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After market fuel mixture screws
Been having issues with popping when decelerating on my 250s. I’m noticing my exhaust is getting hotter than usual. I’m reading this is because the fuel mixture screw is set too rich? Anyhow, I want to adjust the screw in a bit. I’m looking into an aftermarket fuel mixture screw i can adjust without removing the carb but I can’t seem to find any besides some generic ones on eBay and the $20 + $16 shipping* mixture screw sold over at kouba link. Can’t justify the cost for that one. anyone got any recommendations? I don’t want to have to pull the carb out again if I adjust the screw too lean. Would those generic eBay screws effect the performance of the carb negatively? also, will an after market mixture screw made for a kx250 work on the klx250? thanks |
I wouldn't make it any leaner, they run really lean stock.
I don't know anything about the CV... other than one thing. |
Okay, I'll catch the usual crap, but that popping on deceleration is normal. Mikuni America, a company that, along with Kiehin, pretty much made about 90% of the carburetors on motorcycles, says properly jetted carbs will still pop. Look for yourself - Mikuni Tuning Backfire in exhaust They tell you what and why.
You can reduce the popping a bit a couple of ways, two will costs you money, the other doesn't. You can richen the idle circuit (pilot jet and idle mix needle adjustment) beyond what is needed. You might get less popping if you block off the air suction valve (reed valve setup above the exhaust port) by plugging it or installing a block off plate. Last and no cost you can slightly crack the throttle and the popping stops while you slow down, it lets the engine draw a bit more fuel. One question - How can you notice your exhaust is running hotter? Exhaust gasses are around 1200 degrees F, the header will run around 700 degree or so depending on the air flow around it. . It would have to be a dramatic change to be noticeble. I'm thinking most people just become more aware of the heat radiation, with little to no increase in actual temperature. I will say if you let your bike sit for a bit of time idling your exhaust can glow red. Left for a long time it can glow bright red/orange, but within seconds after starting to move, the glow is gone - air flow.. That is the way it is with the single wall exhaust systems. It is the reason you see a lot of cruisers with blued pipes, that's what heat does to chrome. The KLX does have one thing that can cause a hotter exhaust header, that suction system they use to introduce air to the exhaust to burn unburned fuel - pollution. Blocking/removing the system may cool the exhaust a bit. Interesting thing though, the racers who wrap their exhaust with wrap or have them ceramic coated don't do it just about heat/around the engine, they do it to keep exhaust gas speed up to help in exhaust scavenging - aka better flow. The higher temperature keeps the speed of the exhaust flow higher - faster flow - for better exhaust scavenging. Seems that heat is your performance friend. If you really want to cool the radiation heat from your exhaust, get the head pipe ceramic coated. Some headers for cars and bikes are coated. Best known is Jet Hot. Here's a bit of information on coating and exhaust. I had my 650 header Jet Hot coated and the radiated heat dropped dramatically. It also requires no maintenance, no rust or corrosion. . |
Almost all of my bikes that have been tuned for power (some on the dyno) popped on decel
Assuming you dont have vacuum or intake leaks just set the idle circuit using the RPM drop method. Copied this from a motocross forum 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, reignition 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! |
To add to the above, the pilot circuit has very minor fuel input above half throttle and being a little rich or lean on the idle circuit would not be the cause of an overly hot exhaust. It's just not enough to make a difference.
Adjust the idle circuit to have the best off-throttle snap and best no-throttle idle. Don't adjust it for decel popping. As some here advocate, it can be done...sure...but as Mark RageHard note, it's not the right way. |
Originally Posted by klx678
(Post 542594)
Okay, I'll catch the usual crap, but that popping on deceleration is normal. Mikuni America, a company that, along with Kiehin, pretty much made about 90% of the carburetors on motorcycles, says properly jetted carbs will still pop. Look for yourself - Mikuni Tuning Backfire in exhaust They tell you what and why.
You can reduce the popping a bit a couple of ways, two will costs you money, the other doesn't. You can richen the idle circuit (pilot jet and idle mix needle adjustment) beyond what is needed. You might get less popping if you block off the air suction valve (reed valve setup above the exhaust port) by plugging it or installing a block off plate. Last and no cost you can slightly crack the throttle and the popping stops while you slow down, it lets the engine draw a bit more fuel. One question - How can you notice your exhaust is running hotter? Exhaust gasses are around 1200 degrees F, the header will run around 700 degree or so depending on the air flow around it. . It would have to be a dramatic change to be noticeble. I'm thinking most people just become more aware of the heat radiation, with little to no increase in actual temperature. I will say if you let your bike sit for a bit of time idling your exhaust can glow red. Left for a long time it can glow bright red/orange, but within seconds after starting to move, the glow is gone - air flow.. That is the way it is with the single wall exhaust systems. It is the reason you see a lot of cruisers with blued pipes, that's what heat does to chrome. The KLX does have one thing that can cause a hotter exhaust header, that suction system they use to introduce air to the exhaust to burn unburned fuel - pollution. Blocking/removing the system may cool the exhaust a bit. Interesting thing though, the racers who wrap their exhaust with wrap or have them ceramic coated don't do it just about heat/around the engine, they do it to keep exhaust gas speed up to help in exhaust scavenging - aka better flow. The higher temperature keeps the speed of the exhaust flow higher - faster flow - for better exhaust scavenging. Seems that heat is your performance friend. If you really want to cool the radiation heat from your exhaust, get the head pipe ceramic coated. Some headers for cars and bikes are coated. Best known is Jet Hot. Here's a bit of information on coating and exhaust. I had my 650 header Jet Hot coated and the radiated heat dropped dramatically. It also requires no maintenance, no rust or corrosion. . and as for the headers getting hotter .... this is something Ive become aware of when the popping showed up. Maybe its me trying to diagnose the popping w very little knowledge. |
Don, I hope you've enjoyed reading the responses to your problem and your questions - they are the usual "first responses" a member will get concerning decel popping..
Now, lets give you the facts and knowledge you need: Decel is fueled by the pilot system - whose other function is to fuel your engines' idle. There is one other instance where the pilot system plays a role in fueling your bike - whenever you "jab" on the throttle, the pilot system is responsible for providing a little "shot" of fuel to get your engine "cooking" ( a simulated accelerator pump shot that has nothing to do with the fuel screw - only the transfer ports of the pilot system )... As such, decel fueling has nothing to do with "jetting", "properly jetted", "changing jetting", "spark plug color", etc etc etc etc etc etc etc etc.... To reduce or eliminate decel popping, get better throttle response, and improve low RPM power ramping, you must richen the pilot system ( on the KLX). 1. Remove (and cap off) the air injection system. 2. Insure that all exhaust joints are fully sealed and not leaky. 3. Set the fuel screw to 3.5 turns out . Depending on your altitude, you've just eliminated all popping or (at sea level) reduced it significantly. To fully rid any remaining pops, install the #40 pilot jet and put the fuel screw to 1.75 - 2 turns out.. No need for aftermarket fuel screws, etc etc.. You set up the stock fuel screw correctly - and never touch it again.. Remember, all the fuel screw does is fuel idle and decel - nothing else - thinking it does is ignorance and/or delusion.. |
Oh, ok Don.. Your pilot system seems to be failing you... Time to learn the CVK, tear it down and clean it..
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Like I said, the usual crap from people who think they know more than the companies that make the carburetors or motorcycles.
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You know me, I always gotta throw facts at all the fiction..
It's not yall's fault that cleverly written articles fool ya - they fool most.. Properly jetted carbs, dyno tuned carbs, etc etc, can still pop on decel - that is a fact. And this is because shoddy/ignorant tuners don't include the fuel screw / pilot system in their "tuning efforts".. Properly jetted CVK's can decel pop - but none of them have to.. |
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