Author Topic: 86 kx500 resurrection (Read 3927 times)

harveson1 · « **on:** January 14, 2019, 09:27:14 AM »

Hi all. First time post. I am struggling with a 1986 kx500. I bought the bike non running. I had the cylinder re-plated. Put in a new wiseco piston and rings. My ring end gap is .45mm. After the new top end, my compression read 60PSI and would not start, so I had the head modified to reduce the squish to 1.5mm from 2.5mm. The compression jumped to 120PSI and I also learned to jump on it with all i got

. I got it to start once with starting fluid by some act of God. In general all I can get it to do is back fire out the exhaust. I did check the wood drift key. I rebuilt the stator. I did replace the reed petals with new boysen reeds. Now I am thinking about the crank seal because i have what seems to be an excessive amount of oil in the cylinder, on the underside of the piston, oil in the combustion chamber, and oil leaking out the exhaust. I did a leak down test with the piston at BDC and did not see any leakage from either seal on the crank. My biggest leak was from the intake carb boot cracking. My leak down test read 30% at 8-10PSI. Just seeking advice before split the cases to get to the seals. Thanks in advance

sandblaster · « **Reply #1 on:** January 14, 2019, 04:26:27 PM »

120lb is not good.. You need to be in the 180-190 for a new cylinder.
It's a common problem for those putting a newer 89-04 piston in a pre 89 cylinder.

harveson1 · « **Reply #2 on:** January 15, 2019, 08:50:58 AM »

thanks for the reply. I guess i will pray, hunt and kill for a stock 1986 piston, since that seems to be my only option.

sandblaster · « **Reply #3 on:** January 15, 2019, 04:17:29 PM »

Tom Morgan at TMR can cut your head for the correct cc.
Or, you can have your cylinder decked.
Either way will get you the right compression.

harveson1 · « **Reply #4 on:** January 16, 2019, 01:23:06 PM »

Sounds good. Thanks

harveson1 · « **Reply #5 on:** February 02, 2019, 06:21:40 AM »

Well she's a runner!!! Tom Morgan of Tom Morgan Racing cut the head actually increasing the volume. Tom knew exactly what to do and gave great service. I have posted pictures of the head before and after. I also had to change the timing rotating the stator plate clockwise. Thanks for your help sandblaster!

harveson1 · « **Reply #6 on:** May 18, 2019, 02:29:07 AM »

Just an update. I think the true problem with starting the kx500 was me. I was not getting a true reading on compression until I was able to kick it hard enough to get 2 pops of the reed valve. Thats the best way I can explain it. My compression was at best 120lbs until I heard two pops from the reed valve when I kicked it, and then the compression jumped to 150lbs. I am 190lbs with skinny but nice legs.
Other things. I had the oil drain plug welded to repair. I replaced the triple tree fork clamps due to the stop being sheared off. Seems like this is a common problem. I also had to build a bracket for the exhaust due to the mount being destroyed. I cut the exhaust down to fit the bike since it has an fmf gnarley exhaust that was not made for the 1986 kx500. The 1986 kx500 is a challenge to rebuild just from a lack of parts stand point. She is getting close. Just thought since this site has helped me I would do my part to contribute.