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Author Topic: Backfiring at Low Revs  (Read 267 times)

krisztian_andre

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Backfiring at Low Revs
« on: April 14, 2019, 02:30:44 pm »
Hi All,

I bought a second NF until until my old engine is rebuilt.

The new bike startes easily, but it is backfiring heavily at low revs and will stall unless I turn the choke on.

I ultrasonically cleaned the carb and the fuel tap is supplying enough fuel.

Basically is it backfiring at low revs and won't run without choke.

I've searched the internet and it looks like the cause could be anything:
  • Lean condition at low engine speeds (pilot jet problem).
  • Ignition (advance) issues
  • Incorrect valve clearance
  • Burnt valves

I don't know where to start. How do I diagnose this?

Best regards,
Kris

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krisztian_andre

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Re: Backfiring at Low Revs
« Reply #1 on: May 31, 2019, 08:51:12 pm »
Not sure what exactly caused the backfire, but I managed to fix it by:
  • adjusting the points gap,
  • changing the battery,
  • and replacing the spark plug.

I made a timing tool from wood that screws on to the engine and tried to set the dynamic advance using a timing light. The problem is that I measured the point on the flywheel with a continuity tester where the points close and open (AF10), but the timing light shows ignition at points closing(somewhat before AF10) and not at opening.

The valve clearance was 0.3mm exhaust and 0.2mm intake. I left it alone. Will this damage my valvetrain?

banquo

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Re: Backfiring at Low Revs
« Reply #2 on: June 06, 2019, 11:10:29 am »
Hi All,

I bought a second NF until until my old engine is rebuilt.

The new bike startes easily, but it is backfiring heavily at low revs and will stall unless I turn the choke on.

I ultrasonically cleaned the carb and the fuel tap is supplying enough fuel.

Basically is it backfiring at low revs and won't run without choke.

I've searched the internet and it looks like the cause could be anything:
  • Lean condition at low engine speeds (pilot jet problem).
  • Ignition (advance) issues
  • Incorrect valve clearance
  • Burnt valves

I don't know where to start. How do I diagnose this?

Hi Kris,
Sorry; there's not much traffic on here these days, and I missed your original post... :-[

It's important to understand what you mean by backfiring. For me, backfiring is a loud explosion from the exhaust, but it sods to me like what you're describing is actually spitting back through the carburettor?
If so, for me, the most likely is weakness in the idle circuit, although on an unknown bike with unknown history, it's really impossible to diagnose.
However, I've had mine for 17 years now, and have experienced this before. This year, the bike would start easily with the enrichener on (it's not a choke on the Dellorto), and the engine would tick over quite happily, but when I opened the throttle, there was massive spitting back through the carburettor into the air box, so it was unrideable at low throttle openings. However, once past that engine speed, it ran well on full throttle. I stripped the carburettor, removed all the jets, cleaned the float bowl, and blew through all the openings with compressed air, especially the hole where the pilot mixture screw fits. It's been OK since.
In my early days of ownership, it used to spit back when moving from over-run onto throttle. The 'cure' for that was to open the throttle very slowly, and to enrichen the pilot screw setting by half a turn.
Finally, try changing the coil for a known good one (off your other bike?).
I spent a lot of time last year with my carburettor in bits in various muddy fields and at the side of the road, only to discover the coil had failed.
The engine would usually start, but misfired badly. Eventually it started backfiring through the exhaust, and that was what pointed me to the coil.
Regarding your ignition timing, I'd suggest that you set the timing statically for now, and worry about auto advance later.
« Last Edit: June 06, 2019, 08:25:13 pm by banquo »

krisztian_andre

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Re: Backfiring at Low Revs
« Reply #3 on: June 06, 2019, 09:55:25 pm »
Thanks Banquo. I will try a carb swap as soon as I gain access to my other bike. New coil and and leads are ordered.

See the rest in the adrider forum.

Br,
Kris
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