Backfiring / Loud Pop on deceleration

S

slav88

New Member
Hi Guys,

I have an 85 honda GB400, single cylinder, single carb. I recently did an overhaul on my carburetor, due to it leaking everywhere and running too big jets (had #165 main jet, and pilot screw (on fuel side) was only screwed out about 1.5 turns). I replaced the main jet with a #152 and it runs great now, the only issue I have is that when I accelerate over about 90kms and then slow down I get a loud backfire. It is always a single backfire, and it is a loud enough to scare the crap out of a lady that was walking on the side of the road. I don't get any backfiring, or popping any other time, just when decelerating from high speeds.

I'm running stock muffler with the baffle in and stock air box, but new OEM air filter. The jet sizing is stock, and stock settings are 2 1/2 screw turns out.

The bike will run OK, anywhere between 2.5 to 3.5 turns out, with 3 seeming to run the best. But I get the pop at 2.5, 3 and 3.5 turns, when the weather clears up I'm going to try quarter turns (2.75, 3.25 & 3.75 turns).

If this fails, I might try go up 1 size on the main jet.

Anyone got any other suggestions? (I've also adjusted the idle screw, so that it idles at a bit higher RPM as I read this might help - yet to test ride the bike after doing this).

Cheers
Slav
 
naw, you're fine. it's a backfire from building up a rich mixture when the throttle is closing but rpm's are still high, it's not an indicator of a lean condition at all, it could actually be caused more by an over rich mixture at closed throttle. old ladies be damned
 
Decel popping is usually a lean condition. Sounds to me like you have your idle air mixture screw too far out. 1 1/2 turns is always my starting point.

Intake or exhaust leaks can also cause it.
 
running a wideband o2 sensor, decel popping from my experience, was never connected with running lean... it's not something happening in the combustion chamber itself, but in the exhaust pipe
 
Overly lean condition when at higher than idle RPM with closed throttle. Mixture is lean, so combustion is incomplete. Unburned fuel/air mixture is passed on to the exhaust where it ignites.

Some models of Mikuni carbs even have a small diaphragm operated axillary fuel valve that feeds extra fuel when there is high vacuum to preven the decel popping.
 
well you talk about the main jet

the carb you have should be a kehin ve10

stock it should have a 45/110 so i wonder why yours are so big??

and what your other jet is ??

also is your cv diaphragm intact with no tears or holes?
 
Make sure you know which direction turning the air screw does what--- one of my bikes turning it in makes it LEANER, on the other turning it in makes it RICHER.

I understand both sides of the argument here, but from my experience popping on decel is usually a lean mixture condition. I say usually... doesn't always mean it. ALSO- "lean mixture" doesn't mean jetting, can also be cracked intake boots, poor filter seal, bad intake mani gasket, or exhaust leak to name a few. Get your bike to idle and spray starting fluid around all areas of the intake... and by spray I usually use the little red straw and get better aim. With starting fluid, revs will increase when the starting fluid gets sucked into hole. Some use carb cleaner, that will decrease revs when it gets into a hole.

Put the air screws at stock setting, try the starting fluid, check exhaust flange bolts/gaskets if applicable, and also for holes in the header. If they're really rusty it can be hard to see them. Then if that doesn't do it, adjust air screws to make pilot circuit richer. Failing all that, time to take carbs apart. Inspect the diaphragms carefully with a flashlight on the other side of the diaphragm in a dark room. Have the carbs cleaned in an ultrasonic cleaner. Very carefully inspect the air screw to make sure it's not damaged where it seats- this will also cause you probs if there is a lip on the taper or missing o-ring. Some clean the jets with fine wire, I prefer to just replace them if they look suspect. Cheap insurance while you're in there and one less thing to wonder if it's an issue. Make sure floats are set right and don't have holes in them.

Sorry I kinda went on a bit of a carb rebuild sidetrack lol. Best of luck.
 
That's weird didn't get a notification for all the replies... anyway thanks for your responses. Iv'e turned the screw in 2 3/4 turns to make it leaner, the loud pop on deceleration seems to have gone away, and the bike feels a little bit more responsive, but now I get random hesitation, seems the most noticeable when I'm cornering or opening the throttle downhill.

I feel as though the pilot screw is at the best setting now, though I might try go a little leaner, I'm thinking the problem may be caused by 1 of the following. A. The plastic choke nut on the carburetor is threaded, and though it sits flush on the carb when screwed in, it doesn't torque shut, i.e. you can keep turning and turning and it doesn't stop. B. I have a rusted tank and an inline filter, the filter is new so no crap clogging it, but I was thinking that maybe this is restricting the amount of fuel flowing into the carb. The bike seems to run better when the tank is topped up, I'm guessing when it's fuller there's more pressure pushing into the top of the fuel tap.

The carb is in perfect condition, no tears on the diaphragm, new gaskets, O rings, jets, float and float valve (even new OEM screws) . I've sprayed starter fluid around the intake manifold, no air leaks.

Hopefully work it out soon :-\
 
runmikeyrun said:
Make sure you know which direction turning the air screw does what--- one of my bikes turning it in makes it LEANER, on the other turning it in makes it RICHER.
Click to expand...

The ubiquity of the "air screw" actually controlling air leads some people to call a fuel adjustment screw in the same position an "air screw" also.

The GS Mikunis, for instance, have a fuel flow screw in the position where air screws are on other carbs. People still call them air screws but they control fuel, not air. The turning of this screw is opposite (out is more fuel, in is less) of an air screw (out is effectively less fuel and in is effectively more fuel)

Mikey's right, you really need to understand what the adjustment does or you'll be chasing your tail.

Also, very small adjustments can have a big impact so turn it back to the point where it was, then go 1/8 of a turn at a time until you get the popping taken care of.

-Deek
 
Regardless of whether the idle mixture screw is an "air" adjustment, or a "fuel" adjustment, trial and error rarely plays a part in how that screw is adjusted.

On a multi-carburetor set up, carbs are first balanced with a manometer. Then each carb's idle mixture screw is adjusted 1/4 turn or so to get highest idle speed. Idle speed should be adjusted down if engine idle gets above normal. Start at 1 1/2 turns out from lightly seated. After roughing the adjustment 1/4 turn at a time, you can then go 1/8 turn at a time to somewhere in the middle of the range where you get best idle.

Both carbs might not be exactly the same, but the adjustment should be between 1 turn and 3 turns. If you are getting an adjustment outside of that range, you may have to go one jet size larger on your pilot jet.

Yes, you are trying to minimize decel popping, but correct tuning is your goal. A little bit of burble in the pipes on decel is completely normal. You never hear it with stock pipes because it is muffled more. Once you have more open exhaust, you are more likely to hear it on a correctly tuned engine. Adjusting the idle mixture screw just slightly to the richer side of the ideal adjustment will often pretty much eliminate the decel popping if you find a little burble in the pipes to be objectionable.
 
It is just a single carb.

So I've tried every setting between 2 and 3.5 turns (any less than 2 and more than 3.5 and it hesitates during idle), and have pretty much come to the conclusion that it starts to hesitate after riding it at high speeds. As soon as I come off the freeway it hesitates at low speeds (up to 60kms).

I get the pop when the pilot screw is more than 3 turns out, and its a very loud bang, loud enough that I'm worried cops will pull me over for it. I have stock pipe, muffler with the baffle.

No idea whats wrong with it.

Anyone got any ideas?
 
I had a KZ650 that popped on decel due to too rich pilot jets.
I've had others that pop on decel due to too lean mains.
 
So I found out stock jet sizes are 45 & 165, put these in and set the pilot screw to 2.5 turns out.

Ran good, but have the same problem when I come off the highway, it starts to hesitate a little bit then goes away.

Definitely no more backfiring.

Should I try go richer or leaner?
 
Hesitation is usually lean, but not always. I was tuning one bike, and it was so rich at idle that it was literally flooding the plugs when you backed off the throttle. It hesitated, because it took half a second for the plugs to dry off and fire.
 
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