Hey folks,

Just thought I'd tap the wisdom here. I've got an LG4 with an exhaust (no headers...yet) and an open air filter. No other major mods. I've been getting an off idle hesitation and some surging under part throttle cruise, but only when hot. Also the idle was a bit rough. I'd already tuned the carb per the directions on the Pep Boys tuning kit, and replaced a bad TPS a couple months ago. So it seems I needed to do some more tweaking.

Today I tore open the carb again, replaced the float valve and seat, adjusted the float height to 10/32", replaced various seals and o-rings, adjusted the lean and rich stops (5/32"), and also the AIB valve.

The idle has smoothed out tremendously. The off idle hesitation is still there, but not as bad. And the part throttle surge is still there, but again not as bad.

(edit) Oh yeah, the hesitation is not WOT, it's part throttle. WOT response is great. I have adjusted the spring tension for the secondaries and also have different rods and hangers. So this must be a problem with the primaries, right?

Any suggestions?

TIA.

 

That would be correct.

The first thing I'd try would be turning the IAB in a little at a time, like 1/4 turn, & write down how far you go so you can put it back... if that helps but doesn't cure, try backing both the stops out in similar increments. you might want to hook up a dwell meter to the test plug while you do it and see if the ECM notices you doing that, and what the duty cycle is; if it doesn't change, especially if it stays real high (shows dwell numbers higher than 30 degrees on the 6-cyl scale) it may need an O2 sensor.

 

Dwell meter? They still make those??? All kidding aside, I don't have a dwell meter and really would prefer not to buy one if I don't have to.

So what exactly does the IAB do? What part of the fuel delivery does it deal with? Idle to 1/8 throttle? Just idle?

As far as adjusting the stops, If I adjust the rich stop to 6/32" and leave the lean stop as is, does that richen the primaries? Should I move the solenoid stroke up position-wise by the same amount, or lengthen the stroke by adjusting the stops further apart?

Sorry about all the questions. I understand the different parts of the carb, but I don't have a full understanding of how all the different parts work together yet.

Oh yeah, the O2 sensor is only 3000 miles old.

Greg

 

Any answers to the above questions?

TIA

G

 

The IAB regulates the amount of air allowed into the idle circuit to mix with the fuel. It closes when the solenoid is in the up position. Backing it out means that it spends more of its time open (letting extra air in, leaning the idle) and vice-versa. It affects idle, part-throttle at high vacuum, and cruise. It has little or no effect on full-throttle operation or even mild acceleration; the lower the vacuum is, the less the idle circuit is doing.

The length of the stroke sort of sets the range between full rich and full lean. This total range should give the ECM a range of adjustment to the primary mixture from about 13:1 to about 16:1. If you lengthen the stroke, you move whichever end of the range. Usually there's plenty of total range, it's just not centered quite right, so when the ECM tries to set the duty cycle to around 50% like it wants to, the mixture is wrong, and it has to compensate. That's why you use a dwell meter or oscilloscope or something similar to set it: the carb is "centered" around ideal when the duty cycle is near 50% at idle and cruise.

If you move the stroke up or down, you will move it either toward or away from the IAB, so the IAB needs to be moved the same amount to keep the idle the same.

 

Sorry to come in late on this, but I'm learning alot. So if I am understanding what you are saying then once the lean and rich mixture stops are adjusted to "factory" settings, you can do some fine adjustments to the mixture for idle, part and cruise adjusting the IAB up and down. Is this correct? Happiness would be using a dwell meter on the "green" wire and ground for a 50% dwell at idle? Correct?

phil

 

Can some please answer the previous questions? I'm curious whether IAB can be used to adjust drivability as long as the MCS stops are set at a "reasonable" (rebuild kit specification) settings?

thanks,
phil