Mystery backfire/popping through carb

[jakerc02]

My 79 J10 is getting a backfire through the carb that we just can't figure out. It also hesitates/bogs when you hit the gas too fast. Here's a link to a video of it- https://youtu.be/fjM6FAQu5QM it seems to have come on over time and I've done a good bit to the truck to try and help that plus fix other problems. The truck drives good now but just has this problem. Here is some info and things we've done recently that can hopefully help. Any ideas on what this could be would be really helpful, thanks.

-TFI upgrade and MSD box
-Rebuilt the 2150 carb a few days ago which helped other problems but not this.
-Initial timing is 12* and total is about 37* at 2000rpm with ported vacuum advance
-New EGR valve
-Carter fuel pump and added a return line so it's getting good fuel without vapor lock
-21 inches of vacuum
-New spark plugs
-Played with the mixture screws to see if it was too lean or rich but that didn't effect the backfire
-Compression is all between 110 and 120 psi throughout the cylinders
-Did engine clean out with sea foam a few weeks ago

[Stuka]

If it backfired through the carb, you probably blew the power valve, which may be pooring fuel down the intake.

Has it ever run well? Did all of this start after the above mentioned work?

[jakerc02]

It did this before all the work, but I don’t remember if it’s been doing it since we got it in December. At least for a couple months though. We replaced the power valve in the rebuild but could it have already blown it?


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[sirrus]

Backfiring through carb can kill power valve really quickly. Easiest way to check is to pull vacuum hose from power valve and if there is any fuel in it, it's blown

[jakerc02]

Checked it and the power valve is fine. Even though we have good vacuum numbers could we still have a vacuum leak?


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[tgreese]

Mixture screws only affect the mixture at idle. Above idle, the main jets in the bottom of the float bowl control at high vacuum, with the power valve richening the mixture at low vacuum (heavy load). The accelerator pump richens the mixture momentarily as the throttle opens, to overcome the lagging vacuum signal.

Might be worthwhile to use a mechanics stethoscope to listen around the base of the carburetor and around the manifold for vacuum leaks. Your power valve should connect directly to the manifold vacuum fitting nearby on the manifold. You should be able to see the vacuum change in the line to the power valve.

Does the engine backfire while the choke is closed? If it does, I would suspect it's not a too-lean issue.

[HowardT64]

Could the timing be a bit off? Or perhaps a lifter has gone out?

[will e]

That does not sound right.

[tgreese]

A bad lifter or flat cam lobe will clatter. And I don't think it will cause a backfire. Good idea to check the timing again.

How many miles on this engine? If there's nothing else it could be, I would suspect next that it jumped time. Might be worthwhile to turn the crank bolt back and forth with a wrench and watch the rotor to see how much slop is in the timing chain. Replace the timing set is a pretty big job, but something you can do with the engine in the car.

[jakerc02]

I don’t get what you mean by the power valve hooking up to vacuum? Here’s a picture it and the only vacuum line near it is the ported spot to the right of it. The odometer says 72 thousand miles but there’s a good chance it has rolled over. I’m guessing it has looking at the rest of the truck.


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[tgreese]

That's a 2100, not a 2150. A 2150 will have a vacuum port on the power valve can that has to connect to manifold vacuum.

This is a 2150:

J20PowerValvePort.jpg

To test for a blown power valve on your carb, you need to remove the power valve cover and look for gasoline. It should also be a 2-stage power valve.
https://www.carburetor-parts.com/Power- ... _2739.html

[jakerc02]

Checked the power valve and it’s still good.


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[letank]

79 came with a 2150.... do you have the altitude compensation bellow in the back of the carb... someone may have put a non vacuum powervalve cover from a 2100... post pict of the rear facing portion of the carb... and may be the stamp on the driver's side of the carb to see if it is a 1.08 or a 1.21 venturi carb, should looks like this for the 2100



and the back of the 2100



I may as well post the front of the 2100

[jakerc02]

It has the altitude compensation and 1.21 is stamped on the side. Everything I’ve seen online shows it’s a 2150, including the rebuild kit we bought for it.


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[jakerc02]

At least I thought haha


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[jakerc02]

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[tgreese]

TSM79scrap.png

The '79 TSM says that these Jeeps were only equipped with a 2150 when delivered above 4000' (?) elevation. The '79 TSM shows both 2100 and 2150. I don't think the venturi size separates 2100 vs. 2150.

That is indeed an altitude compensation unit (aneroid). A 2150 carburetor body does not include an internal passage for vacuum to operate the power valve. Look at the '79 TSM - free to read and download at the Tom Collins site. Could it be that the rebuilder put the wrong cover on the power valve, and now you have no power valve action?

Or the '79 2150 is a one-year design, with no external vacuum connection? I leaning toward this, considering the vacuum diagrams don't show the expected 2150 vacuum connection from the power valve.

If you were so inclined, you could take the power valve cover off and look for the vacuum passage ...

TSM79scrap2.png

https://oljeep.com/gw/79_tsm/79_Jeep_TSM.pdf

[jakerc02]

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[jakerc02]

Looks like a vacuum passage to me. Also when you move the throttle the accelerator pump linkage doesn’t really move till more than half throttle. Is there any way to fix/adjust this besides just switching up the holes it’s attaching too? https://youtu.be/RlB7GDXqg9I Here's a video of it.


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[tgreese]

Looks like a vacuum passage to me. Also when you move the throttle the accelerator pump linkage doesn’t really move till more than half throttle. Is there any way to fix/adjust this besides just switching up the holes it’s attaching too? https://youtu.be/RlB7GDXqg9I Here's a video of it.


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I have 3 of these carbs to look at (2 2100s and 1 2150) and all of them are backwards from what you show. The accelerator pump action all happens in the 1st half of rotation, and is all done by the time the throttle gets to 1/2. I suspect your carburetor is not put together right.

To get to that area, you remove the throttle flaps from the throttle shaft and slide out the throttle shaft. The flaps are held in place by little screws that are staked to lock them in place. Generally you only remove the throttle shaft if you are going to bush the carb body to cure leakage around the shaft pivots. Carb shop stuff.