How can I tell if my bike is running to lean?

[PE175]

The manual says run the mix at 20:1, If I do that it wont even start. I run about 42:1 for it to even run somewhat decent. I just put in a new main jet last night which dramatically upped the performance. I took the plug out and it was a little dark, but not really. Does this mean I am still a litte rich? Is 42:1 to lean for my old beast? Is there any way to tell if it's to lean other than the plug? It doesn't smoke too badly and doesn't overheat. So it's all good right?
THANKS!!!!!

 

[Peer Lovell]

The ideal color for a plug is medium brown. If its white or light gray you are running too lean. A very dangerous condition!
40:1 or so if o.k. for a newer bike if you are using a top quality premix.
Older bikes run a bit better at 30:1.
I don't know what year your bike is, but older bikes usually recommend 20:1 as oils were not as sophisticated back then.

 

[Danny8785]

The tolerances in old air cooled engines are not as tight as the new water cooled two strokes. One of the reasons 20:1 ratios were recomended was to raise the cylinder compression by filling in the gap between piston and cylinder. I dont know what kind of bike you have, it might help if you tell us.

 

[Smit-Dog]

Looks like you have a vintage 1979 PE-175. I had one of those back in the day. Post some pics if you get a chance. Great memories on that bike!

 

[Layton]

I also had several PE-175's. We ran 50:1 in those bikes. Smit-Dog is right, those were fun bikes.:)

 

[PE175]

Thanks guys, yeah I have a 1979 PE 175. It's a great bike and has been in my family for over 20 yrs. It's 98% origional, and in great shape. I'll have to scan some pics.

 

[PE175]

Hey Smit dog.
I own/have owned 3 of the bikes on your list lol.
PE 175, the DS 185,and a 74 XR 75.
Kinda spooky if you ask me hehe