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
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!