Hi all
Not had this bike long, but it backfires quite badly on the overrun. I know that you're all thinking "exhaust leak" , but this was my first thought too.
I've had the exhaust off, modified the brackets a little to make it fit better, then used exhaust paste on the manifold and Silencer joints. That helped, but it's still doing it a fair bit. For the record:
- Ignition timing spot on
- No carb leaks
- tappets set correctly
Obviously I will go over the above again to recheck and, if necessary reseal, but is there anything else you can think of that I should check?
Yep, check the head to exhaust connection again. Silicone sealant is good for this.
Is the carb slide worn?
My ZB32 used to backfire quite badly on the overrun. It has an alloy head and I think the exhaust port that the pipe slides into was quite worn. Silcone sealant helped but I had a while earlier this year when I had trouble with the magneto and had to remove the exhaust pipe frequently and the backfires were really bad after refitting it.
I used a pipe expander on the end of the pipe where it goes into the head. It needed very little expansion to take up the gap. I was concerned that chrome would flake off, but the expansion was so little and only at the very end of the pipe so the chrome wasn't affected. The result though was no more backfires.
Thanks folks for the feedback. I'll check everything again and reseal the pipe.
However, I've now noticed that it's popping back through the carb too. Something is amiss...
I'll whip the carb off
if there is something coming back through the carb , then the inlet valve is leaking - either valve adjustment too tight or needs a regrind - try not to think about valve timing
Had both backfiring on the overrun,and spitting back through the carb on my B31.
The backfiring was (largely) cured by exhaust putty on the pipe/head joint,and the spitting back was too advanced ignition timing.
Setting the tickover so that the engine stops with the throttle closed fixed the backfiring as there is no fuel going into the exhaust.
Just now need to maintain the tickover with a touch of throttle.
Update.
The exhaust was again a little loose in the manifold. Following the advice of Derek996 above, I used my 3lb 'modifier' tool and a wedge to open up the pipe a bit. It's now a very snug fit.
The carb needle was too high (clip on the lowest notch) and the airscrew too far out. I think that the carb is somewhat worn, but it's now running ok. 20 miles test ride and the popping and banging has now gone, but it does 'burble' on the overrun quite a lot. Did about 8-10 miles at the end, almost all on low throttle so that I could check the plug colour on the pilot jet. It was spot on.
Happy with that 😊
Good to hear that you've got it fixed. 👍