Popping on decel

htown16 · Mar 29, 2020

When I close the throttle I'm getting a popping from left hand exhaust.
Less than 200 miles on bike on new rebuild.
Timing checked (ei so both cylinders the same)
valve lash checked
fuel level in carbs checked
pilot jet cleaned with #78 drill bit
air screw 1.5 turns out

kommando · Mar 29, 2020

1.5 turns out is only a starting point, tune for idle mixture on a warm engine, sounds like you are weak on the left pot.

http://www.jba.bc.ca/Bushmans Carb Tuning.html

The pilot jet air screw may be adjusted at this time, but by very little, no more than ½ to 3/4 turn in either direction. The effective range of adjustment for the pilot jet is 1 turn to 2 turns. This is where a bit of skill & experience comes into play. By turning one screw a little at a time with the engine running (on both cylinders) you can gauge any difference in running and find the sweet spot. When you hear the engine revs race, you want to turn it back a 1/8 to ¼ turn - the sweet spot is not at the extreme. The idle may need to be dropped equally again.

ELLIS · Mar 29, 2020

Sounds like you have an air leak on that side of the exhaust. I assume you have tightened up the rose nuts after the system has gone cold.

ELLIS

maylar · Mar 29, 2020

I'll second the loose exhaust pipe idea. In my experience clogged up pilots show up as low throttle misfire and erratic idle.

CanukNortonNut · Mar 29, 2020

Air leak at carbs. Check the tube between the carbs at the manifold or loose nut/stud.

gortnipper · Mar 29, 2020

My guess would be manifold to head seal. Test with carb cleaner while running.

2nd guess is headers need snugging up. Even if the rose nuts are tight, there still may be a leak there if your crush washers dont seal properly.

Torontonian · Mar 29, 2020

Air leak. Check crossover balance tube/replace . Tighten up manifold screws again. Replace idle screw "o" rings. Check exhaust tightness.

maylar · Mar 30, 2020

Check the muffler clamp too.

baz · Mar 30, 2020

maylar said:
Check the muffler clamp too.

I don't use silencer clamps on my Norton
Work's fine with out them but my pea shooters are a snug fit

EstuaryBoy · Mar 30, 2020

IF you have a balance pipe, don't forget to loosen it off before tightening the exhaust roses (and obviously re-tighten afterwards) but do it EVERY time you interfere with the exhaust/head joint.

htown16 · Mar 30, 2020

Turning each air screw in half a turn has pretty much eliminated the popping. Still can't get it to hold an idle. Baby steps.

Steves · Mar 30, 2020

So just to be sure, the pilot screw adjusts the air in the pilot circuit. The idle screw adjusts the height of the slide. (Stuck in the house and had a few drinks). Set the slides so you are idling at about 12-1400 rpm, tweak the pilot screws for best rpm, turn the idle down and repeat. It will idle down to about 700 rpm (not necessarily a good thing!!)
I do the slide sync by hand/eye with the carbs off the cylinder head, from first movement to wide open. Once all set check with a manometer. Once you get it dialled I. It's like the difference between an out of tune and in tune guitar.

Edit: agree popping is air leak in exhaust side or lean pilot circuit.

Popping on decel

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