Firm

Hi, I am looking for some advice on what to do next to solvemy rough idle issue. Details are follows:

Car is a bone stock 85 LG4, since I got it about a year anda half ago the car has idled rough when hot. It starts perfectly in all weather(even well below freezing), runs great (just returned from a 1600 miles tripand returned close to 30mpg), pulls hard and idles fine when cold. As the carwarms up the idle seems to get worse. If I am at a stop light with the car ingear (automatic) and my foot on the brake it’ll idle around 650rpm and feelpretty rough. If I shift into neutral the rpms pick up at a bit (800-ish) andthe idle smooths right out. That’s typically how I’ve been driving it, but itdoes get annoying, so I’d like to get it resolved.

So far I’ve done a complete tune up; wires, plugs, coil,cap, rotor, fuel filter, PCV valve and air filter. The tune up stuff seemed tomake it better, but it’s still not “right”. I also checked the torque on thecarb bolts, they were pretty lose, but tightening them down didn’t solve theissue.

I guess at this point it’s likely carb related – and I’venever touched a Q-jet before (my experience is with old British carbs…SUs) so Iam hoping y’all can point me in the right direction. Where should I start andwhat should I look for? Any good Q-Jet tuning guides out there?

Thanks

86LG4Bird

iTrader: (1)

I think I know what's going on. I didn't experience this with the stock LG4, but my (daughter's) car does exactly what you're describing with the Vortec longblock and more cam overlap underneath the E4ME carb.
When cold, it's in open loop with the primary metering rods fully retracted (up), giving it a rich enough mixture to idle smoothly. When warm, it's in closed loop, and the MCS is holding the rods too low in the jets, resulting in a too-lean mixture at idle.
First action would be to replace the O2 sensor. If you've already done that, then research on how to adjust the lean stop on the MCS.

Firm

Good advice, thanks.
I didn't think my car had an O2 sensor, but maybe I just haven't seen it yet. I know some Canadian cars were not computer controlled, while others were. I'll try to remember to look tonight when I get home.

86LG4Bird

iTrader: (1)

I'm pretty sure CAN cars had the E4ME carb in 1985, which would require the O2 sensor.

Firm

Checked. No O2 here, so I guess I've got one of the Canadian non-computer controlled cars. I should have included that in the original post, but I didn't know at the time. Sorry.

krisner89

You could test fuel pressure when it's cold and compare it to what the psi is when hot and running rough. It may reveal some vapor locking

zed-028

It could be a few things, i wouldnt have though vapor locking while its running, if it were a full vapor lock it would refuse to restart while hot. I'd probably look at the timing, maybe advance a little at idle see if it changes.

If you're up for the carb tuning challenge then the process is below.

on the bottom of the front of the carb are 2 adjustment screws. its a bit of a balancing act but screw each one inwards (clockwise) bit by bit (half turn on the left one, half turn on the right one) until the engine craps out on you. unscrew each one about 1 turn.

start the car back up, it will be idling pretty crap

start the process again but turn each one bit by bit counterclockwise. youll hear the revs go up, keep going until the revs peak, then theyll start to drop again, stop here, screw them counterclockwise until you hear the revs peak again. This is the sweet spot.