1990 K75RT Gas Tank Weirdness

[Al]

So I took the fuel tank off my bike to repair a leak at the side panel hanger.
Cleaned up between the hanger and the tank.
Cleaned it with acetone.
Plugged it with Permatex fuel tank epoxy sealant. 
Put some fuel in the tank to ascertain that there was no leak.
All was sweetness and light until....

I tried to put the tank back on and it wouldn't seat properly. The tank pins at the back would not go down into their respective grommets on the frame. The tank just rocked from side to side.

I finally removed the tank and looked it over. I saw that one of the "cups" that engage the splined rubber cylinders that stick out perpendicular to the frame was bent up so it couldn't cup the rubber cylinder.

I gently straightened it with a vase grip with rubber sheet protecting the lip from the vise grip jaws.

Tried it again with the same result. The two pins on the back of the tank would not go into their grommets. The tank still rocked from side to side.

Called up my BMW buddy to have look. We slid a piece of cardboard between the bottom of the tank and the central frame top tube. With little bit of pressure on the tank it was apparent that the bottom of the tank is rubbing on the frame tube so that the cardboard strip was hard to slide out.

We could see the frame tube had some scuff marks from the inflation on the bottom of the tank. So we cut away a 1" wide strip of the insulation. Still rocks and pins will not seat in the grommets.

We removed the splined rubber cylinders and rolled some 1 1/8" wide rubber sheet around the steel pins projecting out of the frame. Wound them to about 3/4" diameter, thinking that this would allow the front of the tank to rise up and allow clearance betwixt the tank and frame tube thus allowing the rear to seat in the grommets.  Retried the tank with the same result- still rocks and pins will not seat.

Nothing has been modified- it is all factory original.

I have had this tank off at least 6 times since I've owned it and never had a problem like this before.

I am trying to sell this bike and do not want to spend a whack of money and time to get it working right. It seems the only way to get this tank down the rear would be to cut a strip out of the bottom center of the tank and weld in a channel to allow it to clear the frame. NOT going to happen.

What the heck is going on here? Anyone come across this before?

[volador]

Reads like you need more lube and try pushing fecking harder & get the other dude from your avatar help you push fecking harder too Al

[Al]

Thanks but all that will do is crush the bottom of the tank.

[Past-my-Prime]

Without actually trying to do what you are doing, I have found that the key is getting the correct engagement of the front "cups" onto the rubber bushings. If you are too high or (as it seems in this case) too low you won't be able to get the tank into position to lock in the rear grommets. So I'd try to lift the tank up a bit and ensure that it is sliding properly onto the front bushings.

Sounds kind of obvious but that was my own experience with removing / reinstalling the fuel tank.

[Laitch]

If the tank hooks that engage the front bump stops are configured correctly and if the pins upon which the bump stops mount are correctly oriented—not bent down, forward or askew however slightly—then maybe the tank floor has deformed for an unknown reason. If this were my project, I'd measure from center-to-center of the tank mounting pins then compare that with the center-to-center measurement of the grommet mounting holes. If the tank pin measurement were greater, I might empty the tank, leave the cap open, invert the tank then put an ice pack on the floor to give metal memory a boost to contract the tank floor some.

It's an interesting challenge.

[Al]

Past Prime and Laitch, thanks for your input.

I have gone over the tank very carefully. The cups engage the rubber cylinders just fine. But I can't get the rear pins within 3/16" of the rear grommets. I have removed the front rubber entirely just as a point of comparison and the rear still will not go in because the bottom of the tank hits the frame. The bottom of the tank is flat. It has never been damaged. It worked just fine when I took it off to fix the leak. It worked just fine the previous times I had removed it.

Laitch I might try your ice pack since nothing else presents itself as the problem. Maybe I should post a picture of the front, rear and bottom of the tank to see if some eagle eyed Brick Boy sees something I don't. "An interesting challenge" isn't what comes to my mind. A PIA comes closer to my thinking. After hemming and hawing for the last few years I finally decide to sell it and now I can't because some weird juju has happened to it. It will be worth nothing if the tank doesn't mount properly. The bike only has 78,000 miles on it. If my health and leg strength and bank account would allow I'd be keeping it but it has to go, sad as that makes me. I want it gone.

[The Mighty Gryphon]

Reading this thread I'm a bit puzzled.  Are the pins 3/16" above the center of the grommets, or are they 3/16" to the rear of the grommets? 

On occasion I have had trouble with getting the pins in the grommets on my RT, the usual cause is a connector(one of those big ones for the combination switches) not settling down into that huge mass of wire around the frame backbone. 

I've also had to reposition that big rubber wishbone shaped pad that runs from the headstock down the two side rails of the frame.  That thing can prevent the tank moving forward as well as down.  Truth be told, I have no idea what it's for other than to prevent airflow under the tank.  As long as you're selling the bike, maybe you can remove it and just include it with the bike.

No matter what, when the tank is finally in place, there isn't any spare clearance under it.

[Al]

Sorry, Gryph, the 3/16" is how far the pins are from touching the rubber grommets in the center of the hole. i.e above the grommet.

Yeah, I put some wire ties loosely around the the under seat connector bundles. I can see where the tank is scuffing the frame tube. I'll see if I can post some pix tonight.

Maybe I'll try and remove the wishbone and see if that makes any difference. I doubt it but I'll try. Got nothing to lose at this point. The tank goes into the general position no problem it is the pins at the back that cannot be lowered into the grommets 'cause the steenking frame hit the tank. Or is the steenking tank that hits the frame?

[Martin]

Placing lumps of plasticine under the tank may help reveal where the problem is.
Regards Martin.

[champ7fc]

This is just a thought but , did you pressurize the tank for any reason after the repair? If all you did was remove the tank, repair the leak and then try to reinstall it and face the problem you are describing I cannot think of anything beyond what everyone else has pointed out. If the tank was pressurized off the bike it could have deformed the bottom of the tank, it is fairly thin aluminum, just a thought. craig

[Al]

Martin. I'm pretty sure where the tight spot is becuase I can see the rub marks on the frame but I was also thinking of plasticine to show up in a picture to post.

champ, It has never been pressurized. The bottom of the tank is flat and not bulged out at all that I can see. I didn't pressurize it since the leak was quite obvious. Thank you all for the ideas.

[The Mighty Gryphon]

Place a sheet of newspaper over the frame under the tank.  Push the tank down and try to pull the paper out.  That may give you some idea of where the tank is being held up.

[Al]

Already done did that, Gryph. I put a sheet of cardboard underneath the tank. It is definitely rubbing on the tank and is confirmed by the marks on the heat insulation and scuff marks on the frame tube. I will put some plasticine on the tube and take picture so I can show y'all.

I'll be out turning money into noise this morning with my 6.5 Swede so it won't happen until later.

[Laitch]

Unless you capped the tank after the Permatex treatment then exposed the tank to heat, it seems unlikely that distortion of the tank's floor would occur in sunny Manitoba at this time of year.  It is more likely that whatever shock to the tank bent one of the forward brackets also knocked both of them out of their previous positions and the remedy relies on trial-and-error when re-positioning them. A small change in their forward/rearward angle could have a significant difference in the pivot arc of the tank. Their fit over those grommets should be tight, if my moto is used as an example.

Still, the icing of the tank floor could be fun—dry ice especially.

[The Mighty Gryphon]

Cardboard is too thick.  Putting a piece of paper under the tank and pulling it out will tear the paper where the tank is being obstructed.  This will help locate the problem which, apparently, you still have yet to identify.

[volador]

Too alleviate this rigamarole

'how much do you want for the moto'?

[Al]

I've been thinking $3000 Canadian Pesos.

[volador]

Ive got US$1000 since this moto has got alignment issues

[Al]

I don't think so. Tracks as straight as and arrow. No handling issues or weird tire wear.

This tank issue has never been a problem before now.

New York is along way from Manitoba.

[volador]

I got a delivery dude who handle all the logistics. Let me know when you change your mind. Good luck with the fecking tank

[Al]

Well at a $1700 discount I won't be changing my mind any time soon. I'll cut and weld a channel in the bottom of the tank for far cheaper than that.

[Al]

Here are some pictures of the fuel tank and frame. I hope this works.


Fuel tank Front Cups:


Fuel Tank Front Cups.JPG (213.98 kB . 660x442 - viewed 405 times)



Fuel Tank Underside:


Fuel Tank Underside.JPG (42.65 kB . 385x576 - viewed 400 times)



Front Rubber Cylinders:

Front Rubber Cylinders.JPG (42.04 kB . 385x576 - viewed 411 times)



Frame Scuffing:


Frame Scuffing.JPG (35.38 kB . 385x576 - viewed 399 times)

[The Mighty Gryphon]

Hard to tell from the photos, but that big rubber wishbone doesn't look right.  Have you made sure that the insulation isn't doubled up or folded over someplace?

It's pretty obvious to me that the tank is being held up somewhere and you haven't a clue where or what it is.  The only thing I can suggest is putting a piece of paper a couple inches wide and 11 inches long between the tank and the frame and trying to pull it out while the tank is putting just a little downward pressure on it.  You are using it as a feeler gauge to see where the tank is hitting whatever the F is holding it up.  Once you know what the problem is you can address it.

[Al]

Gryph, I DO know where it is hitting. On the frame just before the relay box. That is why I included the scuff marks on the frame picture. I HAVE put a strip of cardboard between the tank and frame and it catches there.

The tank rocks a bit side to side which also tells me it is contacting the frame and that's what allows it to rock sideways.

I have felt underneath the front and can feel the rubber cylinders go into the cups. I guess I can pull the Rubber wishbone out but I don't that is the problem.

[Martin]

The insulation appears to be bunched up a bit, this could cause problems. When I insulated my tank it took a couple of goes to get it right as I had no references as to how the insulation should be fitted. I ended up using blue fire proof foam sandwiched between reflective foil and 2" wide chrome tape from the local discount store where I couldn't fit the insulation.
Regards Martin.