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Unexpected lean XR400...confused.

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Hello all.

I have a XR400 2002 with standard engine and carb.

Removed air intake funnel, K&N filter(on road only bike), Baffles removed, Header welds removed.

I read everything there is to read about jetting and fitted a 162 main jet...ran lean.

I fitted a 165 main jet (and this is supposed to be rich even with all the Gordon Mods)...ran lean.

So I went to a 170 main jet...guess what...ran lean!

Has anyone ever heard of this?

Any suggestions to what jet I try next?

Cheers

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Are you changing just the main jet? What size pilot and what needle position do you have?

 

Are you saying lean because of plug color or some other reason like the way it runs?

 

You said "removed all baffles", can you expand on that some? 

 

One more question. Are the sizes your talking about stock Keihin (I'm sure I jacked up the spelling on that) jets or from a K&N jet kit? The number system is different on the K&N jet kit. A 165 K&N jet would be like a 155 Keihin jet.

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The number system is different on the K&N jet kit. A 165 K&N jet would be like a 155 Keihin jet.

 

How lame is that?  A 155 Keihin jet is 1.55mm diameter.

Edited by Baja Rambler

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I know. I bought a K&N jet kit probably 10 years ago and it said to use a 175 or 180 main jet with a pipe!! That's when I asked around and found it was not the same number = same size deal. This was on a 400EX quad though and they had different cam and carb from the XR400. Maybe they have changed since then but it confused the hell out of me. Never got it to run right with the kit either. It always had a high rpm miss or got really hot and wouldn't start worth a $hit. I just put stock back in and it was 100% better but still a little lean. Sold it right after that!! LOL!! 1st and only 4 wheeler I will ever own. I just call it temporary insanity now!! LOL!

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Thank you for your replies guys. REALLY appreciate it.

ok...

Air intake leak...my first thought...all seals seem fine.

Baffles: internal silencer has been removed just leaving the fine mesh spark arrestor.

Pilot jet is 62. Pilot system seems fine, starts fine, ticks over fine, choke test passed, fuel screw adjusts idle speed between 1/2 & 3 turns.

Needle position 3.

Main Jet size is Keihin #170

Why I think it's lean: Bike ran fine until hot weather came when it started to run very hot with erratic acceleration, so I checked plug. Very white spark plug. Changed from 162 jet to 170 expecting darker plug. 20 miles journey to work to find only very slightly darker plug, but still lean according to charts.

My first thought is to fit #180...if that runs rich, go back to 175. If 180 still runs lean...then its getting a bit silly! What do you think?

The bike is running fine with 170...no flat spots...perfect acceleration at all revs...just runs slightly hot with white plug.

With 16:42 gearing cruises all day at 75mph, tops out at sat nav recorded 95mph...not relevant I know, but impressive for a humble XR400.

Are K&N filters just SUPER free flowing? or should I recheck for intake leaks?

I really hope I have covered everything.

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What spark plug are you running?

Hot weather usually causes richer conditions. Its the higher air mass/ o2 in the air of cold weather that usually makes for leaner conditions.

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If you still have your stock filter and cage, put that back on for a test run. It won't cost you anything and will be a quick test to see if richer is going to help. The stock filter set up should richen it up quite a bit and you won't have to dig into the carb to swap jets.

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If you still have your stock filter and cage, put that back on for a test run. It won't cost you anything and will be a quick test to see if richer is going to help. The stock filter set up should richen it up quite a bit and you won't have to dig into the carb to swap jets.

 

Partial choke will do the same type of restriction.

 

What about the primary jet and needle position as that is where you spend 90% of the riding time.

 

Good point.  The pilot jet and needle are used for most of your riding.  Also, the main is fairly forgiving in that it can be a couple sizes larger.  I run a rich main in the desert to cool the engine at WOT.  With that, I averaged 66mpg on a recent adventure on an XR250.  I couldn't find gas until I traveled 230 miles.  It was 87 octane siphoned out of a grungy five gallon gerry can. 

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Hello everyone...your answers have made me re-investigate by bike (which I maybe should have done before now).

Thank you.

When I bought the bike I was assured the pilot jet was 62 and the main was 162.

It was a 1 owner bike bought from a friend, so I had no reason to doubt this, and I'd previously confirmed the main jet.

However, I found the pilot jet was a 55.

I have moved the needle clip from standard position (3) to one position richer which has noticeably improved the mid range.

I have also ordered a 62 pilot jet which I am expecting through the post tomorrow.

I will fit them and report on the results.

I really appreciate the knowledge given to me by some of your answers.

I have learned a lot about the XR400 after stripping to the frame: Engine top end, gearing, suspension, oil change intervals etc. all Gordon Mods and now fuelling.

So, if anyone has a question please feel free to ask.

These bikes and the knowledgeable owners are bloody brilliant. I love em!

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Just wish the bikes and parts weren't so USA orientated.

Anyone know where I can find an oil dip stick thermometer in the UK?

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I have had some overheating issues since I swapped for a Mikuni.

On my Mikuni, I lowered the needle clip, raised the needle, and fattened up the jetting. It helped popping on decel and seems to have lowered the heat.

I use to finish off old premix in my XR. perhaps I have some carbon on the piston?

Never touched the needle with the Kehein, but it might help.

The oil stick thermometer was only made by XRsOnly as far as I know.

Trail tech makes speedos with a heat sensor.

I bought a couple thermometers off eBay off eBay, which I haven't mounted yet.

One is for BBQ. It would go high enough is a sensor was added.

One is for a fish tank. The sensor could be mounted under the tank and I could see when above the engine is getting hot.

Also, there are sensors for RC cars. They track heat of batteries. About $20.

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I think I may have sorted  it.

I am now running a 65 pilot, a 165 main and the needle 1 position richer....on a bike with no engine mods and a standard carb and it is now running perfect.

Plus...(with allen float bowl screws) I can swap both jets in 20 minutes with carb on the bike!

I even carried out a bog test and nearly flipped the bike...who needs a pumper carb?

Seriously...forget your big bores, hot cams and pumper carbs...just completely uncork your bike and spend time setting up carb properly...its a smooth-running, arm-jerking, reliable rocket, albeit a loud one!

Thanks again for all your help.

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Why I think it's lean: Bike ran fine until hot weather came when it started to run very hot with erratic acceleration, so I checked plug. Very white spark plug. Changed from 162 jet to 170 expecting darker plug. 20 miles journey to work to find only very slightly darker plug, but still lean according to charts.

How are you checking your plug? Proper plug reading based on WOT conditions followed by IMMEDIATELY shutting down the bike from WOT is the only way to get an accurate reading on main jet sizing. If you're decelerating, idling for any amount of time before shutting it off you'll get readings based more on pilot jet, not main jet. what you have now said, I believe your pilot was suspect being 55 not 65 as you had assumed.

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 Proper plug reading based on WOT conditions followed by IMMEDIATELY shutting down the bike from WOT is the only way to get an accurate reading on main jet sizing. 

 

Are you familiar with the "roll off" method in the Mikuni jetting literature?

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Yes- and it could also help with fine tuning jetting. My question to the OP was how the spark plug was read, under what conditions.

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