850 Running Lean

rmullett · Sep 30, 2011

My 74 /850 was running just fine when it suddenly started backfiring for no apparent reason. I checked the valve clearance... okay. New carbs, checked and very clean inside with no apparent blockage. The engine does respond to the air mixtures yet is still running lean and very rough. I have changed the plugs and still no joy. Any suggestions greatly appreciated.
Robin

renorton · Sep 30, 2011

Have you removed fuel taps or petcocks in fuel tank to check for fuel blockage in screen

grandpaul · Sep 30, 2011

If the carbs do not respond to twiddling the low speed mixture screws, they need to soak for 1/2 hour In Berryman's Chem-Dip.

pete.v · Sep 30, 2011

Check for leaks around the manifolds. Loose Nuts and what not.

Rohan · Sep 30, 2011

As well as airleaks, check that your exhaust is not loose or falling off.

Checked your battery voltage ?
Lot of the leccy ignitions don't like low voltage.
If you have points, that the points are clean.
Remember, 90% of carburation problems are electrical.
And 90% of electrical problems are carburation...

hth.

kraakevik · Sep 30, 2011

One obscure problem may be that the carburetor float bowls aren't cinched up properly--I now carry an Allen wrench to tighten my Amal 932's--which need attention every few months

Tim Kraakevik
kraakevik@voyager.net

rpatton · Sep 30, 2011

Maybe you got a hold of a bad batch of gas. Do both plugs look the same?

Torontonian · Sep 30, 2011

Substitution if all else befuddles ie. put the old carbs back on to isolate cause.