Advice wanted...High speed lean condition.

I took the xj750 on a 100 plus mile ride yesterday hoping to pull the plugs afterwards and see coco brown plugs. The bikes runs strong and doesn't seem to be overheating from running lean but the plugs say it all. They are clean , however not as white as the upper insulator is. So here it is...Stock air filter, stock jets, exhaust and no detectable air leaks. Good transition from idle to throttle, no backfiring or gargling on either acceleration or deceleration. In other words I don't think it runs like it's either rich or lean. I've had bikes that had to be tweaked either way and this one seems tweaked out. So what could cause high speed lean condition? I'm used to seeing plugs of the coco variety on a bike that runs like this but not this one? And another question, will turning my mixture screw out even have the main jets? They are set perfectly right now for idle and if they get tweaked again they will be very rich for idling.

 

My 650 runs about the same, the plugs only start to colour up if its left on idle for a few minutes. When i get back from a run the insulator is white the earth strap is a sort of slate grey and the outer ring of the plug is black. Its been set up with colour tune and i get all the right colours, i have no signs of overheating and if i tweak the mixture screws out slightly it runs to rich. Like yours it runs great so it will be staying as it is.

 

is it losing power or cutting out at max rpm/load? if not, i would say leave it alone as long as you arent boiling pistons wich would show up as glitter on the end of the plug.

 

Move the Pilot Mixture Screws OUT about the width of a DIME!
Run it.

Start checking the Plugs pretty frequently.

Watch each Hole for Plug coloration.
Treat each Hole independently of each-other and make MINUSCULE adjustments to the Pilot Mixtures to achieve getting SOME color to the Plugs.

Examine the Plugs with a Magnifying Glass.
Look at the Ceramic Tip on the Center Electrode.
IF there is erosion or the tip has any traces of stress fracturing ... add some additional Richness to the Pilot Mixture.

Don't get addicted to how very well the Engine LIKES to run in a critically Lean condition!
Damage will be done if the Engine is allowed to stay Lean.

The Lean Mixture burns HOT.
If the Engine is running too hot ... the Valve and Seats don't get a chance to shed the excess heat and will eventually succumb to being overheated and begin to be less gas tight.

You don't need to add much additional Richness to the Mix.
Your sitting on the edge of Fine-tuning.
Tweak the Pilot Mixture Screws out a degree or two until you get some color on the Plug.

Right now ... the Bike should be running like a raped-ape!
Fast and with a lot of power getting out of the hole.
It will stay like that with the addition of some more supplemental Richness dialed-in at the Pilot Mixture Screws.

Get them to be ... at least ... a light tan and you'll be OK.

Commence fine-tuning!
Don't burn it up!

 

I too have a similar issue. Are the pilot mixture screws the ones sealed under a cap. The Haynes manual says "don't even think about touching them!" Paraphrased of course. LOL

Mike

 

Well, Mike ...

We have to do a better job tuning your Bike than they did meeting the EPA Guidelines written back in the late 70's!

To do that you're going to have to get those Caps removed and expose the Mixture Screw tops.

Did you read the procedure?
Are you comfortable that you'll be able to remove the Caps, or do we have to go over the procedure with you?

 

I would be ok with the procedure. Goes something like I need to drill with tiny bit and very carefully into the center of the cap. Then take a drywall screw into the hole just a bit. Then pull it out right? There in one in each carb right? Do I need to take the carbs out to do this? I think I remember a thread that had the procedure. I will do a search again. Do I need to cover these back up?

It runs so well. Although the plugs are super clean. I had looked at them a few times. I did adjust the idle up just a tad when she started to stumble even when warm. But when hot it seems to drift a little high. Then too hot to stick my big hand in there without burns. Could they have up the adjustments in a more convenient spot? :?

Can I just run a little choke? LOL

So I will assume that these screws are more than just pilot mixture. Since based on what I have been reading this affects the whole range not just the idle/pilot correct?

I will try to remove the plugs again and snap a pic.

Thanks,
M

 

In a sense ... they cover the whole range. The Pilot Mixtures primarily govern Idle ... AND ... Off-Idle for the brief duration that Fresh Air precedes the flow from the Main Fuel Jets.

The supplemental Richness you add to the Mixture provides a strong and consistent Idle.
A "Hands-off" Idle that you don't have to bump the Throttles to maintain.

The supplemental Richness Fuels the rush of Fresh Air preceding the Main Jet Flow and allows the Bike to come Off-Idle without "Hesitation" waiting for the Main Jet Flow.

Once you get the Pilots adjusted to provide enough Fuel for both processes; you are really going to like the way your Bike will run.

It's a Joy!

 

would increasing a main jet size, or just adding a half shim(from chacal) to the main jet post make a diffrence?
this is of course appicable when idle is perfect and wot is lean....

 

Sure.
Across the whole range of Yamaha Bikes it's not uncommon to bump the Main Jet up a notch.
You'll get a little more "Pull" when out on the Highway and some have done it just to have their Bikes run cooler on real long ride.

People who are large (like me) actually need to have larger Main Jets just to get the Bike to have a bit more Power rather than needing to keep the rev's up because of the extra weight.

Bumping up a Main Jet size was practically necessary for the XS and XV Bike owners.

About the only down side is Economy.
Don't expect your Bike to go as far as it does now on a tank full.

And, ... Keep the old Main Jets handy in case you find that you'd prefer having faster acceleration over the lesser "Quickness" you might have as a result of running smoother and cooler.

 

I have the same high speed lean going on. My mixture settings on my 85 maxim 700 seem to be a bit rich. The bike stumbles getting out of the hole and when getting the bike up into my backyard (a steep grade) it lacks the necessary power to do so unless I over-rev. The plugs are black down below the threads of my bp8es NGK's but after chopping at 65 or at other speeds the rest of the ceramics are white. I tried upping the pilot jets but that just made my power-lacking situation worse. This bike really wants to run lean! Am I a good candidate for upping the main jets? I love this bike and I don't want to blow it up.

Thanks