KX85 bogs when make starts at 2:nd gear

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8/15/2017 1:42 AM

Hi,

My sons kx85 -17, sometimes bogs when he shall make a start at 2:nd gear. It was worse with the Pro Circuit pipe/silencer so I changed back to the HGS exhaust-system and thought I "solved" it because the bike didnt bog anymore when he did the start training.

But now the problem seem to come back.

I run the stock jetting. According to the manual for our current weather (temp + altitude) Stock should be the right setting. And the bike runs very well on the track, it also answers well when twist the throttle with no gear.

I did tried bigger jets but it didnt solve the issue of boging.

I have cleaned the exhaust valve so I dont think that would be the issue either. (bike has about 30h and the piston about 10) It did bog also right after the piston was changed

Sometimes he can make 3-4 starts on 2:nd and it works fine and then for the next starts it bogs right after he let go of the clutch. Maybe the increased engine temperature makes it bog?

When start at 1:st gear there is no issue.

Any ideas?

//Mikael

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8/15/2017 1:58 AM

try a larger rear sprocket, to close up the gearing, sounds like its right on the edge of being able to pull second gear starts,

it does not surprise me its worse with a PC pipe, they tend to be more top end than mid range,

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8/15/2017 2:05 AM

I would try dropping the needle one position.

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8/15/2017 2:51 AM

Yeah with that pipe you have to keep it high in the RPM range... Both suggestions above are good!

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8/15/2017 3:31 AM
Edited Date/Time: 8/15/2017 3:35 AM

scott_nz wrote:

try a larger rear sprocket, to close up the gearing, sounds like its right on the edge of being able to pull second gear starts,

it does not surprise me its worse with a PC pipe, they tend to be more top end than mid range,

That would most likely help, im only frustrated it varies, sometimes it seems allright and sometimes it bogs.

I dont recall his last bike kx85 -14 acted in the same way.

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8/15/2017 3:34 AM
Edited Date/Time: 8/15/2017 3:38 AM

Yamaknockers wrote:

I would try dropping the needle one position.

I was thinking about the needle aswell, drop the needle (raise the clip) will give less fuel, maybe that will help. I need to test it

Kawasaki actualy also provided a additional needle when I bought the bike, I did not check if it is thinner or thicker than stock one.

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8/15/2017 3:38 AM

Drop the countershaft sprocket 1 tooth, try to burn the clutch up and it will explode out of the hole. 125's do, least mine (when I do it correctly! haha) does. its a sacrifice, but shocking news MX aint won on the straightaways boys & girls.

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8/15/2017 3:49 AM

cvm wrote:

Drop the countershaft sprocket 1 tooth, try to burn the clutch up and it will explode out of the hole. 125's do, least mine (when I do it correctly! haha) does. its a sacrifice, but shocking news MX aint won on the straightaways boys & girls.

1 tooth on front sprocket will change a lot on the all around handling of the bike on the track. My son seem to be pretty satisfied with the stock gearing on most tracks for his current speed.

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8/15/2017 4:08 AM

cvm wrote:

Drop the countershaft sprocket 1 tooth, try to burn the clutch up and it will explode out of the hole. 125's do, least mine (when I do it correctly! haha) does. its a sacrifice, but shocking news MX aint won on the straightaways boys & girls.

Mikey77 wrote:

1 tooth on front sprocket will change a lot on the all around handling of the bike on the track. My son seem to be pretty satisfied with the stock gearing on most tracks for his current speed.

True. And that's your son's preference, none of us knows his bike better than he does.. Drop a tooth a 1st gear turn can turn into a 2nd etc, and a good rider can adapt in many situations.It is a sacrafice. Its just a $14 experiment to try for an easy-to-do 2nd gear start. He!! I didn't even have to cut my chain to try it smile And like your son saw, I did notice different characteristics around various parts of a track.
I personally think it changes the overall gearing more than the handling though, but that's just my limited experience.

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8/15/2017 4:32 AM

cvm wrote:

True. And that's your son's preference, none of us knows his bike better than he does.. Drop a tooth a 1st gear turn can turn into a 2nd etc, and a good rider can adapt in many situations.It is a sacrafice. Its just a $14 experiment to try for an easy-to-do 2nd gear start. He!! I didn't even have to cut my chain to try it smile And like your son saw, I did notice different characteristics around various parts of a track.
I personally think it changes the overall gearing more than the handling though, but that's just my limited experience.

Yepp, you are right, its a cheap test to do. My only concern is that he would end up between 2:nd and 3:d gear in some turns. And as you wrote good (experianced) riders could adapt, but my son is not superexperienced and is not a big fan of feathering the clutch to help when the gear is slightly to high and engine cant power thru that gear on its own.

(sorry for the bad english smile )

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8/15/2017 6:50 PM

The problem surfaced after the rebuild? The first thing I would check is the power valve. Make sure the dot in the arm and the cylinder are aligned. Take the cover off and open the throttle while watching the powervalve to make sure it's opening and closing properly.

If everything looks ok the next thing would be the jetting:
Pilot: 42 or 45
Main jet: 142 or 145
Needle: Nape (oem on 13 kx 85 model)
Clip 3rd
Slide (oem from 17 yz85 which is 1 richer)
Air screw: 1.25 to 1.5
Plug: BR9EG with .22 gap

Good Luck!

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8/15/2017 6:54 PM
Edited Date/Time: 8/15/2017 9:38 PM

Any possibilty its rider error? I mean that with no malice.

Is the issue replicatable when doing practice starts or does it only happen behind formal, race gate drops?

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Copius pattern amounts of phrases like “Anyone have”..., “Anybody know?”.... and their variations.

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Thanks in advance!





8/15/2017 7:07 PM

I don't recall ever changing the front tooth sproket on my 80s. My dad was the mechanic so I don't know. I do know we added teeth to the rear sproket for sure.

80s second gear start is standard and you have to really have the rpms up very high and sorta feather the clutch. Can't just drop it. Takes finesse and technique. But rpms will need to be high.

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8/17/2017 8:03 AM

brocsdad wrote:

The problem surfaced after the rebuild? The first thing I would check is the power valve. Make sure the dot in the arm and the cylinder are aligned. Take the cover off and open the throttle while watching the powervalve to make sure it's opening and closing properly.

If everything looks ok the next thing would be the jetting:
Pilot: 42 or 45
Main jet: 142 or 145
Needle: Nape (oem on 13 kx 85 model)
Clip 3rd
Slide (oem from 17 yz85 which is 1 richer)
Air screw: 1.25 to 1.5
Plug: BR9EG with .22 gap

Good Luck!

thanks for all the tips!

It happen like this also before the change of piston.

Regarding the Power-valve: When I changed piston I basicly remove the locking clips from the axel and then slide the other "arm" of the axel to the side to get free clearence when remove the cylinder.

Then just opposite when re-assemble all. Do you think the standard position of the axel alligement can be altered in this process?

Im not infront of the bike now but the markers should allign when no throttle is applied, yes?

I see about the plug that I use the stock one, wich is called 105 something, wich I think is 10.5 , so that is maybe to hard then? (get not hot enough)

And regarding the jetting suggestions it all gives more fuel than stock, and basicly for colder weather if look at the chart in the manual. We have about 20-22 degrees celsius now in summer, (68-72 F)

But I did also hear from workshop in sweden that the KX85 in stock is equiped with to small jets so your tips is also similar to what I heared here in sweden.

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8/17/2017 8:09 AM

TXDirt wrote:

I don't recall ever changing the front tooth sproket on my 80s. My dad was the mechanic so I don't know. I do know we added teeth to the rear sproket for sure.

80s second gear start is standard and you have to really have the rpms up very high and sorta feather the clutch. Can't just drop it. Takes finesse and technique. But rpms will need to be high.

I dont think I will change front sprocket, maybe add 1 tooth to the back, at the most. actualy on sandy tracks you can drop 1 to run longer on the same gear thru the whole berm not needing to put in the next one in the corner so to speak,

I think my son may release the clutch to quick, I think he "Chocks" the motor and it have not the power enough to handle the sudden amout of torque wich is applied so quickly.

So more training may actualy get rid of the problem,

Only so i Understand correctly: when you say feather the clutch, it meens release it not to quickly?

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8/17/2017 10:58 AM

kzizok wrote:

Any possibilty its rider error? I mean that with no malice.

Is the issue replicatable when doing practice starts or does it only happen behind formal, race gate drops?

x2.
Asking a young kid to nail starts perfectly every time is a tall order. It's perfectly reasonable to believe he just needs more practice, especially since you say the bike runs well on the track and he can do 3-4 good starts at times before you have the problem.

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Braaapin' aint easy.

8/18/2017 12:58 AM
Edited Date/Time: 8/18/2017 1:30 AM

kzizok wrote:

Any possibilty its rider error? I mean that with no malice.

Is the issue replicatable when doing practice starts or does it only happen behind formal, race gate drops?

Falcon wrote:

x2.
Asking a young kid to nail starts perfectly every time is a tall order. It's perfectly reasonable to believe he just needs more practice, especially since you say the bike runs well on the track and he can do 3-4 good starts at times before you have the problem.

I was so sure it was the bike that I did not fully take rider factor into the equation.

He thinks and I observe that it looks like he does similar starts and after 3-4 bikes start to bog,

Neither the -14 kx nor the earlier KTMSX65 -13 acted like this...

I will shortly again change piston and I will try look at all possible mechanical factors to eliminate these as the reason of boging.

Then more practice on the starts and we see where we stand. How about body position? (I think sometimes he climb to much up on the gastank, possibly he is little afraid that the bike will raise out of the gate)

Photo

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8/18/2017 2:02 AM

kzizok wrote:

Any possibilty its rider error? I mean that with no malice.

Is the issue replicatable when doing practice starts or does it only happen behind formal, race gate drops?

Hi,

yes is possible, it seems like this both on practis and at the last race as well

//Mikael

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8/18/2017 2:09 AM

Had the same problem with my kid on his 85sx.
The spring on his air screw didn't have enough tension and came loose.
We pulled the spring set the screw 1.5 turn out and the problem was solved.

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8/18/2017 2:27 AM

Keep starting in first!

Or don't dump the clutch let it slip.

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8/18/2017 2:46 AM

I used to start in 3rd and pulled many a holeshot, this was many many years ago though. cool

Photo

Photo

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-OC
"Feed The Bull"
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8/18/2017 4:04 AM

segers wrote:

Had the same problem with my kid on his 85sx.
The spring on his air screw didn't have enough tension and came loose.
We pulled the spring set the screw 1.5 turn out and the problem was solved.

ok, I checked that and it looks stabil, but thanks for the tip.

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8/18/2017 4:07 AM

kkawboy14 wrote:

Keep starting in first!

Or don't dump the clutch let it slip.

Yes, more clutch sensivity is most likely the key in this matter.

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8/18/2017 4:10 AM

ocscottie wrote:

I used to start in 3rd and pulled many a holeshot, this was many many years ago though. cool

Photo

Photo

Nice! Must have been more power on those old kx:s to pull such Andrew Short starts wink

Cant test anything this weekend becasue we will be in Uddevalla to watch the MXGP racing. Will be nice for sure.

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8/21/2017 1:36 AM

So I check my notes and piston is 20h, not 10 so I took it out yesterday. Whats your thought about the plug?

Some tips above was to use larger jets. Is the plug also telling that? (As I see its darkbrow/almost black)

Photo

And Piston ring clearly is done if look at the color under the ring. What about the top of the piston, is that black normal ? I still use the standard plug NGK R6252K-105 (i think is 10,5 hardness) Maybe as suggested above the: BR9EG plug should be to prefer, 9 Hardness get hotter if understand correctly, but why is that better?


Photo

Photo

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8/21/2017 2:04 AM

we always used to go back to a 9 heat range, but most of the time used BR9EIX irdium plugs, they ran better with the hotter plug,

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8/21/2017 3:21 AM

scott_nz wrote:

we always used to go back to a 9 heat range, but most of the time used BR9EIX irdium plugs, they ran better with the hotter plug,

OK, anyway is not big money to try this plug you used. You also used bigger jets?

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8/21/2017 6:51 AM
Edited Date/Time: 8/21/2017 6:53 AM

Hard to tell how your bike is jetted just from looking at the plug. You can do a 'plug chop' to determine if you have the right main jet, but its damn near impossible to draw conclusions about the pilot/ needle.

To perform a plug chop, warm bike up, find a long straight and run the bike wide open in a tall gear, hit the kill switch without letting off, check plug. Now your plug will give you a decent idea of how your bike is running on the main jet. Black is too rich, White is too lean; shoot for an off white - tan color depending on how hot you want that thing to run. Don't worry if your plug is black before you run this test, it'll clean up at WOT.

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8/21/2017 7:57 AM

OK, Yes I heard about the plug chop so I need try it when bike is put back togheter. I will aslo try the BR9EG plug

Just curious about the top of the piston , I think the blackness (not sure about the english word) is pretty normal, but is quite thick layer , so I was wandering if jetting and/or plug-hardness affect that in any way.

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8/21/2017 8:50 AM
Edited Date/Time: 8/21/2017 8:56 AM

Based on the piston top, it looks rich to me as our pistons do not have that thick layer of carbon build up at 20-hours. But the main jet should have little effect on the starting line bog issue. Certainly lean the needle a clip or two to see if it improves. The NGK BR9EG plugs works well in the KX85. If we go to a deep sandy track, then I install a BR10EG and go up one main jet size.

To me, starting line bogs could be caused by jetting, clutch release (rider), track surface and clutch condition.

On our 85s, after we install a fresh clutch, we usually find slight jetting issues that were hiding behind the slipping clutch.

Adding a tooth the rear sprocket also helps and it doesn't affect the rest of the track significantly.

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