So a few weeks back I did the Dynojet 2152 "upgrade" on my 09 250S with a Yoshi exhaust, pollution stuff removed (all else was stock). I am the second owner and the first owner was not too clear on what the "mechanic" had done to the carburetor when pollution stuff was removed. When I took carb apart, it had stock pilot and main jets, and the factory needle (non-adjustable) with 1 washer under the needle head. The fuel screw access cap had already removed and the adjustment was 2.5 turns out. It got about 65mpg with dirt tires on the street (doing stop and go riding). I live in the Los Angeles area so I'm nominally at sea level.

I follow the DJ stage 2 suggestions of DJ128 main, new spring (with kit), needle on 3rd notch with 2 washers on top, 2.5 turns out on fuel screw, Uni air filter and KDX snorkel.

The bike run great. Its responsive, little popping on throttle release, cruises nicely. That's the good news. The bad news is the fuel economy is terrible. How terrible is terrible, how's 30-35mpg. I was prepared for a reduction in fuel economy but nothing like this and given that the bike runs well, I'm not sure where to even look. I had not thought to look at the float as I did no adjusting, but I hope to look tonight to look for the possibility of fuel dumping out due to an over-full float as I ride. I can say that there is no fuel on the garage floor after I ride so its not so bad that its just pouring out at rest.

I've read as many threads as I could find on this topic and have seen anything that might be a smoking gun. Closest I came was the suggestion that removing the snorkel entirely made someone's economy jump by 10mpg (kind of surprising but possible I guess). I'm not greatly experienced but I have done a few carbs on a few atv's and have never seen anything like this.

So, if you have some (any) suggestions about what I might look for or at for a resolution to this problem, don't be shy. As I mentioned earlier, I don't know where to look. Thanks

 

Poping on decel usually means exhaust leak at header, or running a little on the lean side. Just sayin, but I could be wrong.

 

A little update...I check the float bowl overflow and it was dry both while running and after I shut down. I also checked all of the vent tubes and only one had a hint of fuel in it. It was the one on the left side of the carb that originates just under the diaphragm flange. I need to check and see what that was.

Also I checked the exhaust at the head and it was tight.

Still no smoking gun for the poor fuel economy. I'm starting to think the carb is going to have to come off this weekend for a look-see. Just not sure what to look for.

 

If your exhaust (at head and slip joint at midpipe) is tight, then your decel pops are typical.. The cause is (stock #35 pilot jet + 2.5 turns out + sea level)..
Back out the fuel screw to 3.5 (out), reset idle speed to 1500 RPM and you'll see a big reduction in the POP's.. Replace the pilot jet with the #40 to eliminate all popping.

Before pulling off the carb, use the clear tube method to check the running fuel level in the bowl. You put a clear tube on the carb drain, hold it up next to the carb bowl, open the drain, view the fuel level in the tube - instructions are on a post on this thread - https://www.kawasakiforums.com/forum...s-fixes-43146/

I don't have a clue whether your mileage reduction is due to riding style - now that you have more power - or a fault caused by your carb wrenching session - usually a "borked" carb session results in driveability issues - the above thread will give you stuff to think about..

 

Thanks klxster. The only oddity is that the idle speed varies some between 1500 and 2000 rpm. The no real reason, it's not with temp or warm up. The other oddity is that when it gets near empty, the idle speed goes up to about 2500 for about 1 minute. Sadly that has become my indicator of needing to hit the gas station or flipping to reserve if I'm out away from a fuel stop. I have an IMS tank so I have some range but it's just been a shock that mileage has dropped as much as it has.

I probably will pull the carb anyway to check my work and make the changes you suggest for the popping. I'd really like to get up to 50 mpg again.

 

To the OP, deal with your jetting, ignore the popping under deceleration. Read on to learn why you can pretty much ignore it.

If the exhaust is properly sealed, some popping on decel is normal. Closed throttle creates a condition that doesn't fire properly and can cause popping. It is totally normal. Jetting richer can cut it back some, but now the idle mix is overly rich to fix a non-problem.

There are problems if there are really loud explosive bangs from the exhaust. That's a timing issue and totally not what most are talking about when they mention "backfire" on deceleration.

Here is a bit on the popping under deceleration to maybe get riders to stop "chasing their tails" once they know the exhaust is sealed:

From Mikuni, the carb company, Backfires in Exhaust (link to the site for the following quote):

"It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

Why This (normally) Happens:

1. When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2. The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3. Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4. When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires."

All Things Moto - Deceleration Popping Explained: (link to the site for the following quote):

In reply to:
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Poster: FFRacing79
Subject: Re: DECEL POPPING?

After spending the day on the dyno Saturday, my 450 popped and shot flames out 4 inches after 11,000 rpm runs. The O2 sensor indicated we were a tad rich on the pilot and needle. I attributed it to unburned gases in the exhaust igniting on deceleration and jetting wont help that. Most times a lean condition WILL be indicated by popping, but not always. TW

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This is exactly correct! If you think about the mechanics of what is going on upon decel, it makes perfect sense:
You start out with an engine that is turning high RPM with a carb that is open, flowing freely. Next, you close that slide. This has the effect of creating a tremendous vaccuum in the intake manifold (CHOKE!). This vaccuum does curious things:
1. It pulls air and fuel from wherever it can--including the idle and low speed jetting in the carb.
2. To counteract the vaccuum, air will take the path of least resistance--including sucking exhaust gases back into the combustion chamber during valve overlap (the brief period of time that the exhaust and intake valves are open at the same time.

So what does this mean? Well, now you have a disproportionate rich mixture heading for the cylinder AND what little oxygen is drawn in is further diluted by gases that are coming back into the chamber from the exhaust valve. The result is poor, incomplete combustion! Depending on the temperature of the exhaust pipe and gases, ignition is possible once this mixture is introduced into a more oxygen rich environment. This produces the "pop."

Why does a lean running engine have a tendency to excessively pop? The exhaust gas temperature of a lean engine is substantially higher than one that is running on the rich side ( an EGT probe will show this). Therefore, the high EGT serves to heat the pipe more and the gases themselves become a much better source of re-ignition. By richening things up, you are cooling down the exhaust temp and therefore reducing potential re-ignition sources.

Any 4 stroke engine--even those that are flawlessly tuned will pop on decel. It is harmless, but can be annoying--granted. However, I would never alter the jetting of an otherwise performing motorcycle to get rid of it. Go by the seat of your pants and the color of your spark plug rather than the popping as an indication of the jetting of your scoot!

Seems it isn't uncommon to have popping under deceleration on a well tuned bike. After all, consider the conundrum of trying to jet a bike to run properly at 2000 rpm with the throttle opening up and fully closed, much less trying to get it right at 6000-10,000 rpm with the throttle open and with the throttle shut! If you totally eliminate all of the popping you are most likely fouling up the jetting you are changing.

Here's a bit from Power Commander, which is EFI, but some of the concept does relate:


They can play with the fuel mix in separate conditions that cannot be done with carburetors, kind of proving you can't jet to suit both open and full closed throttle, otherwise they wouldn't have separate mapping for EFI.

Point - popping under decel on a properly jetted engine is normal. Fattening up the idle jet and mix is just wasting fuel, the OP's problem.

If you don't want it to pop, slightly crack the throttle or pull in the clutch and it will stop. It is not a problem. An irritation, maybe, but not a problem. You don't notice it on the stock exhaust because of that huge well baffled muffler, that is part of the task of the muffler these days - baffle the light popping on decel.

 

That's it ! Don't be annoyed or embarrassed by decel pops - be proud of them!


The pilot system has no impact on MPG - it only provides fueling for idle, decel, and rare instances of near idle operations at high speeds.

 

I'm not concerned about the popping since it improved significantly with the DYNOJET setup. I am concerned about the loss of fuel economy with the DJ setup. Thanks for all the info on popping and I will look at the changes suggested for further improvement wrt popping since I'll have the carb off anyway. Still don't have a smoking gun. I don't think it's my riding style too much. Thanks

 

You may have made a mistake collecting the data. Nothing in anyone's experience here would lead us to be able to understand why your mileage would be going down to that degree. Under 40 miles per gallon is just too low for a KLX under almost any circumstance.

I would rerun the gas mileage experiment. Start with a full talk and go about 100 miles. If you just go 20 miles how full the tank was when you start and refill will have much more ability to screw up the test.

One thing that is very true for me is that the more time my bike spends upside down the worse the fuel mileage. Lots of gas leaks out from the carb when it's laid down.

In all seriousness, test again because this seems a lot like a sample error to me.

 

You may be old enough to remember when the Harleys were mainly kick start, maybe really young at the time, but might remember. They had a timing retard using the left grip, like a throttle. Back it off to retard. Some riders would back off the timing under decel because it would pop a little more... they got a kick out of doing it.

I don't really care if my bikes pop, but I did find if I barely cracked the throttle the popping would stop.

Either way, it is the acceleration that is the important part, so jet for that, right?