a loud pop from carburetor

[krumpelsaurusrex]

working on my brakes today i started up the engine and from in the cab I heard a slight ticking or knock noise. Worried its the engine, I got out and revved the engine a few times by hand and got a loud pop from the carburetor. it only happened once, but I don't know why. I shut the engine off and am letting it sit to see what happens. The oil in the engine from the PO is really dirty so I am going to change the oil before I start it up again. Hoping that knock noise is just from really thin dirty oil.

Any thoughts?

69 f-250 w/360

P.S. right now there is no air box or filter on the carb. still need to get one for it.

[MotorDog]

Sounds like you really have two problems, neither of which is serious. Sometimes, when an engine has sat a while, the oil will "bleed down" from the lifters. When started again, you will have a "dry" lifter or two and the clatter is from the lifter being pumped up again with oil. This is common with our trucks. In fact, if you have an Owner's Manual from 1970, you will see on Page 15 a paragraph titled TAPPET NOISE. Here is what it says word for word:

"It is normal for the oil to leak down from some of the hydraulic tappets in your engine during extended shut-down periods (overnight). As a result, these tappets may clatter for a few seconds after the engine starts until oil pressure builds up. This momentary start-up noise is normal and is not detrimental to engine operation"

Regarding the "Pop" from the carbureator, what you probably had was a little backfire from some un-burned gasoline from a cold engine after revving it up and then letting it float to idle. These aren't generally any problem provided you have the carb covered with an air cleaner. Backfires from an un-covered carb have set many a engine fire.....

[george worley]

Change the oil and filter and put a can of Marvel Mystery Oil with the new oil. Run it a week and change oil and filter again. Marvel Myatery oil cleans the varnish out of the lifters and helps clesn the sludge out of the engine.

[NM5K]

A "tick" sound from the valve train area is usually no big deal. That can be
adjusted out.
A "knock" sound is more serious. Usually bearings. A knock will sound much
thicker and heavier. Kind of like someone is whacking the block with a
big hammer. That sound is not good news.
But a tick will sound much thinner, kind of like tapping the top of the
valve cover with a spoon or something.. That's not really that big a deal
in most cases. Or at least compared to bearings.
I consider tappet noise a tick, vs a knock.
When I spun a bearing, I got a pretty good knock noise.
Once that happened it was new engine time. Or rebuild..
I don't have all the stuff to rebuild an engine, so I bought
a rebuilt long block.

[krumpelsaurusrex]

A "tick" sound from the valve train area is usually no big deal. That can be
adjusted out.
A "knock" sound is more serious. Usually bearings. A knock will sound much
thicker and heavier. Kind of like someone is whacking the block with a
big hammer. That sound is not good news.
But a tick will sound much thinner, kind of like tapping the top of the
valve cover with a spoon or something.. That's not really that big a deal
in most cases. Or at least compared to bearings.
I consider tappet noise a tick, vs a knock.
When I spun a bearing, I got a pretty good knock noise.
Once that happened it was new engine time. Or rebuild..
I don't have all the stuff to rebuild an engine, so I bought
a rebuilt long block.

I would say more of a tick then knock based on your post. It does sound like it's near the valve cover, but it's not constant. Ticks at a wierd interval for a few seconds, then stops. It's not constant.

[lee_ford]

Two things…

Go out after dark and fire it up. Look for a spark plug wire cracked and sparking against the engine. This can fool you in day light, but in the dark it will really show up. If you are in a hurry, and do not want to wait till dark, start it up and just grab the plug wires. Run your hand down between the plug wires and the engine. Be sure you are grounded by holding something else with your other hand. When you feel a shock, you found it. OK, just kidding. The spark is VERY potent even on a stock coil. I repeat, DO NOT DO THIS. Wait till dark.

With the motor running, put the working end of a long screw driver against the valve cover and your ear against the handle. If it is a valve lifter going south and causing rocker arm tapping you can hear it and isolate which side it is on.

[lee_ford]

I would say more of a tick then knock based on your post. It does sound like it's near the valve cover, but it's not constant. Ticks at a wierd interval for a few seconds, then stops. It's not constant.

Just reread your post. This sounds like it may be a plug wire or two or more. A lifter should be a steady tick or click.

[krumpelsaurusrex]

I would say more of a tick then knock based on your post. It does sound like it's near the valve cover, but it's not constant. Ticks at a wierd interval for a few seconds, then stops. It's not constant.

Just reread your post. This sounds like it may be a plug wire or two or more. A lifter should be a steady tick or click.

in the month I have had it I have not pulled the plugs yet. Now that my brakes are done I can do that.
Does pulling each plug, chech compression while they are out, replace with new plugs (any reccomendationd on brand or type) and Also replace spark plug wires, seem like a reasonable solution?

Plug wired seem to be in decent shape from the outside, but I know that does not mean much about the inside.

[lee_ford]

Does pulling each plug, chech compression while they are out, replace with new plugs (any reccomendationd on brand or type) and Also replace spark plug wires, seem like a reasonable solution?

Yes. Sounds like a good plan. New plugs and wires are not that expensive. Also, to check compression go ahead and pull all the plugs. The starter will not have to work near as hard. Just make note of each plug wire as to where it goes on the distributor and you should have no problem putting on the new set afterwards.

Plug wired seem to be in decent shape from the outside, but I know that does not mean much about the inside.

Those wires might look good, but if even a tiny crack on the underside next to the engine would not be seen. Like I said, fire it up and run your hand between them and the engine. Again, just kidding. DO NOT DO THIS!!! Look at them after dark. If this sound you hear is the wires, you WILL see it sparking and this sound could be described as a ticking.

Or just replace the plugs and wires and see if the ticking goes away. But I would still recommend looking at it after dark. Looks kinda cool.

[suavematt]

Like I said, fire it up and run your hand between them and the engine. Again, just kidding. DO NOT DO THIS!!! Look at them after dark. If this sound you hear is the wires, you WILL see it sparking and this sound could be described as a ticking.

.... Aw' just stand up on your bumper & pee over the rad' support on to the distributor while she's runnin'
... I'd change the plugs/wires/cap/rotor anyhow, just for the sake of preventative maintenance & for the plugs, I've always had good luck with MotorCraft. But I'm sure everyone has their own "favorite"
Later!

~ Suave'

[krumpelsaurusrex]

It's a lot easier to just ask me to stop posting then to try to off me by truck repairs!

I started the truck up and no sparking came from the wires. I really didn't notice the noise, so really unsure as what it was. I'll keep an eye out though. Before the pop and subsequent oil change, the truck fired right up. It could sit for over a week and fire right up. Now it chugs for a second or two before it starts. I think I am going to start with the plugs first, then work my way to wires, then cap and rotor. The 4th is coming up and I need some beer money!

[fordman]

one thing that can happen when the carb backfires. is that the power valve can get blown out of them. so that could be the chug before the start.

[krumpelsaurusrex]

one thing that can happen when the carb backfires. is that the power valve can get blown out of them. so that could be the chug before the start.

Honestly, I had no idea what a power valve was. A quick google search turned up a decent little video that talks about how to replace it on my Holley Carb. It does seem that all of them are special order, at least from Auto Zone.

http://www.wonderhowto.com/how-to/video ... ve-267173/

It seems simple enough, wish there was a way to find out if that the issue before replacing it. But it makes sense that it may be the reason behind the chug. Before it backfired it always fired right up.

Funny, I took on this project for multiple reasons, one was to learn more about cars. I definitely am doing just that, and quicker then I had expected.
Thanks for all your help guys!

[NM5K]

I've seen lifter noise come and go.. They can kind of oscillate a tad as far as the noise.
I wouldn't worry about that much. It will usually clear up after being driven for a good
while. I kind of doubt it is the plug wires, but that's easy to check in the dark.
It's also easy to see if the power valve in the carb is working. Just shine a light
down into the carb and have someone push the gas pedal, or grab the linkage
near the carb and give it a push. If you see a thin stream of gas spray, it's ok.
If you see nothing, it is probably bad. The only time the valve operates is when
you are actually pressing the pedal to give it gas. It's just giving an extra shot
of gas so you don't feel a "flat spot" in the acceleration. It would run fine at a
steady speed.

[lee_ford]

If you see a thin stream of gas spray, it's ok.
If you see nothing, it is probably bad. The only time the valve operates is when
you are actually pressing the pedal to give it gas.

I think you are confusing the Power valve with the Accelerator pump. If that spray you are talking about happens even with the motor not running it is a pump. The Power valve operates inside the carb fuel flow path to increase the fuel flow under conditions when more fuel is needed than just cruising. The jets are calibrated for cruising and more is needed when the throttle is opened wider to accelerate. The Accelerator pump is needed to give that extra fuel between cruise and the time the power valve can provide more fuel.