Valve Adjustment, ...why did I have zero clearance? Options

[tomeric914]

I had adjusted the valves to .006" before the last track event I went to in April. It was about a 4 hour drive to the track and probably 40 or 50 degrees F. When I got there, the darn thing wouldn't idle (dual IDF 40's, 2.1L). No matter what adjustments I made, I couldn't get it to idle when hot (right off the track) but it would idle if it sat for a while. So I came home from the event, parked the car and started on the flare installation.

Now 3 months later with flares on and driving it on a regular basis, it still didn't idle. Again, checked the synchronization on the carbs, idle mixture, etc. Still didn't idle. Thinking logically, if it won't idle hot, a valve must be sticking open. So I let the car sit for a day and pulled the valve covers to find that all 4 exhaust valves had ZERO (more like negative) clearance! (IMG:style_emoticons/default/WTF.gif)

So what happened?

A. The valves all stretched the same length.

B. All the seats got mashed evenly

C. The valves weren't adjusted at TDC

D. It was about 20 deg F in the garage when I adjusted them last


A and/or B? I doubt it.

D seems the most logical but does the valve grow .006" over 60 degrees?

You would think that finding TDC doesn't matter, but I believe it does. Years ago I tried adjusting them with what is now referred to as the "rocking method" using the opposite side valve as a guide with the theory that a valve closed is a valve closed anywhere on the cam. I found when checking the clearance at TDC that the valve did not have the same clearance as measured when using the rocking method. It ended up being too tight at TDC!

On my 914, the flywheel is paint marked at the groove for TDC and 28 deg BTDC. If I used the 28 deg BTDC mark, I was on another part of the cam that resulted in a different clearance than at TDC. The result being a valve held open when the engine is hot and no idle.

After adjusting the valves at TDC and confirming all adjustment through one rotation, it idles perfectly.

[VaccaRabite]

Don't use the TDC mark when adjusting valves.

Use the "opposing lobe" method.

Also, make sure that when you tighten down the lock nut, you do not further screw in the valve. That is very common to do, and take practice not to do.

Zach

[tomeric914]

Don't use the TDC mark when adjusting valves.

Use the "opposing lobe" method.

Also referred to as the "rocking method" which doesn't always work. That was my point.

The "rocking" or "opposing lobe" methods assume that the cam base circle has zero runout.

Small base circle cams have the lobes ground down to the core diameter to give extra clearance for connecting rods used on stroker cranks. Higher lift cams also have smaller base circle diameters than stock lift cams.

So if the base circle is ground down, then the clearance won't be the same as at TDC resulting in too tight of a valve.

20+ years of adjusting valves at TDC. This a first I goofed on and adjusted all 8 at the wrong mark (I think).

[Don M]

Don't use the TDC mark when adjusting valves.

Use the "opposing lobe" method.

Also referred to as the "rocking method" which doesn't always work. That was my point.

The "rocking" or "opposing lobe" methods assume that the cam base circle has zero runout.

Small base circle cams have the lobes ground down to the core diameter to give extra clearance for connecting rods used on stroker cranks. Higher lift cams also have smaller base circle diameters than stock lift cams.

So if the base circle is ground down, then the clearance won't be the same as at TDC resulting in too tight of a valve.

20+ years of adjusting valves at TDC. This a first I goofed on and adjusted all 8 at the wrong mark (I think).

You are correct that the opposing lobe method is unreliable but not for the reason you gave. The problem stems from the fact that the vavle must be on the seat (closed) at TDC and depending on cam profile, the lifter is commonly still on the ramp rather than the base circle at this point (TDC) which means that setting the valve at the lowest point on the base circle (opposing lode method) can cause it to be held open (slightly) at TDC.