396 tuning advice

[mosaic68]

Alright, I have a 396 with a holley 4150 double pumper, hooker comp headers, flowmaster 40 restrictionless 2 1/2" exhaust, and had the system tuned last September. He found nearly 25% more torque by swapping jets! great!! 316 ft-lbs of torque on the ground.

But! When I drive around town, now, it 'stutters', presumably b/c the primaries are running on the rich side for low end rpm.



My question is, how can I remedy this? Around town and on the highway I'm <3500 rpm, but I would like to keep the tuning improvements.

1 parameter which will change is the exahaust. I'm most likely going to install a pair of bullet perfomance mufflers and H pipe which will add some back pressure. I plan on having it tuned again sometime after the install is finished.

My current idea is to try 1 step hotter plugs for around town. If at the track, I would swap. Are there any other ideas out there? Anybody have experience with this? Hotter plugs the ticket?

[mosaic68]

Talked to a carb fanatic, and according to him, good thing to pay attention to is the amount of vacuum when the engine starts choking - I have a manifold vacuum gage in the car. If the vacuum is to low (smaller number) than the power valve rating, I'll need to replace the power valve with a lower vaccum power valve.

We'll see where this gets me.

The carb junkie also said that I could gain a few horses by removing the choke, and thought it might benefit me to replace the secondary blocks to allow for metering...

making any sense to anyone out there?

[harshman]

I have a 396 and swapped cams to a comp 292h. I couldn’t get it to idle below 1k rpm and it also had a hesitation. After much research I sent the carb (holly 4150 750) to Mark Clark in Mesa, AZ. He did a ton to it and now it’s a screamer. I’d say go in that direction. Make sure you know what cam is in it and get an idea of how much vacuum you are getting at idle – this will help you. Don’t ask me what for but I do know it will help.

[Neil B]

It sounds like the tuner jetted the carb down last Sept. If it is 'stuttering' under light-load/part-throttle conditions now and it wasn't doing this before the tune up, it may be too lean on the primary side. Go up 2 jet sizes on the primaries and see if that helps.

If the 'stuttering' is occurring under load (pulling away from a stop light, acceleration, etc.), it may be the PV or the accelerator pump.

[andrewb70]

Just hook up your lap top and adjust the injector pulse width according to what your wideband oxygen sensor is indicating is the A/F ratio.

Oh...wrong forum...LOL

Andrew