Running on two cylinders

When I started my Trophy 12 for its MOT, I was disappointed to find that she dropped onto two cylinders after a couple of minutes. She was running beautifully the night before. Lovely even beat. What could have happened? I had my appointment booked though and thought it might be some muck in the carbs or maybe something that would clear as the engine warmed up. 

Not so. Warming up didn't make any difference. Maybe one of my coils had failed. Each coil controls a pair of cylinders so it was possible. However, the two cylinders that were not running were numbers 3 and 4. The pipes on these cylinders were cold. One coil runs cylinders 1 and 4, the other runs cylinders 2 and 3: it could not be a coil/ignition problem. So attention turned to the fuelling side of the equation.
 
I had thought that the two fuel lines were to ensure an adequate feed of fuel at high speed, and that they fed a common gallery for all four carburettors. This was an assumption. Looking at the carbs, I now see that one fuel pipe feeds cylinders 1 and 2, the other feeds cylinders 3 and 4.

To cut a long story short, I found that I had pinched one of the two fuel lines that runs from the fuel tap to the carbs. The previous night, I had not bolted the fuel tank in place. Of course, I wouldn't take to the streets with the fuel tank insecure so I had fastened it down the following morning.

To remedy the problem, I took a close look at the routing of the fuel lines. One of them pushes on to the rear of the tap, the other pushes on from the right-hand side. I had used much thicker fuel lines to replace the hardened originals. This meant that they need more room to physically fit in tight spaces and require bends with a wider radius. I had inadvertently routed the bike's wiring loom along the main frame tube exactly where the rear fuel line would need to turn when the tank is fastened. So I solved the problem by pulling the wiring loom further around the frame tube and securing it with an additional cable tie. That gave the fuel line sufficient clearance to bend around the frame tube without too much difficulty.

The result: all working beautifully. The engine's creamy smooth thrust was restored, allowing me to start enjoying the bike in the next few days. It was still very much in shake-down mode for fault finding and rectification. A great feeling.

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.