old KDX, suddenly starts missing at midrange

podfish · 05-24-2008, 06:09 PM

It's an air-cooled '87. I rode it a few miles in the morning, and all was good. It sat all day, while I worked a checkpoint at an enduro. Then it started up just fine, but as I rode down the fire-road it hesitated and missed and stumbled - so I wound it out. It ran fine as long as the throttle was wide open; it starts easy and idles too. What might make the middle crap out? I've just pulled the carb, and it looks fine and clean. I'm a bit worried that it's old electrics, and either the magneto or coil isn't firing well at middle revs - but that sounds a bit far fetched. I stuck in a new plug, which made no difference at all.
Anyone seen this kind of behavior before?

tjscott2012 · 05-24-2008, 07:35 PM

yes, i had the same problem, you might want to make sure your fuel and air source are both clean, which means changing your fuel filter, cleaning your tank, and oiling and cleaning your air filter. oh, and clean your carb out anyways after you do that.

wr250f95 · 05-24-2008, 08:06 PM

perhaps the top end, but its doubtful.
I would check the electrical.

Hardtail · 05-25-2008, 05:07 AM

Leaky jet, cracked intake manifold, bad float, old fuel line, or advanvce maybe?

podfish · 06-11-2008, 09:05 AM

those all are things I'll check out (I've been stuck at work and have yet to get back to the bike!) but there's a couple of things: as far as fuel issues go, wouldn't fuel restriction show up worse at higher rpm, rather than getting better? and the biggy: I don't have any idea how to check electricals on an '87. The CDI testers aren't available any more - shops don't have them, either - so other than using an ohmmeter to check the stator windings I don't know what to check! Any ideas??

turtlemann14 · 06-11-2008, 12:58 PM

electrics are a pain

does the bike being that old not have points?

atvrider93 · 06-11-2008, 10:59 PM

my 4wheeler is doing the same
i think there might be a bit of water in the gas
or i need to rejet it

turtlemann14 · 06-11-2008, 11:37 PM

what color is the exhaust? black maybe?

tigmaster · 06-12-2008, 10:58 AM

You can still check the coil. Just get yourself a multimeter/ohmmeter (same thing) and measure between the plug wire and the frame resistance should be somewhere in the kilo ohm range. Then you can measure the low side of the coil and the frame and it should be somewhere between 20-50 ohms. I would not believe this is the problem but electronics can do some weird things. If you have any electronics questions, let me know.

jc--07 · 08-26-2008, 06:28 PM

i ahate fixin electrics on bikes

i have to do it allday at work aswell then go to my m8s and do his bike :@

05-24-2008, 06:09 PM	#1 (permalink)
podfish C Rider Join Date: May 2008 Posts: 7 iTrader: (0)	old KDX, suddenly starts missing at midrange It's an air-cooled '87. I rode it a few miles in the morning, and all was good. It sat all day, while I worked a checkpoint at an enduro. Then it started up just fine, but as I rode down the fire-road it hesitated and missed and stumbled - so I wound it out. It ran fine as long as the throttle was wide open; it starts easy and idles too. What might make the middle crap out? I've just pulled the carb, and it looks fine and clean. I'm a bit worried that it's old electrics, and either the magneto or coil isn't firing well at middle revs - but that sounds a bit far fetched. I stuck in a new plug, which made no difference at all. Anyone seen this kind of behavior before?

05-24-2008, 08:06 PM	#3 (permalink)
wr250f95 Super Moderator Join Date: May 2007 Location: New Mexico Posts: 7,831 iTrader: (1)	perhaps the top end, but its doubtful. I would check the electrical. __________________ "LIFE'S JOURNEY IS NOT TO ARRIVE AT THE GRAVE SAFELY IN A WELL PRESERVED BODY, BUT RATHER TO SKID IN SIDEWAYS, TOTALLY WORN OUT, SHOUTING'...HOLY S**T... WHAT A RIDE!" this, is why I ride.

05-25-2008, 05:07 AM	#4 (permalink)
Hardtail Pro Mototips Member Join Date: Dec 2006 Location: Southeast V.A. Posts: 1,865 iTrader: (1)	Leaky jet, cracked intake manifold, bad float, old fuel line, or advanvce maybe? __________________ I love the smell of gasoline!! I woke up.....life is good!!!

06-11-2008, 12:58 PM	#6 (permalink)
turtlemann14 Pro Mototips Member Join Date: Oct 2007 Location: West Virginia Posts: 3,986 iTrader: (0)	electrics are a pain does the bike being that old not have points? __________________ If it does not move at first, get a bigger hammer!! Myspace Yamaha 1982 xj550 under bike lights Kawasaki klx250 1979 custom fiberglass silencer+ Honda xr200r 1985 stock = Dakaw Dk250 2008

06-11-2008, 11:37 PM	#8 (permalink)
turtlemann14 Pro Mototips Member Join Date: Oct 2007 Location: West Virginia Posts: 3,986 iTrader: (0)	what color is the exhaust? black maybe? __________________ If it does not move at first, get a bigger hammer!! Myspace Yamaha 1982 xj550 under bike lights Kawasaki klx250 1979 custom fiberglass silencer+ Honda xr200r 1985 stock = Dakaw Dk250 2008

05-24-2008, 07:35 PM	#2 (permalink)
tjscott2012 The Spam man Join Date: Nov 2007 Location: Hurley, Mo Posts: 4,867 iTrader: (0)	yes, i had the same problem, you might want to make sure your fuel and air source are both clean, which means changing your fuel filter, cleaning your tank, and oiling and cleaning your air filter. oh, and clean your carb out anyways after you do that.

06-11-2008, 09:05 AM	#5 (permalink)
podfish C Rider Join Date: May 2008 Posts: 7 iTrader: (0)	those all are things I'll check out (I've been stuck at work and have yet to get back to the bike!) but there's a couple of things: as far as fuel issues go, wouldn't fuel restriction show up worse at higher rpm, rather than getting better? and the biggy: I don't have any idea how to check electricals on an '87. The CDI testers aren't available any more - shops don't have them, either - so other than using an ohmmeter to check the stator windings I don't know what to check! Any ideas??

06-11-2008, 10:59 PM	#7 (permalink)
atvrider93 A Rider Join Date: May 2008 Location: Apex, NC Posts: 869 iTrader: (0)	my 4wheeler is doing the same i think there might be a bit of water in the gas or i need to rejet it

06-12-2008, 10:58 AM	#9 (permalink)
tigmaster Administrator Join Date: Aug 2006 Location: Missouri, United States Posts: 7,526 iTrader: (1)	You can still check the coil. Just get yourself a multimeter/ohmmeter (same thing) and measure between the plug wire and the frame resistance should be somewhere in the kilo ohm range. Then you can measure the low side of the coil and the frame and it should be somewhere between 20-50 ohms. I would not believe this is the problem but electronics can do some weird things. If you have any electronics questions, let me know. __________________ --'03 CRF450r-- --FMF Pipe, Pro-Tech Revavle-- --Vortex Basket With Tusk Disks And HP Springs-- --Pro Taper Triple Clamps And Bars-- --Devol Full SP With Moose HGs-- --Renthal 13/50 With R1 Chain-- --Now It Is The End, Probably...-- --'06 CBR F4I-- --Bone Stock-- Check Our Latest How-To Here

08-26-2008, 06:28 PM	#10 (permalink)
jc--07 A Rider Join Date: Aug 2008 Location: Edinburgh Posts: 602 iTrader: (0)	i ahate fixin electrics on bikes i have to do it allday at work aswell then go to my m8s and do his bike :@ __________________ bikes are gooooooood

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