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Honda Helix Forum installed, March01 2005.
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Author Topic: engine backfire  (Read 8923 times)
« on: April 16, 2005, 03:29:54 PM »

What is solution to engine backfire on deceleration?
« Reply #1 on: April 16, 2005, 09:54:34 PM »

From: Randy Pozzi
 Sent: Tuesday, December 28, 2004 12:28 PM
 Subject: [Honda Helix Discussion] Tech Tip #21 Backfiring On Deceleration

 One 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 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 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 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 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

 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 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. 01/05)

« Reply #2 on: May 21, 2005, 06:13:32 AM »

I cured 100% of my backfiring on deceleration yesterday.....I had a condition described by Randy Pozzi as "baby farts"....not terrible but aggravating nonetheless. I had turned my idle up a while back and that did indeed help, but didn't eliminate all the "sneezing" on deceleration.

A few days ago I had my tires changed out by an oldtimer who owned 2 Helixes and a Silverwing, was 78 years old, and still rode with his wife and worked on them in his garage. He had 2 Dunlop K488 OEM tires that he had not used and were 2 years old in new condition....I verified the age by using Randy's "DOT" number decoding method that is stamped on the tire with the production date. He put them on for me for $140 and I got to see and help in the process.....not a hard job but I didn't want to do it solo the first time out.

I thought everything was fine...and the tires are great...but the backfiring seemed a lot worse on my way home and the bike was a little noisier than before. The rear tire requires the muffler to come off and so I guessed correctly that it was not tight or making a good seal when he put it back on. He didn't have a lift and wrenching while crawling on the floor is not a lot of fun, and I didn't examine the muffler situation other than to shake it to make sure the baffles were still in good shape.

I have a "Handy" table lift at home which lets you sit on a bench, etc, and look right at the rear end at eye level. So I took the muffler back off....4 bolts plus the clamp at the top loosened.One of the bolts had been cross threaded so I re-tapped it and the others to make the bolts go in all 4 holes a lot easier with some added WD40 lubrication.

What I found, aside from the flange at the top where it connects to the head NOT being totally tight against the head, was that the "fibrous" insulating sleeve that is about an inch+ long was deteriorated to almost nothing left....definite leakage going on here. Then I checked for the "crush" washer/gasket condition and at first couldn't find it......I had read that sometimes it is missing altogether. Well, it wasn't....but I had to get a small pick and work it around until I saw the outline of the old washer.....cruched, but still there, and hard to get out.

I went to my local Honda dealer and got the sleeve/insulator and crush washer for $12.....told him they were on eBay for $8.99 + $5 shipping....

Otherwise Honda retail is about $14 for the insulator and 3.50 for the washer...BikeBandit and servicehonda are discounted but I didn't want to wait for delivery since I had it apart and could see right away what  the problem was and fix needed.

I repainted the muffler with high heat black rattle can paint, put it back together, and went out for a test ride.

Wow.......what a difference. Quiet as a mouse and absolutely NO backfiring, baby farting/sneezing noise AT ALL.

 If this had not solved the problem I would have then gone to replace the air cut-off valve which runs about $50+ discounted....but this is the last resort step to fixing the backfiring problem and it turns out I got fixed with all the other steps before this one was needed.

Thanks to Randy Pozzi from the Yahoo board for great tech tips on this problem and other maintenance situations.
« Reply #3 on: May 30, 2005, 05:38:10 AM »

The last time I had a backfiring problem was a few years ago when I got into some contaminated gasoline at a "convenience store".  I ended up have to have the carberator disassembled and cleaned out.  Needless to say I have not been back to that store.  And I've never had any problems since.

Albuquerque NM
'87 Helix (65,000 mi)
« Reply #4 on: July 13, 2006, 04:28:31 PM »

I want to replace the sleeve/insulator and crush washer on my muffler and want to be sure I order the correct parts.  In the picture, I want to order #1 and #8, correct?  Thanks for the help in advance.

« Reply #5 on: July 13, 2006, 10:14:19 PM »

The only piece you need is # 8, the sleeve that slips over the manifold pipe and the muffler opening goes over the sleeve for a tight fit be sure the clamp is tight.
« Reply #6 on: December 18, 2006, 01:26:14 PM »

My 2004 Helix started the coughing mentioned in the string of engine backfire notes.  I let it go until I got around to changing the rear tire.  Upon disassembly, I noticed that the exhaust gasket had deteriorated to nothingness.  Not wanting to make a trip tot he dealer, I cut a one inch wide strip of fiberglass cloth weave and wrapped the exhaust header a couple of times, rubbed a bit of adhesive on the header and the fiberglass as I wrapped just to hold it in place.  Slipped the exhaust/muffler in place, tightened it all down and presto, no more leak and no money spent for a screen gasket from the dealer.
« Reply #7 on: May 06, 2007, 12:49:10 PM »

I just bought a brand new 2007 Helix and I notice this when I go down steep hills, the small "baby farts." So this isn't normal? I should take it back to the dealer and have this adjusted?
« Reply #8 on: May 11, 2007, 01:18:46 PM »

I don't think its normal, mine did not start doing it until about 3 or 4 thousand miles and it was the exhaust gasket; see my post about the fiberglass cloth wrapping.  You should get the dealer to check you ride.  It may be the gasket or one of the other causes.  Xrdaddy
« Reply #9 on: July 09, 2007, 07:03:05 AM »

Thanks for the tips.
I've eliminated almost all of my back firing sounds by checking according to what was written.
What remains is a sneeze when the engine is shut off.
I assumed this was due to the idle being too high so I lowered it as much as I dared without retuning it back to where I started and it still sneezes??? Undecided
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