deadhead

Member
Mar 25, 2001
101
0
Folks,

I plan to build a YZ105 from my son's current stock bore 2003 YZ85. He's running a VForce3 reed valve and FMF Factory Fatty pipe which will stay with the big bore. I want to make sure I have all the pilot and main jets I'll need to tune it already on hand when the cylinder and piston finally arrive. How big will I need to go?

Thanks,

deadhead
 

Vic

***** freak.
LIFETIME SPONSOR
May 5, 2000
4,008
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Generally, big bores require leaner jetting.
 

hoatson

Member
Aug 1, 2005
2
0
YZ jetting

On 85's, knowing how to jet is the most important way to get power & reliability. Don't rely on the jets that someone else uses. 85 jetting needs to be changes when the weather changes - generally richer when it gets cold outside (winter) & leaner when it gets hot (summer). Also, leaner at high altitude.

The trick is to keep notes when jetting - make a table of the jet sizes in your bike and note each change that you make & how it effects the bike.

Riding in mud or sand can be very hard on an engine - richen the jetting a little.

The best thing is to get a friend to time you between two markers - this tells you if the jetting change helps or hurts.

Change one thing at a time, then test it to see if the bike is faster or slower.

Too lean, and the motor will ping & melt.

Too rich, and the motor will be slow.

Because richer is safer than leaner, stock bikes are almost always too rich.

The premix ratio affect the jetting - if you change from 24:1 to 55:1 oil/gas mix, there is less oil in the mix, and therefore more gas in the mix, resulting in a richer mixture. Oil is needed, but does not make power.

The carb jets control the mixture at different throttle openings (not RPM's):

Air Screw - (idle to 1/4 throttle) - turn in to make richer, out to make leaner - it affects idle and early accelleration

Needle Clip - (1/8 to 7/8 throttle) - lower the clip (raise the needle) to make richer - This adjustment has the greatest effect on the bike's jetting. This is rather easy to adjust (no need to buy anything) and affects the majority of the RPM range.

Main Jet - (3/4 to full throttle) - The main jet basically affects wide open throttle only. If the bike pings at wide-open throttle, instal a richer main. Otherwise, try a leaner main & see if power improves.

Don't be scared to jet. You probably will improve power more than a pipe will. Just don't go too lean - temperatures will incrase & the engine will seize.

When you get the jetting good, you will be close to being too lean, so remember to richen the mixture if the weather gets cooler. Also richen when riding in deep mud or sand.

Make sure the bike is in good condition - clean airfilter (not too much oil), new rings at least once per year (once per month won't hurt), clean carb, fresh gas.

I had a new YZ85 that had very little low-end power. The carb bowl has an o-ring gasket that is glued in, and a small piece or the glue got into the pilot jet. Cleaning the carb gave it much better power.

A V-force reed mellows out the powerband hit.

A flywheel weight makes the bike easier to ride, especially in woods, mud, or snow.

Using a longer chain (so the rear wheel is as far back as possible) reduces wheelies if you are getting too much traction. Yamaha makes a longer swingarm that is part of their big-wheel kit.

I have not tried it, but a Pro Circuit 105cc kit (stroke, bore, & port), $1700 probably makes the YZ 85 faster than a 125. It adds low-end torque, smooths the hit, and adds more on top.
 

deadhead

Member
Mar 25, 2001
101
0
Thanks for the great reply. My intention was only to get a starting point for the first time out with the big bore installed. Still no suggestions on that.

Let me ask a little different question... On his stock-bore 85 we're running a #142 main jet and I've got the stock needle installed with the clip in the top notch. The book says that's one full step leaner. The bike really runs well as is, but I can't go any leaner with the #142 main if the temp and humidity creeps up any more. I don't mind playing with the needle clip, but I'd rather not make jetting changes if I don't have to. If I were to run a #140 main with the stock needle and the clip in the third groove (One step richer) can I expect it to perform roughly the same as the 142 with the clip at the top?

Thanks,

deadhead
 

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