Stumble AFTER hard acceleration - JeepForum.com
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post #1 of 20 Old 06-21-2019, 02:36 PM Thread Starter
orange150
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Stumble AFTER hard acceleration

I've been chasing this issue for a while but haven't had the time to real world diagnose things. Work and family...

If I'm driving up a steep hill, 45-50mph, a good load on the engine keeping up with traffic, and come to an abrupt stop (for a stop light or something), the Jeep wants to die. I can "work through it" by staying on the throttle, but depending on the conditions I can't always catch it fast enough.

Googling "stumble after acceleration" doesn't bring up anything, only solutions for while accelerating (usually accelerator pump issues). Acceleration is fine, a slight stumble at the get-go but I know that is tuning issue.

Thinking it may be a vapor lock/fueling issue I've dropped the tank and ran all new lines, new fuel pickup/sending unit, and replaced the fairly new fuel filter.
I recently installed an HEI distributor, thinking the old (I mean OLD) ignition system may be the culprit (wanted to change it anyway ). Runs better but still has the stumble.
I did a quick tear down and cleaning of the carb (mc2150), and it looks good. .50 mains and an 8.5 power valve.

My last thought is that I'm draining the fuel bowl faster than the pump can keep up. I replaced the pump two or three years ago, but I've read a few accounts of people getting sub par pumps from the local parts store.

Is there something else I'm not checking?


'82 CJ-8
258/AX15/D300
OME YJ 2.5"/31s
D30/D44/3.73s/Truetracs
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post #2 of 20 Old 06-21-2019, 05:23 PM
Matt1981CJ7
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I've looked to the plugs for clues to probably 90% of any engines problems.

I'd go out and deliberately duplicate the problem, in a safe place to pull over when it dies. Then immediately check the plugs. They should tell if the engine is starving for air or fuel.

Then, you can further diagnose from there.

Matt


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post #3 of 20 Old 06-21-2019, 06:22 PM
Fourtrail
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Still have a dashpot on your 2150? Issue with the throttle blades slamming shut causing an over rich situation and the stumble you are experiencing.


http://grantorinosport.org/bubbaf250/carb/carb02a.html

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post #4 of 20 Old 06-22-2019, 05:16 AM Thread Starter
orange150
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Quote:
Originally Posted by Fourtrail View Post
Still have a dashpot on your 2150? Issue with the throttle blades slamming shut causing an over rich situation and the stumble you are experiencing.


http://grantorinosport.org/bubbaf250/carb/carb02a.html
No, no dashpot on mine.

Matt, that’s what I need to do, take the time to read the plugs.
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'82 CJ-8
258/AX15/D300
OME YJ 2.5"/31s
D30/D44/3.73s/Truetracs
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post #5 of 20 Old 06-22-2019, 06:31 AM
pedal2themetai
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Hi, OK I see the possible problem.... You've plugged off the bowl vent if the bowl can't vent it can't let fuel in to the motor. the tube at the top of the bowl is the vent tube.. needs to go to charcoal canister or air filter housing.
good luck
tim
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post #6 of 20 Old 06-22-2019, 07:02 AM Thread Starter
orange150
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Quote:
Originally Posted by pedal2themetai View Post
Hi, OK I see the possible problem.... You've plugged off the bowl vent if the bowl can't vent it can't let fuel in to the motor. the tube at the top of the bowl is the vent tube.. needs to go to charcoal canister or air filter housing.
good luck
tim
Actually just did that last week to test if that was causing the fuel to evaporate from the bowl. No performance difference from doing that as of yet.

'82 CJ-8
258/AX15/D300
OME YJ 2.5"/31s
D30/D44/3.73s/Truetracs
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post #7 of 20 Old 06-22-2019, 07:07 AM
pedal2themetai
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Quote:
Originally Posted by orange150 View Post
Actually just did that last week to test if that was causing the fuel to evaporate from the bowl. No performance difference from doing that as of yet.
HI I'm sorry but yes it is .. No fuel after hard acceleration.. the bowl needs to be vented.....
good luck
tim

edit
take a plastic pop bottle put a hole in the bottom fill it with water and watch it drain out now put your thumb over the top and watch it stop flowing and tell me again how plugging the vent in the bowl didn't hinder anything,
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post #8 of 20 Old 06-22-2019, 07:22 AM
gutthans
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Quote:
Originally Posted by orange150 View Post
Actually just did that last week to test if that was causing the fuel to evaporate from the bowl. No performance difference from doing that as of yet.
Could be that, with the vent plugged, you won't be able to fix the original issue. Old school says: Try one thing at a time. If nothing changes, PUT IT BACK the way it was before going further.

Not being a 2150 expert, but wondering if the choke qualification needs adjusting?
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post #9 of 20 Old 06-22-2019, 11:09 PM
Fourtrail
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That is the evap vent that is capped and it only vents when hard on the throttle and the evap vent valse opens. The bowl vents are those two stand pipes in the carb top at the front edge of the choke horn. only late mc2150's had this extra vent for emissions, so having that port capped won't affect drivability.

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post #10 of 20 Old 06-23-2019, 05:53 AM
Dryseals
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Seems to me if it was a fuel delivery problem it would happen as you were driving up the hill. Just a WAG here, driving up the hill the motor is under load vacuum is low, once you get to the top and your foot comes off the accelerator, vacuum should go high again. Maybe your vacuum advance module is sticking and not moving smoothly.
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post #11 of 20 Old 06-24-2019, 12:29 PM
Beachcruiser
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Checked your fuel filter yet?

Fourtrail is correct on the vent situation.

probably out breaking stuff.
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post #12 of 20 Old 06-24-2019, 12:53 PM Thread Starter
orange150
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Quote:
Originally Posted by Beachcruiser View Post
Checked your fuel filter yet?

Fourtrail is correct on the vent situation.
Yea I just replaced it two or three weeks ago, just to be safe.

Drove it around a lot over the weekend, but mainly because the wife wanted to cruise, so I didn't get to check plugs.

But, I noticed it happens most often when I'm stopping hard (let's say, after a hard acceleration ), or compression breaking down a hill which I did a lot of cruising around the neighborhood with the Mrs. I think I'm dealing more with a flooding issue now; my guess is something is going on when the fuel sloshes forward under downhill or hard stop scenarios.

I checked my float last time I pulled it apart and it seemed fine. Might order a new float, needle and seat, accelerator pump diaphragm, and power valve and run through it again just to be on the safe side. It did do a lot of sitting over winter while I moved and did the hydroboost swap.

'82 CJ-8
258/AX15/D300
OME YJ 2.5"/31s
D30/D44/3.73s/Truetracs
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post #13 of 20 Old 06-24-2019, 02:01 PM
Fourtrail
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What is your idle rpm? Does the Idle rpm drop when hard in the brake pedal? Between the throttle slamming shut and the extra load on the engine from the hydro boost doing an abrupt stop. I would bring the idle up to about 900 and see what happens. If that 'fixes' the stumble, then lower the idle back down until it begins to stumble again, if the idle pre-stumble is acceptable, then call it good, otherwise like Matt said, it is time to read the plugs to see why it is stumbling.
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post #14 of 20 Old 06-24-2019, 05:04 PM
pedal2themetai
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HI did you invert the carb to check float drop?? that should show you how much fuel is being kept in the bowl..
pull top, hold float at pivot so it can move but not fall out and invert carb and see where it stops at. up to about half the float would be fuel line aprox.. top of float should not protrude past opening..
good luck
tim
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post #15 of 20 Old 06-25-2019, 10:45 AM
Beachcruiser
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A quick ballpark check of fuel level is to remove the carb top & hold a quarter against the carb side body w/ it's edge aligned w/ the carb body top lip. Fuel should just be touching the bottom of the quarter.

As you suspect, your float needle valve may not be seating well. A quick way to dress the seat (inside the brass fitting where the needle closes off the fuel flow) is to use some toothpaste on the end of a Q-tip. Insert & spin a couple dozen times. Make sure the float moves freely & the hold down spring (if equipped) is in the slot of the needle seat (backside).

Do you still have the stock by pass at the back of the fuel filter that returns fuel/pressure to the tank?

probably out breaking stuff.
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