4.2 258 backfire when decelerating - JeepForum.com
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post #1 of 21 Old 06-12-2021, 01:22 AM Thread Starter
ianorman
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4.2 258 backfire when decelerating

I’ve been chasing a backfire when decelerating down hill then engine dies. I originally had blocked idle jets then a collapsed hose block the fuel return line causing the too much pressure at the Carter bbd carb.

It only does it after it’s warmed up. I’ve replaced the vacuum advance and now just changed the oil (due to the overflowing carb float problem) thinking it could’ve been thin oil causing a sticky valve.

Any ideas

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post #2 of 21 Old 06-12-2021, 06:36 AM
Matt1981CJ7
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Is the backfire thru the exhaust or the carb?

Is it a loud BANG or a popping sound?


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post #3 of 21 Old 06-12-2021, 09:26 AM
bob4703
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If it is a series of pops then it probably is the rumble of an exhaust leak when you let off the gas. Usual culprit is the donut gasket leaking.

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post #4 of 21 Old 06-12-2021, 01:13 PM Thread Starter
ianorman
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It’s coming from the exhaust. A series of occasional pops then usually a loud backfire. I’m starting to think the petrol thinned oil has lined a few valve stems and left a residue causing one or exhaust valves to stick when hot?
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post #5 of 21 Old 06-12-2021, 01:24 PM Thread Starter
ianorman
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I will go check the donut gasket. I did just retighten the manifold bolts an eighth or a quarter turn but it didn’t help. Question my garage warmed me about not having a flexi hose between the manifold and exhaust, has anyone else had a problem with the exhaust or manifold breaking because no flexi hose
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post #6 of 21 Old 06-12-2021, 07:23 PM
pedal2themetai
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Hi,
I might have missed it but what carb are you running? depending on orientation of the float bowl maybe your having fuel starvation when going down hill ,, Float to low?? How steep of a hill are you going down?

good luck
tim
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post #7 of 21 Old 06-12-2021, 09:24 PM
80cj
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Quote:
Originally Posted by pedal2themetai View Post
Hi,
I might have missed it but what carb are you running? depending on orientation of the float bowl maybe your having fuel starvation when going down hill ,, Float to low?? How steep of a hill are you going down?

good luck
tim
Quote:
Originally Posted by ianorman View Post
I’ve been chasing a backfire when decelerating down hill then engine dies. I originally had blocked idle jets then a collapsed hose block the fuel return line causing the too much pressure at the Carter bbd carb.

It only does it after it’s warmed up. I’ve replaced the vacuum advance and now just changed the oil (due to the overflowing carb float problem) thinking it could’ve been thin oil causing a sticky valve.

Any ideas
Carter BBD
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post #8 of 21 Old 06-12-2021, 10:25 PM
JEEPFELLER
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What is your present idle speed?

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post #9 of 21 Old 06-13-2021, 12:01 AM Thread Starter
ianorman
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Ive got the idle around 800rpm. It seems like the timing varies. Sometimes you Rev the engine it dies when it comes back down it dies, other times it keeps running… if I unplug the vacuum advance it doesn’t effect idle? Or should it only effect timing at 1600rpm
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post #10 of 21 Old 06-13-2021, 01:14 AM
pedal2themetai
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HI,

800 is a little fast.. It should be around 650 with a manuel transmission and 750 with a automatic in N. Have you adjusted the air/fuel mixture screw. It should be like 1 1/2 turns out from lightly seated, then adjusted from there to best idle at idle rpm..

Good luck
tim
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post #11 of 21 Old 06-13-2021, 10:29 AM
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Are you using a portable tach (to get RPMs right during timing) and a plain Jane timing gun?

Do you know what the book requirements are for timing and timing RPM?

What year do you have?

Mine's a '77, 258, when timing s finished, I'm sitting at 8 degrees at 700 RPM---distributor vacuum hose temporarily removed and capped during he procedure.

honestly, have you been using meters, or have you just been winging your tuning with no clue of where it should be?

What about MIX Screws? Do you know how to correctly adjust them? all of this works hand in hand to get it all correct. I'll help you, JUST ASK!

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post #12 of 21 Old 06-13-2021, 10:47 AM
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Is your distributor functioning correctly? Is it possibly gunked-up and not moving smoothly internally with the vacuum changes?
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post #13 of 21 Old 06-13-2021, 12:51 PM Thread Starter
ianorman
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I timed it 8 degrees btdc and it made no difference. But for some reason when I unplug the vacuum advance it doesn’t make any difference. Ive got the vacuum advance t connected to the TAC vac at the back as there is minimal vacuum coming off the ported vacuum at the front of the carb.
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post #14 of 21 Old 06-13-2021, 07:45 PM
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See post 8 in here https://www.jeepforum.com/forum/f8/e...kfire-4433651/

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post #15 of 21 Old 06-13-2021, 07:59 PM
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Quote:
Originally Posted by ianorman View Post
there is minimal vacuum coming off the ported vacuum at the front of the carb.
Ported does not produce advance at near idle. It is designed that way to pass emission tests for air quality. Full manifold vacuum will add advance to the idle somewhat, depending upon the specs of the cannister attached to the distributor.

You should take the distributor cap off. Unhook the vacuum tube from the engine and suck on the end to produce a vacuum to the unit. If the can is working, the arm will pull against a small lever that turns a plate below where points or pickup reside, increasing advance. If the arm doesn't move or the plate doesn't turn, or if it sticks and doesn't release easily then it's causing the advance to be off, and that's the backfire.

If it doesn't move get a new vac can as the diaphragm is shot; if it doesn't release, clean the area off with WD-40, a toothbrush, and a soft cloth. Let dry.

Last edited by gutthans; 06-13-2021 at 08:11 PM.
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