Odd valve problem

[1939mars]

I have posed this question to my club's tech advisor but I thought I would get it out in the wider world. I am doing a valve job on my '31 sport coupe. All new valve parts: one piece guides, adjustable lifters, reground C-spec cam.

I have the valve train in and have adjusted the clearances using the method in the Les Andrews book. Everything seems fine except for the 4th valve back. For it, when it is in the measurement position per the book, a feeler gauge will not go in. If I 'rock' the engine (rotate the engine forward so the pistons move about half an inch, then back to even with each other) I measure 15-17 thousandths.

Prior to rocking the engine, when the feeler gauge will not go in, I cannot move the lifter up and down or rotate it. After rocking it, when the feeler gauge goes in, I can both lift and rotate the lifter. When I rock the engine I cannot tell if the lifter or the valve is moving, but it does free up. Everything is well lubed with red line assembly lube.

A friend has suggested I try swapping the lifter with another one to see if the problem moves with the lifter. I am seeking knowledge before I start disassembling.

Thanks,
Scott

 

[nkaminar]

First check to make sure that the 5th valve is all the way up when measuring the 4th valve. I suspect you have checked this and have done the measurement correctly but thought that I should mention it.

If you still cannot get the feeler gauge in, try knocking down the lifter by using some gentle taps with a screwdriver and small hammer. This will test whether the lifter is sticking. I suspect it is not sticking if you can turn and lift it when there is clearance.

If all is in order and you still cannot get the feeler gauge in then the only other thing I can think of is that the cam for the #4 valve was not ground correctly. I hope this is not the case.

 

[rotorwrench]

Moving the cam backward and then moving it back forwrd can cause a variation due to backlash in the timing gears. When moving it up initially, the backlash is forced into the running mode how it will be during operation if you move it back then the backlash will change the cam position depending on how much backlash is in the gear train and it may not go back to the running position until the engine is rotated through the four cycles again.

I would also be curious about the cam bearing bores and what kind of condition they are in. Worn bores can cause problems like this. The rearmost bore is the one that generally had problems due to poor oil feed on early engines but all of them will wear over time. They can be sleeved to repair any concentricity problems. I think I would want to mike the cam lobes and check the accuracy of the grind. That process may be difficult without proper equipment. The only thing I'd be looking for on the follower is whether the face where the lobe runs is true. If the surface is not shaped properly or is tilted off axis too much than that could be a problem.