News

This 1965 Triumph Bonneville came into the workshop with a charging problem, oil and fuel coming from the breather, and a general check-over before its next WOF.

It was still running, but there were a few separate issues that needed sorting before it could be considered reliable.

Fuel and oil in the engine sump

When the bike was first started, it pushed a mixture of fuel and oil out through the breather. Once it was on the lift, around 1.5 to 2 litres of contaminated fluid was drained from the engine sump.

The likely cause was an earlier carburettor leak allowing fuel to pass through the cylinders and collect in the engine. On an older British motorcycle, this needs to be dealt with properly rather than simply topping the oil back up and carrying on.

The old fuel taps were also leaking and had been fitted with the wrong sealing washers. These were replaced with the correct taps and Dowty-type washers, then tested for leaks.

Finding the charging fault

With the primary cover removed, it was immediately clear that the primary case had no oil in it.

The alternator stator was also touching the rotor. There had been a previous repair to the stator, but the running clearance had not been set correctly. A minimum clearance of around 0.008 inch is required around the rotor. Without that clearance, the stator can rub, overheat and stop charging properly.

The damaged stator was removed and replaced with a new unit.

The clutch plates were inspected while the primary was apart. They were still in good condition, so they were cleaned and refitted rather than replaced unnecessarily.

The primary was then reassembled and filled with 350 ml of ATF.

Previous thread damage

Two threads in the inner primary had already been stripped and only had a loose bite. The primary chain adjuster bolt was also an aftermarket item, apparently fitted because the original case threads had previously been damaged.

The existing adjuster was retained, as replacing it would not have solved the underlying damage in the case.

Tracking down the primary leak

After the first test run, a small primary leak remained.

The case was removed again, and a hairline crack was found around the chain adjuster area of the inner primary. This explained why the leak had continued even after the gasket and cover had been refitted.

The affected area was cleaned and sealed using JB Weld gasket sealant before the case was reinstalled and refilled. This cured the majority of the leak, although it will still need to be watched over time due to the previous damage in the case.

Battery drain and ignition switch fault

A replacement battery was fitted, but it was flat again the following day.

Further investigation found that the ignition switch positions had been set up incorrectly, leaving a circuit live and slowly draining the battery.

The switch positions were corrected and clearly marked for ON and OFF. After this, the battery held charge and the electrical system tested normally.

WOF check-over

The bike was also checked for any obvious WOF issues.

The rear brake light was working, but the tail lamp running light was not. The bulb was removed and found to have a blown filament. A new bulb was fitted, restoring both functions.

The remaining lights and electrics tested correctly, and the brakes were in good working order.

The tachometer was also not reading properly. The cable was lubricated and tested, which brought it back into operation.

Final testing

Once the work was completed, the Bonneville started cleanly with no further fuel or oil being pushed from the breather.

The new alternator was tested at 13.5 volts at 3,000 rpm, with the ammeter showing a charge as expected.

A road test confirmed that the clutch was working properly, gear changes were good, the brakes operated correctly, and the bike was charging as it should.

It still carries a few signs of earlier repairs, particularly around the inner primary, but the main faults have now been found and dealt with. Most importantly, it is back to starting, charging and riding properly.

Work carried out

• Drained fuel-contaminated oil from the engine sump
• Investigated the charging fault
• Removed the primary cover and clutch
• Replaced the damaged alternator stator
• Checked rotor-to-stator clearance
• Cleaned and refitted the clutch plates
• Refilled the primary with 350 ml of ATF
• Replaced leaking fuel taps and sealing washers
• Replaced the tail lamp bulb
• Corrected the ignition switch positions
• Replaced the battery
• Lubricated the tachometer cable
• Sealed the hairline crack around the inner primary chain adjuster area
• Tested charging voltage
• Completed a road test and WOF check-over