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Old 12-01-2012, 11:02 AM   #6326
southern xr rider
Joined: Nov 2008
Location: Columbus, GA
Oddometer: 4,154
Originally Posted by MentalGuru View Post
Calm down....

Here are some random thoughts that I'll try to make understandable...

It's all about Suck- Squeeze - Bag - Blow and how the engines brain(camshaft) is effecting it.

From my experences a larger cam makes jetting interesting.

At idle the increase in duration and overlap tends to reduce A/F charge velocity. This decreases the vacuum signal at the carbs venturi at idle. The increased overlap seems to cause the engine prefer a slighty richer A/F charge to run real smooth at idle. The decreased vacuum signal requires a larger pilot jet to be used to offset it. Both are reasons why larger primary jets appear to be needed with the larger cams. The longer intake/exhaust duration shortens the power stroke and compression stroke duration. Basically your engine no longer idles as efficiently, it runs better slighty richer and needs a larger jet to offset the lower vacuum signal/air speed. That's why the idle is slightly rougher. The increased overlap can also allow some A/F mixture to excape at idle making plug readings fun...

At WOT the larger camshaft flows more air at higher rpms increasing the vacuum signal/air speed which will require a smaller main jet to offset the increased suction at the carb venturi. This is because the larger camshaft increases the engine efficiency as the rpm's increase. At least WOT plug chops aren't harder to read.

If you saw a WOT air fuel ratio chart on your bike it would probably start at about 13.5 at low rpm and drop to 12.5 or so after it comes on the cam sweet spot with the right jet.

Don't concentrate on plug chops, Your a$$-o meter is an important tool too.

The needle jet has the hardest job of all. It has to deal with everything in the middle... It is a hard to get a needle spot on. The bigger the cam the harder it is. It runs rich at some rpm/load combos and lean at others. I try to tune it slightly rich when it's "on the cam". I found it to helps keep the A/F mixture from going too lean at lower rpm/high load conditions. The profile of the needle makes the difference.

I say lower that needle any go take an a$$-o-meter reading...

When you start your pig do you kick it fully to the bottom and hold it there until it starts or stops spinning before raising your leg??? Thats how I avoid kickback. I had issues with kickback until I started to do that.

Thats all for now... I hate typing, it hurts...


is gold to thanks so much

lastly I wasnt offended so dont think this guys! just peeved Im always ocding on the damn instead of riding it in any case Im glad I have all the help from you tuning experts and overall great mechanics...

I will take your advice bud and see what will only take a little more tweaking to get this thing kicking ass!

sorry for the longwindidness yet again

cheers dudes!

elsalvadorklr is offline   Reply With Quote