KDX80 won't idle low

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SQLGuy
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KDX80 won't idle low

Post by SQLGuy »

Hi folks,

I recently picked up a 1980 KDX80 for my kids. At the seller's, it was hard to start and idled, but at a high idle. I didn't think too much of that at the time, but when I got it home I found that it was near-impossible to start with the kickstart, and that, once push started, it would run for about a minute before dying.

I checked the carburetor, and it was actually very clean (not even any sediment in the bottom of the bowl). I cleaned it again, anyway, though, and checked the reeds, which were fine. After playing around with it a bit more, I found that, if it were warm, and I set the pilot pretty rich, and the idle kind of high, I could keep it running at the high idle. But cold it still has to be push started. If you reduce the idle to a normal speed, it will idle OK for 20 or 30 seconds, then start missing a little bit, then just die. Once it's running, it has decent power and throttle response, but has the issue idling, and blows some smoke when the throttle is pulled.

I did a compression test, cold (and, BTW, I'm at 6500 feet elevation), and came up with 91 PSI. The service manager at the local dealership told me, under 90, at this elevation, would be a problem. Well, it's not under 90, but it's close.

The question is, do you think this slightly low compression would account for the symptoms I'm seeing, or might there be an issue with ignition as well?

I have a service manual on order, hopefully should get it in a day or two. Also, I do plan to rebuild the top end anyway, since the compression is marginal. I pulled the cylinder yesterday and it looks pretty good... no scratches that I could feel, just the ring of carbon at the top; so I'm hoping (since it has that electrofusion bore) that all it'll take is a new piston and rings.

Thanks,
Paul
KarlP
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Post by KarlP »

It just sounds to me like a tired engine. I bet a piston and rings would breathe new life into it.
'08 KTM200xc
'99 CR/KDX Hybrid with that RB stuff done to it
KX100 for the boy
SQLGuy
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Post by SQLGuy »

Hope so... parts are on order.

However, did I mention that this thing runs points ignition? I had seen some posts on other fora where people had similar problems with worn out points.
KarlP
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Post by KarlP »

Ahhh....Points.
I haven't had to mess with them since I used to ride a Honda CB175 off road.

Still, the fact that it runs better warmed up does point to compression issues. Are new points and condensor available?

I wonder if an aftermarket solid state ignition is available. My brother put such a setup on his '76 yellow yamaha 460(?). It works really well for him.
'08 KTM200xc
'99 CR/KDX Hybrid with that RB stuff done to it
KX100 for the boy
SQLGuy
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Posts: 10
Joined: 06:46 pm Jul 31 2011
Country:
Location: Colorado Springs, CO USA

Post by SQLGuy »

The condenser measured OK, but has a riveted terminal on it that seems loose... I'll probably try cleaning that up and soldering across the rivet.

A new set of the contacts is on order.

Should have checked for a solid state system, though... the points and condensor are actually kind of pricey.
SQLGuy
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Location: Colorado Springs, CO USA

Post by SQLGuy »

I cleaned up the head and cylinder today (soda blasting, mostly). Once it was clean I saw that, at some point, some extra bits were floating around inside the combustion chamber... probably chewed up a ring. (The current rings look OK.) There were a bunch of dings in the dome, one 1/8" or so notch in the bottom edge of a port, and one ding near the edge of the combustion chamber that actually resulted in a raised area where the head gasket should be sealing things together.

Not really wanting to get everything milled, I improvised... we'll see how it goes. I took the dimple down with a couple of strokes of a very fine diamond stone, then put a new sheet of 1000 grit sandpaper on my Silestone countertop and used that as a reference flat surface to sand/polish the head and cylinder mating surfaces. They came out very flat and smooth. I am hoping the result, with a new head gasket, will be good. Any thoughts on this?

Thanks,
Paul
SQLGuy
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Post by SQLGuy »

Well, I installed the new piston, rings, wristpin, top end bearing, head gasket, and base gasket. Had to make the base gasket, because the eBay one didn't arrive in time, and I couldn't wait any longer if we were to take the bike with us camping. I also installed a new set of points, and made really sure that the condenser was well soldered into the ignition circuit. I also cleaned up the old piston, and saw that it had matching nicks and dings to those in the combustion chamber... so, if it was a ring that got chewed up, the piston was reused rather than being replaced - as it should have been. The old piston also has a good gouge on the side of it that looks to have been caused by the previous incident.

The project took considerably longer than it should have because I didn't heed the warnings, and didn't stuff a rag into the connecting rod opening... sure enough, I dropped the wristpin clip in there. Ended up pulling the engine so I could turn it upside-down and drop out the pin. I also learned (the hard way) that you don't get access to the bottom end on these engines by removing the side (clutch) cover.

Anyway, long project short, the first thing I noticed was that the spark appeared stronger while just hand cranking the kick starter. I had to push start it the first time, to get fuel flowing through again, but, since then, it's been starting on the first kick - even cold, even when that kick is from my 10 year old daughter. And it idles perfectly and runs just as long as you want it to.

Not sure whether the ignition work, or the piston work, or both get the credit, but I'm very happy with the results.
KarlP
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Post by KarlP »

Not sure whether the ignition work, or the piston work, or both get the credit, but I'm very happy with the results.
Glad to hear it came together.

One thing I discovered when dealing with points ignition is that after installing a new set and setting the gap (or dwell) I'd never try to clean or file them. I might carefully adjust the gap later on, but the practice of using emory cloth or a points file just shortened the life considerably.

You might like to keep a new set on hand.
'08 KTM200xc
'99 CR/KDX Hybrid with that RB stuff done to it
KX100 for the boy
SQLGuy
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Posts: 10
Joined: 06:46 pm Jul 31 2011
Country:
Location: Colorado Springs, CO USA

Post by SQLGuy »

There is (at least according to the manual) no dwell adjustment for this setup. Only timing is adjustable. (And it's a couple of degrees retarded at this point, so I'll probably still tweak that... need all the advance we can get at 6500 feet.) The way they have you adjust timing effectively alters the dwell. The (IMO) correct way to adjust timing - by moving the trigger plate - requires pulling the magneto, so is not what they tell you to do in the manual.

The points appear plated, which is similar to a lot of more modern pinball machine contacts. For the pinball machine ones, you are not supposed to ever file or sand them. I would expect these are similar. At $35 a set, though, I don't think I'll keep a spare on hand, other than the old set I took out... just in case something catastrophic happens to the new ones.
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