OK-
I have a perplexing issue that maybe someone has experienced or perhaps can just point out the obvious that I may have missed.

I replaced the top end of my 409 SBC this past winter-
I went from:
GMPP large port (cast) 65cc Vortec heads, stock stamped 1.5 Rockers dual plane GMPP intake, 4165 Spreadbore DP Holley (800 CFM) and 2.5" Ram Horn exhaust manifolds.
To:
Profiler 195, 64cc aluminum heads, Scorpion 1.6 Roller Rockers, Crosswinds (Air Gap style) intake, Quick Fuel SS-830 DP carb and 1.75" Hooker Super Comp headers.

I'm running the same Lunati 60140 mechanical flat tappet cam but it is now 4 degrees retarded to prevent detonation and my static compression is about 10.4 running 93 octane.
I also have the stock distributor (new shaft and bushings) running a Pertronix II igniter and coil with 12 degrees initial advance +10 more with the manifold vacuum advance at idle.

The issue I now have is that I can't get this thing to idle well once it gets really warmed up. When I fire it up and run it for about 15 minutes or so, it runs real nice and I have about 12-13" of vacuum around 950 RPM.
Once it really warms up (temp gauge over 200), the idle starts to deteriorate and slows down to the point where it doesn't want to idle and stalls at every light when I depress the clutch.
I have to pump the gas a few times at that point in order for it to start again so it's not running too rich (I actually reduced the IFRs from .036 to .034 because it originally idled so rich my eyes were burning in the car).

It seems like the under hood temps are so high that perhaps the ethanol in the fuel is evaporating or something, causing the poor idle?
When it's hot, the air blowing from the clutch fan is so hot, it can't put my face in front of it. Perhaps the air going into the motor is just too hot? It's like 200 degrees!
The motor makes a lot more power in this new configuration so I expected more heat to be generated but I just can't explain the crappy idle. I never had these issues with the old set-up.
I've measured the plenum temp on the intake and it's about 155 degrees using my infrared temp gun. My copper 4 row radiator has less than 1000 miles on it and I have a 160 thermostat and high volume water pump.
I'm just running out of ideas on what to try next- The car runs beautifully otherwise at anything above 1500 RPM.
Do I have a fuel issue or a temperature issue or a combination of issues?

I've pretty much ruled out the obvious (IE Fuel Pressure, Ignition). While the engine runs hot, it never overheats to the point of pinging nor do I ever have any coolant belching from the overflow.
Sorry for the rambling... I'm just at a loss...
Thoughts/theories/suggestions?
Thank you for your time!
Elm

 

Is your engine based upon factory 400 block?

 

Retarding the cam probably does not do much for solving any detonation issues, and it makes the engine much less efficient at lower rpm. This will contribute to hot running. You can help the issue by getting a bit more timing into that thing: That cam, especially running that far retarded, is going to require initial timing of at least 18 degrees. If it were me, I'd be running 20 plus another 12 on the vacuum advance to get actual timing at idle up around 30-32 degrees. This will really help cool things down and improve idle quality and idle stability. To do this, you'll have to shorten the advance curve so you don't exceed 36 total (run a 16 - 18 degree centrifugal curve).

The other issue with hot running is the importance of a good fuel return system to keep cool fuel right at the carb inlet at all times - I'm installing return systems on all the engines I build now in order to prevent poor hot-idle characteristics.

Finally, make sure idle mixtures are set and optimized when the engine is hot - you'll find that it will richen up quite a bit on that QuickFuel carb when hot if you set the mixtures with the engine just warm.

Lars

 

Yes, this is a production 400 block .040 over.

My dynamic compression was on the bleeding edge with the cam straight up so I retarded it 4 degrees to bring it to more of an acceptable level for 93 octane. Part of my issue is that my quench right now is kinda lousy (.020 in the hole running a .039 Fel-Pro 1014 head gasket).
I ran out of time to pull the block at the last minute to have it '0' decked (yeah, my mistake not to have is done when I built the short block a number of years back) so I am stuck with that for the near future.

Anyway, the simulation software (not something I live by but it gives you a general idea for comparison) indicated my peak HP and Torque would only be about 3-4 off of what it would be with the cam straight-up and at the time, my high DCR was more of a concern than a couple HP/TQ.

The timing brings up a good point though- I originally tried to run it with 12 advanced at idle with no vacuum advance and it would barely pull 10" at 950. When I added the 10 vacuum, I got close to 13" and it ran much better.
My distributor is curved with 22 degrees max mechanical advance and it's all in by about 2600 RPM. I can probably bump up the initial to 14 to keep the max at 36 and compensate by allowing more vacuum advance at idle (I have a restrictor in place to only allow 10 degrees vacuum currently)?
Would the timing as it stands contribute to the deteriorating idle? That's what is stumping me with this whole thing- Why does it run crappier as it heats up?

I did retain the fuel return on the fuel filter from the original L-46 configuration so I think I'm OK from a vapor-lock perspective.

Idle mixture is another head scratcher-
This thing will only idle with the mixture screws 3/4-7/8 of a turn off seat (4 corner idle). Regardless of the IFRs or air bleeds I've tried, anything below 3/4 or above 7/8 turn will stall the motor (even just by just adjusting one of the 4 out of this range). It's the oddest thing. If it's just warm or 210 degrees hot, that is my total useable range of mixture adjustment.
I've been tempted to throw on an old Holley 4779 750 DP just to see if it behaves the same way but I just can't believe it could be a brand new carb causing all this?

I've also verified no vacuum leaks on the intake or PCV fittings etc.
The 12-13" of vacuum at idle is consistent with both the original and current configurations at idle.

So for now, it looks like I can try bumping the timing some more and see what that gets me-
Please keep the suggestions coming!
Thanks!
Elm