Major bogging - - Help

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#1
I recently purchased a '90 kdx200 that came with a pipe,silencer,reeds,air box cover removed and had been jetted accordingly..the bike ran great. Both the pipe and silencer are after market FMF bolt-ons.

I decided to clean the carbon out of the pipe and repack the silencer...Oven cleaner was used to clean both. A propane torch was used on the pipe. I cleaned out all the small holes in the silencer with a small drill bit. Man what a long task that was. Standard household insulation was used to pack the silencer. I tried to match the amount of insulation that was originally stuffed in the unit. The original packing was drenched with spooge and the majority of the holes were plugged.

Now the bike bogs whenever I quickly snap the throttle wide open.The lower the rpms the more the bog. I'm talking total lack of power...as if someone turned off the power switch.
It will bog,then stutter,, then it will finally build rpms as it should.The bog will happen for about 2 to 3 secs.
If I roll into the throttle the problem does not exist.
Help,,,,did I repack the silencer too tight/loose. Do I need to rejet.

I think the key to finding the solution may lay in the fact that this only happens when I'm aggressive with the throttle opening. Slowing rolling into the juice does not produce this prob.

I'm going to take apart the Silencer again. I'm absolutely positive there is not a blockage in the pipe.

Here's a though/question.

If an exhaust gasket is missing or badly dried out will this cause my problem.
Just remembered that I only have one exhaust gasket as one broke while I was taking the pipe off..
I know, I know ( ..... I'll go buy one tomorrow....hehee
thanks.
Bucket

:)
 
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#2
man... your bike is never going to run right for you if you treat it like that! take out all of that insulation crap, and put silencer packing in it, that will help your throttle response. if that doesn't do it, how often do you clean/replace your air filter? clean your air filter, then see how it runs. what is the condition of the top end??? it might be time for a new one. a kdx will never have the snap of a motocross bike, so get used to that.
 

Canadian Dave

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#3
You're definitely running lean down low.

Replace the o-rings on the header section of your pipe then clean it up well. Do the same where it fits into he cylinder. When you reinstall it use some silicone. Run a small bead around the o-rings. You want to use just enough that it squished out from the pipe a little but not so much its forced into the exhaust port. If your pipe is leaking from around the header pipe you'll certainly see a drop in power.

If this doesn't fix the problem take a second and check you jetting then get back to us. We'll need to know what size pilot and main jet you are using as well as what needle and its clip position. That will give us an idea if there is an obvious problem there. You might also want to check and see how many turns out on your air screw. It could be that it was jetting with the pipe full of junk and the silencer plugged and now that you've cleaned everthing up you'll need to touch up the jetting a little

David
 

MADisher

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#4
If it wasn't like this before, it sounds to me like you have something plugging the exaust...

double check the packing, and the pipe.
 
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#5
Hello again....thanks for the replies...
Here is a little more info.

The engine was professionally rebuilt ..Complete top end including re-nikasilled cylinder. The pro shop jetting aswell because previous owned could not get it right..All done prior to my purchase ( oh say 2 months ago)..The air filter is always cleanned and properly oilled. New plug installed BR9ES.
32:1 mixture ratio.
The engine was bang on. Awsome bottom end and a real snapping mid to top end. I checked the jetting myself ( as per Erric Gorr's and Just KDX's web sites) using the plug technique and the feel technique.Absolutely no hesitation at any throttle position.. no crackling and no Boowhaaaaaaa's.( not too lean or to rich through all throttle positions). Plug color... beautiful tan.
All was good.:) And then........ I cleaned the pipe and silencer.:(
The jetting :
Main 150
Slide Cuttway 6.0
Needle Jet R1172N
Clip position 3rd from top
Pilot Jet 45
Air Screw 1 1/2 ( depending on riding conditions)

Thanks again.
Bucket
 

BRush

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#6
In addition to what CDave said, I'd repack the silencer again. This time, don't try to match the exact amount of material, but wrap enough packing so that everything is snug when you install the tube into the silencer shell. Some trial and error is needed the first time. In particular, make sure there are no gaps at the front or rear of the perforated tube. The perforated tube must be covered by packing for it's entire length. I usually cut the insulation an inch or two longer than the tube, then wrap it. The extra length gets compressed when you put the ends on.
 

G. Gearloose

Pigment of ur imagination
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#7
There should only be about 3" of packing on a '90, in the middle section. The reverser and cones should packing-free - is that what you see?
 
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#8
There should only be about 3" of packing on a '90, in the middle section. The reverser and cones should packing-free - is that what you see?


Hello,,,I'm not quite sure what you mean,,,sorry.:(
My silencer is an aftermarket FMF that was packed completely through it's entire lenght. :)
 
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#11
ok,,I've figured out what the problem is......

Here it is

.....ME !:o
Here's what I did/found.
I sealed up the exhaust at the header section of the pipe and where it enters the cylinder.. I repacked the silencer as recommend by the form.
Today was test day...In the morning it was cool > around 15'C < and the bike ran great.. No bog at all...yeehaa!!
As the day and the bike warmed the problem got progressively worse..
I know,,,colder air = more dense air = leaner mixture.(you'll understand :D ,,,read on)
Sooooo,,,I played around with the air screw and had no change what-so-ever. In/out it just did not make a difference.still bogged
I then pulled the choke out, went for a whrill and....TA-DA...no more bog.
And here I was going on about my jetting knowledge...oh well.
Now all I need to do is replaced my pilot jet with a larger one.right???

Any recommendations????(see above for current jetting specs)

Please keep in mind that adjusting the air screw to 1/2 turn out had the best affect..I chose not to turn it in any more for fear of the unknown...I'm sure a fully closed air screw would no be a good thing..I guess I'll buy 45,46,47 and just try them all.....

Thanks again...

Bucket-in training- :)
 
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#12
And again,,,an update..
I've been reading this form alot...everthing from jetting to the kips exhaust system....
and behold...yet another problem was found.
I found the the gears on the left exhaust valve stripped....If I manually operated the exhaust valve operating rod,,the right and main exhaust vavles would work. The left would only operate sometimes.. Parts on order.
I think I still have a lean pilot jet as the engine has a ton more power when the choke is operated and the air screw is almost all the way in..
later.
Bucket
 
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#13
Hello all.
Sorry for taking soo long getting back to the form. (too much overtime I guess)
Here's the latest.
I did indeed find the jets I was looking for.
I replaced the exhaust valves,replaced the #45 pilot with a new 48 pilot jet...buttoned the engine up and went for a ride..
Wow!! what a difference..especially after I dialed in the pilot circuit using the air screw..
I still feel I'm a little lean at WOT...The power really flattens out at the top.
I'm going to install a new 152 main and test...I also have a 155 main if the weather gets cooler...currently we are at about 15-20'c.
When I say wow,,I really mean WOW.. I near scared myself a few times last ride..
The bike pulls amazingly well right from the bottom, through the mid and the unfortunetly flattens out at the very top.
The sound this little monster makes is incredible. It really growls now.
I hope the main will clean the wot problem...if not I'll look at the timing.
thanks and I will keep all posted.
Bucket - in the dirt -:D


P.S. It'd be nice to have someone experienced riding the KDX200 take my bike for a spin...An experienced opion on how my unit is performing would be very advantagous.. I live in the Toronto Canada area and ride maining in the Ganaraski Forest.
Thanks again