Valve Clearance w/Age Leaner or Richer

General question folks... I've searched, but can't find an answer:

Valve specs tighten with wear -- given -- do "tighter" gaps produce a (generally, all else being the same) leaner or a richer run?

So, hypothetically, if one were to never have tuned or changed any aspect of mixture, and assuming there are no fuel/air flow issues or changes over the same time frame, would the bike run leaner or richer as the clearances slowly tighten?

Thanks

 

It is unlikely that wearing valves would have any appreciable effect on mixtures. Mixtures are controlled by the carbs, and the type of air filter you are running. The only way a valve could possibly influence the mixture is if the cam did not allow the valve to remain open for the correct amount of time. There is not much that can cause this condition because the cam determines duration but the cam and the shims control valve lift.

 

You're right, valves won't influence mixture; but valve clearances DO affect timing and duration, which is why it's important to keep the valves in spec.

What happens as the clearances tighten is the motor itself, the actual mechanical air pump, becomes less efficient as they wear beyond spec. Compression will begin to drop, and the bike's fuel economy will suffer as it will now need to use more fuel to perform the same "work."

Eventually, due to the valves spending way too much time open and not enough time in contact with their seats for proper heat dissipation, damage will occur.

 

OK, so, if nothing was changed -- no tuning, no filtration changes that restricts or is less restrictive, no leaks, no exhaust changes or exhaust flow changes, no fuel/air flow issues through the carbs, etc. --

then if I understand this it would not be running leaner NOR richer, but would make less power.

 

You're not likely to notice a valve out of spec at all for the brief period before you ruin it.

Should you check them? YES! But if you've got an issue it's probably something else.

 

Thanks, this is pure hypothetical -- I don't have any issuesthat I am debugging resulting in this question -- I wanted to know the answer from a pure operating knowledge perspective? And, I assume if the answer we've arrived at is that neither leaner nor richer will occur (under the hypothetical conditions I asked about), then a plug color will ALSO not povide any information on the in-or-out-of-spec condition of a valve.

 

Pretty much.

Except that as the valve clearances tighten to the point of being out of spec, a whole bunch of the factors you just listed DO change: Tuning, exhaust flow (controlled by the exhaust valves,) fuel/air flow through the carbs (controlled by the intake valves,) it all changes.

Those valve-controlled changes to intake and exhaust flow are why it's necessary to do a new vacuum sync after you've adjusted the valve clearances; and why it's equally important to have the clearances in spec before doing a vac sync.

Keep in mind that at 6000RPM, the intake and exhaust valves are doing their job 50 times per second. At that rate, if everything isn't operating within its specified ranges, you can see how the results would quickly deteriorate.

Is this leading to another one of those convoluted rationales for not needing to regularly check and adjust valve clearances? Because you just gotta do 'em, and every 5000 miles. Period.

 

Thank you this is very good information

 

No, no, but thanks again

 

OK, whew.

I still like "a fat disgruntled old guy said they're fine" as a reason to not check clearances on a (likely untouched) bike with 14K miles on it. "Because they're bulletproof."

 

Wellllllll......... atleast PART of what you accuse me of is true -- LOL!!

 

No accusations; just a reminder that this is something that needs to be done repeatedly and at the specified intervals, during the course of XJ ownership.

The good thing about taking the time to read and record ALL the shims the first time, coupled with accurate record keeping on subsequent adjustments, means that you can almost PREDICT which valves may need re-shimming, and have the appropriate shims on-hand before you ever pop the cover.

A little "initial legwork" can turn it into a one-step process rather than the usual two-step. And it gets easier every time.