KTM cough

[Poppagofast]

I have a 2006 ktm 250SX-F starts cold very well with choke on, once choke is off bike coughs at first crack of throttle, if I SLOWLY rev it she will pick up and go, but any semi quick crack it will cough and die...which SUCKS in a turn.

Any suggestions would be greatly appreciated ! 

 

[MXT]

Have you tried blipping the throttle while the choke is on? You're not necessarily supposed to, but that'll tell you which way you need to go on the pilot jet.

You're saying it's stalling out in turns? I believe that bike will be running an FCR-MX carburetor - KTM's are notorious for not having a leak jet in the bowls, which causes the accelerator pump to actually flood the engine out - if your rear tire is spinning with the bike it actually serves to make your low end response better but will stall out on the bench.

[Poppagofast]

Thanks for reply, if I leave the choke on, throttle response is crisp 

[MXT]

Okay, interesting.

That narrows it down to the bike needing to be richer on the low end; however, this is most likely either of two issues: 1. your accelerator pump is not functioning, or 2. you are too lean on the pilot jet.

To make the bike stumble and stall from lean bog is something that a lean pilot jet/setting wouldn't necessarily cause very easily, unless you were running ridiculously lean. That then leads me to think that potentially your accelerator pump is not functioning: can you remove your air boot and look into the intake part of your carburetor? When you do so, go ahead and open the throttle on the bike and check to see if the accelerator pump shoots fuel into the intake, the accelerator pump nozzle is on the air filter side of the slide.

What's your current pilot jet and what is your fuel screw currently set at?

[Poppagofast]

Please forgive me for not knowing the info on pilot jet or fuel screw setting...just purchased the bike...not very familiar with it yet! just getting back into riding after a 30+ year hiatus, I will definitely pull the air boot and take a look for fuel, my friend did the top

end rebuild on the bike last year, I'm sure he didn't change/mod carburetion so I will ask if he has info 

[MXT]

What kind of fuel did you leave in the bike over winter? Ethanol-free or ethanol fuel? Did you run a stabilizer?

[Poppagofast]

I'm sure he didn't put any stabilizer in the fuel I live on the Gulf Coast of Alabama, we ride pretty much year round, normally run premium pump gas with ethanol 

[MXT]

Okay. Once again I have the feeling that the accelerator pump is not functioning correctly, although if you're able to see it function then we'll have to figure out what else it could be.

[Poppagofast]

I will research that tomorrow for sure, thank you for getting me this far with the problem, I'll check back after I pull the boot and see what is happening ! 

Thanks again 

[Poppagofast]

MXT are you still there? 

[MXT]

Yep I'm here!

[Poppagofast]

Couldn't stand it, pulled the air filter and can see fuel is pumping when I twist the throttle 

[Poppagofast]

Couldn't stand it, pulled the air filter and can see fuel is pumping when I twist the throttle 

[MXT]

Okay. Next step is going to be to clean out your pilot jet and main jet, so go ahead and pull them out and clean them with carb cleaner.

Make sure you don't get carb cleaner on anything else in the carburetor! It'll swell the rubber parts and ruin your seals!

Another thing to do is to pull out your fuel screw, make sure you check how many turns it it goes before you back it out, so that we have a baseline on your fuel screw. Does it have a spring, an O-ring, and a washer? The o-ring and washer may likely stay in the carburetor but the spring should easily fall out.

[Poppagofast]

Okay, that's gonna be tomorrow when I'm fresh ; )

Thanks for all your help!

[FORMA MOTO]

If it idles, it's not the pilot jet

If it reacts positively to slow throttle, it's not jetting (more on this)*

If it reacts negatively to quick throttle, it's not jetting, its the apump system that has failed.

If you motor is stock, use the stock apump components from KTM.

If it is modified at all, use the Honda version, as it is more agressive and works better with pipes/cams/porting. This requires the linkage spring upgrade, which should be done regardless.

http://tokyomods.com...arburetion.html

  http://www.crfsonly....roducts_id/3932

 

*Poor valve clearances can create strange 'jetting' symptoms, so check this first.

So can a worn out needle jet (emulsion tube), slide plate seal, and slide plate (all maintenence items)

So can improperly adjusting your fuel screw (by starting with the idle too high to begin with, and creating a condition that is like a mini-choke circuit)

[MXT]

7 minutes ago, Krannie McKranface said:

If it idles, it's not the pilot jet

If it reacts positively to slow throttle, it's not jetting (more on this)*

If it reacts negatively to quick throttle, it's not jetting, its the apump system that has failed.

If you motor is stock, use the stock apump components from KTM.

If it is modified at all, use the Honda version, as it is more agressive and works better with pipes/cams/porting. This requires the linkage spring upgrade, which should be done regardless.

http://tokyomods.com...arburetion.html

  http://www.crfsonly....roducts_id/3932

 

*Poor valve clearances can create strange 'jetting' symptoms, so check this first.

So can a worn out needle jet (emulsion tube), slide plate seal, and slide plate (all maintenence items)

So can improperly adjusting your fuel screw (by starting with the idle too high to begin with, and creating a condition that is like a mini-choke circuit)

KTM's are notorious for having accelerator pump systems that don't allow the engine to run under quick throttle bursts unless the wheel is on the ground and the bike is moving. You can find videos on this on YouTube, and most people were able to fix it by running something like an R&D Powerbowl or a Boyesen QuickShot (with the adjustable leak jet)

I've also had my pilot essentially completely clogged and the bike still ran - when I purchased my bike the previous owner had just basically removed the pilot screw from the bottom of the carb to allow enough fuel to get by. I found this out through symptoms similar to this, which I fixed by unclogging my pilot.

KTM's recommended starting point for the throttle slide plate is 1mm for the idle from the intake side, but then of course you'll have to bump the idle a little to get it to idle where you want it to be.

The fact that it's running off "idle" on choke real well indicates to me that the bike is running too lean on the bottom, and that additional fuel from the choke circuit was what allowed the bike to not stumble; a non-functioning accelerator pump would lead to these types of situations but if he's seeing the a-pump squirt fuel on throttle openings, then I don't know if we can diagnose a bad a-pump just yet.

When my bike was having issues with its throttle slide plate not sealing correctly, it definitely was running lean on that idle end due to a vacuum leak, and would NOT stall out when the throttle was whacked on the stand, hence leading to a sign that the accelerator pump is potentially flooding the engine out normally with too much fuel.

[FORMA MOTO]

19 minutes ago, MXT said:

KTM's are notorious for having accelerator pump systems that don't allow the engine to run under quick throttle bursts unless the wheel is on the ground and the bike is moving. You can find videos on this on YouTube, and most people were able to fix it by running something like an R&D Powerbowl or a Boyesen QuickShot (with the adjustable leak jet )

I've also had my pilot essentially completely clogged and the bike still ran - when I purchased my bike the previous owner had just basically removed the pilot screw from the bottom of the carb to allow enough fuel to get by. I found this out through symptoms similar to this, which I fixed by unclogging my pilot.

KTM's recommended starting point for the throttle slide plate is 1mm for the idle from the intake side, but then of course you'll have to bump the idle a little to get it to idle where you want it to be.

The fact that it's running off "idle" on choke real well indicates to me that the bike is running too lean on the bottom, and that additional fuel from the choke circuit was what allowed the bike to not stumble; a non-functioning accelerator pump would lead to these types of situations but if he's seeing the a-pump squirt fuel on throttle openings, then I don't know if we can diagnose a bad a-pump just yet.

When my bike was having issues with its throttle slide plate not sealing correctly, it definitely was running lean on that idle end due to a vacuum leak, and would NOT stall out when the throttle was whacked on the stand, hence leading to a sign that the accelerator pump is potentially flooding the engine out normally with too much fuel.

Adding choke does not make it run richer, as it is not actually a choke, it is a throttle bypass. You get more fuel and more air at the same time. 

If the bike ever ran well with the jetting it has, the problem he has not is NOT jetting. Jetting requirements do not magically change over time......but the possibility of COMPROMISED jetting is very high: you need to clear the passage above the pilot jet with strong solvent and fishing line or copper wire, or the pilot circuit, even with a highly recommended brand new pilot jet, will still act lean. 

The quickshot and any adjustable leak jet will not solve an apump problem other that lack of fuel, which is usually not the problem. Usually the problem is lack of 'pump' duration and squirt pressure, which is soley controlled by the diaphrams throw distance, and pump rod length, and linkage spring tension.  

Keihin FCR MX carbs were updated in 2008, and we made available to all companies, but only Honda used them. This version of the carb is much better in the apump section. Companies like Tokyo mods, R&D racing and others can sell you kits to replicate the upgrades in a retrofit fashion (see the links I provided). This upgrade will eliminate the issues  with fuel delivery 100%, but it won't stop cavitation. That requires the Powerbowl system from R&D, but is typically only an issue in MX racing.

 

 

[MXT]

8 minutes ago, Krannie McKranface said:

Adding choke does not make it run richer, as it is not actually a choke, it is a throttle bypass. You get more fuel and more air at the same time. 

If the bike ever ran well with the jetting it has, the problem he has not is NOT jetting. Jetting requirements do not magically change over time......but the possibility of COMPROMISED jetting is very high: you need to clear the passage above the pilot jet with strong solvent and fishing line or copper wire, or the pilot circuit, even with a highly recommended brand new pilot jet, will still act lean. 

The quickshot and any adjustable leak jet will not solve an apump problem other that lack of fuel, which is usually not the problem. Usually the problem is lack of 'pump' duration and squirt pressure, which is soley controlled by the diaphrams throw distance, and pump rod length, and linkage spring tension.  

Keihin FCR MX carbs were updated in 2008, and we made available to all companies, but only Honda used them. This version of the carb is much better in the apump section. Companies like Tokyo mods , R&D racing and others can sell you kits to replicate the upgrades in a retrofit fashion (see the links I provided). This upgrade will eliminate the issues  with fuel delivery 100%, but it won't stop cavitation. That requires the Powerbowl system from R&D, but is typically only an issue in MX racing.

 

 

Okay. So confusion of terms, but generally in my area when anyone refers to "jetting" it's more a "check the jets and clean them out" which is what you're saying to do.

In terms of the enrichener circuit on these carburetors, they have a separate metered starter jet which in turn will actually add a richer mixture if the starter jet is set up that way, so although it is adding both fuel and air, that ratio is much more fuel intensive. 

I don't understand how you're saying the adjustable leak jet won't fix the issue: if the engine is flooding out from being too rich because the a-pump is pushing too much fuel, then an adjustable leak jet should allow the excess fuel pressure from the squirt to leak back into the bowl, no?

[FORMA MOTO]

7 minutes ago, MXT said:

Okay. So confusion of terms, but generally in my area when anyone refers to "jetting" it's more a "check the jets and clean them out" which is what you're saying to do.

In terms of the enrichener circuit on these carburetors, they have a separate metered starter jet which in turn will actually add a richer mixture if the starter jet is set up that way, so although it is adding both fuel and air, that ratio is much more fuel intensive. 

I don't understand how you're saying the adjustable leak jet won't fix the issue: if the engine is flooding out from being too rich because the a-pump is pushing too much fuel, then an adjustable leak jet should allow the excess fuel pressure from the squirt to leak back into the bowl, no?

When you pull the 'choke' you effectively (not actually) bypass the carb's idle circuit (pilot jet) and go to a new circuit that includes the starter jet and more air. It does not 'add more fuel' to the pilot circuit, it adds another circuit entirley, with more fuel and air, hence the much higher idle. 

An adjustable leak jet just adds more or less fuel to the pump system. If the pump system is set up wrong, the leak jet adding more or less fuel will have no change.

Almost all 'race' bikes are delivered with the apump system set up 'soft' for the stock tuning. This means large leak jet, long pin diaphragm, short push rod, and no linkage spring enhancement. Changing the leak jet in this senario will have only a very minor influence of the squirt pressure and duration.  Adding more fuel, alone, is not the solution.

If you 'tune' the apump system for performance, this means a smaller leak jet (there is no practical reason for it to be adjustable), a short-pin (longer throw) diaphram, and a long (longer throw) push rod, coupled with a stiffer linagae spring so the throttle linkage and the apump linkgare are not 'connected'. If you try to do this with an oring or safety wire you will get 50% of the benifit, and it will be temporary and likely to fail in 20 hours or so. In the case of using safety wire, it can actually cause the throttle to hang or stick open.

If you go back to 2002-2006, Factory Honda, Yamaha, Suzuki, and KTM all used the R&D racing Powerbowl system to solver their Apump squirt and Cavitation problems.

You can do the same thing today with the Powerbowl kit, or just the parts in the other links.