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:
3076
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:
3077 3078
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:
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(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):
3080 3081 3082 3083 3084 3085 3086 3087

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

[stevedmc]

[QUOTE=Farrar;10969]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./QUOTE]

Dang. I was hoping to look at your fast idle screw this weekend. I'm not sure mine is working correctly due to my own fault. I was loosening it a while back and it came out. Well, I put it back in but I'm not sure I put it back in correctly because nothing seems to happen when I turn the thing.


Its funny how a carb with a few screwed up settings will still start every time and get me from point at to point b (with 25 mpg thank you). I'll see if I can figure out what I screwed up with the fast idle screw after work today.

[Farrar]

No problem. I plan to test the otterstat wiring mod later this week. Since the engine will be hot enough to turn on the radiator fans, I will be able to put my gloves on and set the idle the way Bill recommends. This will be done before Saturday, so you can still take a look and see how my car is set up.

Farrar

[content22207]

... nasty, icky, gooey mess out of the air cleaner housing (only to have it reappear later, of course).

Is your PCV valve clogged? If the PCV valve isn't working, the engine will suck actual oil (PRV valve cover baffling sucks) backwards through the breather hose.

Bill Robertson
#5939

[Farrar]

Is your PCV valve clogged? If the PCV valve isn't working, the engine will suck actual oil (PRV valve cover baffling sucks) backwards through the breather hose.

I replaced the PCV valve last year when my old one ceased rattling when I shook it. This one should be fine, but I will check it when I get home this evening... if it doesn't rattle, I will hit it with brake cleaner and see what happens.

FYI, it looks like the same goo that I cleaned off the bottom of the breather cap a few months ago. It was impossible to get the thing entirely clean but I did the best I could. I would order another one, but they're unavailable.

Edit: Could this be an intake valve issue? RockAuto is currently having a blowout on B28F intake valves -- less than $6 each, but when they're gone, they're gone -- should I go ahead and buy replacements?

Farrar

[Ashyukun]

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
Oops... I did exactly the same thing... and have occasionally had problems with it trying to diesel when it shouldn't be anywhere near overheated. Looks like I may NOT have a problem with the temp gauge and instead need to learn how to properly adjust my carb. :reading:

[content22207]

The curb idle screw has its place and purpose (for example, I used it for the pics I took of my car's current draw at 2,000 RPM ), but as stated, shutting the engine off with the throttle plates cracked open causes two problems:
- fuel will be drafted through the throttle circuit (venturis), not the idle screws
- ignition timing will be advanced

Bill Robertson
#5939

[congerz83]

I also have an idle problem... as well as an acceleration problem and a cold start problem... but I digress. My car will idle high when first started then go back to normal. Then, even after warm, it'll idle at about 2,000RPM until I punch the gas quickly. I'm guessing this has something to do with the choke. Also, at times, after awhile of driving, the car will idle very low 450-500RPM it will also stall at times in during this "mode" of operation. I'm also going to start keeping better track of my mileage because I think mine has dropped drasticly.

[stevedmc]

Tonight I decided to rebuild my carb hoping I might figure out why my fast idle screw wasn't working.

I think I figured out my problem. I was messing around with my choke a while back and it looks like I did something wrong when adjusting it.

Upon putting the carb back together and adusting the choke (following the instructions in the url below) the fast idle screw seems to be working fine now. When the choke is closed I can tell the throttle plates are cracked. When I hold the choke wide open the throttle plates close and the fast idle screw doesn't touch anything.

If I have time tomorrow I will put the carb back on the car and see how things act now. I won't have much time to mess with it in the morning and its too dark to do anything now.

http://www.therangerstation.com/tech_library/pdf_documents/motorcraft_2150_carb.pdf

[content22207]

Upon putting the carb back together and adusting the choke (following the instructions in the url below) the fast idle screw seems to be working fine now. When the choke is closed I can tell the throttle plates are cracked. When I hold the choke wide open the throttle plates close and the fast idle screw doesn't touch anything.

That is normal. That also is why you need to tap the accelerator pedal lightly before starting the car -- the fast idle screw in that position (warm choke) will prevent the choke mechanism from rotating back when the choke spring cools/contracts. After you held the choke wide open, you should have been able to slightly move the throttle linkage and see the choke mechanism rotate and the choke plate snap shut.

Bill Robertson
#5939


http://www.therangerstation.com/tech_library/pdf_documents/motorcraft_2150_carb.pdf

Pretty good writeup, but note:
- The rods in a 2150 are not "metering rods" as most people are used to thinking of them (gasoline never touches them) -- they actually change the size of the air bleeds at the top of the venturi assembly
- The idle adjustment screws are not "fuel/air mixture screws" -- you can not change overall fuel/air metering by turning those screws, only by changing the size of the jets (idle and throttle circuits both share the same jets, fuel wells, and venturi tubes -- the circuits divide at the top of the venturi assembly)
- The Choke adjustment text (p. 43) is for an Autolite 2100, not the Motorcraft redesign (you don't have a plastic nut at the top of your choke rod, a piston inside the choke assembly, etc). The text does not match the illustration on p. 44. Weird.
- As the choke spring warms up on a running engine (p.45), the mechanism will never move until you take tension off the fast idle screw. You can stand there all day watching it, but it will stay in the same position until you slightly move the throttle linkage.

Bill Robertson
#5939

[sean]

Take it to the tangent thread guys.

[content22207]

... My car will idle high when first started then go back to normal. Then, even after warm, it'll idle at about 2,000RPM until I punch the gas quickly. I'm guessing this has something to do with the choke....

My earlier reply has mysteriously disappeared, so I will try again (this *IS* important information...):

Make sure this spring is in place, and that it has not stretched:
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Practical experience has taught me that Motorcraft 2100 throttle plates on a PRV will not reliably close all the way without it, hence its addition to every conversion I do.

Bill Robertson
#5939


If I have time tomorrow I will put the carb back on the car and see how things act now.

I find it easiest to use a dime rather than a screwdriver to turn the fast idle screw when the carb is in place on the car. Remember that the threads are facing you (lefty tighty, righty loosey).

Bill Robertson
#5939

FYI for Steve (and anyone else who cares -- it's raining outside so I can't do much else):

This is the choke mechanism they were talking about in the Ranger rebuild article:
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It's an earlier version of what we run (notice the old pulloff cylinder is still in our housings -- just no piston inside of it). The plastic adjusting nut is very prone to snap with age -- you can buy brass replacements. The adjusting nut does the same thing as Screw #5 on p. 44.

Bill Robertson
#5939

[sean]

Remozed the [bill voice] nonsense[/bill voice]
Squeaky wheel got oiled.