Backfire and puff of smoke through the vent line? By skooter (Read 3,235 times)

[skooter]

Ok, next issue.... It starts, it runs, but if im at WOT and then back off and cruise it will sometimes pop.

What can be causing this?

Is it time for a new top end?

13K original miles on 50cc's seems like a lot.

[Alleyoop]

Skooter,
Here is a check list for backfiring on Deceleration:
Alleyoop
Backfiring 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 Some Culprits That Can cause Backfiring on Declaration:

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
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
enriches 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. Richening The CV Jetting May also solve backfiring on

deceleration by re-jetting one size larger on your Main Jet.

[skooter]

Thank you I'll check those things out.

[Bashan]

I've had the backfiring problem twice. One was my header pipe flange was bent and leaking, and the other was the cut valve. I run a clear vac line to my air cut valve and if the little rubber diaphragm in the valve is leaking you'll see gas in the line. The red arrows point to the vac line in this pic. The auto-enricher is removed in this image:

[skooter]

What is a cut valve? I know silly question.

I just ordered a new 3 way fuel valve. I have a feeling the diaphragm is leaking fuel into the carb. So I wanted to install a new 3 way just to be sure.

My stock airbox does have a crack in it where the square part connects to the round tube part. Could this be messing something up?

At 13K miles Im sure all kinda of things start to wear out.

[Alleyoop]

Yes, Tape up that Crack on the air box, any Extra Air coming in will cause a Lean condition. You can use Duct tape that works great.
Alleyoop

[Bashan]

Mar 20, 2011 16:29:14 GMT -6 Alleyoop said:

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 enriches the pilot circuit on deceleration by means of a rubberized piston.

There's two screws that hold that plate that the vacuum line hooks onto. Take those off and take note of the parts that come out. There will be a rubber diaphragm and spring. Inspect the diaphrgm and if it looks brittle and/or cracked it's leaking and causing a lean mixture on decelleration=backfire. I don't even know if you can buy those, it'd probably be simpler to get a new carb. At 13K it could sure be worn out. Alley's correct also, if you have holes in your intake system pre-carb your getting too much air=lean=backfire. Hey, I gotta ask, what vent line does the puff of smoke come out of?! Rich

[skooter]

OK thanks everyone for the helpful info.

The carb is actually brand new I just installed it a week ago.

The puff of smoke is coming out of the crankcase vent tube that runs to the cover over the valves.

I did find that my air box was cracked and I used some silicon to seal it up, with tape for good measure. It was missing a bracket and when being ridden the weight of the air box must have loosened up the crack. it is now secured and no more movement.

I'll go over all of the vacuum lines again to make sure nothing is loose as well.

Another question...

What are stock variator weights?

I am going to order a top end kit this week and thinking I may order new weights as well. I'm not going to a 100cc kit because the engine has high miles. But maybe a 72cc or basic 50cc rebuild kit. Whichever I can find the best (cheapest) deal on.

[Bashan]

Mar 21, 2011 17:14:36 GMT -6 skooter said:

The carb is actually brand new I just installed it a week ago.

Well, there goes that theory...

The puff of smoke is coming out of the crankcase vent tube that runs to the cover over the valves.

You can see that when you're riding? I stick the crankcase breather hose up one nostril to keep it out of the way; plus I get some killer head rushes out of it . You shouldn't be getting a puff of smoke out of that hose. The times I just had it tied off near my leg I could feel a little air come out of it occaisionally but that's it. Does your scoot use a lot of oil? I'd suggest running a compression test and check your rings and valves.

What are stock variator weights?

Look in the tech library on the CVT board, there will be a couple tutorials on how to take your variator off. You'll find these little weights (small cylinder below the yellow dot) that are supposed to move up and down on their ramps:

If they get turned sideways like the red dot, or they get really dirty, they quit moving and your scoot won't "shift" properly and/or it'll vibrate.

Oh wait, you wrote "stock" not stuck! Sorry, I misread that. The weights in that picture are stock meaning they're from the factory, not aftermarket. Rich

[skooter]

I have the crankcase vent tube zip tied upwards near the front of the seat as was suggested by another board member. I guess this lets the oil run back into the crankcase.

I'm gonna check the variator weights and see if they are clean or worn. They are stock and with a dinosaur scooter I have and 13,112.3 miles she's lived a long life.

I'm going to do a top end on the scooter soon as soon as i find a good deal.

I think with that many miles oil will find ways to burn, drip, and leak. It could be blow bye going around the piston rings too...who really knows. But that's quite a few miles.

[runningslow]

Mar 22, 2011 9:53:43 GMT -6 skooter said:

I'm gonna check the variator weights and see if they are clean or worn. They are stock and with a dinosaur scooter I have and 13,112.3 miles she's lived a long life.

Factory weights with 13K miles on them... yeah, those will have flat spots.

[skooter]

What is the factory weight? I want to stay within 2 grams, but get more get up and go.

[Alleyoop]

From the factory they will put anything they have available in them. So you will have to weigh them, now they will probably weigh a little less than they should due to the wear on them so if they are 9gram weights they will probably show as being 8.? so round it up and that should be what gram they where originally.
Alleyoop

[sprocket]

>>>Well, there goes that theory...

Not so fast!

You need to remove the air cut valve front plate and make certain you can blow through it. There is a very tiny hole at the bottom of the barb and they sometimes are not drilled all the way through...

Mine was like that...took months to find...