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Old 03-28-2004, 05:11 PM
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Default What makes an engine backfire through the carb?

MC2100 304

When I give it a sharp snap to the throttle off idle (while in neutral) it coughs fire out the carb. I advanced the timing to 10* and it stopped. So I drove it an hour away (I needed to bond w/ the Jeep, you know, gain some trust both ways) and an hour home. Once, on the way home just after shifting into 2nd it coughed and stumbled just like before. So I'm guessing it aint fixed. What makes an engine do that????
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Old 03-28-2004, 05:34 PM
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Default Re: What makes an engine backfire through the carb

Does it do it all the time or just when its cold or just when its warm? Any other info you can give will help.

Do you have the Carter Carb?
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Old 03-28-2004, 05:47 PM
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Default Re: What makes an engine backfire through the carb

It used to be that it only did it when it was cold. But today it did it after warm up to. It wouldn't do it if I hit the gas like a normal person, but a sharp snap off idle and it would gasp, blow fire and then idle high for a few seconds. And then it happened once when it was under load, low RPM, fully warmed up.
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Old 03-28-2004, 06:16 PM
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Default Re: What makes an engine backfire through the carb

I am not expert with carbs and hopefully some of the Jeep gods on this board will be able to shed some light on it for you. But since most of those guys are out wheeling while im stuck in my office today, see if this helps.

It could be that your a little to lean. Im guessing you have the carter BBD carb. If that is the case, when was the last time you cleaned it and cleaned out the tubes on the top of the carb. If it was mine I would:

1. Clean it thoroughly
2. Lean the Carb out another 1/4 turn

If the previous doesn't help then (and I am leaning towards this to begin with) you may not have the distributor firing incorrectly. You could:

1. Pull the #1 plug and using finger move to #1 position
2. Look under the dist. cap to see that the rotor is near the #1 plug position on the cap.
3. If its not adjust dist. or cap accordingly. (maybe your off a tooth on the dist.)

See what that does. re-post results

I have found backfires through the carb have been fuel and fuel/air mixture related but have seen that it can be timing related as well.
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Old 03-28-2004, 06:40 PM
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Default Re: What makes an engine backfire through the carb

It's a motorcraft MC 2100 1.08
I had just cleaned it and readjusted the float (lower).

To adjust the carb I have always turned the front screws in until the idle lowered and then back out a 1/4 turn. Back and forth with them a few times to make sure it's right. I don't have a vac guage. Maybe it's too lean. ......hmmmmm

</font><blockquote><font class="small">In reply to:</font><hr />
(maybe your off a tooth on the dist.)


[/ QUOTE ]

Would the timing light still show correct timing if this were the case????


EDIT- I'm not trying to argue, just some thoughts and questions I have. Thanks for your replys.
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Old 03-28-2004, 07:15 PM
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Default Re: What makes an engine backfire through the carb

First I definately don't take offense to questions because I am certainly not an expert as I stated.

1. Question... are you getting a "pop" noise up through the carb or is it and actual fire thats coming up... I should have asked this first.

2. Yes... the timing will read correctly if the dist. is set correctly. My understanding is your reading is at 10 degrees advanced your firing 10 degrees before TDC. I was always taught to pass emissions retard the timing. To get more power advance the timing as far as you can without getting any pinging (backyard mechanix [img]images/graemlins/tongue.gif[/img])

As a layman... If your getting actual fire up through the carb then too much fuel but if your getting a air pop then too lean. I was assuming an air pop which I used to get all the time on my Carter BBD.

Of course I have been told that... "I'm as usless less hen**** on a pump handle" so... go figure [img]images/graemlins/tongue.gif[/img]
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Old 03-28-2004, 09:45 PM
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Default Re: What makes an engine backfire through the carb

yup flames ....... along w/ a loud crack

I'm thinking I need to see if I have a leak in the vac advance line
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Old 03-28-2004, 10:09 PM
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Default Re: What makes an engine backfire through the carb

I'm sure somebody will correct me if I'm wrong, but if you backfire much (sometimes just once) on a MC2100, it can damage the power valve. That will lead to more backfiring. I've got that same carb on my CJ5 (304) and the CJ7 (258). The 258 hasn't given any recent probs, but the 304 gave me fits when I first got it (backfiring BAD--there were exhaust back pressure problems at the same time that made it run awful) and they went away when it was rebuilt and a new power valve put in.

Just a thought.
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Old 04-01-2004, 12:25 PM
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Default Re: What makes an engine backfire through the carb?

I'm no expert either, but there almost always has to be a spark for there to be fire, quite possibly? when you hit the peddle hard the timing is moving and the intake valve is still open when the spark plug fires.

maybe slack timming chain

Clint
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Old 04-01-2004, 01:43 PM
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Default Re: What makes an engine backfire through the carb?

</font><blockquote><font class="small">In reply to:</font><hr />
maybe slack timming chain

[/ QUOTE ]

that wouldn't be the only slack thing in that Jeep... [img]images/graemlins/grin.gif[/img]

Actually Aaron adn I were talking abou the same thing... the intake still being open when it sparks.. I was thinking timing, didn't cross my mind about a slack timing chain.


brent
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