68S vs a 68 slow jet for a XR650R


22 replies to this topic
  • philipstjohn

Posted November 05, 2006 - 04:45 PM

#1

I have seen the difference between the two, and wonder what I shoud expect if I change from a 68 to a 68S. I am at sea level, running a 68 slow, 175 main, B53E comp needle in the 3rd seat, open air box, uni filter, drilled stock tip. The bike starts first kick most times, if not the second kick. Please tell me why I should go to a 68S if you can. Thanks guys...:mad:

  • pigryder

Posted November 05, 2006 - 05:22 PM

#2

I actually went from the stock 65 to the 68 and noticed no difference... I honestly dont think you'll notice a difference in the S

  • philipstjohn

Posted November 06, 2006 - 04:51 AM

#3

I actually went from the stock 65 to the 68 and noticed no difference... I honestly dont think you'll notice a difference in the S


The 68S as I understand creates a richer mixture, which most say helps start the bike and helps it run cooler? My bike starts first kick most times, and since I re jetted I have not had any over heating issues. Just wondering if there is a potential performance gain...

  • pigryder

Posted November 06, 2006 - 05:17 AM

#4

actually it emulsifies the fuel better because of the hole size difference on the shaft of the jet, If your bike starts and runs that good I'd say dont bother

  • eastreich

Posted November 06, 2006 - 09:38 AM

#5

The better emulsification properties of the 68s should also give you better throttle response, but like was mentioned earlier, if you aren't having problems, don't worry about it. The s jets are only available through Honda and are therefore a little more costly!!!

  • philipstjohn

Posted November 06, 2006 - 09:58 AM

#6

The better emulsification properties of the 68s should also give you better throttle response, but like was mentioned earlier, if you aren't having problems, don't worry about it. The s jets are only available through Honda and are therefore a little more costly!!!


I've tried to get a 68S thru two local Honda dealers- and both stated they could get a 68 but not a 68S- XR's Only told me they have a 68S available and I believe they stated $12.50 for the jet FYI
So you think I will notice a little better throttle response with the 68S? I noticed alot of black soot on the exhaust tip yesterday when I pulled it off, and did not noticed this when I first inspected the tip prior to re jetting? Could I be running too rich?

  • pigryder

Posted November 06, 2006 - 10:16 AM

#7

sounds a little rich but better rich then lean, and the pilot and or slow jet only works up to 1/4 throttle....basically idle speeds and just over.....more to do with starting..

  • eastreich

Posted November 06, 2006 - 10:18 AM

#8

You have two local Honda dealers that are lazy and don't want to look up the part numbers... I've gotten s jets from my Honda dealer. But my Honda dealer also rides a 650R, so that might have something to do with it.

If you've got black soot you are in all likely hood running a little rich. Do a plug check to find out where the fuel circuit is fat (main or pilot) and rejet accordingly.

  • eastreich

Posted November 06, 2006 - 10:29 AM

#9

The Honda part number for the jet should be 99105-MBN-0680. It is listed as simply a 68 pilot jet, but the MBN moniker means that is is specific to XR650R model. This should be the 68s pilot that you would be looking for, as opposed to a generic Keihin pilot. An example of a generic Keihin pilot jet part number from Honda would be 99103-399-0550 (55 pilot). This jet is the same style as the one in the XR650R, just not the 's' series.

  • philipstjohn

Posted November 06, 2006 - 02:24 PM

#10

You have two local Honda dealers that are lazy and don't want to look up the part numbers... I've gotten s jets from my Honda dealer. But my Honda dealer also rides a 650R, so that might have something to do with it.

If you've got black soot you are in all likely hood running a little rich. Do a plug check to find out where the fuel circuit is fat (main or pilot) and rejet accordingly.


Can you explain how to do a plug check that determines which fuel circuit is fat? I know how to read a plug for possible signs of potential problems, running rich or lean, etc. But how do you know if its a slow jet or main jet issue? Also, for clarification, how does a perfect running XR650R plug look? Please fill me in if you can. Thanks :mad:

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  • eastreich

Posted November 06, 2006 - 06:46 PM

#11

To check the low speed circuit (pilot), ride the bike around at low throttle settings (not necessarily lugging the motor, but not full throttle blasts either). Try to keep the bike at or below the transition range where the main jet kicks in fuel to the engine. Pull the plug (while it's hot) and inspect the tip. If it is black (sooty) the pilot circuit is in all likelyhood a little fat. If it is a light brown, it is closer. If the ceramic is white, burndown is imminent.

To check the main, the opposite is true. I usually find a nice spot of open desert or long stretch of dirt road to run 'er up in top gear and stay wide open for a little bit. It is getting all the fuel from the main that it can at that point. Then chop the throttle and kill the motor. Let it coast to a stop and check the plug. The same color scenario from before applies.

If you run a pumper carb, you can visually tell if the accelerator pump is too rich by a jet of black soot exiting the pipe when you whack open the throttle at idle (I need to fix that one of these days...).

The perfect running XR, or any engine for that matter should have a plug that has the ceramic tip a nice vanilla white to a very light tan. Not too much fuel to loose power, but not lean enough to do any damage.

This is a little different for the 2-strokes out there, as they have one distinct problem. At high engine speeds, they lose more fuel out the exhaust port than at lower engine speeds. When help wide open (as across a dry lake bed in SoCal or in the dunes) for an extended period of time they can lean out to the point of siezure. That is why the savvy desert racers tap the kill switches on their 500's across the lake beds. This trick cools the cylinder and piston below siezure levels and provides a little more lube with the unburned oil and gas. If you are in a high speed desert race with a two-stroke, it doesn't hurt to have a main 1 or 2 sizes larger than you normally would. I had a friend of mine brain fart in a Hare and Hound one year. He made it about 1.5 miles across a 2 mile dry lake. His CR500 got to ride out in the bed of a truck. I did the kill switch trick with my KX500's and never had a problem.

  • Billahjack

Posted November 09, 2006 - 04:55 PM

#12

XR's Only told me that a 68s is pretty close to a 70 regular pilot jet in function. The BRP websites show the difference on the emulsificaiton holes with pictures. I'm running a 68s.

If you need full choke to start up when cold, then you are running lean-ish. If you don't need the choke to start up when the engine is cold, then you are running rich on the pilot. I was running too rich and switched from a 72 to the 68s that I have now. Bought it from a Canadian E-bay vendor that sells the complete Baja jet kit (a couple mains, the needle jet set and 68s pilot). Send the seller a message asking for just a 68s. It was about $5 for just the 68s with shipping included and arrived about 5 days later.

  • philipstjohn

Posted November 09, 2006 - 05:14 PM

#13

XR's Only told me that a 68s is pretty close to a 70 regular pilot jet in function. The BRP websites show the difference on the emulsificaiton holes with pictures. I'm running a 68s.

If you need full choke to start up when cold, then you are running lean-ish. If you don't need the choke to start up when the engine is cold, then you are running rich on the pilot. I was running too rich and switched from a 72 to the 68s that I have now. Bought it from a Canadian E-bay vendor that sells the complete Baja jet kit (a couple mains, the needle jet set and 68s pilot). Send the seller a message asking for just a 68s. It was about $5 for just the 68s with shipping included and arrived about 5 days later.


Do you remember the name of the vendor?

  • drop

Posted April 12, 2007 - 05:34 PM

#14

Here is the vendor: http://cgi.ebay.ca/H...bayphotohosting

I'm a little wary about this one though. The auction says that "the jet is stamped 68 but is a 68s". Sounds fishy to me. Has anyone bought from this vendor?

  • Kawabuggy

Posted April 12, 2007 - 06:17 PM

#15

The correct 68S will MOST DEFINITELY have the "S" stamped into it. Don't settle for a jet that does not have the correct stamping.

I ordered a 68S from Xr's only and was told that they had what I was looking for. I received the jet and the invoice showed a "68S" but it was just a regular 68. When I called them about it, they basically told me that I could return it for my money back if I wanted... So, don't expect to order it from them and get what you really want.

The Honda dealership is the way to go. They will get you the "S".

I think trial & error is really the way to go with this. Start with the 68S, and then do the plug test as indicated above. That will tell you what is really happening.

  • philipstjohn

Posted April 12, 2007 - 06:25 PM

#16

Many have posted that the Honda number for a 68 pilot is actually a 68S. The 68S is actually a Honda spec, according to some who have posted on this issue. I went to two Honda dealers, none had it, but a buddy of mine found one in his search for another jet. FYI

  • Billahjack

Posted April 12, 2007 - 06:35 PM

#17

http://www.siriuscon...com/default.asp

They shipped a 68s style jet to me for $6. It was not marked S but I verified the emulsification holes with my 65s and a regular 70 and it looks like an S. Works great too.

  • zook350

Posted April 12, 2007 - 10:13 PM

#18

my local dealer didn't know about the s jets either but just have them order the xrr part number and the s jet will come in. most dealers will not stock the s jets so you will have to order it.

  • motopsycho650

Posted April 13, 2007 - 02:40 AM

#19

Trust me, try a 68S pilot jet, and you'll see the difference immediately. Depending on where you ride, and how the bike is set up, you may even need a 70S.

With my cut out side panel, I had to up the jet to a 70S here at 5000 Ft. I've got low end response like you wouldn't believe. No hessitation, very little de-accel pop, easy starting even when it's cold.

  • Thumpage

Posted April 14, 2007 - 05:24 PM

#20

The guys are right; Many Honda dealer parts guys don't know about having to actually look at the XR650R carb parts screen and ordering the 68S pilot jet from the carb parts listing. The dealers do not normally carry these in stock and have to order them. The Honda part number does not have the 's' within the number but it is in fact a 68S pilot that will be stamped as such. The 's' pilots seem to be Honda specific versions.
You can get away with a standard 68 keihin but it should most likely be a little leaner in effect at idle.
This is what I have gathered. I believe the 68s kind of falls between a standard 68 and 70 pilot. The 's' and non-s pilots just seem to 'act' slightly different.
The air emulsion holes on the standard pilots are bigger which should allow more air to be emulsified with the fuel being drawn from the tube which may result in a somwhat leaner emulsified fuel mix. So how this turns out, the standard 68,(with its bigger emulsification holes) should provide a slightly leaner emulsified mixture than the 68S. If you use a standard 70 pilot it will naturally provide more fuel but also a generally higher amount of air emulsified with the fuel being supplied. In the end, it seems the standard 70 is just a little bit richer than the 68S and a little leaner in effect than a 70S, *at idle*.
You also have to consider how the pilot jet works when you are deccelerating. When you chop the throttle off, the AJCV (Air Jet Cutoff Valve) will activate when the vacuum increases. The AJCV cuts off the air that would otherwise come through the pilot jets side emulsion holes. This causes the un-emulsified raw fuel from the pilot circuit to richen up the air/fuel mixture as the throttle is closed. In turn, this does a couple things; It prevents engine popping from the lean condition that occurs when the slide is suddenly closed and it softens engine braking. Once the vacuum drops and the bike is back near or at idle, the AJCV opens back up and the pilot jet meters the fuel back to it's original state.
The difference in size of the emulsion holes may slightly effect the way the transition from decceleration to idle feels between the 's' and non-s pilot jets. 'S' & non-s pilots should differ a little in their effect at idle but may be very similar when the slide is closed and deccelerating, as their is conceptually no air being supplied to the emulsion holes for a period of that time. You may or may not feel the difference in transition.
Remember, if you change your pilot, make sure to go through the fuel screw tuning procedure again to dial it in. This helps keep your bike idling in it’s proper range. If the pilot jet is right, it will fall anywhere from 1.5 to 2 turns out.





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