Strange thump

Ok, my 68 Chevy has been sitting the better part of the year, since I was gone for work.

Dropped a new battery and a couple fresh gallons of gas into her, and she started up with a little help from some ether and a good dose of patience.

Last time I had it running, around New Years, I replaced the plugs, wires, rotor button, and distributor cap. Also did some oil changing. Everything ran fine.

Now, that I got her cranked with the help of my dad, there is a little pop sound as it idles, also a loud thumping noise and vibration.

Idles fine, but if you step on the gas, it's like there is a thump and a miss, couple with a sputter. If you ease on the gas, the only thing you notice is the little pop.

The pop seems to increase in speed as the rpm's ramp up.

Firing sequence is correct for the 250.

Any suggestions?

Went out just now, and tried to take it for a spin around the block. The pop sounds like a metallic tap with the doghouse closed. Kind of like tapping on an aluminum can, and the bigger thump sound is really bad when I give it gas. I tried to ease out the clutch to take off and both noises are really bad, and the engine sounds like it is trying to bog down. I was only able to move it back and forth a few feet each way.

so,,,,,,with the parking brake on and the van in neutral, is the noise there,,,,,does it change when you step on the clutch,,,,take the oil cap off,,,,,is the tapping in the valve cover,,,,,,let it idle, pull off each spark plug wire at the cap or plug,,,,,does anything change or feel different between any of the 6 cylinders? With the parking brake on, watch the engine, try and take off a little in 1st or reverse,,,,,,,,does the engine move or something clunk?

it would be helpful to pin point the source of the noise, especially the loud thump. Get a mechanic's stethoscope, (or even a wooden dowel up to your ear), and try to listen to see where the noise is coming from.

I would check the bottom end, and make sure the thumping is not a rod knock.

I'm goin in Doni's direction on this one and suggesting that the engine is lifting off of a broken motor mount (probably left)......Engine/trans movement could also cause all the hard linkages to clang and bang, or even scarier, make them bind..... You wouldn't really see that the mount rubber is separated while the engine is setting on it.... One cool thing about the early vans is that you can troubleshoot under the hood while a buddy drives....

OR.....you can jack up the engine with a floor jack (and wood block) at the oil pan to see if it readily lifts....

The idea of a motor mount had crossed my mind, but I'll definitely give it a go.

So went out today and did a review of my firing sequence. I had just retraced what the PO had when I put in the new plugs and wires. When I looked this evening, I found my mistake. the PO had it as 153642...

Switched the 2 and 4 plugs around... Fired up nicely, no thumps.

Still the pop sound, but only when I let off of the accelerator. Took it around the block, and the pop still picked up as the rpm's picked up, but only when I stomped on the pedal. Gradually increasing speed, the pop wasn't as pronounced.

Lifted the lid on the dog house, and the pop sound actually sounded as though it was coming from the air intake. Pulled up in front of the house, revved the motor while I was parked. The pop noise happened as I let off the gas. Also, there was a few backfires.

Could my carb use some tooling?

did you take out and look at those 2 plugs that were misfiring,,,,,,take some carb cleaner,,,,,spray down the carb at idle,,,,,if it speeds up,,,,spray the outside and the intake manifold and see if the engine speeds up,,,,,,,,,are the points and timing adjusted?

No, I didn't take out the plugs. I just swapped the wires for the 2 and 4 cylinders. I guess the sequence was off, since that fixed the issue of the thump. I'll have to pick up some carb cleaner tomorrow.

Haven't adjusted the timing. Everything is pretty much the same as the PO had it. He stated that he had it rebuilt and the mechanic bored out the cylinders a little. In his words, "added some pep to it." I don't know about that.

What am I looking at if the engine speeds up once the carb cleaner is applied?

since those plugs were not firing at the right time, they could be fouled,,,,,compare them to one of the other 4 good ones,,,,,,if the engine speeds up, you got a lean condition,,,,,not enough fuel or a vacuum leak,,,,,,you are looking for clues for us,,,,,so you dont get 100 guesses,,,,,,,is the Dwell 31-34 degrees,,,,timing 6 BTDC,,,,,,,is the vacuum and centrifugal advance units working? you got to make sure the basics are correct......

Went out today, and sprayed the carb cleaner.

The initial spray bogged the the motor a touch, but the idle remained the same, once the motor came out of the bog. No speed increase that I could tell. The pop sound is definitely coming out of the oil can looking device that sits on top of the carb.

I wasn't aware at first, but my dad told me that the choke was stuck open, and he had to force it to close. Thought I'd throw it out there to offer more clues.

I have a timing light somewhere at my grandparents, I'll have to go look for it this afternoon.

so you talking about the air cleaner that sits on top of the carburetor,,,,,is it like this or some other kind,,,,,,then the sound is coming up from inside the carburetor,,,,,run the engine with 1 spark plug wire off at a time,,,,,see if the noise changes when any 1 of those wires are off,,,

That's the set up. Just not a clean as yours.

I pulled the number one plug wire from the distributor, and took it around the block, no noises, no pops.

Reattached the number one plug wire, once I returned to the house, and the pop reappeared. No pop with any other wire.

Pulled the plug on the number 1 cylinder... I'm thinking it is fouled. Below are the pic of the air cleaner/carb setup, and two pics of the plug.

Nice detective work Don!

also,,,,,,,that popping could be from the #1 intake valve leaking or open,,,,,,,you need to take a compression test,,,,,,

Ok, I'll have to pick up a compression test kit, or see if my dad has one that I can borrow.

Also, how much trouble would it be to get on of those smaller air filters that goes directly on the carburetor? How would I go about plugging the hole in the side of the valve cover, or would it be better to just get a new cover?

you got the wrong oil filler cap on the valve cover,,,,,thats supposed to be sealed,,,,,,,that pipe out the side has an anti backfire valve in it,,,,,,probably clogged up,,,,,,,it provides the air to the PVC valve,,,,that air cleaner set up is for Smog controls,,,,,,,peoples take it off and put little hot rod air cleaners on the carb.....I probably got the part number for a rubber plug for that side tube,,,,,,,

Well, no luck with the compression test kit at my dad's. I'll have to pick one up tomorrow.

I went out and put a new plug in the number 1 slot. Still had the pop. For some reason, I sprayed some more carb cleaner into the carb, and the pop stop for about 5 or 6 revvs of the motor, then returned.

That chrome plug on the top of the valve cover is how I received it from the PO.

Ultimately I plan on dropping a 350 once I get back from my next trip at sea.

Also, just wanted to say thank you for all of the help!

I think Autozone or Oreillys will let you borrow one,,,,,,,the popping could be mechanical,,,,a valve, rocker arm, pushrod or lifter problem,,,,,,,or electrical, a distributor problem,,,,,,,,,see what the timing light says on the #1 and 6 wires,,,,,,they should say the same thing on the timing plate,,,,,,

Started up the van, and the radiator blew... Story of my life.


Oh well, just as well. I'm planning on dropping a 350 in when I get back from my next trip for work, and was going to upgrade the radiator anyways.

Could be the # 1 cylinder valves are out of adjustment. If the valves are not opening enough it could create a backfire. ReSet the lash on them and make sure it's right.

How do I reset the lash?