hello I had a 83 jeep Laredo dropped off for popping out the carb under a load.the customer put a 85 4.2 with weber carb and gm style distributor.so I compression tested 155 all cylinders,set timing tdc compression #1 all timing marks lined up and rotor pointed to #1,firing order is correct,plugs good,smoke tested found leaks at carb adapter plates replaced re tested checked good,set timing at 9degrees btc and idle 650 as book states,still popping only under load ,swapped carb with a known good one same thing,messed with distributor timing could not get it to stop popping so put back to stock specs,choke is fully open,electric fuel pump has good pressure,so sitting idles perfect can get it to pop sometimes hitting throttle fast not always.but when you try to drive it pops through carb and falls on its face.i am not sure wich direction to go now so I am hoping a jeep expert or someone here can point me in the right direction.thank you for any help/ideas
Need some expert Jeep help please
- Thread starter jeepssrepair123
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If it is popping through the carb guaranteed there is a timing issue. Double check the distributer is not off a tooth. Check the firing order again with the wires. It is popping through the carb because an intake valve is open on a compression stroke.
Double check everything Distributor, wires, firing order, tdc, timing, and cam timing. Even look for tracks in the distributor cap. Check the spark plugs. Something in your ignition system is not right.
Bent/stuck valve will do it but you said it has good compression so that is probably not it, but what about a bad timing chain? If it jumped a tooth that could do it, and possible still give good compression readings.
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sgtjdgelinas
TJ Enthusiast
I had a GM that popped through the carb during start up because of corrosion in the distributor cap. It ran like crap then wouldn't start at all.
thank you for your replys.so yesterday I rebuilt the carb did not change anything.but when I was messing with the timing if I set it to 26degrees btdc it ran down the road without backfiring does have a hesitation under quick acceleration but is best it has ran.so im thinking the cam could be off the only thing that does not make sense to me. is on tdc compression stroke #1 the distributor rotor points to #1 on the cap so that makes me think the cam timing is correct since distributor is cam driven.and when I check timing chain play by turning crank slow until I see the rotor move. it has 8 degrees of play wich would be normal if stock timing is set to 8 degrees btdc.and jeep manual says timing chain is worn out if it has 15 degrees or more doing that test.so I am a little confused.any ideas thank you for your help
Pull the plugs out and see if they are all consistent look for ones that might be dirty when others are clean or clean ones where others are dirty. Pull your distributor cap off again and look for moisture/carbon tracks. There is something wrong with your timing. I guaranty it. If it is popping in the carb it is not the carp it is compression leaking in the intake. That means a valve is open when it shouldn't be.
I don't know what your issue is I am just trying to give you suggestions. Some may seem obvious while others may seem obscure. Just trying to give ideas without looking at it.
But I will tell you that you have a valve open at the wrong time, and you need to figure out why.
Or you have a plug firing at the wrong time. Meaning it is firing when the valve is open.
My guess would be that you have a couple wires swapped. I would trace your wires again just to make sure you have them on right again. It is not your carb. Even if the carb was bad and dropped a lot of gas you would see black smoke form the tail pipe. A lot of it, and say it leaves gas in the intake. No plug should be firing when an intake valve is open. So if it is, the timing is off, and that is where you are now.
You are firing gas with an intake valve open. Now trying to figure out which one and why is the trick.
Does that make sense? If not, I can try and explain it another way?
A worn intake valve ? Even with good compression, plus, is the distributor driven by the cam? If its high mileage you could have excessive play there ... is a valve job a consideration?