Sudden idle problem
 

Truck idled perfectly up until last night, I did a compression test of the cylinders and right after that it started idling like piss. 70's era 305 (not 350) , quadrajet 4 barrel carb. HEI. All plug wires are connected where they should be. Plugs themselves are new less than 20 miles on them.

I started my trouble shooting by messing with the idle mixture screws and I discovered that I can now turn them both all the way in without any ill effects other than slight increase in rpms. Is this always an issue with the carb? Just the other day the engine would stall if I turned them in.

I don't have a tach but I definitely feel the rpms are higher than normal idle would be, this is NOT being caused by throttle linkage or idle speed set screw. I'm not sure if something such a vacuum leak could increase the rpm's resulting in getting no response in the idle circuit or whatnot, could it?


Either I disturbed something that caused this or the carb just happened to act up in between starts. No vacuum leaks found as of yet anyway.

EDIT: To be clear, I didn't do the compression test due to the poor idle. It was unrelated and afterwards is when the poor idle came about.

Re: Sudden idle problem
 

Sounds like vac hose knocked loose somewhere

why turning screws the other day ?

Re: Sudden idle problem
 

Thats what I was thinking but haven't found a single bad or disconnected line.

And as far as I why I messed with it the other day, just because I just bought the truck a couple weeks ago wanted to tinker.

Re: Sudden idle problem
 

You cracked a sparkplug!

Re: Sudden idle problem
 

I pulled them all out and they all look fine. Or is it not always something you can see with the naked eye? Oh it looks like they can crack internally and thats probably what you meant.

Re: Sudden idle problem
 

Agreed!...or you disturbed something in the spark plug wires, maybe at the cap vs. the plug end.
You have to plug by plug, then wire by wire start doing some thorough trouble shooting.
There is no instant magic bullet point we can give you...but the troubleshooting will usually find something simple behind the problem.

my two bits.
Coley

Re: Sudden idle problem
 

Try starting it in the dark and see if you spot any electricity leaking. Sounds like a bad plug wire but usually they will miss. You may also have tweaked the choke adjustment by accident.

Re: Sudden idle problem
 

Replaced all the plugs, no luck there. Turned all the lights out and see no sparks. Choke has been disabled so I don't think it could be the culprit. Exhaust definitely smells very rich though. The wire going to the HEI which I think is the power wire, if I wiggle that wire without disconnecting it, the engine will stall. Not sure if thats normal but figured I'd add that in here just in case.

Re: Sudden idle problem
 

You should make sure the power connection for your HEI is rock solid.
Then, you should get an electrical tester and make sure you have 12 volts to the distributor....not 8, not 9......12 volts.
If you don't, the distributor may not fire correctly and it will give the impression of a bad tuning or carb problem....and this degradation can happen within the wiring. (ask me how I know)

Coley

Re: Sudden idle problem
 

12.4 volts engine off. 11.4 volts while running.

Re: Sudden idle problem
 

+

= bad

I'm in agreement with Coley

Re: Sudden idle problem
 

12.6V is a charged battery. If your system doesn't have a minimum of 13.5V while running, you are not charging your battery, and not supporting the rest of the system.

An alternator/regulator/drive belt are in your future. I vote drive belt tension first.

Re: Sudden idle problem
 

I do have 13.5v+ at the battery while running, but only 11.4v coming into the distributor.

I did also notice that the alternator has the little clipped in plug that goes into the side of the alternator, it looks like it should have 2 wires attached to the clip but I only have one wire on mine.

Re: Sudden idle problem
 

I hooked a new wire up to the distributor and am now getting 14 volts to it. I think the idle is possibly smoother but still too high and I can still turn both idle mixture screws in without the engine dying.

Re: Sudden idle problem
 

Ground wire come off to the engine?

Re: Sudden idle problem
 

Where’s the vac advance plugged onto?
Is the vac advance pot good?
What’s your initial timing?

The reason you can adjust the mix screws all the way in and it not quit is that you have the idle on the carb turned up enough to totally expose the idle transfer slot and you’re idling on the power circuit.

There’s info here about the transfer slot. On a Holley but the info is the same for yours.

http://www.crankshaftcoalition.com/w...EI_distributor

Re: Sudden idle problem
 

Maybe a carb. base gasket or intake leak...

Re: Sudden idle problem
 

Vac advance is new I've also disconnected it to make sure it wasn't leaking already somehow. Initial timing is at 10° right now but I've retarded it to test it but it didn't help.

Re: Sudden idle problem
 

Adjust your timing up to 14 initial.
And hook your vac pot to manifold vac source.

You need to speed up your idle with more timing. Then you can close the primary enough to get the mix screws working.

Re: Sudden idle problem
 

Even though it worked just fine before my compression test ?

I disconnected all the fuel lines that day and opened the throttle fully while I cranked it for the compression tests. Is it possible that that somehow did something negative to the carb?

Re: Sudden idle problem
 

Possible.
Maybe you got something in a bind holding the primaries
open a bit.

What did you use to hold the carb open?

Re: Sudden idle problem
 

What were the results of doing a complete check for a vacuum leak?
Remember, this isn't a quick 'visual' test or cursory check.
You have to identify everything on the top of the engine that can 'leak' air...from the intake manifold, to the carb insulator gasket, to the small ports on the carb, to the power brake vacuum line, every line....cracked hose?...vacuum tube pulled out?..leaking main brake booster?
Sometimes a pair or pliers will allow you check each and every hose on the upper part of the engine. You have to clamp every one of them and hold them for 5-10 seconds to see if there is change in engine speed or the way it runs.
How old is the Quadrajet?....have you ever had it off and cleaned it thoroughly with carb cleaner?....or run a full can through it?
I like Quadrajets but the best thing you can for them every couple of years is to fully remove them from the vehicle and vigorously clean them on the work bench with carb cleaner and compressed air.

...my two bits.
Coley

Re: Sudden idle problem
 

What if anything did you use to hold the choke open too.

Re: Sudden idle problem
 

That is a great call, or you messed up a plug wire.

Grab each plug wire (while the engine is running) till you get shocked, then that is the problem cylinder.

Actually, disconnect and reconnect each plug wire at the plug using insulated pliers. When you pull one off and the idle doesn't get crappier, that's the one with the bad plug or wire.

Re: Sudden idle problem
 

Another way I do the 'bad plug or plug wire' test is to fully pull up all the protective wire 'grip boot's at the distributor on all the wires....but keep the plug wires inserted fully down into the distributor.
Then I use a simple light/12 volt tester (grounded to the firewall, etc) and touch it down into each of the (8 or 6?) distributor wire connection points.
Its easy to get in there with the 'pointy end' of the tester and essentially 'short' out that plug/wire. You can quickly work your way around the distributor several times.
If you find one that yields no change in the way the engine is running...then that wire or plug (or cylinder) is dead.
What I like about this method is the very limited chance of ever getting a shock by 'pulling' off wires then attempting to reinsert them.

....more food for thought.
'
Coley.