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#Post#: 2606--------------------------------------------------
Backfiring at Low Revs
DIR By: krisztian_andre
Date: April 14, 2019, 8:30 am
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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
#Post#: 2625--------------------------------------------------
Re: Backfiring at Low Revs
DIR By: krisztian_andre
Date: May 31, 2019, 2:51 pm
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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?
#Post#: 2630--------------------------------------------------
Re: Backfiring at Low Revs
DIR By: banquo
Date: June 6, 2019, 5:10 am
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--- Quote from: krisztian_andre link ---
>
> 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?
>
--- End Quote ---
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.
#Post#: 2631--------------------------------------------------
Re: Backfiring at Low Revs
DIR By: krisztian_andre
Date: June 6, 2019, 3:55 pm
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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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