1968 340 push rod issue

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Slantsix64

Slantsix64

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Hey guys working on my buddy’s 340 dart he just got, pretty much stock 68 340 compression heads valve train,etc only thing done on the rebuild is they used a size bigger Mopar purple camshaft (as were told)

Issue is that I believe the push rods might be too long, why I think this is because, when he got the car there was a loud ticking and we found the valve lash was extremely loose, the adjusters were getting backed out. So not knowing the lash, and to get this thing running and out of my yard we tightened it to ZERO lash.

A couple weeks later he tells me the ticking is back, we bring it to my yard sure enough some of the rockers backed out once again.

I was suspecting the push rods are too long, causing this issue, also the car likes to idles around 900RPM and it’s a 4 speed.

I feel that his cam is not as radical as it sounds, just because the push rods are adding more lift to it being the wrong. We’re going to get my push rod checker and see what going on.
 
Um no, the pushrod length has nothing to do with the nuts backing off.
The only thing that can cause the nuts to back off is that they, the adjusters, or even the arms, are no good.
But the thing that causes ticking on rockers in the which the nuts are NOT backing off, is another matter. You got three/four choices;
1) the pushrods are hitting the tunnels in the heads, or
2) the lifters are not working properly, or
3) the lobes are falling into the pan,
4) Zero lash on hydraulics is too loose; any wear occurring anywhere in the valve gear will immediately show up as ticking.
Typically, we tighten hydraulics 3>4 turns. But some of us run at 1 turn or even 1/2 turn after break-in. ; once the lifters are "broke-in". At half a turn, I might have to adjust them maybe every two years.
I run at this minimum lash, because if the lifters ever pump up, there is no chance that the pumped-up lifters can ever drive the valves into the pistons. Thus, I can blissfully rev the engine until the rods fly out the side. My stock rods seem to be good to 7200, so that's when I shift. Your results may vary.

Therefore, if this engine keeps developing ticking, on rockers in the which the lash-adjusters are good, you gotta suspect lobes going down.
Ima thinking it's time to inspect the lifter feet.
 
Last edited:
Slantsix64 said:
also the car likes to idles around 900RPM and it’s a 4 speed.
Click to expand...
That is likely a tuning issue. My guess is that she is running too much Idle-timing, and if you close the Idle speed-screw, she wants to hesitate at take off.
The cure for both is to run less Idle-Timing.
If you brought that car to me, I can make it idle at 550rpm, in gear and pulling itself on flat/level/hard ground; with any cam up to a 292/292/108, the biggest that I have tried. The only tricks I will use are idle-timing of 5*, and a couple of tiny holes in the primary throttles. Plus
If the piston ring-gaps are too tight, I will change that.
If the engine consistently runs over 210, I will change that.
If the carb is sucking hot underhood air, I will change that.
If your WP speed is too slow, I will change that.
Leave it with me, and when you get it back, it will do what I said it will, even if I have to pull that crummy cam outta there.
There is no issue with idling at 900 with a 4-speed, except you can't drive very slow. If the car still has 3.23s and a 2.66box, then the slowest speed she can drive is about 8 mph@900 rpm. which sucks. If you want to drive slower, you gotta ride the clutch, and in the meantime; with excessive idle-timing and a nearly closed throttle, the doggone car is getting jumpy. The cure for that is less Idle-timing and more throttle. And if the engine is unhappy, it is because she wants some bypass air.
One size bigger Mopar cam, to me means the 276/284, or possible the 284/.484 cam; both of which can be had in LSAs of either 108 or 114s, and in 110s aftermarket. So there's a lotta variables right there.
As for Idle-timing, that 340 will like big numbers, in the range of 25>30 degrees. But, as you are finding out, you can't run it there with a 4-speed.
Try 12>14, and reset the speed-screw/mixture screws accordingly.
Make sure;
>the secondaries are fully closed.
>the PCV system is functioning as designed.
>If you have a metering-rod carb, make sure the rods are staying down.
>If the idle is not smooth, Temporarily introduce a bit of bypass air as may be required, until the mixture screws like to be in the center of their working range; about 3/4 turn on a Holley-type, or 2.5 on a Carter-type. I do this into the PCV hose or into the PCV itself . Once I got her running sweetly, I measure the hle I drilled in the PCV system, calculate the area, devide it in two, recalculate for two holes, then drill each primary T-valve the appropriate size. I put the holes on the transfer-slot side, between them and the Idle-discharge port, about 3/16th from the edge. I usually drill 1/32 smaller on the first test. Then go back and plug the hole(s) drilled in the PCV system.

If you drill the throttle-holes too big, don't panic. Chamfer the holes, warm them up and solder a slug in there, making sure it overlaps the chamfer nicely. Then dress it down just a lil.
I have not had much success drilling the solder plugs, so I just move over a lil, with a smaller hole this time.
Some guys will/may tell you that drilling the plates is the wrong thing to do. Lemmee tell you a wrong thing to do; wiping from the back.
 
AJ/FormS said:
That is likely a tuning issue. My guess is that she is running too much Idle-timing, and if you close the Idle speed-screw, she wants to hesitate at take off.
The cure for both is to run less Idle-Timing.
If you brought that car to me, I can make it idle at 550rpm, in gear and pulling itself on flat/level/hard ground; with any cam up to a 292/292/108, the biggest that I have tried. The only tricks I will use are idle-timing of 5*, and a couple of tiny holes in the primary throttles. Plus
If the piston ring-gaps are too tight, I will change that.
If the engine consistently runs over 210, I will change that.
If the carb is sucking hot underhood air, I will change that.
If your WP speed is too slow, I will change that.
Leave it with me, and when you get it back, it will do what I said it will, even if I have to pull that crummy cam outta there.
There is no issue with idling at 900 with a 4-speed, except you can't drive very slow. If the car still has 3.23s and a 2.66box, then the slowest speed she can drive is about 8 mph@900 rpm. which sucks. If you want to drive slower, you gotta ride the clutch, and in the meantime; with excessive idle-timing and a nearly closed throttle, the doggone car is getting jumpy. The cure for that is less Idle-timing and more throttle. And if the engine is unhappy, it is because she wants some bypass air.
One size bigger Mopar cam, to me means the 276/284, or possible the 284/.484 cam; both of which can be had in LSAs of either 108 or 114s, and in 110s aftermarket. So there's a lotta variables right there.
As for Idle-timing, that 340 will like big numbers, in the range of 25>30 degrees. But, as you are finding out, you can't run it there with a 4-speed.
Try 12>14, and reset the speed-screw/mixture screws accordingly.
Make sure;
>the secondaries are fully closed.
>the PCV system is functioning as designed.
>If you have a metering-rod carb, make sure the rods are staying down.
>If the idle is not smooth, Temporarily introduce a bit of bypass air as may be required, until the mixture screws like to be in the center of their working range; about 3/4 turn on a Holley-type, or 2.5 on a Carter-type. I do this into the PCV hose or into the PCV itself . Once I got her running sweetly, I measure the hle I drilled in the PCV system, calculate the area, devide it in two, recalculate for two holes, then drill each primary T-valve the appropriate size. I put the holes on the transfer-slot side, between them and the Idle-discharge port, about 3/16th from the edge. I usually drill 1/32 smaller on the first test. Then go back and plug the hole(s) drilled in the PCV system.

If you drill the throttle-holes too big, don't panic. Chamfer the holes, warm them up and solder a slug in there, making sure it overlaps the chamfer nicely. Then dress it down just a lil.
I have not had much success drilling the solder plugs, so I just move over a lil, with a smaller hole this time.
Some guys will/may tell you that drilling the plates is the wrong thing to do. Lemmee tell you a wrong thing to do; wiping from the back.
Click to expand...
Thanks AJ ! Right now it currently has a 1405 600 eddy, i have a stock AVS for a 340 laying around, the car has 3.55 gears
 
Slantsix64 said:
Hey guys working on my buddy’s 340 dart he just got, pretty much stock 68 340 compression heads valve train,etc only thing done on the rebuild is they used a size bigger Mopar purple camshaft (as were told)
Click to expand...
All right, first off you need to post some pics of your rockers. Stock '68 340s did not have adjustable rockers, so we need to know what you've actually got.
At this point I'm going to guess stock '64-'67 273 mechanical rockers. You didn't say what the replacement cam is- hydraulic or solid? If hydraulic, the stock 273 pushrods cannot be used and custom length rods need to be checked for and ordered. If it's solid, the pushrods MAY be able to be used, depending on the brand of lifter used- again, checking and ordering the correct ones if necessary.
The other issue with these rockers is that the adjusters are an interference thread, and after 60 years of use they lose the "tightness" in the threads and no longer maintain their adjustment.
The common fix is to chase the threads in the arms and spot face the top of the arm; then install a jam nut. Crane used to offer upgraded adjusters but since being bought out by Comp Cams they have not been available... Alternately, some people will just give the rocker bottoms a couple taps with a hammer (to close up the threads) and re-install the adjusters.
 
And get with 3B Engineering to correct the geometry, he knows of what he speaks.
 
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