81 kdx175 no powerband!

[kdxgeoff]

its an 81 kdx175. bought it used and ran into all kinds of problems. first the coil went. got a new one. then the stator and flywheel. replaced those too and the bike ran great. fouled a couple plugs trail riding(not riding it hard enough) and then all of a sudden one day i was riding, everything was great, i stopped for a smoke kicked it back over and it felt like i had a flat spot that i couldn't throttle through, hence the bike never hit its powerband. i'm not sure how i did it but i ended up running it hot enough or something and got over the flat spot. boom powerband again. everything ran great rode about an hour and put it away. came back to it the next day and same thing runs ok with a little throttle but try to gas it for powerband and you get brr brr brr. but no whine like powerband. it sputters. seems like its flooding. i got lucky again this time and slowly let it climb and it hit powerband, once it hits powerband once its like its hot enough and no problem. but when you stop and shut it off when you start back up its sputters. this last time i was not able to get it past the sputter. no powerband, no power, just sputter. any ideas. thanks

 

[reelrazor]

Try cleaning and repacking the silencer and/or spark arrestor, at least taking a look at it.

 

[kdxgeoff]

it has a silencer i think. i took it off and tried running it with no luck. it looks like its filled with tar. i figure its fuel but its jet black. if i am too look at it what am i looking for.this is my first dirt bike and first 2-stroke. i've done some work on old streetbikes and got 2 of em running good again but i'm kinda stupid when it comes to the dirtbike. seems like it should be easier, only one cylinder and all. i searched the forums and found a few topics on this. they talked about power jet and an accueator or something in the pipe that might be siezed. i dont know what these are or where to find them. i know a little something about carbs. it appears that my carb has a main jet and i assume a slow idle jet like on my streetbike. all i remember is two jets. but back to the silencer. what to look for? if i took it off should it hit powerband then or does that screw up the backpressure or something. any ideas about the black tar looking fuel dripping from it? thanks

 

[RoostRider]

The black tar looking stuff is most likely 'spooge' from running too rich of carburator jetting.(you are correct about the carb being similar to your street bike. It has a main and pilot jet) Which is going to directly contradict my next sentence. Sounds like the main is running WAY lean.

Have you checked to see that all the parts you worked on are still on and in place properly?... If the key that holds the flywheel or the stator on shears it can act really strange. Check the timing on the coil too (some are adjustable)

Another possibility is that it started running lean because something plugged your main jet (dive in and clean it up if you know streetbike carbs) OR that your needle is not pulling out of the main jet when your slide goes up. (a clip should hold it into the slide at a set position)

Check those easy things first. Let us know what you find

 

[reelrazor]

That black tar is fairly normal for a two stroke exhaust that has been neglected. It wouldn't hurt to remove the entire pipe and check to see if you have carbon built up thick in the exhaust port. You will also be able to get a look at the bore that way.

Check the basics, air getting in(clean air filter), air getting out, compression first. Pull the carb and get any gunk out of it. Especially check the air bleed passages in the throat of the carb and that the jets are clear. (or if you already did clean and oil the air filter, it may have been overoiled and is causing the airbleeds to be, well, oil bleeds).

 

[RoostRider]

oh, if it's the jet being plugged, you may already be in trouble on that one, it sounds like the top end is blown in that you say that it is worse now... IF that turns out to be it, it won't run well even once you clean it up. You'll need to hone and new piston and rings most likely... perhaps bore it over, if it's real bad

hopefully it's just the timing and no extra damage was done

 

[kdxgeoff]

i checked the flywheel. the key is still holding, timing looks right at least the same place i put it. i cleaned the carbs the first time it did this, lost the c-clip on the slide in a gravel driveway and had to drive an hour for a new one. i tried moving that clip from the middle where it was to down a notch(richer right) thats when i took it out and was able to slowly inch it into powerband. it ran great for 1/2 hour. shut it off to wait for a friend and about an hour later started it back and put, put, put. couldn't get it to rev high. really frustrating cause i knew all it had to do was get over the hurdle once so to speak and it would go. missed out on some real good riding at a place i had never been. you did lose me by saying the black crup is from it being too rich, but then sayin my main jet seems way lean. help me understand. i think you said spooge. thats kinda how its feels. like theres spooge clooging up something and if i get it revved enough or hot enough to burn that out then as long as i keep it running hard everthing is fine until i shut it off and let that spooge settle again, wherever it is and then i have to burn it out again. when i cleaned the carbs they were pretty damn cleane to begin with. i'm pretty sure they aren't gummed up. as far as being properly jetted i doubt it. the guy i got the bike from said it was overbored but the jet wasn't stepped up. that might definately be a performance issue down the road but it ran great for a week. thats all. i've had it for 2 months and have spent more time wrenching on it and pulling hair and spending money than i have riding it. i got all excited because after the new stator and flywheel it ran great for a week until this happened literally out of nowhere. like i said ran great for 4 days, stopped for a smoke during one of my rides, kicked it over and sputter sputter. thanks again for your help and patience

 

[kdxgeoff]

reelrazor could you explain the air bleed passages in the throat to me. are they the 2 hoses sticking out on either side of the carb. when i lay the bike on the side where gas flows out of?

 

[uts]

Does this bike have a power valve. I'm not familiar with this bike. But if it does it may be full of carbon goo. When its hot the goo is kinda melted and the power valve can slide. When it cools when you stop for a smoke it gets harder and stops the power valve from opening properly, hence no power band.
Just a thought

 

[kdxgeoff]

uts, thats exactly what it seems like. but i don't know if it has a power valve cause i dont know what it is or where to look.

 

[RoostRider]

it would be a valve system on the side of the cylinder.... a bulge in the cylinder perhaps... usualy has a cap on it that can be removed for inspection.

As for the lean/rich thing... I am just saying that this is the charachteristics of a bike running WAY lean on the main. It could be other things, but check to be safe. The spooge in the pipe and silencer can't be helping, clean them both (a search here will tell you how). clean out the powervalve (I think you have one), because that sticking could kill top end too. (and it makes more sense given the condition of your pipe).

 

[kdxgeoff]

i dont think it has a powervalve. i dont remember anything on the side of the cylinder to be opened or adjusted. i can't actually check the bike til tomorrow..its at a friends house, but i did look it up on a parts fiche and it doen't look like it. i will clean out the pipe and silencer and check compression. thanks and keep the ideas coming please.

 

[reelrazor]

No, it doesn't have a powervalve.

The air bleed(also called air jet) circuit supplies air to mix with the fuel to atomize it. When you adjust the pilot air screw you are changing the air to fuel ratio slightly by allowing more or less air to mix with the fuel that comes through the pilot jet. Some carbs have two airbleeds, one does the pilot circuit, one does the main/emulsion tube circuit. On carbs that have one air bleed, that one supplies both.

The air bleeds are the in the lower part of the upstream(airbox side) carb throat(at the leading edge of the venturi). Dis-assemble the carb, including taking the needle jet(emulsion tube-it is the 'nozzle' that the main jet screws into. It is brass and will come out the top of the carb when the throttle slide and main jet are removed) out of the body. Then shoot carb cleaner into that airbleed, it should froth in the space that the needle jet came out of, and into the pilot jet well-if it has one airbleed. If your carb has two airbleeds one will pass to the emulsion well(usually the one more 'centered' in the throat), the other to the pilot well. Whilst you have it out, make sure all the little holes in the side of the emusion tube are clear. This is a classic spot for aluminum corrosion to happen(and not get cleaned). If your emusion tube shows any sign of corrosion soak a pipe cleaner in carb clean and swab the well that the emulsion tube sits in also.

When you get no airbleed, the carb will dump large droplets of (poorly atomised) fuel that are hard to burn and will give a false rich condition. It produces symptoms like you are describing, including the intermittant nature and ability to run decently once up to speed.

It is very common for air filter over-oiling to 'clog' the airbleeds, especially after a decently hard run pulls oil out of the filter and then you take a short break. Overnight sitting will also do this as the oil will pool up and since the air bleeds are in the lowest point, they collect it.

 

[kdxgeoff]

thanks reelrazor i'll try to dig deeper into the carbs first. i must admit that you have me lost as to the terms like venturi and upstream air throat. i'm not sure what the troat is. there are only 2 adjustments that i know how to make. one being a screw on the outside of the carb, i assume is similiar to the carb on my street bike whereas i used that screw and turned it in and out in order to sync my carbs to the same vacuum. that screw i turned all the way in then backed it out 1 1/2 turns. the other adjustment i know is to move the circlip up or down onthe slide needle. down towards the p[ointed tip making it a richer mix and up to the flat end going lean right. i wont ask you to explain any further the terms or construction of the carbs. i will look more into that myself and see if i can aquaint myself better with what you mean. what i will do is pull it off and make sure that air blows freely through all holes that i can find. if anything seems wrong with what i have done so far let me know. ie. turning out that screw 1 1/2 or moving the circlip. what exactly does that screw on the side do? add air to the mix? turning out for more air?

 

[RoostRider]

there should be two screws. The idle adjust and the airscrew. You are correct on the needle clip (rich and lean). The air screw is typically 1 1/2 turns out.

Venturi is the narrow area in the carb throat. The hole he refers to is visible from the airbox end of the carb.

Since your willing and anxious to see how this all works, im going to go find you a great link that demonstrates it. be right back.

 

[uts]

I'd say it does have a power valve. If you look at the cylinder, at the front near the top where the exhaust comes out(looking from the side of the bike) there should be a little cover that is bolted on to the cylinder. It will be roundish or oblong in shape. Kinda hard to describe. Anyway this is where the power valve is. It restricts the height of the exhaust port at low revs and opens at higher revs to make the exhaust port bigger. It is meant to broaden the power range. If this gets stuck closed your higher rev power will die. You will need a manual to pull this apart and clean it. Keep in mind it could be any of the other problems that the guys have listed to but I suspect a gooed up power valve. The spoog is another indicator cause spooge is what chocks up power valves.

 

[uts]

Sorry, I was doing a slow post and you guys snuck in on me. No power valve......ingnore my last post.

 

[RoostRider]

http://justkdx.dirtrider.net/

go to- tech tips- carb tuning

there is a thorough explaination there, with an animated carb cut away in action and everything. Happy reading

 

[kdxgeoff]

yeah your right i forgot 3 adjusments. after thinking about it more i think i understand most of the places in the carb your taking about, ie. the throat, venturi. i got those. but the emulsion tube, or emulsion i'm not following you on.

 

[kdxgeoff]

hey thanks for the link!

 

[RoostRider]

You're welcome. I don't mind helping out someone who actually wants to learn.

Read it up and you'll know all the workings of your carb. :yeehaw:

 

[kdxgeoff]

alright thanks guys i have a place to start working and things to look into. its late and gotta go to work tomorrow so it might be a few days before i can get back to it. hopefully friday. it seems like i'm having a hard time at about 1/2 throttle so i'm gonna clean it all up, carb and pipe and make sure the airbleeds are clear and try moving the clip down. i'm thinking it might be running lean seeing that the guy i got it from said the main jet needed stepped up but it shows signs of running rich. i understand the post about the airbleeds might be clooged and not properly atomizing the gas letting in droplets, spooging up the exhaust fouling my plug and giving a false positive so to speak for it running rich when its actually lean. i just hope i didnt try to much to get it running and burnt the rings or piston. i guess i'll find out soon enough. didn't know anything about engines or motorcycles a year ago but after buying to streetbikes from the early 80's and now a dirtbike from the early 80's i'm quickly learning. i managed to get both the streetbikes running with the help of guys like you on the forums so i plan on getting this one going too. its just too much damn fun to ride to let it sit around. besides e-bay is a godsend for anyone with an old bike. if i need it, i'll find it and if its broke i'll fix it. so thanks for the help. i'll be back in a few days so don't forget about me cause i'm not giving up on this thing. thanks

 

[reelrazor]

yeah your right i forgot 3 adjusments. after thinking about it more i think i understand most of the places in the carb your taking about, ie. the throat, venturi. i got those. but the emulsion tube, or emulsion i'm not following you on.

Okay, on that link that Roost Rider gave ya? In purple it says 'air screw' and points to the air bleed passage. The little green bubbles flow into it at the right, past the air screw and the next stop is the needle jet(needle jet, emulsion tube, and nozzle are terms that get interchanged by different manufacturers/manual publishers).

The main jet actually screws INTO it. It is what the slide needle lifts out of. If you take the throttle slide and main jet out, it should FALL out of the carb body when you invert it. Most don't, especially if the carb has been 'cleaned' without removing or cleaning it.

The needle jet/emulsion tube/nozzle has a hole all the way through it. The main jet controls flow of fuel into that hole. It also has a series of holes drilled sideways into it. These holes allowo air to be mixed with the fuel. These holes get plugged easily. The carb hasn't been cleaned until it, and its' bore have been cleaned.

 

[magneto]

Are you getting a good supply of gas to the carburator? You may be running the bike out of gas at the higher throttle settings. Take the fuel line off of the carb and check for good gas flow. Take the gas cap off and then blow compressed air back up through the line into the gas tank and then check gas flow again. (Don't use too much air pressure, maybe 30 psi max.) (I fixed a poorly running Ford Workmaster tractor this way.)

 

[kdxgeoff]

ok. i understand what the emulsion tube is and i did not take it out and clean it when i cleaned the carbs. so i1ll start with that. thanks. just in case if i did blow the top end is compression test gonna tell me? low compression, add a tablespoon of oil and if it goes up its the rings? if it doesn't go up its the piston? is this right? thanks i hope i didn't blow it