Camshaft Knock? 305 V6. Please advise...

[restorevival]

SO.... Where to start... Not at the beginning because the story is too long. I'll start with my current frustration and see where the discussion goes from there.

I recently had the 305E v6 in my wife's '66 GMC rebuilt. Since getting it back from the shop and putting in lots of time adjusting, tuning, etc., it finally seems to be running right.

However, when idling, there is a distinct but fairly faint knocking sound. I drove it back to the machine shop and the guy that did all the machine work and put the engine back together says that the knock is coming from the camshaft. Here's his explanation: "These engines have no thrust plate on the camshaft, so at slow idle, the slack in the timing chain is allowing the camshaft to float and creates the knocking sound that you hear." He also said something about the movement of the camshaft being related to helical gear of (I think) the distributor? I may not be remembering that last part correctly.

His solution was to increase the idle speed until the knocking went away, which he did. The idle seemed a little fast to me, but I don't have a tach installed (yet) to know what it was idling at exactly. I drove it around a bit and it seemed fine, but then started dieseling/running-on whenever I turned it off...

I am starting to get into another topic so I will get back to the point. --Does the above explanation of the camshaft knock make sense?? At this point the conversation with the mechanic has gone beyond my understanding of the inner workings of this engine, so I lack the knowledge and vocabulary to question him further on it. I would really like some help from the community on this one.

My gut instinct is that the knocking is a problem, and that increasing the idle is only curing the symptom. I would love it if someone responded by saying that my instincts are wrong on this one, and that I have nothing to worry about... and then be able to explain why/why not.

Thank you in advance for your feedback.

[geezer#99]

Find a new mechanic!!
The cam will walk forward at high rpm but not move and rattle at idle.
Turn your idle back down so it doesn't run on and disconnect the vac line to the distributor and plug it off and see if the knock goes away. Sounds like low idle detonation. You might need to lower your timing to cure it.

[restorevival]

Yes, I have already decided to move on to a new mechanic, but still talking with the first one because I want this engine running right. It was NOT cheap, and I want him to make good on the warranty if that's what it takes.

Can you explain more what makes the cam "walk" forward or not? I assumed that the cam should be very stable, and the knock I hear indicates significant movement. Is there any weight to the thrust plate idea?

Is "low idle detonation" the same as pre-detonation or pinging? If so, I would say this is not pinging. This is a much lower tone, and sounds to me like a rod bearing knock.

As for the run-on, I adjusted the point gap slightly, then reduced the idle and adjusted the mixture which stopped the run-on. I will try removing and plugging the vacuum line next week; am out of town for a few days.

Thanks for the suggestions geezer#99. Any other thoughts?

[geezer#99]

Cam walks because of it's design. The lobes aren't flat, they're machined on a slight angle so the lifter's will rotate. The angle combines with the lifter rotation causes the cam to walk rearward. The back of the cam gear limits it. It won't show up at idle but might walk forward at higher rpm. A cam button in the cover stops that forward motion.
Low speed detonation is pinging but at a much lower rpm (like idle) than you'd normally hear at a higher rpm under a load. It's sounds like a dull rattle like a rock in a metal can.
I've had motors that had low idle detonation but didn't ping at higher rpm. It's caused by too much initial timing combined with too much vac advance at idle. You likely only have manifold vacuum available for your dizzy vac pot.
I like your mechanics statement that the slack in the timing chain causes the rattle. There shouldn't be any slack. He just rebuilt it. It should be new.

[restorevival]

I gotcha, that makes sense on the cam movement.

The sound quickly goes away with increased rpm. It is a very consistent somewhat slow rhythm, unlike the disorganized clatter of what high speed pinging sounds like. Does that follow your description of low speed detonation?

Timing is currently set at the 10 degree mark on the pulley. I'm pretty sure I only have manifold vacuum available (Stromberg carb), but will double check when I get home. On that note, I recall disconnecting the vacuum line to the distributor (why, I don't remember now) and there was no idle speed change, making me think that maybe the vacuum advance diaphragm is ruptured. Would that make a difference in this current scenario? (clearly needs to be addressed soon enough, but one thing at a time...)

Thanks again, geezer. This is all very helpful.

[restorevival]

Originally Posted by geezer#99

I like your mechanics statement that the slack in the timing chain causes the rattle. There shouldn't be any slack. He just rebuilt it. It should be new.

I was also thinking that there shouldn't be any slack in the timing chain...

[geezer#99]

That sounds about right on the detonation. When you're driving it and completely release the gas pedal while at 10 mph you might hear it even more. That's due to the higher vacuum when you suddenly close the throttle and you get max vac advance.
Double check on your vac pot when you get back. If you suspect it's shot just hook a vac hose to it and suck on it real hard, put your tongue over the end and then release it. You should hear it snap back. Or you can pop the dizzy cap off and put a vacuum on the hose the same way and watch for the point plate to move.
If you can't figure it out, just post again and we'll troubleshoot it for you.
A good way to pinpoint the source of noises in your motor is use an old broom stick. Put one end behind your ear lobe and put the other end on the block in various places. You'll be surprised what you can hear. Watch out for that fan though.

[restorevival]

I'll be back online late next week. Thanks again!!

[restorevival]

I was back under the hood yesterday and today, and here's what I've come up with...

Originally Posted by geezer#99

Double check on your vac pot when you get back. If you suspect it's shot just hook a vac hose to it and suck on it real hard, put your tongue over the end and then release it. You should hear it snap back. Or you can pop the dizzy cap off and put a vacuum on the hose the same way and watch for the point plate to move.

Vacuum advance is definitely not working. Nothing happened when I tested as above, so I pulled the points plate off and the arm of the vacuum advance was immovable. I ordered a new one, will pick it up in the morning. Should the points plate pivot easily? Mine is pretty stiff, so I'm considering taking it apart to clean and lube it.

Originally Posted by geezer#99

A good way to pinpoint the source of noises in your motor is use an old broom stick. Put one end behind your ear lobe and put the other end on the block in various places. You'll be surprised what you can hear. Watch out for that fan though.

Tried the broomstick method today, but my results are sketchy. I seem to hear noises everywhere. Things are louder when listening to the bottom of the block around the edge of the oil pan, but I'm not sure it's the same sound that I'm hearing without the broomstick...

I drove around a little today trying to hear any changes when I let off the gas, but couldn't detect anything. I'm assuming that's because the vacuum advance is not moving, and therefore max vacuum advance cannot be achieved...

I'll try all this again tomorrow after installing the new advance.

[66customcamper]

The stethoscopes they sell at the parts houses for about $10 work remarkably well for pinpointing noise sources.

[restorevival]

Alright, I installed the new vacuum advance, which pushed the timing at idle to about 14 degrees BDTC, so I adjusted timing at idle to 7.5 degrees. The engine definitely runs a little smoother now, but the knocking sound is still present.

Listened with the broomstick again and now with a slightly slower and smoother idle, things do sound noticeably different through the stick. The knocking sound is much more pronounced and clear on the passenger side of the engine (cylinders 2/4/6 side), with the clearest sound coming from the wall of the cylinder head, but no distinguishable difference between front (#2 area) and rear of engine (#6 area).

I retested the vacuum by plugging as suggested above by geezer#99, but again no change in sound. I drove around again for a bit trying to pinpoint any changes in sound with changes in rpm, load, etc, but nothing definitive.

Any other suggestions? Anything else I should try that would help me narrow it down?

Thanks again for the suggestions geezer#99 and 66customcamper.

[Sharps40]

Whether or not its shaft type rocker arms or hydrolic lifter with the stamped rocker arms....

22 Million dollars (which I ain't got, but if I did I'd lay it on the dash) says its a rocker arm tapping or if hydraulic system, a lifter induced tap....betcha at higher rpm its either hidden in the general woosh of noise or oil pressure is higher/parts under load and the noise don't exist anymore.....

Do a hot engine valve adjustment, regardless of the type of lifter/rocker. In any event, if its a rocker, often the simple pressure of a finger on the offending rocker will eliminate the tap of the valve seating a bit too quick/hard and you know....it needs snugged down or lifter replaced, etc, etc.

Could be as simple as worn out rocker shaft or rocker studs, etc....specially if valve train parts like shafts, studs, nuts/balls/keepers was reused.

[restorevival]

I have been away and unable to work on the truck for a couple weeks, but will be able to get back under the hood this week or weekend.

Originally Posted by Sharps40

Do a hot engine valve adjustment, regardless of the type of lifter/rocker.

To clarify on the "hot" valve adjustment, is that exactly what it sounds like? Do a valve adjustment after the engine has warmed up? I have only done cold valve adjustments, what does the hot version provide- more slop or movement of the parts?

Thanks for the suggestion, I'll try it and report back my findings.