Engine backfires when I shut the car off ***Carbed Car***

[stevedmc]

Does anyone have any suggestions on what might cause this? I started noticing this problem about 6 months ago. Normally this wouldn't bother me too much but because my car is carbureted I have my fuel injector holes plugged. A couple of weeks ago one of my injector plugs flew out when it backfired and I would like to see if there is something I can do to prevent the backfiring.

[content22207]

Make sure your throttle plates are closing all the way. The 2100 uses airflow through the throttle bores to switch between throttle and idle circuits. If your throttle plates are cracked open when you shut the engine off, you will still be drawing fuel through the venturis rather than through the idle screws.

Bill Robertson
#5939

[sean]

Moved the thread here since the question is about a modified fuel delivery system.

[content22207]

Remember also that cracked open throttle plates will advance ignition timing (the edge of the driver side throttle plate itself is the on/off switch for the vacuum port as it covers it up) -- overadvanced ignition leads to hard shutoffs.

Bill Robertson
#5939

[stevedmc]

This has got to be the dumbest question in the world but am I only supposed to use the fast idle screw to adjust my idle speed? I have been using the screw that is on the throttle cable assembly.

[content22207]

Steve has been a very bad boy:
IdleSpeedNoNo.jpg
That screw cracks open the throttle plates all the time -- they will never fully close because that screw is forcing them open.

If you want to change cold idle speed, use the screw that presses against the fast idle cam:
SteveChoke2.jpg SteveFastIdle.jpg
Use a dime to turn it. Remember that the threads are facing you: clockwise decreases fast idle, counter clockwise increases fast idle.

If you want to change warm idle speed, use the idle adjustment screws on the front of the carb, underneath the fuel bowl:
SteveIdleMixture.jpg
(Those screws are bypassed when the engine is cold because the fast idle screw is cracking the throttle plates open).

This is how a 2100 switches between idle and throttle circuits (and meters fuel overall):
AMC2100Manual1.jpg AMC2100Manual2.jpg AMC2100Manual3.jpg AMC2100Manual4.jpg AMC2100Manual5.jpg AMC2100Manual6.jpg AMC2100Manual7.jpg AMC2100Manual8.jpg

Bill Robertson
#5939

[stevedmc]

Well that sucks. I guess I need to figure out how to get that screw back like it should be.

[content22207]

Once your carb is set up, that screw will be backed off all the way. You only need it to force the throttle plates open so the engine will run while you make the other adjustments.

Bill Robertson
#5939

[sean]

Rather than merge this with bill's carb thread I decided to create some modification forum prefixes. You'll see it used here. Figure as more unique carb issues start popping up it would make searching them easier.

[Farrar]

Once your carb is set up, that screw will be backed off all the way.

Well, dooky. If that is the case, then mine is also set up incorrectly... a few weeks ago my engine was idling low after a drive, so I bumped the idle up to 900 RPM using that screw and haven't touched it since.

I guess I will try to set it up properly once I clean all that nasty, icky, gooey mess out of the air cleaner housing (only to have it reappear later, of course).

Farrar