Me too, this is probably the answer......

 

Did you mess with the air cutoff valves at all. My understanding is that they deal with a lean condition under deceleration that causes popping. At least that's what they taught me in m/c mechanics school some 25 years ago.

 

It seems counterintuitive but backfiring on deceleration it is usually caused by a lean condition actually. I'd check for an exhaust leak. Cleaning the carbs couldn't hurt, as well as checking the air filter. Is the idle mix screw on the intake side of the carb or the engine side? I can't remember. If it is on the intake side turning it out leans the mixture. Since it just started though I would look at other things besides carb settings first.

 

Thanks guys. Just cleaned the carbs when I took the bike out of storage, but Ill hit them up while I'm poking around in there. I'm just so confused by this. Bike still runs strong, but now stumbles a bit if I crack the throttle open at idle. Any further thoughts?

 

Wrong.

I taught "Tune-up and Emission Control" classes for 34 years. Instruction included 20+ hours of lecture on carburation alone, up until fuel injection took over, and complete instruction on automotive emission control systems in detail, including operation and diagnosis of all the systems.

I've explained this in detail before, but here goes one more time:

1. Decel backfire (exhaust backfire) is caused by a rich condition which develops when the throttle plates close while piston speed is high.

2. This happens when the driver "suddenly" takes his foot off the gas (or releases hand throttle).

3. When the throttle closes suddenly, a very high vacuum (22" Hg to 25"Hg) is created by the fast piston speed.

4. This high vacuum (over 21" Hg) pulls excess fuel through the carburetor's main metering and idle/low speed circuits, creating a "rich exhaust" condition, loaded with hydrocarbons.

5. Upon hitting air containing O2, when the exhaust exits to the atmosphere, the hot, HC rich exhaust "explodes" creating the backfire.

6. An exhaust leak can also "suck" air into the hot exhaust stream, creating backfire on decel.

As early as the 1950's some auto manufacturers controlled this condition by adding various devices to the carburetor or intake system.

GM and Ford, in particular, used a "dash pot" which would slow the closing of the throttle slightly. The dash pot was improved upon, starting in the '60s, by using a "throttle positioner", which offered even more slowing of the closing of the throttle plates.

Japanese manufacturers used a throttle positioner on nearly all of their Aisin, Hitachi, and Kehin automotive carburetors from at least the early 1970's until EFI took over in the mid to late 1980's.

I've worked on thousands of these carburetors, and sometimes one of my students would turn the wrong screw while attempting to increase idle speed. The result would be that after taking the rpm over 2500, the throttle positioner would not allow the throttle to close, keeping the idle up near 2500 rpm. I would then have to locate the throttle positioner adjustment screw (in very crowded conditions on the side of the carb) and back off the adjustment to get idle back to normal. The student usually didn't know which screw they turned. Those Japanese carbs had many "adjustment" screws all over the carb linkage.

There were a number of other devices used to prevent decel backfire, including a vacuum controlled "antibackfire valve" used on pre computer controlled air injection systems (AIR).

One type of antibackfire valve was called a gulp valve. The gulp valve would actually channel air from the "smog pump" directly into the intake manifold to lean out the air/fuel mixture during deceleration. The gulp valve was triggered by vacuum of 22" HG or higher.

When the gulp valve failed, the car would often run so lean that it would not even idle.

An intake backfire (carb fart), on the other hand, is caused by a lean condition (it can also be caused by an ignition or valve timing problem). Just talk to anyone who has worked on a car with a weak or defective "accelerator pump" in the carburetor. If they were leaning over the engine with the air cleaner removed, then snapped the throttle open, they probably got singed eyebrows.

I was ASE certified in Engine Performance for 20 years (1990 through 2010), and Advanced Engine Performance (L1) since ASE started offering the L1 certification in the mid '90's, when OBDII was coming on board. I'm letting my last ASE certification, the L1, expire this June without renewal, since I retired over 2 years ago.

And the idle mixture screw, or pilot screw, on most carbs will go richer as it is turned out. Turning it in (clockwise) will make it go leaner.

There are a few exceptions, such as some Holley carburetors, where turning the mixture screws out (counterclockwise) will lean out the idle mixture. I used to love working on Holley's, they were so technician friendly.

 

I'll buy that. Lol! That makes perfect sense. The only part that still confuses me is the sudden onset of it, which makes me think vacuum leak. But leaning the mix a bit probably isn't a bad place to start.

 

Carburetor Air Cut-Off Valve - Japanese Vintage Motorcycle ...
Carburetor Air Cut Off Valve Kit. Includes Diaphragm, Spring and O-Ring. Helps prevent "after burn" caused by a lean fuel mixture from a malfunctioning air cut-off valve ...
Carburetor Air Cut-Off Valve - Cached

 

Both are correct. However, it seems to me that if you have been riding the bike under the same or similar conditions and this was indeed sudden onset then I would definitely look at the most simple answer, which to me would be an exhaust leak most likely at the exhaust gasket on one or both cylinders. It is far more mechanically intensive to pull apart the carbs and clean them throughly than it is to replace the exhaust gaskets at $5/each.

Cleaning the carbs is a good idea. I just think that tackling the simplest things first is the way to go.

 

That too!

 

Thanks for the info. I'll be checking those gaskets in a day or so. Hopefully that's the answer.

 

Couldn't tell if the exhaust gaskets were leaking, but ordered them anyway. While poking around the bike with it running, I tried to see if I could hear any air leakage. I did hear pulsating air escaping near the airbox. I pulled the top of the box and discovered air coming out of that white box with the 2 hoses on it. It was coming from the hole on the right side. Is that supposed to happen? Could this be my problem? What is that box? Like a PCV valve on a car?

 

How did you solve this, I have similar issue