Carb vacuum/idle

flipper35 · #1 (**permalink**) 12-26-2002, 12:47 PM

I was checking the vacuum on the 4160 to try to get the highest vacuum reading while adjusting the idle mixture. Turning the idle mixture didn't really change anything, but when I pulled the plug on the vacuum port to put the vacuum guage on, the idle went up 200 - 300 rpm.

I get about 9.75 inches at 1100 rpm idle, which makes me think there may be a vacuum leak.

Cam is 292 duration on both lobes and 106 degree centerline.

The idle is fine, doesn't get hot, but it does seem to load up a bit. I am happy enough with it, and this may be the best it gets with this cam.

I mostly want to know why the idle would go up when the vacuum port is open?

xlr8or · #2 (**permalink**) 12-26-2002, 01:01 PM

I believe it goes up because you are leaning out the mixture by opening the port, there by pulling less fuel in.

Scott

Mr.Fixit · #3 (**permalink**) 12-26-2002, 02:12 PM

You allow it to pull more air by giving it another place for air to sucked in. Engines want to run fast, you slow them down to idle speed by choking off their ability to suck air in.

flipper35 · #4 (**permalink**) 12-26-2002, 02:18 PM

So basically this is normal then? OK.

Mr.Fixit · #5 (**permalink**) 12-26-2002, 02:38 PM

Yes, it is effectively the same as opening the throttle blades a tiny bit more.
You are likely idling too rich as well. That cam might want drilled throttle blades on the primary side.

flipper35 · #6 (**permalink**) 12-26-2002, 02:52 PM

I thought it was a little fat, but turning the idle mixture screws in didn't seem to affect anything, including vacuum.

I think I will wait on the drilling. My cousin is supposed to send another cam for me to try out. More duration, lift, 108 degree separation. I am not sure if it is worth the trouble, though the Mopar dyno says it's worth 50 horse for my engine. The engine only has 8000 miles on it with this cam.

I miss the bottom end from the Carter carb. Maybe the new cam would help. Price is right. Shipping from WI to CA!

Mr.Fixit · #7 (**permalink**) 12-26-2002, 03:51 PM

More cam duration will hurt bottom end torque. Try advancing your cam a few degrees to get the bottom end power you want.

Screwing the idle screws in all the way does nothing to your idle quality?

flipper35 · #8 (**permalink**) 12-26-2002, 04:28 PM

I didn't try all the way in . Only a couple turns. I thought the 108 degree separation would help torque. I guess the increased duration would negate that though.

Mr.Fixit · #9 (**permalink**) 12-26-2002, 04:40 PM

turn them all the way in (gently), then back them out 1.5 rotations each, for a starting point.

Bob In Ct · #10 (**permalink**) 12-26-2002, 06:04 PM

Brent:
Get the book "Super Tuning and Modifying Holley Carburetors". It goes from basic concepts to high end stuff. If you read it you'll be one of the few who really understand accelerator pumps, power valves and the rest.

By the way, letting more air into the manifold will increase the RPM in the same way opening the throttle does, except the resulting mixture will be leaner since the extra air did not go through the carburetor. Also, be careful about adding HP (high RPM) at the expense of low end torque!

Bob

Hotfingrs · #11 (**permalink**) 12-27-2002, 03:02 AM

One other thing to consider...you need to adjust your idle vacuum at under 900 rpms...then you can reset the idle to 1100 rpms if that is where it needs to be....At 1100 rpms you started to uncover the transfer slots, and won't get a true idle adjustment

One other thing...when you pull the vacuum line....if the idle goes down, the carb is set too lean, and if the idle goes up more than 50 rpms or so, it's set to rich. There should be very little change when you pull the line....

flipper35 · #12 (**permalink**) 12-27-2002, 12:42 PM

It won't idle for very long at 900 rpm. I was looking at the Carter carb and I think the bottom end was better because of the small primaries, does that make sense?

My HP peak is at 5300rpm and the torque peak is around 4400rpm. Basically a warmer Crate 380 (the old one, not the new 380 Magnum). Better heads and cam.

The new cam is 320 degrees duration and 108 degrees separation and I wasn't sure how much better/worse the cam would be. That's where I depend on the experience of you people.

It stays pretty clean with the Jacob's Ignition, I just wondered about the rings with a fatter idle mixture. I will try what Mr. Fixit says since I turned them 2 full turns in and they hadn't bottomed out.

I have read several books, but I could read a book on how to deliver a baby too, and without some practical experience and someone to train me that has experience when things don't always go by the book, I get a little lost. I am one of those hands on , visual learners.

Thanks again for your input guys, especially Mr. Fixit for the help in the past also.

On a side note, my flywheel is 47# and I was looking at a lightweight 30# steel one (Mopar says the 11# aluminum will warp with street use). Will this feel like a 10, 20, 30 ft/lbs increase in acceleration?

flipper35 · #13 (**permalink**) 12-27-2002, 01:05 PM

Also, if I drill the throttle plates, shouldn't that move them out of the transfer circuit. And do I drill which side of the plate? Near the circuit or the other side?

Mr.Fixit · #14 (**permalink**) 12-27-2002, 03:43 PM

Near the idle slot. You want to increase airspeed past the slot to draw properly. Then you can lower the throttle blades back into their idle slots and get the same idle speed. get the book, it will be interesting even if you never touch a carb.

flipper35 · #15 (**permalink**) 12-30-2002, 02:41 PM

OK, the book isn't available locally, so I ordered it through Amazon. I'll let you know how it works out.