lorenzo Posted May 1, 2020 Report Share Posted May 1, 2020 Prior to rebuilding this long-term (dismantled) project, I've begun dragging out and examining parts, with a view to doing a trial assembly before parting with serious money on chrome plating, powder coating, etc. Starting with the frame, I found two sawn/ground off welds on the seat rails either side of the air filter (arrowed in pic.) :- Anyone out there know what was there originally ? Brackets/ lugs/ a bridge across the filter ? ....and do I need to replace it ? Outer edges of the air filter body appear to stand slightly proud of the frame rails, and there are marks on the underside of the seat part of the tank/seat unit. Bike is a UK supplied and registered example from new (Aug. '73) and came to me with Homerlite fuel tank, which I assume is original -(this would have replaced the usual fibreglass item fitted for other markets, I believe). I know these are notorious for splitting the welds between tank and seat panel; I'd like to get the fit right. Ideas ? Quote Link to comment Share on other sites More sharing options...
vintagenut Posted May 1, 2020 Report Share Posted May 1, 2020 Hi Lorenzo, I'm wondering if the previous owner had welded the airbox support tabs to the frame to keep it solidly in place? I've attached a borrowed pic showing the tabs I'm referring to. When mounted, the OEM fiberglass seatbase would have secured the airbox into position but, if running a different seat and tank, perhaps the airbox was flopping around beneath the seat without the pressure of the OEM tank unit to keep it solidly down. Definitely one for the, "What were they thinking?" file! I'd just grind/file and sand them smooth as original. Have fun! Christian 2 Quote Link to comment Share on other sites More sharing options...
gooey Posted May 1, 2020 Report Share Posted May 1, 2020 Hi Lorenzo, My 85 has got these tabs on the frame, I think they are to support the original fibre glass tank seat unit. I've replaced mine with an aluminium unit and it sits on the tabs fine. Hope this helps. Guy. 1 Quote Link to comment Share on other sites More sharing options...
feetupfun Posted May 1, 2020 Report Share Posted May 1, 2020 Here is what the lugs would have looked like originally. My photo shows an M85 frame. My other M85 frame (the pretty one in the photo) has nothing there at all and a M80 frame on the other side has no lugs either. The lugs do line up with the side mounts on the airbox like in vintagenut's airbox photo but what purpose they are intended to serve I don't know. As far as I know both my M85s were sold new in Australia and came with fibreglass tanks. 1 1 Quote Link to comment Share on other sites More sharing options...
feetupfun Posted May 1, 2020 Report Share Posted May 1, 2020 Here is a photo that shows how the seat/tank unit rests on those lugs 2 Quote Link to comment Share on other sites More sharing options...
rcgods Posted May 2, 2020 Report Share Posted May 2, 2020 None of my M85 have these lugs and neither have my M80 or M91 frames. First time I've seen them. Quote Link to comment Share on other sites More sharing options...
vintagenut Posted May 2, 2020 Report Share Posted May 2, 2020 It's interesting to see how those added frame lugs function, feetup - thank you for the closeup photo. I recently let go of a m85 before picking up another m80 and neither had/have those tabs. Perhaps Bultaco encountered some variations in their production process due to material supply or finish/fit irregularities (ie. the top subframe tubes accidentally cut too short or welded in a few mm too low... leading to some tank/seat units having an unsupported gap ahead of the airbox that could lead to breakage under rider weight) and then added the top tabs and modified airboxes to correct the issue? If so, how great is that? Some very conscientious quality control folks and skilled builders at the factory recognized and corrected those variances to insure the best quality product went out! Another thought along the same line... maybe the original tank had to be replaced at some later point and the tabs were in the way of a proper fit? Again, who knows?... But that does circle us back around to Lorenzo's initial question of what to do. Lorenzo, if you haven't since already done so, I'd recommend taking a look at the fit of your fiberglass tank/seat when mounted onto your frame via the angle feetupfun provided in his photo. If you see a significant gap at the front, then definitely address that issue. Thick hard-rubber blocks could be used - as mounted to the bottom of many Japanese steel seat bases from that period (ie. Honda Elsinores, for example) could possibly be used in lieu of replacing the original tabs. Otherwise, I'd personally opt for having a skilled welder (which I am definitely not!) replace the tabs. A bit more expensive, yes - but you can't go wrong keeping it stock! Good luck and keep the updates coming! C. Quote Link to comment Share on other sites More sharing options...
lorenzo Posted May 4, 2020 Author Report Share Posted May 4, 2020 (edited) Gentlemen, thanks very much for your response - I'm almost overwhelmed. How did we ever manage, "pre-internet" ? Vintagenut : no tabs on the air filter, and no sign they were ever there. Same thing with my mod. 92 (also '73). gooey : Alloy tank unit.........do you have a pic. ? feetupfun : Thanks again for your detailed shots - exactly what I was looking for (and was unable to find elsewhere). So it seems that the tabs were to support the front of the seat section of the fibreglass tank unit, and assuming the original "long" seat is used, support at the rear is provided by the seat resting on the frame loop ? Perhaps someone can confirm........... Additionally, could they be to maintain a clear "breathing" space above the filter intake ? This leads me to ask if anyone knows what type of seat was fitted to the UK bikes with Homerlite alloy tank unit.......my bike has a short trials type seat which may not be original, and anyway doesn't extend rearwards as far as the frame loop, meaning any weight placed on the rear of the seat would provoke bending/cracking of the alloy, assuming the tabs were still in place. Edited May 4, 2020 by lorenzo missing text Quote Link to comment Share on other sites More sharing options...
woody Posted May 4, 2020 Report Share Posted May 4, 2020 That's the correct size for the trials seat although not the correct cover, the back end was supported by resting on the mudguard. Can't help with the Alpina version, never seen one Quote Link to comment Share on other sites More sharing options...
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