Engine: lark knock?

[1961lark]

I have a1961 lark (obviously) with a 259. I recently cleaned the carburetor and gas tank and now my problem is that it seems to knock. I'm guessing it's a main bearing? it's only noticeable under acceleration. my question is how hard is it to change out the main bearings? I suppose the transmission would need to come out also? Does the engine need to come out? Will the crankshaft need to be ground or will standard bearings work? this is very frustrating to just get it running well and have this now. the previous owner even said that it was rebuilt.... *sigh* I suppose the first step is to plastigage it....

[StudeRich]

The crankshaft will have to be inspected for damage & out of round before you will know if it must come out and be "turned". If it has been "Rebuilt" the Crank would already be undersize on Rods & Mains, and may need to go smaller yet.

It is most likely a Rod Bearing as Studebaker Mains rarely give any trouble.

It is very important before doing any disassembly or buying parts etc. to make sure it is not a fuel Pump or if Auto. Trans., a Flex Plate making the noise.

[55 56 PREZ 4D]

I had a slight knock/noise in my 259, until the fiber timing gear let go. [the original gear only lasted 57 years and 100,000 miles]
New fiber timing gear and the knock went away.

[jackb]

.... on your back: you can drop the oil pan and inspect the bearings/journals...... but that's on your back. No fun. In the end, the easy end, you will wrestle with installing the pan gaskets and rear main seal which can give you fits. I've done it twice, but now will only pull the engine and perform on the stand. At this time, pull the plugs and clean inside block..... I'd be skeptical of a "rebuild" comment at this time. Could be a simple shop or install mistake or maybe it wasn't the only quickie rebuild item done. GL

[1961lark]

how would I be able to tell if it's a fuel pump or timing gear? It's an automatic transmission, so a flexplate isn't an issue. I recently replaced the fuel pump, but I've noticed that when the car gets warm, the oil warning light will flicker. but as soonas as its revved up its fine.

[jclary]

how would I be able to tell if it's a fuel pump or timing gear? It's an automatic transmission, so a flexplate isn't an issue. I recently replaced the fuel pump, but I've noticed that when the car gets warm, the oil warning light will flicker. but as soonas as its revved up its fine.

A flexplate is only present in automatic transmission cars.

Another possible source of a knock is if a small piece of carbon deposit has broken loose in a cylinder and banging around between the piston and head.

[1961lark]

oops, my mind was on other things, I meant to say manual transmission. if it is carbon, while it eventually just break up or will the heads have to come off? if it iscarbon, which seems unlikely. the plugs looked perfect.

[jclary]

The last time I had a carbon piece knocking around, it was a rather loud knock. It was also with a Chevy. 350 V8. I poured some ATF down the carb while revving the engine. The smoke was horrible...but it burned the carbon out and ran fine until I sold the vehicle years later.

[1961lark]

so basically, you "seafoamed" it.

[warrlaw1]

If it only is noticeable on acceleration, I doubt that it would be a rod bearing or a main. We used to be able to drive an engine with a knocking rod bearing by ACCELERATING and coasting...then the noise would go away. It would be most noticeable at constant speed. Something else is going wrong.

[Spinneyhill]

What petrol are you using? Try lower octane gas?

Have you changed (advanced) the timing during all the tune-up activity?

Perhaps the bush mechanic's screwdriver stethoscope might pick up something at idle?

[Chucks Stude]

A high mileage Studebaker engine might have a piston skirt slap. I have heard that before, and had it confirmed by one of Smokey Yunik's students. He runs a racing engine shop up near Dallas.

[1961lark]

ok, I'll hopefully be able to look at it this weekend and get a more exact diagnosis. I did set the timing, I only set it to the IGN mark on the dampener. I always run non ethanol 87 octane gasoline. Will piston slap cause any damage? I know some later chevys had it and it was considered normal for them...