OK so I've been chasing a mid RPM(aprx. 1000-2500 RPM) knock for a few months now. Last time I had frustrating issues with this motor you guys pointed me in the right direction(I had a miss adjusted vacuum advance) and I'm hoping ya'll can help again. OK so back in June I got the motor running correctly and drove it around and the world was happy. Then I had to do some work to the truck and it was down for a few months. I'd go out and start it and let it idle a bit, but couldn't drive it around. Got everything fixed, drove it around some more, then it sat for about a month cause I just didn't need to drive it. So I was moving last month and needed to use the truck on the short trips it was fine. Then I had to take it out on a long trip and about 3/4 of the way back the sky opened(literly and figurtivly) and it's been nothing but trouble ever sence.

What it's doing is between 100-2500 RPM I get what sounds like spark knock. While driving it likes to cut out when I let off the accelerator, but while in the driveway it won't cut out after I goose it. Also it seems to not like full throttle in 4th gear. Twice on test drives when things seemed fine I'd go from a stop to full throttle 2nd and 3rd gear pull good the engine sounds good, but when I shift into 4th and get back on the throttle *POP* backfires up through the carb so bad that it blew the choke flap over the air horn. I've checked for vacuum leaks and can't find any. I've changed fuel filters and drain the tank refilling with 93 octane plus Lucas octane booster, just in case. I've replaced the timing chain and the distributor, also just in case. I've taken all advance out of the vacuum advance. Put in new plugs, wires, cap, and rotor. I've got the timing backed down to 6*, but in teating and checking have backed it below 0* on the balancer. I can get the motor into this "knocking rpm band" and just retard the dist until the engine starts to "fight" and the knock doesn't go away. At this point I feel like all that's left is to conclude that it's either a rod knock or a piston pin.

Here's the engine specs: 1963 Ford 352 .040" over, Sealed power pistons 246NP .010" milled from the deck, .020" milled from the heads, stock intake and exhaust valave sizes(C1AE 6090A heads), Weiand dual plane intake, 1" open carb spacer, Holley 600cfm 6.5" power valve & #72 jets screws out two turns, shorty headers, Clevite RV cam: Lift 484/510, Adv. duration 270/280, Duration @ .050" 204/214, Lobe sep. 107/117, Professional Products balancer(80009), crank turned 10/10. This motor has only about 150 miles since the rebuild. And is in a 1968 Jeep M715 backed by a NV4500, NP201 t-case, and 5.88 geared axles.

Thanks
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Not to take my word for anything but that seems like a lot of material removed from the heads and deck. I could see some high compression there and possible issue with the pushrod length. That along with some lift perhaps valve spring tension. What rockers and springs are you running? I am guessing you are running on Premium?

 

Running stock rockers and springs, nothing high-performance or special. I am running high octane fuel and have thought about the compression being an issue, so someone did some bench top math for me and compression shouldn't be too high. Here's what he gave me:
Let's say the pistons are Sealed Power #246NP-040.
Compression Distance: 1.816"
Head Type: Flat Head w/ 4 Valve Reliefs - 9.0:1 Compression Ratio
Minimum Piston to Wall Clearance: .0015" At 2.150" From crown of piston
Pin Diameter (Inches): .9751"
Pin Diameter (mm): 24.768 mm
Ring 1 Qty: 1
Ring 1 Width: 5/64"
Ring 2 Qty: 1
Ring 2 Width: 3/32"

Let's say the block deck height was stock at (+/-)10.170", then you cut it 0.010" for a new block deck height of 10.160".

Let's say the rods were stock length at (+/-) 6.540".

Let's say the crank was stock stroke at (+/-) 3.500", giving a radius of 1.750".

We know the compression height on the pistons is 1.816".

If we add the stack of parts, 1.816", 1.750" and 6.540", we arrive at a stack of 10.106".

If we deduct the stack (10.106") from the new block deck height of 10.160", we find a piston deck height of 0.054". Now, if we add a nominal gasket thickness of 0.041" to that, we find we have a squish of 0.095". Not quite what one would want in a motor that needs to run on pump gas with any respectable static compression ratio. HOWEVER.....

This motor does not have a respectable static compression ratio. If we figure the chambers at 75 cc's originally (they always pour larger than published data) and you took a 0.020" cut on the heads, we might guestimate 1 cc for each 0.007" cut and call the chambers 72. A bore of 4.040" with a stroke of 3.500" calculates to 735 cc's in the cylinder. The pistons are flat top with 4 valve reliefs, so we will figure 7 cc's in the eyebrows. The bore of 4.040" and piston down in the bore 0.054" at top dead center calculates to a piston deck height volume of 11 cc's. A nominal head gasket diameter of 4.400" with a thickness of 0.041" calculates to 10 cc's.

Adding up the volumes with the piston at bottom dead center and dividing that by the volumes with the piston at top dead center calculates to a 8.35:1 static compression ratio. (735+72+7+11+10) divided by (72+7+11+10).

Even with a non-existant squish, in my opinion, the motor should not be pinging on pump gas at 8.35, even with the short cam you're using.

I played around with the motor all morning fiddling with the carb and timing. And also hunted all around the motor with a long screwdriver listening for the knock. It's coming from the rear of the engine, almost sounds like it's coming from the bellhousing. So I did a cylinder cut out test and found that the knock goes away when I pull spark from cylinder #4. So it looks like it's engine internal.

 

Nice. I will get some mileage digesting that.
You know I was convinced that I had an internal knock comming from deadening #6 (Pulling the plug wire). Not just me but others who "listened in" It was a exhaust leak right next to the plug hole. Very small, unseen drove me nuts. I couldn't find it until I took a 3/8 fuel hose to my ear and followed all the gaskets. Tried Steth, long screwdriver, disconnecting stuff you name it but the hose picked it right up in seconds dead on. If it is a leak that is.
I hope a easy fix for you.

 

You don't mention what your advance curve is. This sounds like classic too much timing in to fast. What is your max timing at what RPM? What is your initial timing?

 

Base timing is 6* total advance is 30* at around 3400rpm. Knock comes in at around 1400rpm with or without the vacuum advance hooked up.

 

If it does it with the vacuum advance disconnected, it sure as hell will do it with it connected. With your gear ratio that shouldn't be to much advance. The only other thing I can think of to check is to make sure the damper has not slipped. Manually set the engine to TDC without looking at the damper, either with a dowel in the spark plug hole or off the #1 rockers. I have a piston stop set up that I can manually set the piston against and then rotate the engine around against the stop again and that will give you TDC half way in between the two marks. If the damper has slipped the timing marks on the damper tell you nothing in relation to what the rotating assembly is actually doing.

 

Yeah I've spent weeks checking that. Everything short of pulling the head back off. Just finished putting the second timing chain on Friday. I did several test and check fits of the crank to cam, piston position, rocker arm position, even cover, pointer, dampener relationship. Near as I can tell the dampener didn't slip. I've been chasing the "advance ghost" for about two months now. But it it was an advance issue, why would the knock go away when I pull the cylinder #4 wire from the cap(see my 2nd thread post)? In theory if it was strictly a timing issue it should be off on more than, so one cylinder pulled might lessen the knock but not eliminate it. I really wish I had an old Sun lab scope, I think that would show me more information on the ignition.

Right now I feel like it's still an advance/general ignition issue, because that's what everyone else keeps pointing at. But I'll be dang'd if I can find one. I think I need to do more visual inspections, but that means pulling the eingine or the trans to get the pan off. I could be over reacting on the rod issue too cause I recently diagnosed a a similar sound/condition situation in an '03 E350 5.4L 40k. The two techs working on it thought it was everything but an internal noise, so while they had the trans down we pulled the oil pan off and found a spun rod on #8. So it could all be in my head.

 

If you line up the marks on the time gears then you should be looking at 0 on the dampner. Unless you have a factory keyway off set (EPA crap) 360, some 390 and 460 or use a plus or minus keyway on a multi keyway set. (building in advance or retard). Should be straight up on compression. Correct me if I am wrong.

That kind of noise will make you crazy. I wish you could have heard my exhaust leak it sounded just like a wrist pin.

 

Have you recheck the firing order on the dist cap? Your discription says it could be a couple of swapped wires or even cross fire between plug wires or arcing in a dirty or cracked cap.

 

I messed up this post. Dang! Next one.

 

A tooth off on the distributer could have your plugs off too! rotor at #1 plug

 

That would show up on the timing marks on the damper with a timing light.

 

Yeah I got ya. I was considering what you said about the dampner moving. A long shot.

 

Didn't mean to give the idea of swapped wire or crossfire. I have checked for those and couldn't find anything. I even put another new set of everything on just to be sure, Bluestreak cap, rotor and points and Autolite wires. And yes the rotor is pointing at #1 consistently.