Is hesitation usually caused by starving for gas? On my rebuilt q-jet I am getting some part throttle hesitation, should I screw in the IAB in some more?

thoughts?

thanks,
phil

 

Exactly how does this hesitation occur: when you are depressing the throttle, or when being held at part throttle?

If the former, look to the accelerator pump first. Something may not be quite right.

How did you adjust the IAB when you put it back together?

 

Hesitation is when I push on the throttle.

I adjusted the IAB by starting with 4 turns up from bottom-out. Then I have been trying to use the Dwell, after the car is warmed up, but changing the IAB doesn't change the dwell. It stays solid. Replaced O2 sensor and adjusted the TPS to .5V at "no choke" idle.

thanks, phil

 

I just saw in another post that if you have an automatic you have to be in Drive (wheels blocked) to check dwell. Is this correct?

phil

 

That's correct.

If moving the IAB doesn't affect dwell, put it at about 1/2 way and adjust the idle mixture screws in the throttle body (evenly) until the dwell comes in. Or, set the idle mixture screws at 7 turns out and try the IAB adjustment again. You need to let the dwell "settle" for several seconds between each small adjustment.

 

Thanks for the response. I will try it tonight. Hopefully that will get rid of the hesitation.

There seems to be some confusion on the board about duty cycle above and below 30 degrees. I assume that if the dwell is above 30 then the MCS is cycling faster than 50% of the time which means that the ECM is trying to lean out the mixture. Are both of these assumptions correct?

In addition, I saw some posts that said there dwell wouldn't change because even after they warmed up the engine the o2 sensor would cool off at idle and then force the ECM into open loop. It this is happening, how do i keep it warm enough? Part throttle?

thanks,
phil

 

Well I think I solved it! My problem was that the dwell would be soild at curb idle and no matter what I did it wouldn't change. So after reading dozens of post someone mentioned that the GM manual suggested that you may need to go part throttle to keep the 02 sensor hot enough to stay in closed loop. I have just replaced my 02 sensor so I don't think it is faulty. So when I put the engin at a constant part throttle (~1500rpm) the dwell meter started to bounce a little and the IAB started to make a difference. I adjusted the IAB to have dwell bounce around 30 degrees and the car seems to be running great with no hesitation. I was running way to rich. I have the lean stop 3 1/2 turns up and 4/32 travel. The Idle Mixture Screws are 7 turns out. All those settings required the IAB to be very high. Almost flush with the top.

Hope everything continues to work great. Also hope that setting Dwell at 1500 rpm will work for everything.

thanks for all your help.

phil

 

Well I did solve it this time. After further investigation I found out that the 7 turns out for the Idle Mixture Screws is too much for my carb (83 Z28, LG4). It was to rich even with the IAB flush with the top and forced the ECM into open loop at curb idle. That is why nothing I did with the IAB at curb idle changed the dwell. I moved the Idle Mixture Screws to 4 turns out and then i started to get the dwell moving at idle.

thanks again for everyones help,
phil

 

Phil,
How do you know where to set your lean stop? Why did you pick 3 1/2 turns?

 

There is a measurment (reported in the service manual) that is taken from the top of the primary jets to the small plate on the M/C Selinoid when the selinoid is pushed down. It turns out that 3 1/2 turns is a very close approximaition to that measurement. I think it was also listed in my rebuild kit as 3 1/2 turns. You can buy a small tool set from Napa that has a gauge that is placed between the jet and the small plate to get the exact height.

NAPA prt #bk7001134

phil

 

Oh yeah, that. I have one of those tools. I've adjusted mine that way, I just didn't know it was about 3 1/2 turns. I don't like the idea of adjusting it this way, it seems too "benchtop" to me. I wish I knew a more empirical way of setting it, but I don't. The rich mix stop, I would imagine you'd want to set to the best E.T.
Thanks,
Homer