KX 420 lacks power

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cjreeder

Member
Dec 23, 2002
37
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After cleaning out the carb on my '81 KX 420, it runs really rich and has even less power than it had before. When I bought the bike last month it would at least wheelie in first gear when it hit the power band. Now it sputters a lot and no power band wheelies at all.

Air filter has 90W gear oil on it, is this restricting the air enough to cause such problems?

Gearing is 14/48.

Pilot jet is 35
Main jet is 165
Needle jet is 371 / R-8

All in all she burns a lot of gas, puffs a lot of smoke most of the time, only idles when first started, and fouls a lot of plugs.

What should the fuel height be set to?

Anybody have a picture of the flywheel timing marks, and how far can you generally stray from stock timing? I unbolted this once, but then couldn't decide for sure what was supposed to line up with what.

Do I need to get real filter oil before going for smaller jets?

How many gears should this thing wheelie in just from the powerband when I get it running right?

Thanks,

Chris
 
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Studboy

Thinks he can ride
Dec 2, 2001
1,818
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As bscottr said, get a clean airfilter properly oiled, repack your silencer and clean all of the core holes. If this does not fix the problem, I would take apart the carb again and inspect, Maybe you accidentally bent your float so the level is too high? I would leave it at stock timing until you get other things sorted out. Good Luck.
 
C

cjreeder

Member
Dec 23, 2002
37
0
Originally posted by Studboy
As bscottr said, get a clean airfilter properly oiled,
Click to expand...

Okay, I finally got the factory service manual for the bike. Timing is fine, but the fuel bowl level was actually too low. According to the manual, I cleaned the airfilter and oiled it with 30 wt (ok, I used 10W-40) oil and squeezed all I could out onto rags. Tried that, and bike was still really sputtery through the whole range. So I turned the gas off to let it run leaner. When it fot lean enough that it finally had great throttle response, I shut it off and opened the carb. Barely a tablespoon left in the bowl at that point. So as a temporary fix I leaned the float bowl as much as possible with the float valve still being functional, and went riding. It's the best it's ever run for me, but it's still sputtering. So when I get the money I'll get an aftermarket airfilter with some sticky oil, put the float level back up where it belongs, and start dropping the jet sizes.

It's rich everywhere, so I want to change the pilot and main jets. But dow far do I drop the jet sizes as a starting point? Pilot is 35, main is 165. Since it's rich now even with a way-under-filled float bowl, and I'll be fixing the float bowl level when I get new jets for it, I figured it needs to drop size quite a ways. But how far is quite a ways? I was thinking to go 30 and 155.

Originally posted by Studboy
... repack your silencer and clean all of the core holes.
Click to expand...

Pardon my ignorance, but what does a repacked silencer have to do with the carburetor? And what are core holes? My silencer is welded shut, and the exhaust O-ring is gone.

Thanks,

Chris
 
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Studboy

Thinks he can ride
Dec 2, 2001
1,818
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I would get some real air-filter oil. Using 30wt (or any other regular motor oil) will not stop the dirt that well and will always suck some into your engine (the oil) resulting in poor performance. It also drips off the filter after a while. I would try and go down 1 on the pilot and 2 or 3 sizes like you said on the main.
A plugged up or unpacked silencer affects your jetting (due to backpressure and flow) and also kills your throttle response! Not to mention adding excess noise. Any reputable person will tell you to get a clean airfilter AND repack and clean your silencer before jetting.
 

gwcrim

~SPONSOR~
Oct 3, 2002
1,881
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For a quick test, just remove the air filter and take it for a very brief test ride. That will certainly help eliminate or confirm that variable.
 
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Jonny426

~SPONSOR~
Apr 30, 2001
84
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Check to see if your float has fuel in it. Remove it from the carb, and give it a shake(brass float),or just replace it if it's plastic, as it can abosorb fuel over time. Does the needle seal completely? leave the petcock open for a few mins. and be sure no fuel drips from the overflow tubes. Everybody is right as far as real filter oil and silencer condition.
 
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kdx42081

Member
Oct 27, 2002
3
0
i have a 81 kdx 420 hence the name mine kinda done the same thing found the silencer (muffler) plugged with carbon gunk cleaned it out now screams like brand new make sure u check it quick way to check is to remove it and run a SHORT test run an see if power comes back
 
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Luft

Member
Dec 24, 2002
105
0
Try Maxima FFT or Twin Air foam filter oil. The 90 wt can be used for chain lube but not the air filter becuase the air will not flow through it properly. 10W40 is not a good idea because it does not have tackiness modifier and will be sucked right through the filter into the engine and oil foul the plug. Oil the air filter properly, restore the correct float height, install a new and properly gapped plug, and repack the silencer. Start with the main jet and make one adjustment at a time going smaller on the main and test for plug color and engine pull at full throttle in top gear by holding throttle all the way open and holding the kill switch and coasting to a stop. Weigh how the plug looks versus how the bike pulls. If it blubbers it's rich, if it bogs it's too lean. Have the needle in the middle or stock position and and the air screw at the stock setting.
 
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Studboy

Thinks he can ride
Dec 2, 2001
1,818
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"...test for plug color and engine pull at full throttle in top gear by holding throttle all the way open..."
He he he...what fun on a 420. You'd better get ready because you are going into hyperspace!!!
 
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luvtolean

Member
Oct 3, 2002
172
1
Originally posted by Studboy
"...test for plug color and engine pull at full throttle in top gear by holding throttle all the way open..."
He he he...what fun on a 420. You'd better get ready because you are going into hyperspace!!!
Click to expand...

Once he gets the muffler and filter sorted. ;)
 

David Trustrum

~SPONSOR~
Jan 25, 2001
1,396
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Hows the top end look?
Click to expand...

Big, black, with lots of fins. :moon:


Lots of good ideas above.

To add; Have a real good look at the needle & the needle jet. On older bikes, especially if they vibrate (!) the needle jet will get worn, possibly oval & replacing it may be the answer to your problems.
 
C

cjreeder

Member
Dec 23, 2002
37
0
Funny you should mention that. I remember now that one little piece of the needle jet shroud that sticks up into the airstream has been tweaked and slightly broken. I don't know what else has been done to it or even why. Do the jet needle and needle jet come as a set or individually? The bike can take either an R-8 or an R-6 needle jet, and either a 6F31 or 6FL28 jet needle. What's the difference between these?

The muffler is glass-pack style, no restriction.
 
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cjreeder

Member
Dec 23, 2002
37
0

The floats themselves seem fine, but the float valve does not seal completely. I used to think it was normal for them to not seal completely, why else would we have to shut off the tank valve when we shut off the engine? But I've noticed recently that when the tank gets down to only a half gallon or so in it, the bike starts running LEAN on top end instead of rich. The float valve seems like the most likely culprit, unless you guys think it might be the petcock. Is there any way to repair a float valve, as opposed to just replacing it?
 

David Trustrum

~SPONSOR~
Jan 25, 2001
1,396
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OK well this sounds like it needs to be fixed first off.

To repair a fuel float bowl jet is not easy.

Best option is to buy a new pair of jets. Ok now for what I would do being impatient & totally too cheap to buy new bits where might be avoided.

Remove the jet. Check for gunk behind it. Drop a small ball bearing down the jet & give it a tap with a punch to make a flat seat. Have a good look at the float needle. It will have a bit of a ring but we will try to reuse it. Test refit. Turn gas on & wait. Shake bike a bit.

OK didn’t work? Using a drill press drill the jet the next size up from inside to outside. Ball bearing tap again. The float needle will sit further down in the bigger hole sealing on a new area of the needle & necessitate the float be adjusted to compensate.

As far as float height the general rule is; adjust it down till it starts leaking, then back up again till it doesn’t (this of course assumes the jet seals which you are hoping to fix).
 
C

cjreeder

Member
Dec 23, 2002
37
0
Originally posted by David Trustrum
Best option is to buy a new pair of jets. Ok now for what I would do being impatient & totally too cheap to buy new bits where might be avoided.
[/B]
Click to expand...

Cheap and impatient-- glad I'm not the only one. I noticed that the float tang doesn't meet squarely with the float valve, so maybe the tang pushing sideways on it is messing things up, too. I'll check it out tomorrow and try the ball bearing thing if this doesn't work.
 
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cjreeder

Member
Dec 23, 2002
37
0
Some success at last!

I took the carb and the tank apart. The float valve is fine, but the petcock was partially clogged, probably the reason for lean runs on a low tank of gas. Also, the starter circuit had been stuck partially open, probably making the mixture overly rich. With those problems fixed, I set the fuel level back up where it's supposed to be, cleaned the gunk out of the 165 main jet, and started it up. It ran rich as ever, bottom end and top, with lots of sputtering. Ran no different with or without the (now fixed) starter circuit engaged. Tried it without the air cleaner, which made some change but was still really rich. Air filter back on, dropped main jet from 165 to 145. Throttle response is crisper. Still sounds and smokes like it's rich under 1/4 throttle, and it won't idle until you run it out of gas, but from half-throttle on up sounds and acts slightly lean. 15 second plug chop WOT on a used plug (can you do that?) gave a chocolatey ceramic tip (better than black like I had before), but I couldn't see deep into the plug to get any real main-jet info. Changed oil mix from 50:1 to 30:1 or so to help lubrication, and went riding, staying off WOT when possible. Next trip to the LBS will try 155 main and change pilot from 35 to hmm, maybe 30?

So, about pilot jets, how do you choose those? The Kawi service manual doesn't say anything about how to select the right size. At this point it never idles except right before you run it out of gas.

As for needle jets, how would I tell if it were worn out? It still looks round to me, is there some tolerance to look for, like should the jet needle totally fill the needle jet when it's down or what? Or is the jet needle supposed to shut off fuel flow at low throttle positions by plugging the top of the main jet itself?

What is the throttle slide supposed to bottom out onto? Does the throttle cable stop it, or does it bottom out onto something else? When mine is down, there's still 5/16" or so gap, and there's no free play in the cable. Wondered if their should be. I have to give the bike about 1/8 throttle to get it to idle.

Thanks,
 

David Trustrum

~SPONSOR~
Jan 25, 2001
1,396
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Good sounds like some progress then.

Sorry can’t be bothered reading back all the post, but try drop the needle to lean mid throttle position out a smidge. Was a bit scared by large change of the main. Little steps.

Putting more oil in the mixture leaned it out a little of course. If the needle itself looks ok with no damage, well maybe the jet is ok, maybe not, might be worth trying a new std, possibly smaller jet or thicker needle. No they don’t block off fully when closed.

Just try smaller pilot jets. When it gets to the stage in neutral (properly warmed up of course) where you can blip the throttle & it hesitates then start winding the airscrew in to improve it & possibly go up to the next size. Then try it riding it, it will most likely need richer still under load. Up a slight hill crack it open from closed should net hesitate.

If there is supposed to be an idle adjuster on the side of the carb (probably RHS) then it would bear on the underside of the slide & there will be a wear mark.

Check the fuel line is not kinked, old line does making it hard to detect & fuel filters are straight when the really need to be curved IS SOMEONE IN THE AFTERMARKET BUSINESS LISTENING? Try opening the drain screw & see that it trickles for a reasonable time, you might waste a $1 of gas but you will be sure that the flow is ok.
 

David Trustrum

~SPONSOR~
Jan 25, 2001
1,396
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Yes they can, just don’t expect to predict what their value is afterwards.

Some are based on flow & some on bore but if you must whatever you do file the old value off so the next time you pick the jet up you don’t go ‘oh great this must be a 230 I’ll just try one size smaller’

Oh & 'cheap' has an ‘a’ in it. ‘e’s are more common but [insert some clever analogy about scrabble scoring which I won’t think of until tonight]
 
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Studboy

Thinks he can ride
Dec 2, 2001
1,818
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Originally posted by cjreeder
Okay, we're getting really cheep here, but can main jets be drilled?
Click to expand...

Not expecting any sort of good result,  that is unless you have some really precision equipment and know what you are doing! :think:  
 

David Trustrum

~SPONSOR~
Jan 25, 2001
1,396
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that is unless you have some really precision equipment and know what you are doing!
Click to expand...

Hey I once tuned a roadrace bike with a broken 1.5 mm drillbit , battery drill & handheld jet.

Someone had turned up to a far away meeting, the night before decided that ripping the airbox out has to be a good idea & didn’t take it with him. Fearing it was too lean & would lock up I was forced into helping, none of my jets were in the right range & we were desperate.

Defying odds -it worked great, the bike had never run better & stayed like that all meetings thereafter. He thought I was some sort of magician & I couldn’t believe how ‘bush’ I had been.

. . . Until another friend said the hot setup tip for going down a jet size when you don’t have it is to jam a bit of fuse-wire in the hole. 2 bits for a larger change.

Oh good grief!! :ugg:
 
C

cjreeder

Member
Dec 23, 2002
37
0
Originally posted by David Trustrum
Oh & 'cheap' has an �a� in it. �e�s are more common but [insert some clever analogy about scrabble scoring which I won�t think of until tonight] [/B]
Click to expand...

Ah, but it sounds so much cheaper when it's spelt like a cheeping bird:)

So is a 145 main jet 0.145mm in diamater?

Good idea, filing the number off.
 
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smarttoys

Sponsoring Member
Apr 29, 2001
199
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Jets can be drilled out with fairly good results. You should used a good number drill set. The kind that furnace guys use work well, the drills have brass handles on them. When I have been in a real pinch, I have used acid core solder to fill in a jet and then drill it out to a smaller size. I don't recommend this but it works in an emergency. :thumb:
 
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