Q Why is my Bonnie rich on the right?

If we don’t know the an­swer, we’ll find the per­son who does

Motorcycle News (UK) - - GARAGE -

I have a 2009 Tri­umph Bon­neville 865 EFI SE that has de­cided to run rich on the right-hand cylin­der. The bike does have an af­ter­mar­ket ex­haust fit­ted, but ran fine for a week, so I do not think that is part of the prob­lem. I only no­ticed it when I re­moved the Sec­ondary Air In­jec­tion (SAI) system and checked the plugs. It seems to be only at low revs or con­stant throt­tle when it causes the plug to go sooty. The 12,000mile ser­vice is due in a few hun­dred miles. Oddly, the left-hand side runs per­fectly. If it was a prob­lem with the SAI or lambda sen­sor would the en­gine light not come on? Pete King, email


An­swered by Clive Wood, Clive Wood Tri­umph It’s pos­si­ble the lambda sen­sor has failed, but it’s more likely to be be­cause the SAI has been dis­con­nected. SAI has been around the Tri­umph range since 2001 to clean up hy­dro­car­bon emis­sions by bleed­ing air into the ex­haust system. On the Bon­nies it’s a reed valve block un­der the fuel tank that’s con­trolled by a so­le­noid. Most own­ers of carbed Bon­nies re­move it and blank off the pip­ing be­cause it’s an added com­pli­ca­tion, can blue up the ex­haust and there’s no emis­sion test at Mot-time (yet).

But on a fuel-in­jected bike it’s linked to the ECU, which is also get­ting data from the two lambda sen­sors in the pipes with dif­fer­ent modes for low, medium and high revs to work in tan­dem with the cat­alytic con­ver­tor. The sen­sors will ex­pect to see 14.5:1 air/fuel ra­tio as that’s where the cat is at peak ef­fi­ciency, but if the SAI is dis­con­nected it might see 13.5:1 so it tells the ECU the AFR is too rich and leans it out. Take it back to stan­dard first to di­ag­nose the fault.

Dis­con­nect­ing the SAI can cause a Bonnie to run badly

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