), it slowly gets worse and worse throttle response, becomes less crisp, idles rougher, etc. Power-wise it's there, this is primarily a driveability issue. I'm not saying that the engine starts sputtering/hunting, etc; it simply becomes less crisp and "in tune". The strangest thing is that the motor is most crisp after running for about 5 minutes after initial start-up in the morning (T-stat ~ 150F). Then it slowly drops off from there and becomes "lazy".

I've taken it to multiple tuners (some of which I have high confidence in) and they are able to make the power, but this issue just will not go away. I've also used multiple carburetors, all to no avail. This leads me to believe that it may be an external issue, perhaps relating to E10 fuel (or something else?). Here are the carburetors I've tried so far:

New Quickfuel SS-680-VS
New Sean Murphy Induction custom 3310 750cfm VS
New Sean Murphy Induction custom 4150 780cfm DP (using currently w/ 1" Wilson Tapered Spacer)

I've tried different spacers, shielding the fuel line with quality insulation, etc.

Timing is around 16-18 Initial/36-38 Total @3200ish
Not sure on the jets, I can verify if necessary.

My AFR goes from ~14-15.0:1 when it's warming up/crisp to ~12.0:1 (+/- .5) once it is warm (this is under light cruise/part throttle).

I'm a college student without much in the way of time to work on my car, and have been getting more and more discouraged by this problem. It makes the car much less enjoyable to drive. If anyone has any ideas whatsoever, please let me know. I'd really like to keep this carburetor and not have to spring for fuel injection/LS if I can get away with it.

Thanks for the assistance,
Matt

it picks up slow but is fine after 2800?
what have your tuners said about it?
a digital box should get good power. fix it if you can
what distributor?
i agree about the vac leaks. make sure you use a new base gasket each time.
you should know your jets sizes.
float levels?
how are your plugs. sounds like you are super lean up front.
are you getting vac at your advance hose at idle? should be off idle.
are you pulling idle circuit at your secondaries? while idling, stuff a rag in your secondaries. Should maintain idle.
how is your power valve?
your at 180* but have a 150* stat? run a 180* and it should cycle properly. Your stat should be able to shut to allow the radiator to cool the water in it.

If its doggy under 3000 rpm and your truly at 38* then I'd lean towards carb issues. Your digi box voltage should not be an issue or you'd most likely see it at top end.
I'd start by pulling your plugs, then go into the carb tune. if all is fine then go into the carb.
check that stuff and see what you find.