Hesitation / Misfiring between 1000 and 2500 rpm

[AzzuraAzul1980]

I have a 1980 FI and recently started to hesitate from stop up to about 2500 rpm and then engine acceleration would take off with good output. Recently installed new electrical pick-up, new plugs (old ones looked black like running rich). Just removed MAF and flap seems fine with full stroke movement. Checked and reset timing to 12 degrees BTDC at 900 rpm. Sniffed the engine with propane looking for rogue air leaks and found no leaks. Elec. resistance across water temp. RTD about 375 ohms with engine at 190degF and I wiggled wires and connector at water temp. detector with engine running and no change in engine output. Checked fuel pressure and runs btwn 25 and 30 psi which seems on the low side but doesn't explain good engine output above 2500 rpm point. For the most part, engine idles at 850-1000 rpm just fine but intermittantly engine will want to stall after cranking warm engine back up. In other words, intermittant poor idle at engine start in warm condition and I have to keep goosing accelerator to keep engine running and eventually gets to idle smoothly again.

Hoping that someone has had similar issue in the past and can help.

Thanks.

[narfire]

AFM..
In the AFM there is an arm with two pins on the end. They travel along a track as the rpm increases. What seems to happen is the car spends most of its time in the 900-3200 rpm territory so the track gets worn at the lower rpms setting. Common problem with the FI. Pop the black plastic top off the AFM box, easier said than done, with a box of band-aids close by. (use a box cutter to start).
You'll see the arm and the track with two lines on it. Back the screw off that tightens the arm to the shaft and push down or pull up just a tad so the pins move the half a mm to a clean part of the track.
Clean the track and button it back up. Don't muck with the springs .
Should give you smoother running at lower rpm and I also found it easier to set my idle.

Much thanks to Ron Luxmore and others for this evolution.
Chris

[RRoller123]

Just an FYI: When the car is idling, I have noticed that the AFM arm datum point seems to sit right around the "34" area of the Thick Film resistive material shown in picture. The part way to the right almost never gets used while the car is running, because the door flap is always being pushed open somewhat. On mine, the arm would sit just to the right of the TF area when off, and I fiddled with adjusting it to be just on the TF, all a waste of time.

Note that it is recommended to put a small downward bend on the pickup arm if you LIFT it on its shaft, but this is not necessary if you push it DOWN on its shaft. Either way should get the arm to contact better TF material. It made a huge difference in my car, it had an occasional stumble at certain speeds, and adjusting the arm down and cleaning the track area completely eliminated it, and immediately.

[AzzuraAzul1980]

Hey, thanks for your replies. The screw 'that tightens the arm to the shaft' looks like the screw to adjust the fuel pump switch arm to keep the switch from closing when engine off, correct? The arm with the two pins is coupled to the black plastic piece which slides on the central D-shaft.

When I loosen the screw and pull up slightly on the arm/black plastic piece, I am indeed able to get the pins to move slightly, however, as I retighten the screw everything wants to go back to original position. The black plastic piece coupled to copper arm/2-pins finds it's original position as screw re-tightened.

Seems like I have to independently bend/deform the copper arm with two pins without monkeying with the screw.

What I am missing?

Thanks.

[RRoller123]

Just to be sure: It is the horizontal bolt in the photo below, not the screw, that we are talking about loosening to raise or lower the contact arm.

I also notice in the photo below that there is what appears to be a dead spot on the inner track wear path, right at the location where my arm would typically sit while idling. Might just be dirt or whatever, but it may be an issue.

[AzzuraAzul1980]

AOK. Thanks for the clarification. Will get back.

The pic is not mine but I will clean the area.

[RRoller123]

Give a very slight abrasive cleaning to the under side of the gold contact that sits atop the arm too. This is the gold colored disk in the pic. It is important that that be clean and low R.

[AzzuraAzul1980]

SUCCESS - at least 97% of the way.

I loosened the 7mm horizontal hex bolt screw and was able to get the two pins on new tracks by push/pull action on the shaft. I cleaned the Thin Film material track with fast drying electrical component cleaner (from Radio Shack) and lint-free cloth. While manually moving the throttle back and forth, I noticed some stick-slip action of the two pins on TF track. I tried to lessen the pressure force of the two pins on surface by slightly bending up the copper arms. I also lubed the surface with dielectric TEFLON lube and took her for a spin.

DAY AND NIGHT difference. Definite cause and effect and thanks for the lead on this MAF issue. I say 97% of the way there because once in a while during the cruise I detected a bit of hesitation while RPM's were coming down from 3500+ into the 2000-3000 rpm range but I am not complaining as the driving experience is back.

Regards.

[RRoller123]

That's great, except that the low resistance electrical connection of the arm to the TF is key, so clean off that Teflon lube completely. No surface treatment, just a clean surface is called for. That might get you the final 3%!