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1985 xr80r mostly new, carbon fouling

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I have done everything I can imagine on this little bike:

New Head & valves

New cam and rockers

new oil pump (not that it matters, but to be thorough)

New coil & condenser

Fresh Bore & hone + new piston & rings properly indexed

Valves adjusted to spec

New points gapped to spec

New spark plug (standard) gapped to spec

Air filter is new

Carb has been disassembled and thoroughly cleaned & dried with compressed air multiple times.

All of this and I am still fouling plugs. (dry black carbon)

Air screw adjusted per the clymer manual at 1.5 turns out

clip is 4th groove from the top as called for in the manual.

I tried moving it to the 3rd groove from top lowering the needle a bit to see if it would help... it didn't.

I have mentioned all of the above to my local tech which left him scratching his head.

I don't want to band-aid the problem with a hotter plug until I determine & fix the actual cause.

I have already checked float level and plan to recheck this afternoon

float valve is new (it came with the rebuild kit)

I'm really open to suggestions at this point because I feel like I have covered anything it could be...

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'85 eh? post a pick in the OLDER XR thread! sounds like its had lots of TLC

you may want to pull the flywheel and make sure the advance mechinism has not worn grooves causing it to hang.

if its not getting full advance I think that might cause extra carbon from not burning the fuel mixture as completely as possible.

I mix grease and anti sieze together and then dab it on the areas where the advance mechanism moves as to try an limit the wear in that area.

I dont know if thats your problem but it seems you have covered everything else and thats the only other thing I could think of. it is an old bike and probably has worn groves there but even if it has and you dont have a better replacement the grease will work for quite a while.

the plus side to having points ignition on these is that when they are right they advance more and quicker than CDI

also consider an irridium spark plug, they are hard to foul even with oil burners.

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Thanks for the reply...

The advance mechanism is free moving with no noticeable wear.

I was using the iridium spark plug in my latest testing, though I carry 2 spare standard plugs on our trips.

Where is this "OLDER XR thread" I was not able to locate it with search....

I assumed it was in the photo section.

Is it titled "OLDER XR"?

Since I bought it, I have basically rebuilt & painted the entire bike. Still has old tires with some life left in them. And the fuel tank has that aged & faded look as well. I learned the hard way that you cannot paint a plastic tank but luckily I was able to just scrape all the paint off with my fingernails 👍 but not before I got some nice pictures (pre-blistering)

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you have to disassemble it to notice the wear incase you didnt know.

the photo thread is here in the 80-200 section:


you'll see an '85 I had that was near mint condition. I still have three other '85s (my favorite year)

you can sand the tanks with an orbital sander to get the color back starting with 220g if its really bad progress in stages to 600g

or if youre handy with a straight edge razor blade you can scrape off the faded layer of plastic by holding the blade perpendicular to the surface and pulling it accross, move in fast motion so it dont jitter or skip and it will stay smooth then polish out when done

a heat gun will also restore faded plastics but better practice on a junk peice first because you have to move the heat just right so it dont melt. make sure the tank is empty and dry with no lid on it.

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  • 2 weeks later...

It turns out a huge part of my issue was due to a worn needle jet, but I'm still having slight issues I want to resolve...

I disassembled the spark advance mechanism and all appears to be smooth, but I did notice it seems to be barely under the tension of the springs at the farthest point of it's travel...

I can't find any inspection guidelines for this part in my manual or online...

Does anyone know how to inspect the function of the spark advance to ensure it is operating within spec?

I am assuming that when I twist it, it should snap back to one single starting position under the tension of the springs...

but on mine the tension on the springs is loose allowing me to twist it almost 2/3 of its travel before any tension is put on the springs

(it will stay put if I let go of it at this 2/3 point without snapping back),

then for about the final 1/3 of the travel... it is under the tension of the springs.

I do not know if this is the way it is supposed to be or if it needs adjusting or new springs....

If I were to bend the tangs to which the springs are attached to remove the slop, would this screw something up?

Any ideas on this would be awesome...


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