backfiring through carb

[pdg]

I have a twin carb long stroke A7. I have fitted repair kits to both AMAL 275 carbs. The right carb is running well, with the spark plug the right colour. The left carb is back firing through the carb and the spark plug is whiter (running lean). I've turned the pilot air screw fully in and moved the needle up one notch to 4 from the top. I can't see any obvious air leaks. Does anyone have any advise what to try next, please.

Many thanks, Peter.

[Triton Thrasher]

Get it idling and spray WD40 at the joints between the carburettor and the head, to betray the air leaks.

What throttle opening causes the backfiring?

[pdg]

Small throttle opening, giving 2000-2500 rpm.

[morris]

Are you running a magneto? Then maybe there's timing difference between left and right cylinder.

[Triton Thrasher]

Small throttle opening, giving 2000-2500 rpm.

Then probably not needle clip position.

More likely air leak or blocked pilot jet.

Can you swap the carburettors over, to see if the problem follows the carburettor?

[trevinoz]

Have you checked the pilot jet?
Total strip down required to do so.

[pdg]

Many thanks for all the replies. When I tickle the carbs I normally wait for the breather hole in the main body (in the threaded section below the pilot screw) to fill with petrol. I've noticed that the lean carb takes a long time (compared with the other side) to fill to this hole compared with the one that's running fine. I was planning on checking the float chamber for any blockages to see if there is something preventing sufficient fuel to get through, and the position of the float needle to see if the float's too low. Does this seem reasonable?

[Triton Thrasher]

There's no doubt that odd float levels on twin carbs will give you tuning problems.

[pdg]

... looks like blocked pilot circuit was the problem 

[a101960]

... looks like blocked pilot circuit was the problem

It is amazing how much trouble a blocked or partially blocked pilot jet can cause. I now work on the basis of having two or three pilot jets. One in service in the carb, and a couple of spares that I keep clean ready to swap at the slightest hint of trouble. I always change the pilot jet after the winter lay up, it saves a lot of grief. In my experience even if the carb is completely drained at the end of the season there is a good chance that when you start the bike up after the winter lay up there is good chance the pilot jet will be blocked or part blocked.
John