Too much pedal = pop

[alg]

Have a 73 Thing, all original and stock.

If I am at higher revs (like 45 in 3rd or 55MPH in 4th) and back off the gas completely and then push the pedal down about 3/4 of the way I hear a pop in the back. (usually only do this when I have to 'jump' into traffic or to suddenly change lanes to avoid getting hit by someone on a digital phone)

I know a Thing will not leap to speed like my 6.0L V8, and the engine does not die when it happens, but it seems odd.

Carb is rebuilt and rebushed. All timing is spot-on and ignition and fuel systems are checked.

Could it be that accel pump-looking thing? Not much info on how to adjust.

[emersonbiggins]

Do you have any exhaust leaks?

[alg]

No exhaust leaks.
Have a Danish replacement for the stock exhaust. It was installed about one year ago.

Its not really an exhaust pop. It sounded more like a pop in the carb. It was muffled, and came from the engine compartment. On beetles when you have an exhaust leak you can get a pop when you roll off the throttle, and its not that same noise. Mine happens when you let off the throttle and then really lay into it.

I was thinking it may be related to running rich, and then when you hit the pedal you got a squirt of fuel that drips down the intake and ignites.
But the car does not run like its running rich, and there is no real mixture adjustment for the main jet. Odd.

alg

[Ian Epperson]

Perhaps it's not coming from the engine? Something that's banging on acceleration - like maybe the back seat fold-down supports?

There's been too many times that I've hunted down an engine noise only to discover something physically rattling around the engine compartment (dip stick, oil filler cap, screwdriver, etc.)
_________________
VW-181 shirts and stickers.
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[alg]

Thanks for the info.
I know its not something just loose. I do most of my ACVW troubleshooting by ear, and nothing is rattling in my Thing.

The reason I think its the carb is that it reminds me of what happens to a Holley 4bbl carb on a Ford 289 when you first install the carb.
You have to install the Holley and then fire it up to get it dialed in.
Usually it runs very rich when you dial it out of the box.
You bolt it on the motor and if you hit the throttle before it is warmed up and adjusted, you get a pop and some flame comes out of the carb.

With the limited info on adjusting a 34PICT3 I cannot find out how to adjust the carb besides the idle. I have done the idle adjust, and the carb runs perfectly at 900RPM when warmed up. (actually runs perfectly when cold as well)
But I just have this sense that the Thing is running a little rich. I get this feeling that the pop is a side-effect of the rich condition.

So, if the choke is ok, and the carb is adjusted for the proper idle, is there any other adjustment I may need to make? Does one have to adjust that little lever that pushes on the throttle linkage? (not the fixed idle screw)

alg

[emersonbiggins]

[kubelmann]

This is a very general automotive rule. If you back off the gas and it make an audible noise (as in bang pop or pow), you have an exhaust leak. You can use carb cleaner to detect the leak by spaying all the joints and looking for the most obvious evidence. This procedure takes two tech to detect. Senor Biggons called this one imho.. K-mann

[nightphorge]

If you have a custom exhaust the popping can also come from not having enough back pressure....

[alg]

I have validated it is definitely not an exhaust or intake leak.

The pop happens after you press on the gas, not when you let it up.

Since the carb was rebuilt and rebushed I think the issue is the accel pump.
(it has not been touched and is the only remaining adjustable thing in the carb that is not in a known state)

I read that the accel pump can be adjusted, but the Bentley manual and a reference on a web site only have this statement:
"The 34 PICT-3 carb's have a bell-crank with an adjustable [accel pump] segment."

Does anyone know how to adjust this on a bone-stock 73 Thing? I'm just not finding the info anywhere, even in the RMTOMThing CD I own.

alg

[kubelmann]

Good ole Rob And Dave to the rescue:


go to:


http://www.vw-resource.com/accelerator_pump.html#adjust

[kubelmann]

Adjust the Injection Quantity

(34 PICT/3 Carburetor)
Loosen the retaining bolt on the adjusting segment (on the accelerator pump mechanism, right side of the carburetor).
Turn the bell-shaped adjusting segment counterclockwise (so that the retaining bolt moves clockwise in the slot in the adjusting bell) to increase the injection quantity; turn the adjusting segment clockwise to decrease injection quantity.



Accelerator Pump Adjuster
~~~

Please note carefully: It is the ADJUSTING SEGMENT that you turn counterclockwise to increase the injection quantity, NOT the retaining bolt. If you do it backwards, you'll have a terrible "stumbling on acceleration" problem. (Once again, "Voice of Experience!")

Tighten the retaining bolt and recheck the injection quantity.
Note: To compensate for the flat spot inherent in the 009 distributor (but ONLY this distributor), adjust the accelerator pump injection quantity to the maximum; i.e., move the adjusting segment counterclockwise as far as it will go.

~~~

[alg]

Thanks KMann.
I use vw-resource like a bible and never saw a link to the page you listed.

I just read the article about the fuel injection in the latest Hot VW's and its making my carb issues seem like a time-consuming pain in the ass.
EFI would cost more, but it would take me about 4 hours to install and I would be done with it.

Oh well, its worth the 7 months of carb problems to have my Thing. Ya, that's the ticket. (sigh)