4150 Lacks Throttle Response When Hot, Help Please

[kryptik]

Hello everyone,

It's been a few months since I've posted/attempted to tackle this issue once again...

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

[Denvervet]

May try a smaller Carb. My 572 came with an 850 cfm.

[Gitter Dun]

What size motor? If you are running a dual plane manifold, they tend to like bigger carbs. I personally like to see my AFR's at around 13.

[astroracer]

Vapor lock? Where/how is your fuel line run?

[John Wright]

Electric fuel pump? maybe the pump is heating the fuel?

[kryptik]

Hey guys, thanks for the responses.

To answer your questions...

1. It is a dual plane manifold (Edelbrock Performer RPM Air-Gap). The motor displaces 383ci; the detailed specs are listed on my profile.

2. Vapor-lock/pre-vapor-lock was definitely a consideration. The fuel line is run along the top of the frame rail (similar to the original steel hardline); it is a braided Teflon -8AN unit with a high quality heat-resistant sleeve around it. We wanted to run the line through the frame, but there was no exit point that we could find on the 2nd Gen. The attempt was to keep the line as low as possible to maintain pumping efficiency on the mechanical pump.

3. The pump is mechanical.

Thanks again guys. Any and all help is welcome.

-Matt

[TheMeat]

I have the EXACT same issue with my 73 Camaro but to a further extent. I actually see vapor lock in the warmer months and my car leaves me stranded. Winter time, I am good to go. I also have the stock fuel line routing with mechanical pump. I ended up wrapping the header tubes for cylinders #2 and #4 as they are closest to the line prior to the pump. This helped quite a bit but still goes into vapor lock.

This spring I plan on doing an electric pump with return and well insulated line and just be done with it.

[sporter]

It looks like you're carb is rich on the primary side under cruise. What kind of air filter do you run? I had an over oiled K&N air filter choke up an 850DP on my 455 and it acted like yours, ran better cold than warmed up. Removed the filter and the car cleaned right up. After I cleaned and "correctly" oiled the K&N, problem went away. The beauty of the Holley based carbs is how easy it is to tune them. Of course, the curse of Holley based carbs is how easy it is to over-tune them. If your air filter is good, I'd try droping down a few jet sizes and upping the air bleeds. Too much pump shot can make for lazy response if you're already a bit rich. Too high of a float level can be an issue too.

[johngross]

Could your intake be developing a vacumn leak as it warms up?

[slowcamaro]

Could your intake be developing a vacumn leak as it warms up?

Kind of my thought. Put a vacuum gauge on it cold when running well and then warm when the problem occurs. Perhaps the gauge will lead you in the correct direction.

[kryptik]

Hey guys, thank you for the replies.

Relating to your questions:

1. I run a K&N oil filter, but I am still on the factory oil (could it still be over-oiled?) The AFR at idle is around 12:1-13:1, that is the leanest I can make it. If I go any leaner to where the motor is happier (14:1), when I'm driving and I push the clutch in, the motor falls beyond its idle speed and stalls. I don't think there is excessive pump shot, it actually needs a bit more I think (because of a hesitation upon cracking the throttle).

2. Intake vacuum is up at almost 20.

Also, do you guys think that a damaged DIGITAL ignition box (or the fact that it is not wired DIRECT to the battery) can cause these issues when the box is heated up?

Thanks again for the help. Cheers,

Matt

[campindog]

Can you try a different coil? Maybe it's getting close to failing.

[kryptik]

Brand new coil...car makes very good power and runs well past 3000rpm.

[anthonys 69]

If im correct your carb is not running a chock. Wright?. And you initial timing is with vacuum line disconnected correct?. Your engine set looks pretty good a little rich but not bad. So im going to through so ideas at you. .is your gas tank vented?. Is your exhaust building up back pressure? Are you running a stall converter? Is your trans fluid getting to hot?

[kryptik]

Anthony:

1.The choke is pinned open...so essentially no choke.

2. Initial timing is with the vacuum line disconnected yes.

3. Gas tank has a non-factory vented cap.

4. Exhaust should have minimal backpressure (2.5" straight through magnaflow kit w/ x-pipe)

5. Transmission is a TKO600.

Thanks

[anthonys 69]

Hook up a hand held vacuum pump and check the operation of the diaphragm in vacuum advance . I’m Very curies if it’s ok.. see if it hold vacuum or looses vacuum.

[MonzaRacer]

I have a question, do you have room to run a plain old HEI? not MSD box?
If so get an advance sprig kit, install the lightest springs on stock weight at first, make sure vac advance is good.
Install at same 16- 18 initial, set basic idle speed/mixture, then hook up vac advance, reset all settings.
Check idle AFR. IF it wont lean out you either need to plug off and redrill the idle mixture restrictions on metering block, OR increase the idle air bleeds a tad,, may only need few thousandths but that tuning. Generally Holleys are close but idle circuit runs rich.
Im curious just whats being missed hot.
Either something is breaking down or others just havent found issue yet. It appears to be heat related. The clean cold idle but rich hot just seems to make it heat related.
Another question is what spark plugs are you running brand and part number. Have my theories.

[Notalent]

Is it a 110GPH fuel Pump?
Are you running a regulator?
Do you have a fuel pressure gauge? Whats it show for fuel pressure?

[kryptik]

Pump is a 120GPH carter super street. No regulator. Last time I checked pressure is around 5.5 idle. Drops a tad when doing a WOT pull on the Dyno.

[Madjack]

TRY A MANUAL CHOKE! could be fouling a plug or two on start up! my LS6 IS GOOD FOR THAT, it has a780 on it too w/75cc squiter on primary!

[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.