Valve Clearance? Options

[last337]

Can someone explain this to me? I have been fighting with this thing for some time now. I had so many problems for so long with my engine just dying and after pulling my tank and changing fuel lines and filters I am back running again.

Now I am having another issue where I will get my dual carbs tuned just right at operating temp and be running just fine. However, after a while I start to hear backfiring again. The only thing I havent done in a while is set my valve clearances. Actually it has been a long while since I have done that and I am suspicious about it being the culprit.

I understand how if my timing was off or my mixture was off I could hear pinging or backfiring through the exhaust. But I dont know why my valve clearance would do that only as the temp of the engine changes or just seemingly by itself at times.

Can someone explain how proper valve clearance is so critical?

[r_towle]

Valves set to tight will remain open longer than they are supposed to.
Creates hard starting, low power, and backfires up the intake.

Valves too loose creates lots of valve tappit noise and lower power due to not opening enough.

Valves are set cold because as the engine warms up, the aluminum push rods get longer...so you end up with zero lash when hot.
Setting them hot sucks, but can be done....wear gloves.

If your carbs are getting worse as the motor gets hotter, you need to be looking at possibly intake leaks, and possibly linkage binding up when hot.

Here is how I put gaskets on the carb setup to prevent boiling the fuel when the motor gets hot.
Another "car runs worse when hot" issue.
It also prevents intake leaks.

head
rtv sealant
gasket
phenolic spacer
gasket
rtv sealant
manifold

then
rtv sealant
gasket
spacer (sometimes, not always...unless I have having boiling issues)
gasket
rtv sealant

Now, I use the sealant very very thin.
Its just smeared on both sides of the gasket to ensure a tight seal to the head and the spacer.

I have found that most of my leaks are head to manifold and not manifold to carbs...but that is my experience.

Sometimes I have needed to trim the gasket and the spacer to get around and away from the cylinder head tin which can intrude on me trying to get a tight seal down there...

One more issue when hot.
Distributor advance plates with OEM grease from 40 years ago stop moving.
The grease is more like glue now...
Fix is to remove advance plates, take them apart, clean and re-grease.
Put it all back in place...
Its another super hot thing when the motor is running...cant touch the distributor.

RIch