Tech Tip #21 Backfiring On Deceleration
One of the most asked questions pertaining 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 question entirely.
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 gases 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 altogether.
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 mis-connected or leaking carburetor vacuum hoses can also encourage backfiring. That means the carburetor 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 mis-connected 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. The limiter cap is designed to allow counter-clockwise movement (lean mixture). Removal of the limiter cap will allow clockwise movement (rich mixture). 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 jet enrichment.
Randy Pozzi (Rev. 01/2005)
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