My 360 has a disturbing knock sound when the engine rpms get around 2000 rpm, but goes away if accelerator is pressed. When cruising on the high way I can hear it knocking until I accelerate. I am driving a 1968 2wd F250 with a manual 4 speed transmission, and manual everything else. It seems like it is knocking when the manifold vacuum goes up at higher rpms. It seems to have begun after purchasing a distributor from the local auto store. Is it possible the centrifigul advance is incorrect. it times fine with the timing light. When i first noticed it i just thought it was a rod knocking and figured I would end up replacing the short block soon anyways once i completed the build up of another block i have. However, time has stalled the replacement block and the engine in the truck has been doing this for a year now. It is hard to believe it is a rod. I'm thinking it is a timing problem at higher rpms with higher manifold vacuum. If I open the throttle up while it is knocking, the knock goes away until the engine rpm catchs up to the new throttle position. Any suggestions as to what it may be caused by. If it is the distributor how can I check the centrifigal advance and adjust if needed.

 

It sounds like it's got too much vacuum advance. For a test, try pulling the hose off the vacuum advance unit, plugging it (to prevent a vacuum leak) and tucking it out of the way. Then try driving it and see if the problem is still there.

I dont think it's the centrifugal advance causing it.

 

I will try this tomorrow. The timing was set with the vaccum disconnected and plugged, per the manual. I guess the next question is how much should the timing advance at idle when it is reconnected. Is there another method to adjust vaccum advance and how much should it be set at? The distributors vacuum advance diaphram has a single vacuum hose connection and it is hooked to manifold vaccum. It does not knock at low rpm were vaccum advance is the primary mechanism, that is why I was questioning the centrifugal advance combining with the vaccum advance at higher rpms when the vaccum has caught up to the cruising rpm.

 

The primary function of the vacuum advance, is to add extra timing at cruise, to compensate for the relatively slow burning caused by a thin mixture. The vacuum advance doesn't care what RPM you're running at, it will try to add timing whenever there is vacuum. There should be a set screw in the diaphram that allows you to limit total vacuum advance. I'm not entirely sure how you get to it though.

As I'm sure someone will point out, having the vacuum advance hooked to full manifold vacuum is not correct. There should be two vacuum ports on the carburetor. The top one is usually the ported vacuum for the vacuum advance. Try hooking it there. The ported one disables the vacuum advance at idle.

 

takes a certain size allen head-count your turns!

 

I am with rusty70f100 with the vacuum advance being hooked up wrong. At work we were working on a 70 Cougar Eliminator with a 390 FE and the customer had the vacuum advance hooked up to the manifold and it seemed to hesitate and spark knocked a little, although it didn't help that the timing was off anyway. I would try hooking the vacuum hose to the carbeaurator and see what happens. I don't think it is a bad idea to unhook it altogether and plug the line and see what happens too though.

 

I have disconnected the manifold vacuum and the knock went away. The only other vacuum port I could find was at the base of the carb. It was hooked to the aircleaners intake hot air diverter diaphram. I swaped them and the problem came back. I did not see any other vacuum sources to try. the distributor was the single vacuum diaphram style.

The original distributor needed a new vacuum advance diaphram when I bought the truck. When I put the new diaphram on I adjusted per instructions and everything worked fine until the engine kept dying mostly when I turned left. The shaft was so sloppy I was loosing my point gap as the shaft wobbled around. The new distributor took care of the intermitent ignition, but identified another timing problem I guess.

I believe the vacuum advance is currently set at about 8 to 10 degrees when the manifold vacuum is applied at idle. The timing is set at 6 degree BTDC.

Any other suggestions?

 

Well even then it shouldn't be pinging. Try this:

Bring cylinder #1 to TDC, on the compression stroke. Then, make sure the timing pointer points to 0 degrees on the balancer.

 

The pointer is on the 0 degree mark at TDC.It also has had a new Timing gear and chain approximately 5,000 miles ago, which was the same time I installed rebuilt heads. The knocking started about 2000 miles ago when I installed the rebuilt distributor. The knocking is not the customary pinging associated with premature ignition.

 

I'm not suspecting the timing chain and gears. I suspect that the harmonic balancer may have slipped.