1975 F250, 390, 4bbl, comp 268h kit, stock heads, headers, msd.

I have been chasing a noise for a few months now and am at my wits end. I rarely and barely hear it when the motor is cold, once warm/hot the noise is most noticeable at 1500 rpm with my foot just resting on the gas, no load. I hear it on decel from cruise, 2-2.5k; as the tach sweeps down past 1500rpm the noise fades in and then back out once around 1300 rpm. I have now begun to hear it faintly at around 2k, also with no load and foot resting on the gas. I do not hear it under hard acceleration. Its a pretty healthy "tick" and almost sounds like a really sharp exhaust leak.

its definitely coming from the drivers side of the motor.

i have replaced the header gasket 3 times now, all with no evidence of a leak big enough to make half as much noise.

i have run the motor with the valve cover pulled and can't pinpoint the noise with a steth or a rubber hose. all lifters/pushrods were spinning, though some more sporadically than others

i checked lash on all valves 5-8 and they held my .147 stack of feelers snuggly (lifters compressed, pumped back up immediately after releasing them)

i have checked and rechecked timing, jetting, plug gap.

i have no idea where to look next, oil pressure, temp and overall feel are all great. sorry for the long post, any help is appreciated.

 

Try using the stethescope from under the engine with a friend doing the gas. Check the block walls at each cylinder to pinpoint the location. You may have a loose wrist pin or a piston slap starting.

 

wouldnt something in the bottom end make noise any time the motor is running? also, a compression check showed an even 150 on all cyl's if that helps.

 

I'm suggesting that you listen at the walls of the block, not the skirt. A piston pin or piston slap would be in that area. Also they are two different noises a pin would be a clicking or tapping noise and a piston slap would have a more hollow, sort of echoing sound.

 

Mel has the right idea, a wrist pin or piston noise will not make noise under hard acceleration and can fade in and out like what you're describing.

 

i stethed the cylinder walls and they all sounded the same. i did still notice more noise coming from number 5 exhaust valve AREA, when raising rpm to where i always hear the noise, maybe my steth is crappy but i didnt notice anything out of the ordinary on the cylinder walls.

 

i looked and listened to as many wrist pin and piston slap videos i can find and none of them are close to the sound i'm hearing. all of these videos presented more of a knock than a tick and i'm sure the bigger internals in a 390 would give a deep knock. i did manage to pull out the lifters for 5,6, and 7, all of which looked perfect, along with the cam lobes. rocker tips are perfect, push rods straight. tried pulling spark plug wires to see if one killed the noise but none of them did. i tried stething from underneath and again, nothing from the block wall and it still sounds like its coming from the head area.

 

Well, another possibility would be a sticking valve on #5 and the exhaust would be the first place I would check since that is where you heard the noise. It will get a little messy if you do it the way I suggest here. With the valve cover off and a timing light on #5 plug wire, start the engine and see what is happening with that pair of valves as the noise starts sounding off. If it's a sticking valve or broken spring you will see the valve start going out of synch with the rpm.
Let us know your findings.

 

argh!, i just put the valve cover back on and cleaned up. i'll research a sticky valve and pull the cover off tomorrow. i just had a valve job done maybe 4-5000 miles ago when i installed the comp kit and everything is very very clean everywhere my eyes can see. if its a sticky valve are there any fixes that dont involve pulling the head?

 

Not that I know of.
Is there any chance you over revved the engine? A valve/piston collision could bend a valve enough the cause sticking and still run pretty good, for a while at least.
Pray for a broken valve spring.

 

no chance of over revving, i'm way too cautious and have never revved past 4k. i have checked vacuum recently and it was more or less steady at 21-23 inches at idle, no big jumps or anything. cursory inspection of valve springs (looking and trying to move any of them) turned up nothing, everything seemed fine. i'll see what tomorrow turns up...

 

Good luck with it.

 

When you say "268H kit and stock heads" does that mean you installed the recommended valve springs too?

 

Yes, the springs came with the kit from comp

 

well, after using three different methods of listening to the valve train, i have pin pointed the noise to the #5 exhaust valve, specifically the rocker tip/valve stem area. yesterday i pulled and checked this lifter and it looks picture perfect, smooth shiny surface on lifter and cam lobe, feels just like all the others when compressing it, releasing and then immediately re compressing it, they all pump right back up. the lash on this and all other valves is still right around .15 (.147 stack of feelers) and watching the pair of valves with a timing light shows nothing out of the ordinary, they are bouncing away like all the other valves. my springs say to be installed at 1.9 inches and they are all right there. again it is most noticeable when hot.