Alienlefty

I have been posting in the wrong forum, so let's try here.

I have an LS2 crate engine in a 64 Corvette. It has an edelbrock intake with a Speed Demon carburettor. I am having horrible trouble trying to get the engine to idle at a sensible rpm. I have checked for the obvious air leaks and found none. After a decent warm up, it idles at 1500 rpm. I have the pcv plumbed into the vacuum outlet at the rear of the carb base. If I pinch the pcv line, the rpm drops about 150 rpm. I have removed the carb and made every adjustment I can find to make sure the butterflies are fully closed, although I must say that I am suspicious that the secondaries do not fit perfectly. But I have tried stuffing the secondaries with a towel, and the engine will stall after a few moments. I wondered if I have enough initial advance, so I am running the number 6 pill in the MSD box. This makes the engine more responsive, but still no effect on the idle rpm.

Any suggestions would be appreciated.

Pop N Wood

iTrader: (1)

I had trouble getting my LS2 to idle at a decent RPM until I backed off on the initial timing. At idle speeds (~800 RPM) I am running 10-12 degrees of static advance and another 10° of vacuum advance. When I had more static advance I had the butterflies against the stops and it still idle over 1000 RPM. By the way this caused AFR problem when coming slowly off idle.

I notice the #6 pill sets static advance of 22° at 0 RPM and 32° at 800 RPM. That means you are running 32° 1500 RPM. I can almost guarantee that is your issue. That is too much advance for a stock cam.

Ditch the pills and put in a custom curve. You can ramp the timing back up once you are off idle, so you can keep the #6 curve from 1500 RPM and on if you wish. Backing off the idle timing will make the starter's job a lot easier too.

speedtigger

iTrader: (16)

Check the linkage between the primaries and secondaries. I have seen carburetors where the link rod had too much bend in it, so when the primaries are closing it actually pulls the secondaries open a little. Worth checking.

Alienlefty

I finally found some time today to take a look at programming a custom curve. But I have 4 pc's and a Mac in my house, and not one of them has a connection port that will take the cable supplied by MSD. Thwarted again! Why couldn't they just fit it with USB like everything else these days?

Pop N Wood

iTrader: (1)

All you need is a $9 usb to serial converter from Amazon. Radio Shack has them for $30.

I hope there next version has bluetooth and a smart phone app.

speedtigger

iTrader: (16)

It just occurred to me that you might have the Performer RPM intake. Those intakes have 4 mounting holes for the ignition module. One of them has to be plugged if the ignition module is not mounted on the intake as it is a vacuum leak.

A-Dub

iTrader: (6)

Yes this is most likely your problem. The MSD pills don't have enough timing to cause it to idle that high.

ZONES89RS

iTrader: (25)

That Dan hole caused me a headach before I figured out it went all the way through.

Alienlefty

I found that hole some time ago in my earlier search for vacuum leaks. I have been over it several more times since, convinced there must be something I have missed, but I cannot find any more leaks.

Pop N Wood

iTrader: (1)

I don't know, 32° advance at idle seems excessive to me. I had problems with mid 20's.

speedtigger

iTrader: (16)

I would close the secodary adjustment throttle screw all the way and then remove the secondary linkage actuator rod and then try it.

Alienlefty

I have done that too. I need to check out this advance curve first. If I pull the vacuum signal to the MAP sensor, would that give me an indication?

speedtigger

iTrader: (16)

I don't think your problem is timing related. If you are absolutley certain that all of your carburetor throttle blades are 100% closed and you still have a high idle, there must be a vacuum leak somewhere.

Do you have power brakes? If so, did you try disconnecting the booster and plugging the line? Other than that, the only the thing that comes to mind is a pinched intake manifold o-ring gasket.

Pop N Wood

iTrader: (1)

The #6 pill is only adding 4° of vac advance. If you pull it you will see the idle drop some, but maybe not much.

For a quick check put in pill #1. It will drop your idle timing from 32 to 20 (plus another 6 vac advance). That much timing will drop your idle RPM.

I'm telling you guys I was running 15° static advance and another 12° vac advance at idle with the exact same motor. I had both the primary and secondary throttle stops backed all the way out so they weren't even touching and the motor still wanted to idle around 1200 RPM. Pulled some of the static timing and had to turn the screws in to put me back at 800 RPM. If he is running 32° static and another 4° vac advance then it just makes all kind of sense he will idle faster.

bad67ls

iTrader: (2)

check your carb to manifold base gasket. it is dark in color (damp)?

edelbrock seems to be producing nothing but **** lately. a friend of mine got one that the grooves for the o rings were cut too deep on one bank of the runners, and i got one with a carb flange that was over .015 high in the middle of the front surface.

they also sent me 2 intakes boxed wrong before they finally sent me the correct one.

the company has really gone to hell in a handbasket in recent years...

Alienlefty

I found an adapter for the serial to usb on ebay and it turned up today. So I am about to go out and try reprogramming the MSD box. I did try pinching off the vacuum line to the MAP sensor, and the idle dropped significantly. So I am optimistic that there is a potential solution in the reprogramming.

Alienlefty

I have had a play and with a little success. I feel kind of ignorant because I don't really understand what I am doing. But when I connect the laptop to the msd and start the engine, I get 3 green dials. The middle one appears to be ignition timing, and it jumps around from zero to 25 degrees whilst idling. The only way I found to drop the idle speed was to zero the MAP curve. Since the MAP curve doesn't appear to be RPM related, I am not sure what effect this will have. I am going to drive the car now to see how it feels, but the idle is still around 1000 rpm. I am positive there are no vacuum leaks.

Alienlefty

I have driven the car now with the modified (Zero'd out) map curve. It all feels OK, but obviously it can't stay that way. The idle is more stable at around 850-900rpm, but it still seems abnormal to me that with everything backed right off I can't get any lower than that.

As a general question, the MAP sensor replaces the vacuum advance on a distributor. So why wouldn't I programme a flat curve into it, and connect the map sensor to the ported vacuum source on the carb base plate? Thus eliminating map issues at idle, but bringing them back in again under other driving conditions.

Pop N Wood

iTrader: (1)

You shouldn't have to do that. When things are set up right you should want the advance at idle/low RPM. Vac advance is timing added or taken away based on the MAP pressure. I believe MAP pressure is on one of the green dials. Typically the lower the pressure, the more timing is added.

I don't know why your timing is bouncing around on the green dial. That should be rock solid at idle.

Do you still have a pill plugged into the side of the box? Because you can't adjust your timing if that is in. You should get a graph showing how the timing varies with RPM and a second with MAP reading. What sort of RPM timing are you set for at 850 RPM?

S10xGN

iTrader: (11)

To me, that "bouncing around" idle smacks of a vacuum leak...