Rough Idle

opeysgarage · #1 (**permalink**) 09-02-2003, 08:42 PM

I have been trying to get this thing to idle a little smoother for a while now. It is a freshly built 383 with edlebrock rpm heads and intake manifold. It has an edlebrock 750cfm carb and a few other tricks. I do not know how radical the cam is. I have bought and tried a vaccum gauge. It has good vaccum at the intake manifold but it does oscilate slowly about one or two PSI. I adjusted the edlebrock by use of the two screws in front for max vaccum. I think I ended up close to 15 at idle. Under throttle the engine responds well and runs smooth but at idle it shakes like it wants to come apart. When I put it in gear it almost wants to stop unless I set the idle higher than normal in park.
Please help... I am still learing the hot rod stuff.
Thanks..
Opey

Bumpstick · #2 (**permalink**) 09-02-2003, 09:17 PM

Well boys and girls the word for today is "Vacuum leak"

Or atleast thats what is sounds like...? The roughness and dying out in gear... With the amount of vac that you have and the surging of it is almost a dead giveaway...

opeysgarage · #3 (**permalink**) 09-02-2003, 09:25 PM

Liek I said before, the engine is fresh. 100 or so miles on it. Where should I look first for this intake leak? retorque the intake manifold and carb? All of the vaccum lines and caps seem to be in place..
Thanks again..
Opey

Silver_Camaro · #4 (**permalink**) 09-02-2003, 10:07 PM

Get you can can of carb cleaner and spray around the carbuerator first.. Especially where it bolts down to the intake. If you notice it rev up and bog down at all then you've found your vacuum leak... Be sure to focus where the carb bolts to the intake.. some times the gaskets just aren't quite the right size.. (this happened with my freshly rebuilt carb... Bolted it on and have a vacuum leak like you wouldn't imagine! ended up puttin the old gaskets back on and sealed up nice and perfect hehe.)

78SilverShark · #5 (**permalink**) 09-03-2003, 10:40 AM

I would also say that it is a vacuum leak, becuase that's why you are fine in the higher RPMs, where vacuum is little to none. Idle has the highest vacuum. Which is why it might be the spring in your carb. But that probably isn't it. Hmmm....

78SilverShark

**Deuce** · #6 (**permalink**) 09-03-2003, 11:11 AM

I will disagree with what been said............

I believe (with 15 inches vaccum) that you have a fairly lumpy camshaft............and the reason it dies when you put it in gear is that you NEED a higher stall converter.

Just my 2 cents........and I could be mistaken.......

gt2betubbed · #7 (**permalink**) 09-03-2003, 11:21 AM

To go along with Duece, when you drive, do you notice that the motor "wakes up" in the higher rpms, and is a little sluggish in the lower? This could let you know a little more about the cam, and do you know what stall you have currently?

opeysgarage · #8 (**permalink**) 09-03-2003, 12:47 PM

I dont' know anything about stall. I have a turbo 400 three speed auto in it right now. No tricks to it, just stock. I have looked all morning for intake leaks or anything loose enough to cause one. I tightened **** up and took several vaccum readings. I have a shakey 12 to 13 inches of vaccum. With higher engine speeds the vaccum climbs and rests pretty steady. On the road the car does great. I climb into it at any speed and 100 mph pops up on you quick. I don't know what cam I have in there yet. The builder is suposed to be sending me a build sheet. I have reason to beleive that I need new wires. Would the wires and cap give any symptoms that would resemble what I have?
Thanks for all of the info..
Opey

gt2betubbed · #9 (**permalink**) 09-03-2003, 01:32 PM

Did you try the carb cleaner trick? Some leaks aren't visible.
Try calling the builder and asking for a cam spec sheet, along with a stall speed. Did he build the tranny as well? If the stall is stock, you are going to need to know the specs of the cam (along with some other things) in order to get the correct stall on your converter.

opeysgarage · #10 (**permalink**) 09-03-2003, 02:16 PM

I did spray all of the sealing areas in hopes of finding the leak that way. We use the same little trick on the bikes so I know what I was listening for. I never heard it though. I continued and tried to turn a few bolts anyway. No improvement. Just in case you need to know, we are working on a 1973 vette. The trany is stock to my knowledge and was not built by the engine builder. He worked just the engine. I do think that the trany may have a shift kit in it. I don't know the 400 but it shifts in to second hard enough to bark the tires with this engine.
I'll try to contact the builder today with the cam specs and stuff..
Thanks again.
Opey

s15laynrubba · #11 (**permalink**) 09-03-2003, 02:22 PM

Quote:

I will disagree with what been said............

I believe (with 15 inches vaccum) that you have a fairly lumpy camshaft............and the reason it dies when you put it in gear is that you NEED a higher stall converter.

I also agree.Just had the same problem very recently with a larger cam. I also had 15 inches of vacuum with a little bounce to it and it was the 1800 rpm stall i had. I changed to a 2500 rpm stall and presto...It was cured...Jay.

opeysgarage · #12 (**permalink**) 09-03-2003, 02:24 PM

one more thing with the stall converter though is that the engine isn't idleing smooth in park either. I mean if it were smooth in park and then dropped to nothing in gear it would be different. It idles rough no matter if it is in gear, reverse or park. I'd bite on the heavy cam thing if the engine had a rythum to it, but the idle is pretty chaotic and surgy while in park. Am I way off here? Could these symptoms be ignition based?
Thanks again..
Opey

gt2betubbed · #13 (**permalink**) 09-03-2003, 03:21 PM

There's a difference in a rough idle and a lumpy cam.
Does it sound as if you can almost hear each cylinder poping off? A duration cam doesn't necessarily sound smooth and lumpy, it can surge all over the place (not too far) depending the size.

To me, that's too coolest damn sound in the world!!!!!!

opeysgarage · #14 (**permalink**) 09-03-2003, 09:34 PM

What should I expect to pay for a higher stall converter if that is what I end up needing?
I got rid of the PCV and replaced it with some pop in filters. That seemed to help a lot. I'll throw a vaccum on it again in the morning and I'll know for sure.
It is still idled kind on high in park for a desirable idle in gear. It drops when I put in gear still but not as bad. I like the way it idles in gear now.
I bought a new set of 8.5mm wires and a new cap and button o through on it tomorrow too. I am not sure that it will help smooth things out much but if I get this out of the way I will not have to do it for a while.
What kind of Hp and TQ do you all think I should be getting out of a 383 with 9.6:1 compression, edlebrock rpm heads, edlebrock rpm intake manifold, edlebrock 750 CFM performer series carb, hooker headers with 2 1/2" true duals, an oil pan with a windage tray, and a high flow air cleaner. It is cammed to but I do not know which edlebrock cam was matched with the set yet. Any guess at hp and tq are welcomed...
Thanks again..
Opey

opeysgarage · #15 (**permalink**) 09-04-2003, 05:55 PM

I think my cam is the edlebrock with 244@.50
I am still running rough at idle and droping considerably when put in gear. I don't have a tach hooked up to it yet but I am working on it. I put a new cap, rotor and wires on it this afternoon. 8.5mm silicone wires with the accel premium cap and button. It removed the thought that there may be a miss because of the corroded up cap or cracked wires. Could my timing be slightly retarded? With increased displacement an engines timing should be advanced a little right?
I'm still guessing at it. I have about 13 inches of vaccum right now. It fluctuates between 12 and 13.5.
Thanks all..
Opey

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