Today I ordered a mild web cam and 40 thousandths over piston kit from Wiseco (My motor is definitely in need of it). Parts unlimited said that Wiseco didn't offer 20 thousandths over kit. Anyway, do I need to rejet for more gas to enter and maybe adjust fuel mixture screw. Also, does anybody know how many cc's a 40/1000 over piston makes? Just wondering if I will be anywhere near a 405EX? Thanks

www.geocities.com/hulioni

 

Do a plug read to check jetting, it'll probably need it.

Yes, .040 is the first overbore, see the Engine Mods 400ex post for a list of overbores and their resulting effect on engine volume.

.040 over = 1mm over = 406cc

 

Changing the dynamics of the cam and increasing the volume of the cylinder shouldn't affect the ratio of air to fuel. You are not easing the restriction on air; you are just allowing your engine to take in more of the same ratio of air to fuel from your carb.
Now, if you're planning on piping and/or removing airbox lid, drilling holes in airbox lid, etc., then you need to rejet.
To be on the really safe side though, I guess I would make a couple of quick plug checks anyway like gabe said. Theory works good on paper, but not always in the real world.

ogre

 

The real world is what I was referring to. Because you are not only are changing the timing, of when and how much air/fuel is pulled in, but also the efficiency of the exhaust and intake system that were both designed and tuned to work with the stock cam and engine volume, which can end up changing the various dynamics and pressure differentials that occur inside the carburetor... Too many variables, best to check and be sure.

 

There's the "gabe" post I was looking for. I often find a concise and informative post attached to your name - this time it was a simple "yes" answer. So I thought I would extend it a bit. I must admit, I'm the first to cringe when a poster claims they haven't rejetted yet after putting a pipe on, etc.; wondering if they have to do it at all. Every once in a while, I will post something on this, but the overwelming onslaught of questions about this issue leaves me baffled on how these folks can't search the subject, or go over to the tech connection. Anyway, I was tickled pink when this topic was addressed, because it dealt with the effects of internal demands on carburetion, rather than a reduction on constriction question. Either way, checking jetting after any motor mod is candy for your engine.

ogre