53000 miles: Fuel leak and carb clean

Yellow petrol residue on the alternator

I noticed I was arriving at home smelling of petrol. Although I find a bit of a petrolly whiff has its own appeal, it is not something my wife appreciates. There came a point when it was obvious that something needed to be done. That point was seeing petrol dripping onto my alternator. Not good.

I thought it was due to a faulty float bowl seal on the carburettor for number two cylinder so it was a case of getting the bank of carburettors off the bike and giving them a bit of a birthday. I figured the air filter could do with a clean anyway and, after 53000 miles, I decided it would be wise to change the cam chain (next post). So Ruby was going to get a big birthday.

24 years and 52,995 miles

As it happens, I think it likely that she was made in July or August in 1991. I don't know for sure. She was registered 1st of September 1991 but with chassis number 544, production having begun in March and building up to a rate of about 50 machines per week during the year, my guess is that she was constructed any time from mid June to Mid August. So happy 24th birthday, Ruby. She would be having a reasonable run out afterwards because I had an international trip to do for work, flying from Heathrow, and I had decided to ride there, stowing my bike gear in left-luggage for the duration of my time abroad. FWIW, that trip worked out well - free parking for the bike and £20 for left luggage.

So the full list of jobs were: diagnose and solve fuel leak, clean out carb float bowls, clean and re-oil air filter, change air box for a better second-hand part I'd bought from Trevor, change cam chain and check valve clearances. The first four are here.

I have found that the top of the motor tends to accumulate road grit over time. It surprises me because I have inner panels that sit in front of the exhaust pipes and fill the gap above the oil cooler and below the water radiator. Anyway, it is helpful to brush off the cam cover before removing the plug leads. I remember I'd got the coil low tension connectors muddled up when I first tried to restart the engine back in 2012. So when I removed the coils, was very careful to keep note of which connectors belong to which coil. 

It is always a bit of a fight to get the carbs out. There is not much room between the airbox and the inlet stubs, and the rubber is ageing. I don't rush at the job, straddle the bike so I can get a good grip on both sides, and put a block against the front tyre to guard against the risk that I'll roll the bike off its stand in my pulling and pushing. The air filter was satisfyingly dirty. 

I have had a bottle of RAMAIR foam filter oil for years. I got it for my old GS650 Kat. It has a solvent that evaporates to leave a tacky coating. Good stuff. It just needs engine degreaser to clean it up. I was lucky enough to have a sunny day so the filter dried out quickly and then took the RAMAIR oil very well. 

 The old air box was severely cracked and my attempt to repair it with filler rod and a soldering iron had finally given up. The replacement box was in great shape and, I realised, had a slightly different design from the broken one. It has a plastic frame that retains the filter and this frame has recesses for four long stainless screws that clamp the sides of the box.

Replacement airbox, with filter frame including
recesses for through-bolts on each side. 

 The carb float bowls contained a fair amount of a red dusty residue. The seals showed a bit of alloy fur but nothing that looked like a leak. Perplexing but I cleaned them up and reset the floats 1mm higher than before (15.5 instead of 14.5). The motor had been running a bit rich, from the colour of the exhausts, so I was happy to make this change regardless of the leak.

The carburettor floats being reset to 15.5mm.
Fuel pipes visible beneath 

 To cut a needlessly long story short, I reassembled the carbs, installed and tested them for leaks. The leak was still there - but this time, with the tank out of the way, it was clear that the leak was actually from one of the fuel pipes, just above one of the blue plastic T-pieces I had installed. So I replaced it with some double-walled 5/16 fuel hose. That really did fix the leak.

View of the reoiled filter through the left end of the airbox

Carburettors and fuelling

I was confident that my Trophy's misfire was due to an ignition fault. However, there was a chance it could be related to fuelling. My general thought was to do the best I could to get all the engine systems in tip-top condition, from the starter motor through airbox and carbs to spark plugs. The age of the machine (approaching 21 years) probably meant there'd be plenty of muck in the carbs and that the floats would benefit from attention.

The 125BHP 1991 Trophys are fitted with a bank of four Mikuni BST36 flat slide constant velocity carburettors with the serial number 1240010T0301.

• Cylinders one and four both have SE60-3 jet needles and 112.5 mains.
• Cylinders two and three both have SE77-3 jet needles and 117.5 mains.
• Needle jets (0.8) and pilot jets (40.0) are common to all four carbs.

There was evidence of fuel seepage, a golden-brown glaze, on the edges of the float bowls and staining underneath, and of course plenty of grime everywhere in the induction area.

The carbs strip down without much drama, as long as the bank is firmly supported so you can get a good grip on a Philips #2 screwdriver to release the float bowls. I found that corrosion had crept between the float bowl seals and the carb bodies throughout. I cleaned it off with a blunt screwdriver and WD40. The float levels needed minor adjustment only (a millimeter or two). There was a fine rust-brown deposit in all the bowls.

There are two plastic mesh filters fitted into the fuel line spigots as standard. These showed no evidence of contamination. I think the stuff that had collected in the float bowls was too fine to be stopped by the plastic filters. In fact, the petrol tap also incorporates a fine plastic mesh filter - still too coarse to have kept this silt from the carb bodies. An old toothbrush and carb cleaner sorted this out in a thoroughly satisfactory manner.

The carbs are fed by two fuel lines, each being 7mm bore preshaped plastic pipes. Both had hardened over time - quite understandably - making me think it would be unsafe to reuse them. I bought a meter of 7.6mm bore fuel injection hose and some new hose clips to replace them, along with two inline fuel filters with paper elements.

The fuel injection hose is double walled which means it is robust and will resist kinking. The downside is that they won't tolerate tight bends. I allowed about 5cm of extra length on each pipe, plus the fuel filters each being about 5cm long, to accommodate the wider turns the injection hose would need. There is plenty of room around the front and rear sides of the petrol tap. Unfortunately, the same is not true to the rear of the tap where one of the hoses and a vacuum line attach. Judicious juggling and careful routing of the main wiring harness are needed to make it work. More on that later. The additional length of the hoses means that I can uncouple the low fuel sensor wire, release the front of the petrol tank and rotate it 180 degrees to rest on the rear subframe for engine testing and tuning.

That's right: the startup moment is upon us.