Suspension modification problem

Howdy, as you can tell I’m brand hammer new to posting on this forum. I’ve been around for a bit and have been able to get a lot of great info/help from just searching. But I’m actually very stumped at the moment. I got back into the bike scene after about a 25 year hiatus about 5 years ago with an xs650 and then an XS1100. However I picked (what I think) is an XJ beauty queen this past spring that was nearly 100% original. The bike runs great and I love it so much I decided to dump some money into the suspension. Sooo, I ordered a race tech emulator, springs and progressive shocks for the rear. I upgraded the brake system with an EBC floating rotor, sintered pads and stainless line. I also put new shoes on the rear. So now that I have done that the bike seems to be a little out of wack. When cruising along at approx 40 mph it seems to act like the tire is a little out of round. I get an up and down motion in the front. It seems to fade away the faster I go. However, I’ve been Leary of going much above 55-60 out of concern for safety. I spoke with my buddy “Jetmechanicmarty” who gave me some advice on where to start looking but I’m still stumped. He had suggested it could be the damping but as far as I can tell they are working the way they should? The ride is pretty plush and the sag seems to be right.
So I’ll begin with what I have done so far:
1) checked the tires the best I can for balance, pressure and trueness. Tire is in good shape and on the newer side. Took front tire off and put on jack stands to balance.
2)unassembled everything including the forks, put a lighter weight oil in (10wt) and reassembled ever so carefully. To get both fork oil levels to the 150 mm height I did put a little more in one fork.
3)when up on the center stand rear wheel seems to spin pretty freely. When front tire is lifted it seems to spin nice and freely while seemingly true.
So unfortunately I did all of these mods at one time so it’s hard for me to isolate it. Could it be suspension? Brakes? Tires? Could the combo from the rear shocks to the front be incompatible?
Obviously I didn’t notice this problem before the mods but could it have been there and just not noticeable until the suspension upgrade?
Sooo, thanks in advance for any help/insight as well as taking the time to read this novel!

 

By the way she’s a 1982 xj650 j. Thanks again!

 

I agree with your approach but I'd take it to a shop with a dynamic balancing machine.
How old is the tire? I had a brand new front tire that behaved this way and it was replaced under warranty.

 

Going by the stamp on the tire (kenda challenger) they seem to have been manufactured in 2011. I can’t imagine they have more than 1-2000 miles on them but other than the cost, that would probably be one of the easiest remedies. I just didn’t want to throw any more money at it without really knowing. However it kinda looks like based on the age of the tires alone it wouldn’t be a bad idea anyways.

 

Unlikely to be brakes
Was it doing it before the suspension mods?
If so try putting the old rear shocks on and take it for a ride, try the old front springs with the emulators and then with emulators removed, try only one thing at a time though

 

I will try the old springs first and then the rear shocks. I will say that most information out there talks about setting up the emulators with straight rate springs. The company I got the “package” from sent me progressive. Any chance that would be an issue?

 

Agree with Simmy have balance checked .

 

Did you see and evidence of "cupping" on either of the tires . They can be very small and cause problems with vibration. Run your palm flat over the tire tread, you can feel them if they are there.

Are you going to the Iron Horse next weekend? I'll be there.

 

I am hoping it’s the tires because I went ahead and picked up a set of Dunlop 404s today. I’ll get them mounted and balanced professionally over the next couple of days and hopefully she’ll be smooth as silk. I’m crossing my fingers! I was kinda hesitant to do that because the Kendas on it were in nice shape but they were a little aged. And yes I’ll be at the rally! I’ve been working hard to get her ready. Worst case though, the ole xs650 is always good to go. I’ll bring both just in case but I really love riding the xj. Thanks for the advice though and I look forward to seeing you at the Iron Horse. I think it’s gonna be a great turnout.

 

I bet new tires will solve the problem. I'll have my XS11 and Venture there at the rally. See you next Friday. I'll be there Thursday and ride some. I like to start early.
Look me up when you get there I'll be in the white trailer right in front of the lodge.

 

Oh yeah, we rode together last year. We did the loop through the dragon then over through the Smoky Mtn National Park (after a little detour). Ha ha, that was a great ride. I wouldn’t mind doing that one again this year with maybe a little detour into gatlinburg for lunch? Hoping we have that same great weather!

 

Mlew, will you have the same crew with you?

 

Most of them plus my wife

 

Update: new tires mounted and balanced seemed to be the answer! Got them back on Monday and got a very short ride in before the rain came. Hopefully be able to get her out today for a longer ride before loading up for this weekends rally. Can’t wait! Thanks for everyone’s insight.

 

Be careful with new tires , they can be slick especially in the rain for first few miles or so.

 

Thanks jet fixer, good advice!