NX650 bottom end rebuild

The only reason the cases had to come apart was the worn-out splines on the gearshift shaft. The Dominator is actually pretty simple to strip and reassemble if you have the factory manual. You don't need any special tools beyond a clutch centre holder, a very long 5mm Allen key which you can hook up to a torque wrench (for the engine casings), and a universal flywheel puller. A handy substitute for the Honda cam chain tensioner holder tool is a small punch, or even a ground-down nail would do.

NX650 cylinder head

The only way out of the morass of worn parts is to spend money.

Repair options first: Phil Joy at Joy Engineering, Ware, no longer repairs cylinder heads as he did for many years. He suggested one person who might do the job, but then pointed out the scored central journal is not critical. That reminded me of a stripdown Honda UK did of a 1989 Dominator used by Performance Bikes. Nine months from new the central journal was scored, but it went as well as ever and there was no other engine damage.

This gave me hope, but we still had a cylinder head crack to deal with. I soon gave up looking for a second hand head. They aren't available, for obvious reasons, and used engines on eBay or from breakers sell around the £650 mark. Not a risk worth taking. They could have cracked heads too. New heads are £800-£1000 as far as I can tell.

The Cylinder Head Shop told me they could pop out the valve seats, grind the crack out, fill it with weld, remachine and then refit the seats for about £350. As a bonus, they would thicken the plug area to take a long reach plug. I sent it off. Six weeks and many calls later they hadn't started the job, so I got it sent back. They apologised and offered to get it done pronto, but I had lost all faith. The highly respected SRM Engineering reckoned a repair was not worth doing. Can you see daylight through the crack? No. Was it running OK when you got it? Yes. Well, you could always try putting it together and running it as is, they suggested.

I got a third opinion from Ducati bevel expert Tony Brancato, who I went to see on a job for Bike. Tony said he could repair it, but it would never be as good as a new. That settled it. I decided to replace the worn cam parts and re-use the old head. The cracks happen, apparently, due to overheating. I will make sure this engine never runs low on oil again.

Dave Silver Spares quoted £203 for a new camshaft. I found a cam billed on eBay (usa-motorcycles-inc) as an XR650/89 Dominator fitment in the USA for about £75 and went for that. Dave Silver supplied two followers and two sub-rockers for another £70 or so.

What goes wrong with 650 Honda singles

After 30,000 miles and a reported running-out-of-oil incident at some point the bottom end, piston and gearbox seem fine. The rest is a bit of a mess:
– knackered gearshift splines
– worn clutch shock absorber springs
– scored central cam journal in the head
– severe wear on two cam lobes and followers
– coked-up exhaust ports (it was burning oil)
– suspected worn rings (yet to be measured)
– a crack in the cylinder head.
I will explain how I propose to solve these problems, and what it costs, in future posts. For now here are the pics. And clearly I was robbed when I bought the bike for £31.

Carb refurb

Carbs, I have discovered, are often the first thing to wear out on an old engine. Slides, needles and main jets are all affected by the gas shuttling back and forth on tickover and at low revs. The result is crap fuel consumption and wooly low-speed running. There's also the problem of fuel residues building up during laid-up periods.
Under the filth this Keihin carb looked OK after 30,000 miles but the odd auxiliary slide lifter mechanism was seized. The bike ran like a pig, partly due to the lifter but mostly I suspect due to the inlet stub having a large crack.
Fed up with carb trouble on my ZZ-R I sent the Domi carb to Mike Davies at JRS in Swansea (www.jrs.uk.com, 01792 402458), who refurbishes mainly Keihin, Mikuni and Dell'Orto to what I consider to be the best standard in the UK. Japanese carbs are either unobtainable or very expensive, so worth taking care of. Mike's service was very good. He rang me to confirm he'd received the carb, told me what he was doing and estimated the final cost. When a pilot screw didn't shift he rang me to say the machining and spares (from Dave Silver) would be a bit more.
It came back clean, set to Honda's recommended values and with a superb fact sheet. Cost was £110.
The stripdown shot shows the strange auxiliary lifter, which uses a ramp on the throttle pulley to help lift the slide under sudden throttle openings.

Chassis finished

After a lapse of months, I've done the chassis. Nothing too tricky, just lots of cleaning and repainting. The rattle can Simoniz Five Wheel Steel which goes on so nicely and dries pretty hard is, I've discovered, not resistant to brake fluid. But then what is? The oily clag all over the top frame tube, ignition coil and loom due to clumsy refilling with oil (the engine burned a lot) took hours to clean with solvent (usually white spirit) and toothbrush.

The rear brake shows a sensible (tight-fisted) mix of repainted bits and cleaned-up old stuff. With summer use rust isn't going to be a big problem.

Suspension linkage gets another shot at life

Instead of working, spent a dreamy afternoon assembling parts onto the newly cleaned up chassis. These pics show how even the most unpromising bits and pieces can sort themselves out. Blasting by Pete at Summit, Paint by Simoniz Three Wheel Steel rattle can, bearings by Dave Silver spares. Unfortunately the four genuine Honda chassis bearings in this linkage were an odd size (24.5mm OD) and thus £73, plus another £18 for the seals. The swing arm bearings, exactly the same thing but 26mm OD, were a tenner for two at Anglia Bearings in Peterborough. The other good source of suspension bearings is Wemoto's website, but they didn't have the full set for the Dommie so I didn't bother this time.

Shock rebuilds

Most people would regard a knackered old oe shock as scrap, and it is – but a decent quality new one starts at £300. So rebuilding one for £180 is worth thinking about.

I've had five shocks rebuilt by Falcon Engineering (falconshockabsorbers.co.uk): a TDR250, VFR750, ZZ-R1100, RRV Blade and now this. Robin Packham opens the cylinder, cleans everything up, and puts in a better-than-new PTFE piston ring, new oil and usually a new damper rod and seal too. He also cuts a thread in the cap and screws it back on. A final gas-up to get the pressure back and the job's done.

I've had the best results with non-adjustable shocks. The rebuilds on the Blade and ZZ-R altered the damping range somewhat. As the Dommie's shock is straightforward (once you've hacked off the plastic outer cover which eventually keeps the dirt and moisture in – note the blocked drain hole in the rusty 'before' pic) I got it blasted and sent it off. It came back quickly and with only a few paint chips on my rattle can-sprayed spring and shock body. On this shock there's nowhere to put the gas valve except the top portion of the thread (Honda must have assembled the original in a pressure chamber). So I lined up the best place and marked it with some tape before packing it in the box.

Robin replaced the spring and fitted the preload collars before Loctiting the valve in and gassing up. I can never change the spring now, or reduce preload beyond a certain amount, but that's OK.