Mystery Engine Knock
 

Been chasing an engine knock for a few weeks in my roadster. This is on a newly rebuilt engine with almost 1000 miles on it. Along with the knock a cylinder is misfiring (either 3 or 4, they both change a bit when shorting out). It is definitely coming from the back of the engine. The knock is not consistent in pattern and is quite loud. It is definitely at camshaft speed and in the top end.

Here is what I have checked:
Timing gear - no broken teeth, nice and tight;, plunger spring good
Compression test - all four cylinders are putting out over 60 pounds
Oil pan - taken off and crankcase inspected, nothing
Valves - valve cover taken off, clearances checked, no stuck valves (at least at time)
Just for kicks I put some marvel mystery oil in the gas to see if anything would happen, no change.


Me and many other guys have not been able to pinpoint where this noise is coming from' just on the top end in the back of the engine. The thing that seems to be making everybidy scratch their heads is that it is really loud at idle but once you increase the throttle it quiets and turns more into a rattle.

Anybody have any ideas as to what it is? We are all stumped!

Re: Mystery Engine Knock
 

How about a rod hitting the dip tray?

Re: Mystery Engine Knock
 

No witness marks on the dip tray and the knock is not at crankshaft speed.

Re: Mystery Engine Knock
 

What about a visual inspection of the valves and piston heads on 3 and 4 via the spark plug holes?

Re: Mystery Engine Knock
 

CHRS...,


Is it conceivable that there is something loose in the clutch bell housing?

Re: Mystery Engine Knock
 

Does it seem like it could be coming from the starter ? Maybe a problem like Fred had.

Re: Mystery Engine Knock
 

The book Model A Ford Mechanics Handbook, Vol. I has a Troubleshooting Chart 4.6, Engine Noises on page 4-8.

Number 2 is Valve Noise which can be caused by loose valve stem or guide or stuck valve. The book recommends trying to squirt oil into the carburetor while the engine is running to try and loosen a stuck valve. I know you have looked at that but it could be that a valve is sluggish and not completely stuck so that it only happens when the engine is running.

Re: Mystery Engine Knock
 

I have yet to look at the top of the valves, probably should pull out my small little inspection camera and look at that.

Is it possible that it could be from the bell housing? Seems weird that that would cause a misfire?

It is coming from the passenger side of the engine for sure.

Re: Mystery Engine Knock
 

Check your "Private Messages" in the top right corner of this web page.
Marshall

Re: Mystery Engine Knock
 

Loose valve seat? I had that once, drove me crazy finding it.

Re: Mystery Engine Knock
 

I just had that this winter. Didn't manifest as a knock, but did lead to odd symptoms.

Re: Mystery Engine Knock
 

I don't have valve seat inserts.

Re: Mystery Engine Knock
 

Another possible source of a knock is a tight distributor shaft or the screw that secures the distributor being too tight. But I don't think that is your knock. If there is a miss associated with the knock than I would suspect a sluggish valve. This may not work but I would try squirting some PB Blaster for other solvent type oil into the carburetor while the engine is running at a good clip. Lead used to lubricate valves. You might try some lead substitute, available at the auto parts stores.

Re: Mystery Engine Knock
 

Possibly a broken piston

Re: Mystery Engine Knock
 

UPDATE:
Ran the engine with a modified valve cover that has the top half cut off to see what the valves are doing. One of the valves on cylinder #3 is sticky and was closing hard. Time to pull the head and take it apart.

Re: Mystery Engine Knock
 

I would suggest that you run some MMO or other top end lube for a while before pulling the head.

Re: Mystery Engine Knock
 

Since you know exactly which valve it is, I wonder whether you could remove the manifold (a lot easier than removing the head) and then apply a penetrating oil spray to the opened valve from underneath. Might dissolve whatever's making it stick.

Re: Mystery Engine Knock
 

Before I took everything apart, I tried taking the cover off and spraying the stem with WD40 and also through the spark plug hole. I also sucked some MMO through the vacuum and in the gas. Didn't do anything.


So I took it apart yesterday and didn't find anything wrong with the valve itself, not bent. Nothing really on the stem of the valve. It fit perfectly in the valve guide with a little clearance on each side. I used some lapping compound on the stem and put it back together with lubricant.



Still have the sticky valve for whatever reason. I ordered some ball hones and I am going to take it back apart and hone a few thousandths off of the valve guide. At least that's the plan.

Re: Mystery Engine Knock
 

I didn't know that they made ball hones that small.

BTW, is it an intake or an exhaust valve that's sticking?

Re: Mystery Engine Knock
 

Could it be a weak valve spring?