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View Full Version : Need help tuning my edelbrock 1405, idle mixture screws have no effect

01-14-2003, 11:44 PM
I can't seem to get my carb adjustment together. I've got a 1973 K5 with a 327 swapped in and an Edelbrock 1405 with electric choke on top. Since the rest of the truck was pretty much slapped together with no attention to detail and seems to be a little lacking in power, I figured I'd make sure the carb is adjusted properly. So I searched this board and learned to do it with a vacuum guage, as opposed to with a tach like the manual says. So I hook up the vacuum guage, engine warmed up and choke open. I screw both mixture screws in all the way (they were about 2 3/4 turns out), and then I proceed to adjust back off the screws, making it richer. From what I understand, I'm supposed to back the screws out one at a time until I get maximum vacuum (or rpm if using a tach) all the while resetting the idle speed after each adjustment. Well, I have screwed both screws ALL the way in and All the way out and I cant tell any difference in vacuum. It stays at a constant 22" Hg no matter how rich or lean I make it. So I borrowed an OLD tach from my dad to do it that way, same deal. The needle moves but it is so slight I dont see how I can do this with any degree of accuracy. The tach has a red and a black wire, I put the black to - on the battery and the red to the tach terminal on my HEI. It reads 600 rpm at idle in Drive and 1100 in park. I tried adjusting the carb at both shift settings. Am I retarded? well, probably, but I have mucho mechanical ability yet I can't figure it out. I know I am running rich now cause of the exhaust smell, but how do I get it right? My timing is at 8* btdc BTW. Any help is sooooo appreciated. /forums/images/graemlins/1zhelp.gif

01-15-2003, 05:46 AM
Ok, the screws that you are twisting only affect the idle mixture. Sounds that you are idling high enough that the idle circuit is not in effect anymore and the regular circuit is taking over. Turning those screws should have a drastic effect on the idle. You should be able to kill the engine with them. Try lowering your idle speed screw some and try again. Look like you will have to play with it some. Good luck with it.

01-15-2003, 07:29 AM
Yup, curb idle is set too high, or you have a vacuum leak.

01-15-2003, 07:48 AM
I had a similar problem with my idle mixture screws....they're supposed to kill the engine (or almost) when you screw them in. I couldn't get any change at all no matter how I turned them.

The "short" version of the story is that I had a mechanical fuel pump that was failing and putting out something like 20 psi!!!! So no matter how little fuel I was trying to "set"....the pump was overwhelming the carb with fuel and it would run even with the idle screws closed.

New fuel pump installed.....and everything is normal. Idle screws now behave as they're supposed to! /forums/images/graemlins/thumb.gif

I'm not saying that this is your problem too, but it had a couple of mechanics really puzzled...the failure was intermittent so it took a while to find and fix.

01-15-2003, 06:12 PM
Thnks for the replys fellas. I have a brand new fuel pump I just installed after Christmas. It's the two outlet type, where the old one only had one discharge, so I teed the two together, maybe this causes too much pressure, ill check it out. Also, I tried lowering my idle but I damn near backed the screw all the way out with no lowering of the idle speed. I doused it with carb cleaner to make sure the linkages and pivot points weren't gummed up, but that didn't help. I think I'l look at the fuel pump, maybe try plugging that second discharge outlet. I'll put a fuel pressure guage in next paycheck too. Thanks again guys.

Storm Trooper
01-15-2003, 06:29 PM
Don't forget the vaccumm leak possibility already said. Make darn sure the choke isn't keeping the idle up either. Thoose screws do not really effect the overall performance as much as the needles, seats, springs & rods do but you do need to start with the basics. While your cleaning it, you should check the float levels and clean the inside too.