KDX 200 1988 - smoking problem & Idle

[omrik88]

Hi everyone,
I'm new to this forum.
Got a 1988 kdx 200,white. It smokes alot - when engine is cold and hot. Can't get it to Idle - engine stops if i don't hold the throttle. The engine seems to work very well when riding !
It's an old bike and i'm working on it, so i'll try to give as much information as i can :
1)Fuel mixture - 2.5% oil to 1 liter of gas(95 unleaded).Should i use leaded fuel?
2)Airbox-new filter,boot is dry&cracked - coated it with RTV silicone outside.
3)Carb-Keihin 36mm,modified by DG performance, needle clip position-3(middle).Cleaned it. It's an old carb - there are signs of ware(scratches etc.).Idle tuning&Choke are on the same screw - Idle dont seem to respond.
The carb Boot is also cracked. coated it with RTV as well.
4)Cylinder- modified by Freddete. I have not opened the head,but i know for sure that it's dirty and the top of the piston as well. I can see some scratches on the piston,both rings are on.Reeds- checked them,cleaned,tightened - look ok but will need new ones.
5)Expansion chamber - by PSI, in good shape, i put RTV where it connects to the motor,so it seals better.
6)Silencer - by DG, cleaned and repacked it.

I thought of getting the stock Carb back on (keihin 35mm ?), but since it's a modified engine, isn't there a risk of the piston getting siezed due to a wrong mixture or something?
I hope that's enough info.

Thanks alot for taking the time to read this!
I would really appreciate your help.
Omri. :)

 

[mudpack]

It's a two-stroke.
It will smoke.
A lot.

You don't need leaded fuel.

The stock carb will work fine on a modified engine.

Turn in the idle screw(throttle stop screw) until the bike idles to your satisfaction.

 

[Joburble]

Re: Idle. Take mudpack's advice

1) I think the correct mix should be around 32:1 or 3.125%
2) Silicon is there to seal up the cracks. You could pull the old stuff off and re silicone it to your satisfaction until you have a new one (if available).

It may be worth getting someone who is good at jetting to take a look at it, because at 40:1 or 2.5% I would not think there should be a lot of smoke (depends on what you think a lot is). Don't try and cut the smoke down by putting less oil in.

What do you mean "Idle tuning&Choke are on the same screw"?

 

[omrik88]

some more questions...

Thank mudpack and Joburble,

I'm still learning stuff, so i don't know all the terms. got the Clymer manual,it's very usefull.
*I'll try that silicone spray, is there a certain type that i should look for?
* so i actually need to put 3% instead of 2.5% of oil? That's a bit confusing, cause it already smokes alot. I will not reduce the oil - i know that's not something to play with on a 2 stroke engine.I thought an old engine might need slightly less oil (?). I'm guessing the Carb is the bigger issue here.
*About "jetting", i'm not sure what that term means exactly,
my pilot jet is no. 48(pretty sure),the main fuel jet says 36.
The choke&idle adjust are using the same mechanism - up&down is Choke and turn right&left is Idle adjust (this knob/screw is on the left side,so it's hard to tune it).
Does jetting mean that i could change the pilot jet to different sizes/numbers to get the right mixture? (along with other variables like: oil/fuel ratio,air screw adjust etc.)
*About the air screw - i turned it according to the manual, but since this is not a stock carb and not a stock cylinder, maybe the tuning should be different. Could this also be part of the problem?
I'm just trying to think of all the options...
will post some pics later.

Thanks a lot guys!
Omri.

 

[sr5bidder]

I woud take care of that old rubber first, then freshen up the top end with new piston and rings

as far as the carb goes I have never seen one that adjusts like that....did someone tell you or is it in the manual

the main jet should be numbered in the 100's like 136 or 148 perhaps they have a mikuni main jet in there and it would be numbered like 250, 360 etc.

find that throttle stop screw mudpack mentioned

 

[omrik88]

Deleted Pic - Will Post New One

Hi,
I really just want to get it running good for a start. later on i will probably have to get some new parts... would rather not spend too much money if i can work with what i have.

I guess DG performance did change some things on the carb, but i'm not sure what. this exact carb is not on the manual cause its not a stock, but its close.
you can see on the pics that at the same place where the choke&idle screw is - there's a metal cap that shuts something just below, so maybe originally some other screw was there...i don't know.

*I DELETED THIS PIC WILL ADD NEW ONE ON ANOTHER POST*
Thanks.

 

[omrik88]

can't get more pics on...

there seems to be a limit on file sizes here...will try to get more pics later.

oh, on the left side of the carb there is this serial code:

PJ 36DGC GG

( "PJ" is in a bigger font if that matters...)

hope this helps.

 

[Joburble]

I'll try that silicone spray, is there a certain type that i should look for?
* so i actually need to put 3% instead of 2.5% of oil? That's a bit confusing, cause it already smokes alot.
Thanks a lot guys!
Omri.

I know the oil thing may be confusing but you need a place to start, and that place IS the correct oil ratio. After that you work on jetting, again make sure you get very good or expert advice from someone who knows what they are doing.
The silicone I was meaning was not the spray type but the RTV type (same as what is on there at the moment).
In regard to the choke, I have never heard of the choke being turned left or right as the idle screw, I just don't think that is correct. Post a pic of the other side of the carb if possible, that's where the idle screw should be.

 

[Joburble]

there seems to be a limit on file sizes here...will try to get more pics later.
.

Here is what I posted to someone else regarding pictures.

You will be safe uploading any .jpg's however this site appears to have a 100Kb limit in total for each person, so your pictures will have to total less than that. If you need a free program to reduce the size of the pictures you can use irfanview to do it. http://www.irfanview.com/

Or you can put your photos on photobucket.com and simply post the link into the message. On photobucket it even has a direct image link box for you to click in and copy the link ready for pasting into your message. If you want to know what I mean then right click on the pics of sr5bidder's mudfilled bike parts and select "copy image location" (if using firefox), then open notepad and click 'edit/paste' and you will see that the link is actually in photobucket.com, give it a try and see if you can figure it out from there, if not, ask again, Hope that helps. Oh, if you are using IE when you right click on the picture click 'properties' and it will show you where the address is too. Then to show the picture in the message you just have to write

at the end.

 

[julien_d]

FYI - My buddies 90KTM has a keihin carb that adjusts idle by turning the choke seat in/out. I am not sure how this works inside the carb, but that's how the idle adjustment works on it. Looks like the same carb as you have there. However, I would like to see that "spot below" that you have the green mark through. That would be the correct location for a separate idle adjustment if there is one.

All these guys are right on. First things first, check/service the top end. It's impossible to jet the bike correctly if compression is low. Leaking crank seals can cause excessive smoke as well. After these things are verfied, you'll need to work on the jetting. The correct oil ratio will NOT cause too much smoke, but an overly rich fuel/air mixture will. Yes, jetting is changed by installing new jets of a smaller or larger size.

 

[jth]

My buddies 1988 cr250 also has a keihin carb that adjusts idle by turning the choke seat in/out. A separate idle adjustment is on the left side of the carb.

 

[mudpack]

My buddies 1988 cr250 also has a keihin carb that adjusts idle by turning the choke seat in/out..

Yes, this is a fairly common thing on Keihin carbs.

 

[Joburble]

Yes, this is a fairly common thing on Keihin carbs.

I learn something every day :cool:

 

[julien_d]

I used to learn something every day. If I still do, then I forget 2 things every day!

 

[omrik88]

Some more new details and a pic

Hi again,
I checked Keihin website and DG performance. there is alot of info and charts about what i should do and what carb parts are available to buy. this is very complexed tech stuff so i'm not sure what to make of it.
Some new details that i have learned:
*This carb is of the "PJ" series by keihin, and from what i see,the idle adjust&choke are originally on the same screw.
*the "Main" jet(the one where the needle goes in)- is fixed in and cannot be removed - and from the keihin charts it seems that this jet is suppose to be removable! , maybe that's one part which was modified.
*the pilot jet/slow jet(if i got the terms right) shows the number 48 with a "star" mark next to it.
*the biggest jet where the fuel comes in, shows the number 3.6, with the same "star" mark. i'm not sure if it's removable or not.
*on the Left side (choke side)there's a code: PJ 36DGC GG
*inside the carb,above the float bowl,is marked the number 20 or 2.0 , if that means anything.
*I know for a fact that this carb was bought after the original(35mm) was finished, and worked great on the STOCK cylinder, even better than the original carb - more power,higher rpm,faster response. The problems started when the cylinder was changed to a MODIFIED Fredette Cylinder.
*I don't know if the current setup in my carb is the originall one ,or someone might have changed it. (i had this bike years ago,then sold it and bought it back...it has a long history).
*plus, there's also the PSI chamber and DG silencer, which probably affect tuning.

I hope this info&the pic will make things clearer.
Any ideas where i should start? any info that i should be asking from Keihin/DG?
Thanks for the help!!! :)

 

[jessica456]

Hi everyone,
I'm new to this forum.
Got a 1988 kdx 200,white. It smokes alot - when engine is cold and hot. Can't get it to Idle - engine stops if i don't hold the throttle. The engine seems to work very well when riding !
It's an old bike and i'm working on it, so i'll try to give as much information as i can :
1)Fuel mixture - 2.5% oil to 1 liter of gas(95 unleaded).Should i use leaded fuel?
2)Airbox-new filter,boot is dry&cracked - coated it with RTV silicone outside.
3)Carb-Keihin 36mm,modified by DG performance, needle clip position-3(middle).Cleaned it. It's an old carb - there are signs of ware(scratches etc.).Idle tuning&Choke are on the same screw - Idle dont seem to respond.
The carb Boot is also cracked. coated it with RTV as well.
4)Cylinder- modified by Freddete. I have not opened the head,but i know for sure that it's dirty and the top of the piston as well. I can see some scratches on the piston,both rings are on.Reeds- checked them,cleaned,tightened - look ok but will need new ones.
5)Expansion chamber - by PSI, in good shape, i put RTV where it connects to the motor,so it seals better.
6)Silencer - by DG, cleaned and repacked it.

I thought of getting the stock Carb back on (keihin 35mm ?), but since it's a modified engine, isn't there a risk of the piston getting siezed due to a wrong mixture or something?
I hope that's enough info.

Thanks alot for taking the time to read this!
I would really appreciate your help.
Omri. :)

its a 2 stroke so its gonna smoke alot and u should use klots mixing oil and probably whats wrong with your bike is u dont mix the oil right

 

[omrik88]

Thanks Jessica.


Any more ideas after i added pics and more info, anyone???

 

[julien_d]

Looks identical to the carb on my buddies 90 ktm. You are correct, no other idle adjustment.

Very first things I would look at, considering this:

4)Cylinder- modified by Freddete. I have not opened the head,but i know for sure that it's dirty and the top of the piston as well. I can see some scratches on the piston,both rings are on.

Compression is within service limit, and leak down test to be sure it's not sucking transmission oil.

 

[omrik88]

Thanks Julien,

Like i said before, i'm trying to solve this problem without having to spend too much money, if i can.

I don't have a compression gauge ,and a visit to a mechanic will not be cheap.
The overall feel that i get from the bike - when i kickstart it i feel a good resistance from the Kick, it's not soft. the engine feels strong. Could i "assume" from this that the compression is ok?
I don't know how to perform a Leak down test. Could a condition of "sucking transsmition oil" happen due to a worn out piston and/or piston rings?
Could i improve/fix this situation by changing the piston rings(only)?
or would i have to change the whole piston kit?
I would rather keep using this piston if i can get the engine to work right...
* About piston/rings, how do i know the correct measurments that i need? i know they are sold in different sizes(bores).
* If i clean the head and top of the piston,could this improve anything?

I hope i'm thinking in the right direction...

 

[julien_d]

The only way it's going to suck transmission oil into the crank case is through a leaking crank seal, not through worn rings. The rings could improve a situation of low compression. You can not assume the compression is in service limits by feel of the kickstart lever.

A good quick check for leaking crank seals is to pull the stator cover and see if any gas/oil residue is present around the flywheel. This is no guarantee, but a good place to start looking.

The top of the piston will be stamped with either STD or an oversize mark. You need a service manual to get correct measurements for ring end gap, and piston clearance.