BMW R80.....................................................

Gear­box prob­lem? Fixed. Electrics? Sorted. Stephen Her­bert now turns his at­ten­tion to petro­chem­i­cal mat­ters as his ‘bar­gain’ eBay Boxer gets set to re­turn to the road. Time to seal the tank and bal­ance those carbs

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Gear­box prob­lem? Fixed. Electrics? Sorted. Stephen Her­bert now turns his at­ten­tion to petro­chem­i­cal mat­ters as his ‘bar­gain’ eBay Boxer gets set to re­turn to the road. Time to seal the tank and bal­ance those carbs

Did I men­tion that Hi-Yo (the sil­ver R80) came wear­ing a tatty-but­solid tank from an RT? I know it’s from an RT be­cause of the fair­ing fret-marks on the side of the tank near the BMW roundel… and be­cause the PO told me. In­cluded with the sale was the cos­met­i­cally su­pe­rior but pin-holed orig­i­nal tank, sans fair­ing scars. Which to choose, how to pro­ceed?

Af­ter a bit of prod­ding and pok­ing I de­cided on a plan of ac­tion: use the orig­i­nal tank, seal it with Pet-Seal, and flog the RT tank on eBay to get some money back. Which is ex­actly what I did. I’ve done tank seal­ing be­fore, but only as a pre­ven­ta­tive mea­sure. I’ve never ac­tu­ally sealed a rusty, leak­ing tank so I was a bit ap­pre­hen­sive about my own abil­i­ties, and of the abil­ity of the cho­sen sealant to do its job. I chose POR-15 from Frosts, mainly be­cause I’ve used it be­fore (other tank sealants are avail­able). I read the dis­truc­tions dili­gently be­fore pro­ceed­ing, par­tic­u­larly with ref­er­ence to seal­ing pin-holes and the like. Be­fore us­ing the stuff I glued some be­spoke bits of ally over two of the pin-holes that had got a bit big­ger in stor­age. I used bog stan­dard Araldite for this, rea­son­ing this should stick well and that the POR-15 would pro­tect it from petrol degra­da­tion.

So I set to work, first with the de­greaser in the POR-15 kit, then with the metal treat­ment fluid and then with the POR-15 sealant it­self. All pro­ceeded ac­cord­ing to plan, but sev­eral thoughts struck me as wor­thy of not­ing from my ex­pe­ri­ence:

Th­ese BMW air­head tanks don’t lend them­selves to be­ing emp­tied of flu­ids. This is be­cause of the way they’re con­structed – try tip­ping petrol (or any other fluid out) and you’ll find that the filler and the pet­cock ar­eas col­lect liq­uids in var­i­ous nooks and cran­nies so heat and air are re­quired to be used in greater quan­tity than ex­pected to fully dry th­ese ar­eas

The in­side of the tank for the most part

was rust-free (apart from the rusty bits, ob­vi­ously!). This is be­cause of the OEM brown paint ap­plied at the fac­tory, which sticks re­ally well to the metal. The POR-15 in­struc­tions call for bare metal in­side the tank be­fore pro­ceed­ing so I tried to re­move said paint… and even­tu­ally gave up. I had poured sev­eral hand­fuls of nuts ’n’ bolts in­side and rat­tled away till my eyes hurt but still that paint stayed on. In the end I con­cluded that the paint was there to stay and that it was a good ’un and the POR-15 would stick very well to it

I bought one large ta­pered rub­ber bung and two smaller ones to push into the filler and pet­cock holes to re­tain flu­ids dur­ing the process. This was a wise in­vest­ment and worked well

De­spite fill­ing the known pin-holes (see above) be­fore start­ing, the de­greaser and metal treat­ment flu­ids were very good at find­ing new ones. This caused some mi­nor leak­age which I tem­po­rar­ily stemmed with duct tape dur­ing the treat­ment process un­til the POR-15 liner sealed them at stage 3

I in­vested heav­ily in a ‘be­spoke dry­ing jig’ to re­move any traces of damp be­fore ap­ply­ing the sealant. OK, I ac­tu­ally used my trusty Black and Decker hot air gun on a stand plus my work­bench to do this. The hot air gun was po­si­tioned so that the hot air would go straight up into the tank, through the filler hole in the in­verted tank and emerge through the pet­cock holes. Heat rises y’know.

The point is that you can’t get rid of damp­ness by waft­ing the wife’s hairdryer at the tank for five min­utes. It takes a se­ri­ously long time to do this crit­i­cal part of the job, and any trace of damp means the sealant won’t seal.

Af­ter com­plet­ing the long job of seal­ing the tank, I turned to more cos­metic as­pects of the job, T-cut­ting the blue over­spray off the tank (see last month) and ap­ply­ing new BMW roundels to its flanks. Th­ese two ac­tions didn’t re­sult in a per­fect fin­ish but did trans­form the

ap­pear­ance of the bike while keep­ing a cer­tain char­ac­ter.

Be­fore re­fit­ting the pet­cocks, I stripped them and cleaned out 30 years’ worth of gunge. It struck me at this stage how good the engi­neer­ing is on th­ese bikes. Not only are the pet­cocks them­selves very well en­gi­neered (with the pos­i­tive ‘click’ stops men­tioned in a re­cent air­head ar­ti­cle in th­ese pages), but the way they are at­tached to the tank is well thought out. They use spe­cial nuts con­tain­ing both right- and left-handed threads so you can get them ‘just so’ in terms of tight­ness and at the same time fac­ing in ex­actly the right di­rec­tion. Clever stuff!

In the end, all my hard work paid off as the end re­sult was a com­pletely sealed and at­trac­tive tank on the bike and £120 in my PayPal ac­count from sell­ing the tatty tank.

On to the leak­ing pushrod tube which ini­tially I’d planned to re­place. Read­ing the BMW fo­rum, how­ever, it seemed to be a big­ger job than I ex­pected, since the tubes are an in­ter­fer­ence fit in the heads and need to be po­si­tioned just right in or­der to ap­ply the right pres­sure to the seals. In ad­di­tion, it seems that there is a risk of dam­ag­ing the crank­case open­ing for the bar­rel if the con­rod were to drop un­ex­pect­edly in that area, re­sult­ing in a big­ger oil leak than the pushrod tubes.

Af­ter much con­sid­er­a­tion I took the wimpy op­tion and used Loc­tite black sil­i­cone to seal around the ex­ist­ing seals, at least as a tem­po­rary mea­sure. Be­fore ap­ply­ing the sealant I squirted the area with car­bu­ret­tor cleaner to im­prove the like­li­hood of the sealant seal­ing… which for now seems to be work­ing.

I may have men­tioned in pre­vi­ous in­stal­ments that the front tyre was not hold­ing pres­sure. I de­cided to put new tyres on front and back due to the age of the ex­ist­ing black stuff, so I needed to tackle this prop­erly.

Air­heads of this vin­tage have cast wheels and tube­less tyres and I was ad­vised that poros­ity can be a prob­lem so I ap­proached this with trep­i­da­tion. I took the front wheel to my lo­cal MoT sta­tion-cum-bike guru and asked Si­mon’s ad­vice. He took the tyre off and, rather than im­me­di­ately fit­ting the new one, ad­vised me to deal with the prob­lem of cor­ro­sion.

It seems that win­ter salty wa­ter gets into the gap be­tween tyre and rim, caus­ing the al­loy to ox­i­dise. It’s not clear to me whether this rough sur­face against the tyre was the source of the air leak or poros­ity of the cast­ing as pre­vi­ously sug­gested, so I wire-brushed the in­side of the rim and painted it with my old faith­ful Aldi paint. When I took the wheel and new tyre back to Si­mon a week later, he fit­ted them to­gether and I haven’t had any prob­lem since. So I still don’t know for sure… but do I care?

I didn’t do too much else to Hi-Yo at this stage, be­cause it was ap­proach­ing the start of the rid­ing sea­son and I fan­cied a ride. With this in mind, I did all the usual main­te­nance stuff about check­ing / chang­ing air, oil and other flu­ids and headed off on a VMCC run. Th­ese air­heads are ideal for VMCC runs as most now fall within the 25 year rule and they’re less tem­per­a­men­tal than some of the older stuff. I’m part of VMCC Cheshire Cats Sec­tion and one of our early-sea­son runs is the Big Break­fast and Llanuwch­llyn Run, which in­volves meet­ing at Prees Heath just south of Whitchurch, then rid­ing over the hills to the south end of Bala Lake in North Wales. It’s a lovely run through great scenery and I was re­ally look­ing for­ward to it – and it didn’t dis­ap­point, and nor did Hi-Yo, com­plet­ing the 160 miles with­out a prob­lem.

How­ever, if I were crit­i­cal (and I of­ten am), the bike’s car­bu­ra­tion wasn’t spot-on. So

a carb strip-down, clean-out and tune-up was re­quired. The main prob­lem was rough run­ning in the mid-range and a cer­tain hes­i­tancy when com­ing off tick­over. Th­ese air­heads use Bing CV carbs, which op­er­ate in a sim­i­lar way to the SU you might have had on your old Mini, or the Stromberg on your old Tri­umph car. They have a but­ter­fly throt­tle, a vac­uum-con­trolled pis­ton / spring / di­aphragm / nee­dle thingy, and a sep­a­rate ‘choke’ which is ac­tu­ally a small sep­a­rate car­bu­ret­tor mounted on the side of the main one for en­rich­ing cold starts. Help­fully, the BMW spares ven­dors do a re­build kit con­sist­ing of re­place­ment gas­kets, seals, O-rings and di­aphragms. I or­dered one forth­with.

Strip­ping and re­build­ing was rea­son­ably straight­for­ward, apart from the mix­ture screw on one carb which re­fused to let go un­til I’d given it a se­vere talk­ing to, plus co­pi­ous use of pen­e­trat­ing fluid, blowlamp (gen­tly!) and torque. It took about three days, but emerged rel­a­tively un­dam­aged. The prob­lem was even­tu­ally traced to a com­bi­na­tion of a rock-hard O-ring plus Al O jam­ming up the 2 3 thread – th­ese mix­ture screws are in in­di­rect line of salty wa­ter spray from the front wheel in win­ter.

There was quite a lot of hard, pow­dery de­posit in the voids be­tween the main jets, atom­is­ers and nee­dle jets too. Not sure what this was – it didn’t seem to be white (Al O ) 2 3 so could be a re­sult of the dreaded ethanol. Any­way, it’s gone now. I re­placed all the O-rings and the di­aphragms, clean­ing out as I went, then re­assem­bled with new float cham­ber and starter gas­kets from the re­pair kit. I also took the pre­cau­tion of ap­ply­ing a smear of cop­per-ease grease to the mix­ture

screw threads to pre­vent fu­ture prob­lems.

In the mean­time I’d read on the BMW fo­rum about chang­ing the throt­tle springs for lighter ones, and mine felt quite heavy on the run. I sent off for some from a guy in Cal­i­for­nia trad­ing as ‘EZ Pull Springs’. He does lighter springs for all air­head carbs, and mine ar­rived within a week.

To tune up, I set the var­i­ous screws to nom­i­nal po­si­tions as laid out in the BMW man­ual and went for a ride… …and it was EVEN WORSE! I prob­a­bly should have ex­pected that, as the nom­i­nal set­tings are there just to get the en­gine warmed up in preparation for tun­ing, which was the next stage. When I had my pre­vi­ous air­head, the R45, I had made a dif­fer­en­tial manome­ter to bal­ance the carbs. It con­sists of a one-me­tre long wooden bat­ten, some plas­tic tub­ing and a small quan­tity of ATF fluid. You can no doubt dis­cern that this is a high tech, en­gi­neered tool. It cost me less than £5 to make and sim­ply con­nects to the vac­uum take-offs un­der­neath each car­bu­ret­tor, so you can ad­just the throt­tle bal­ance us­ing the manome­ter to check the dif­fer­ence in vac­u­ums in the in­lets.

The se­quence of ad­just­ing is as fol­lows (as­sum­ing the en­gine is warm): 1. Back off all throt­tle and starter ca­bles 2. Set the throt­tle stops to nom­i­nal set­tings 3. Set the mix­ture screws to ¾ turn out from fully in 4. Con­nect a bal­anc­ing tool of your choice (for me, my bit of wood and plas­tic pipe; oth­ers op­tions are avail­able) 5. Start up and ad­just both throt­tle stops to bal­ance at tick­over, keep­ing the revs to 900rpm 6. Ad­just both mix­tures sep­a­rately to achieve fastest and most sta­ble tick­over (re­peat steps 5 and 6 a few times as mix­ture ad­just­ment af­fects tick­over speed) 7. Once you’re happy with tick­over bal­ance and mix­ture, ad­just throt­tle ca­bles to give 0.5mm play 8. Restart en­gine and run at (say) 2000rpm and bal­ance ca­ble ad­just­ments to en­sure both carbs are equally open at that speed 9. Check tick­over again 10. Ad­just starter ca­bles to sim­i­lar 0.5mm clear­ance and lock up ad­justers

The dif­fer­ence was in­cred­i­ble. Not only had I got rid of the rough run­ning re­sult­ing from the ‘nom­i­nal set­tings’ ear­lier, I’d also ironed out the prob­lems about pick-up and rough mid-range ex­pe­ri­enced on the run. The lighter throt­tle springs seemed bet­ter too, but th­ese need to be tested on a longer jour­ney to see if wrist fa­tigue is elim­i­nated. Th­ese carbs are great when set up prop­erly and it’s very easy to ig­nore them, you do need to fet­tle them to stay on top of their per­for­mance.

So in con­clu­sion, I’m now look­ing for­ward to us­ing Hi-Yo for what he was in­tended – rid­ing and en­joy­ing. My ex­pe­ri­ence with this and the pre­vi­ous air­head I owned is that you don’t re­ally need an­other bike – it’s so versatile. So much so, that I’ve sold my mod­ern BMW F800GT. That was a re­ally ca­pa­ble bike, lovely ride and per­fectly re­li­able… but ever so slightly bor­ing.

Hi-Yo re­mains my cur­rent ma­chine of choice be­cause I know it will start and just keep on run­ning. Mind you, the two En­fields aren’t bad ei­ther. And there’s room for an­other sta­ble­mate now the F800 has gone… Pass me the lap­top and fire up eBay. It’s a dis­ease, you know!

Pho­tos by Stephen Her­bert

This was the petrol tank on the BMW when Stephen pur­chased it. The marks near the BMW roundel were from a fair­ing when it was pre­vi­ously at­tached to an RT

Some­one had pre­vi­ously re­paired a hole us­ing Araldite or sim­i­lar so he ground down the paint near the grow­ing hole…

…and glued on a patch of al­loy in the same way

Bal­anc­ing the carbs

Stephen’s home-made dry­ing rig utilises a hot air gun on a stand be­low the work­bench. This blows hot air up into and through the tank to dry the in­sides prior to seal­ing

An­other be­spoke tool, this time a vice hold­ing a piece of al­loy box sec­tion to hold the carb body. This al­lowed Stephen to ap­ply more pres­sure when try­ing to re­move the jammed mix­ture screw

Stephen stripped each carb in se­ries, to avoid mix­ing up parts from both of them

Ox­i­da­tion of the wheel rim af­ter wire-brush­ing and prior to paint­ing

New BMW roundels com­plete the ap­pear­ance

A big rub­ber bung was em­ployed to re­tain flu­ids in the tank while it was sealed

Pushrod tube painted; de­greased around the seal area prior to seal­ing with Loc­tite black sil­i­cone

One clean and re­con­di­tioned carb

The BMW’s first run af­ter its re­build

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