TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. > Project Classic Motobricks

K75RT leaky tank fix + generic maintenance

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samih:
So my newly purchased '92 K75RT turned out to have a leaky fuel tank which I discovered when assessing slight bubbling at the bottom of the tank. As promised in the introduction section going to document the fix here.

This how the tank looked like at the start of the repair.

Mostly harmless:



Not really pretty with a bit of previous tampering present:



Yes, the previous tampering was a bit more serious than expected:



What's going on here? There were localized splattering of what looked like polyurethane foam underneath the paint, both here and also in some places underneath the tank. The reaction of PU with the top coat had turned some areas of the tank into a bad imitation of a balloon. The alumin(i)um underneath was perfect with just a bit of red PU foam remaining. The paint coat here was very thick so most likely the tank was respayed at some stage.

There was also an old patch visible with no signs of attempting to paint over it.



Inside the tank looks largely solid with just tiny pinholes in present on couple of places.



These are the two main culprits:



I'm planning to use HTS-2000 to patch the tank with continuous flow of air through the tank to mitigate the chance of a thermal event. Filling it with water is another possiblity although I'm afraid it might mess with the adhesion. I'm currently waiting for these brazing rods to arrive from UK. As there are multiple potential locations for future leaks I'll also seal inside the tank with epoxy.

The final image shows the bottom of the tank after light sanding and debubbling it. I had to peel off the partially fuel saturated insulation as those darn bubbles were present pretty much all throughout the bottom of the tank. For the repair metal to work the surface needs to be totally fresh alu so plenty of work needed here just before the brazing attempt.



But yes, I'm getting ahead of myself here as usual. The first job will be to practice with the HTS-2000 at the bottom of the tank when those rods appear by post. After that it will be plenty of grinding and sanding to get the tank ready for the paint. I'll need to call to the paint shop next week to order a spray of suitable base coat compatible with bare alu tank and also some high build primer. Then in stage n it will be time for the final colour "675" and a matching clear coat. The inside epoxy coating will probably be added right after HTS before the painting process starts.

The tank was a bit faded anyway and has a scratch on the right hand side so might as well just spray it all.

Apart from this tank saga everything else seems pretty good. The front brake light switch was broken and under the seat, ABS is flashing constantly if not for other reason than this missing switch. The driveline splines were in perfect condition and well greased sometime in the past. All three spark plugs were under torqued which I guess is better than over torqued. Brake fluid looked murky and coolant good. I also took out the thermostat which I'll test later on.

Laitch:
Thanks for the useful photographs. Members here will certainly be interested in the outcome of using HTS-2000. Good luck with this task.

samih:
Thanks Laitch, will definitely keep this thread up to date. In general I'm super impressed about the quality of everything on this old girl. The only difficult fastener so far has been the speed sensor screw on the final drive and even that came out with plenty of penetrating oil and patience. Looks like my bike was hardly ever used in "winter" which helps a lot.

samih:
Still no sign of the HTS-2000 delivery but I have been "productive" and have largely reassembled the bike apart from the side fairings and the tank of course. I have been running this contraption for the past week or so to get rid of all of the remaining fuel vapours.



It pushes in the fresh air and exhausts it via the fuel sender aperture where I inserted a length of flexible tubing snaking into towards the front of the fuel tank. This is to ensure that all the volume of the tank is ventilated. The exhausted air doesn't have any smell of fuel now after a week which fills me with confidence that the tank won't explode on me later on.

I also disassembled cleaned and resealed the gear position switch. As reported here the rubber covering of it had started to fail and had lead to water ingress with sign of slight corrosion within. Apart from dropping 2/3 of the pins and springs on a dusty concrete floor and used 45 minutes searching before I found the last spring, I sealed the switch using Loctite anaerobic instant gasket gel (which is really meant for metal surfaces but that's all I had) and copious amount of self vulcanizing repair tape and it was at least initially working when tested. To make the life easier for the said switch I installed a rear mudguard extension that may or may not do anything but in theory will help the water making it all around the guard to safely exit below the swinging arm/gearbox as opposed to just behind them.



The rear edge got the the same treatment as there were unwanted splashes on the lense of the rear light after my initial journey on the bike. It can be muddy here in Ireland.

The other jobs apart from splines and fluid changes: I reseted the ABS fault code for low voltage (# 5)  and as I now have a new front brake like switch fitted (was not fitted at all) hopefully the system will be operational now. At least the brake light tests now result in the appropriate sequence of the ABS/bulb fault indicators and also the ABS test socket middle pin now reads constant voltage indicating no faults present.

Is 10.8 VDC expected btw on this pin with a battery voltage of 12.8 Volts or do a have a bad connection somewhere? I suspect all is good as the ABS now seems to work without the low voltage indication. Haven't of course had a chance to road test it yet to see if the wheel sensors are gapped correctly but all the systems at least pass the POST.

Now just need to patiently wait for the HTS to arrive...

samih:
Still no sign of those brazing rods but I contacted the vendor and they were definitely posted. There seem to be a post Brexit related slowdown with the post between UK and Ireland.

As this is moving more on the way of generic maintenace: Checked the valve clearances last night. The #2 and #3 inlets are a little bit tight but decided to leave them be for now and will check again on the next service. The spec in millimetres is 0.15-0.20 mm for the inlets and these both were a very loose 0.10. The others very right at the mimimum at 0.15 (in.) and 0.20 (ex.). In general all is looking great under the valve cover and had I not known better I would have guessed that the lobes were machined about 6 months ago:



I think in general it was just a question of blowing away the cobwebs from this fine motorcycle:



As mentioned the splines were looking great:

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