I just installed a FSW header and KLX 300 muffler on my 351. This should make it run leaner. I will get some spark plug reads later...but if its too lean now could I fry my motor before I get the plug read? Are there any engine signals that could tell me I'm too lean while riding so I could abort? And it a new 351.

Also how far or long do I have to ride to get a plug reading?

 

You should be ok to run it and see how it acts as even if it is bad lean like not running well you will notice that. A 4 stroke is way different than a 2 stroke as far as lean goes and they will tollerate it alot more without grenading as they don't depend on the charge for lube also.