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?

Re: Mystery Engine Knock
 

Also look at the lifter.

Re: Mystery Engine Knock
 

It was an intake valve on cylinder number 3. I fidn't know they made hones that small either, they come even smaller than 11/32 too.

Valve springs were checked, they are all brand new and very strong.

Re: Mystery Engine Knock
 

It was an intake valve, so the problem has been fixed ?

I don't think you'll find a hone that small, but, you can find a reamer.

I've loosened them up using grinding compound.

Most of the time the valve is fitted OK, maybe a tad tight, and will start hanging up because of 'crud' [ technical term] usually from sitting around not being used.

Re: Mystery Engine Knock
 

They do sell ball hones that small, specifically a company in California called Brush Research. I bought some from them and I am waiting on them to come.

I verified it was an intake valve by watching the engine run with a valve cover that has the top half cut off.

Re: Mystery Engine Knock
 

Update: still have the sticking valve. Mechanically it does not make any sense as the guide has plenty of clearance for the valve, a different valve and spring were put in with thr same results, and the lifter works fine. I am unsure of where to go from here as I have run out of ideas and gotten sick of tearing the engine apart after 4 or so times!!

Re: Mystery Engine Knock
 

Check your private messages again.
Marshall

Re: Mystery Engine Knock
 

Why keep sending private messages?
We all benefit from each others experiences and shared knowledge.
If you have something to say, please just post it so we can all benefit from it.

Re: Mystery Engine Knock
 

After you've been on here a few years and you get tired of the "professionals " picking your posts apart you'll be sending private messages too.

Marshall has more Model A experience than most of the members on this forum and is very helpful.

Bill

Re: Mystery Engine Knock
 

Interesting, i have not heard of the cut valve cover before.

1: Did it stop the oil splatter?

2: How high up did you cut the cover?

I ask because I need to do the same thing, but instead of cutting a cover I will make one from sheet metal.

Re: Mystery Engine Knock
 

A picture would be great too. [emoji106]


Sent from my iPhone using Tapatalk

Re: Mystery Engine Knock
 

Chris, I am sorry you are having so many problems. In my mind, the only thing that makes sense is a sticking lifter. Unfortunately fixing a sticking lifter will require some work. You will have to take the pan off and remove the cam. You can keep the lifters up with cloth pins. You will have to remove the valve keepers and springs.

How have you determined that the lifter "works fine"?

The only other thing that I can think of is to use a heaver valve spring on that valve. I assume you have checked the spring. You could take the heaver spring out after a few thousand miles. I have stretched springs to increase the spring force.

Re: Mystery Engine Knock
 

The valve cover is cut in half horizontally more or less, maybe little taller than half. When running, there is not any oil splatter but the galley does fill up almost to the top of the valve cover which is why you have to be careful when you cut it.

Btw, Marshall was suggesting the valve could be bent but the original one and the new one was checked for straightness. Plus they are stainless so they are pretty hard.

Re: Mystery Engine Knock
 

I spoke to an engine rebuilder who told me to the check if the lifter comes up and down fine and turns in the bore. Sometimes what happens is when people remove the old lifters they can nick the hole.

Btw, I should mention that the valve guides and valves are modern and the lifters are the adjustable type.

Re: Mystery Engine Knock
 

Possible problem w/the camshaft?

Re: Mystery Engine Knock
 

It just seems like it has to be the lifter or something in the lifter bore. I don't wish that frustration on anyone.

Re: Mystery Engine Knock
 

I am trying to think of how you might loosen up the lifter without taking it out. It may be possible. Try removing the valve cover, removing the valve spring keeper and spring, and tying up the valve. Then lift the lifter with the camshaft by moving the engine into that position. Then put a heavy-duty rubber band around the lifter and turn it by hand or with a drill motor until it loosens up. May take a long time. You can make a heavy-duty rubber band by cutting a slice out of an old inner tube.

Re: Mystery Engine Knock
 

The lifter works great, and is not stuck what so ever at all. I just don't see it being that.

Re: Mystery Engine Knock
 

This is a real mystery, isn't it. I have run out of ideas. If it was my engine I would try a stronger valve spring.

Re: Mystery Engine Knock
 

Hmmm, this is a real wonderment.
Valve and lifter are good.
I can't wait to hear about the fix.

Re: Mystery Engine Knock
 

Are you running a new cam, reground cam or original cam? A bad lobe might overheat the lifter?
Can you lift the lifter up and spin it or does it get tight at the top of its travel?
Does the adjuster on the lifter show wear like a hollow in it? Maybe it is not hardened?
Could the valve spring or keeper catch on anything (doesn't seem likely).

I am just trying to trigger a thought that will point to the cause of the problem. It seems like you have ruled out the valve end of the problem.