Carburetor problems with '86 Helix

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Carburetor problems with '86 Helix

Postby anny_dragon » Wed Mar 31, 2010 2:36 pm

I have been the proud owner of an 86 Helix for the past 4 years. Recently, the bike has been having some problems, mainly backfiring during deceleration and rough idle/ stalling after start.
The backfiring usually only happens after the bike is warmed up, and mostly during times of more rapid deceleration. The backfiring consists of 1-3 popping sounds.
The rough idle/ stalling happens after the bike has been sitting for more than an hour. The bike will start right up, and warm up fine on the kickstand, but as soon as I center the bike to ride the engine rough idles and sometimes stalls. Even after riding down the street the bike exhibits a rough idle and can take 15 or 20 minutes to warm up completely.
From what I've read the culprits could be the air intake valve and the float adjustment. What I'm wondering is should I just invest in a new carb altogether, since the bike is an 86 and has 20k miles? Or is there something that can cause both of these problems?
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Postby dneilson » Wed Mar 31, 2010 3:48 pm

Here's the Service Manual--maybe some of Troubleshooting will help.
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Postby crazychile » Wed Mar 31, 2010 5:43 pm

So did this just start happening after you started riding it after the Winter? Maybe your carb is gunked with some old gas.

Its not a fix all, but I would start with using some Sea Foam in the tank if you arent already.
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Postby anny_dragon » Wed Mar 31, 2010 6:17 pm

I actually drive my scoot year-round, and got an engine clean with last service so it shouldn't be too gunked..... I saw a carb on ebay for 60 bucks, is my scoot old enough to where I will be needing one soon anyways?
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Postby alexkram » Wed Mar 31, 2010 10:41 pm

I seriously doubt your carb is worn out.

What you have is a lean condition at idle. It could be caused by a dirty pilot jet (extremely common on any carbureted small engine) or a vacuum leak like a crack in the intake boot (also common on an old helix). It starts right up initially because the Helix has an automatic choke which richens the mixture for a few minutes after startup.

The pilot jet is easy to unscrew and check once you have the carb float bowl open, it's the smaller one off to the side of the main jet. Use a can of carb cleaner with the red straw to clean it out (when it's unscrewed). An air compressor helps to blow it out too. Don't get carb cleaner on the rubber parts inside the carb, it's not good for them.

That said, I have a 1987 Honda 200 Fat Cat which had a lean idle condition I could not fix with cleaning. So I bought a brand new chinese carb on ebay for $29 and it fixed the problem. A new pilot jet might have also done it but for $29 it wasn't much more for the whole thing.
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Postby zozman » Sat Apr 03, 2010 11:11 am

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.

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Postby batilzown » Sat Apr 03, 2010 1:32 pm

mine has the same backfire you speak of. And from what i read it seems to be the exhaust gaskets. So i went ahead and ordered some and hopefully have them in a day or two. Anny i will respond if it solves my backfiring problem.
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Postby Alarmnbob » Wed Apr 07, 2010 8:30 am

Most of the time it is a leaking part be it exhaust gasket or intake sleeve.
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Re: Carburetor problems with '86 Helix

Postby inthecheeks » Fri May 24, 2013 11:00 am

I know this is an old thread:

Mine is doing the same thing now, I get more than two or three. I get a loud then a couple of snapping sounds then a pop pop pop pop when I decelerate. Now if I I stark braking quickly and decelerate with the brakes I don't get it. This usually only happens after the bike warms up.

When you changed the gasket did it fix it?
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Re: Carburetor problems with '86 Helix

Postby slonaker » Thu May 30, 2013 12:23 am

inthecheeks wrote:I know this is an old thread:

Mine is doing the same thing now, I get more than two or three. I get a loud then a couple of snapping sounds then a pop pop pop pop when I decelerate. Now if I I stark braking quickly and decelerate with the brakes I don't get it. This usually only happens after the bike warms up.

When you changed the gasket did it fix it?

I doubt you will get an answer. The original poster and the one who said he/she ordered a gasket each just posted a couple of times. I don't even remember either of the names, so they haven't been around in a while. That said, when my Dad had the popping on his Elite 250, a gasket fixed the problem.

Take a look at zozman's response. He knows what he is talking about.

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