of the most asked questions pertaing to the Honda Elite CH 150/250 and CN 250
Helix involves a backfire through the exhaust on sudden deceleration.
An engine is not supposed to backfire and, when it is persistent, It could be
harmful to the engine. If there is an explosion in the exhaust pipe when the
cylinder is trying to exhaust its spent combustion products, a back pressure is
created which interferes with the next cycle. This could result in incomplete
extraction, irregular charging with the fresh mixture,
and overheating. In addition, valves are not designed to seal against pressure
from their backsides and a broken valve rattling around in the combustion
chamber is a real possibility. While the explosion sound ranges from a baby fart
to a gunshot, its annoying to say the least. In almost all the situations
studied various causes and remedies have been found with none answering the
In general, backfiring on deceleration (as opposed to acceleration) is generally
caused by a lean condition in the pilot circuit. What happens is that the
mixture leans out enough to where is fails to ignite consistently. This, in turn
allows some unburned fuel to get into the exhaust pipes. Then, when the engine
does fire, these unburned gasses are ignited in the exhaust pipe, causing the
backfire. Then, the classic diagnosis is too lean a fuel mixture. The real
mystery is where that lean condition is coming from.
Here are the suspects:
1. Low Idle. Some folks cure backfiring by turning up the idle slightly.
This is the screw with the spring on it in the right front of the CV carburetor.
Recommended rpm is between 1100-1500. Many scooters never run at the low end of
the recommended idle rpm range anyway. Turn up the idle first.
2. Exhaust Gasket. This seems to be one of the most frequent causes of
backfiring problems and an item that has been seen to fail quite often. Air is
sucked back into the combustion chamber causing a lean condition at ignition and
the backfire. The exhaust gasket is a small metal rimmed fiber edged part that
people seem to forget to inspect when servicing the exhaust or leave it out
3. Leaking Carburetor Intake Boot. The CV carburetor is fastened to the intake
with a stiff rubber intake boot. Sometimes it is not sealed properly or gets a
small crack in it enough to allow excess air into the intake. The extra air from
the split boot will cause the popping. To check your intake rubber, you can
spray WD-40 in the suspected area while the engine is running. Any decrease in
engine revs confirms a leak. Another method is
to use an unlit propane torch, just barely cracked open, and see if the idle
changes when the gas gets sucked in. Don't get carried away or you will flame
the scoot! Less messy than using WD-40.
4. Leaking Vacuum Hose. Any misconnected or leaking carb vacuum hoses can also
encourage backfiring. That means the carb fuel mixture is also slightly leaner,
more prone to backfiring. Check for cracked or stiff bending hoses and replace
if necessary. Putting the hoses back correctly may help with the backfiring
without having to adjust the idle screw or pilot screw. The misconnected hoses
can also degrade low end response slightly.
5. Pilot Screw. On the back of the Keihin Constant Velocity (CV) carburetor is
the pilot adjustment screw also known as the enrichment circuit adjuster. On the
CH 150, it is covered with a plug which has to be removed. On the 250 cc motors,
this screw should have a limiter cap on it to vary the adjustment. For a rich
carburetor adjustment, turn the adjustment screw clockwise (in toward the carb
body) a tad to eliminate a lean running
condition. All adjustments should be made with a warm engine. See Tech Tip #7
Pilot Screw Adjustment & Fix for a more detailed service).
6. Air Cut-off Valve. This component is the prime suspect. On the back of the CV
carb is the air cut-off valve. The air cut off valve enrichens the pilot circuit
on deceleration by means of a rubberized piston. One hose to the valve (nearest
air filter) is the air tube connector and the other (nearest the autobystarter)
is the vacuum connector port. If air pushed through the cut-off valve by a
pressure pump leaks by the vacuum port connector while vacuum is initiated with
a vacuum pump, replace the air valve. I replaced a faulty air cut off piston and
my backfiring ceased. The air cut-off valve is expensive (about $70) and should
be replaced after all other suspects tested have failed.
7. Enrichening The CV Jetting. I solved a similar deceleration backfire on a
CH250 by removing the 112 (lean) main jet on a CH 250 and using a 115 or 118.
The CN250, which uses a leaner 110 main jet, could also benefit from the main
Randy Pozzi (Rev. 01/2005)