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Noob to jetting. Looking for guidance

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I am very new to Jetting.  Last bike I owned I had an old flat track racer assist me and get it jetted but since I lived and rode in the same elevation I never really touched it from there.  Just picked up a new 2018 KTM 250 XCW and I want/Need to boost knowledge and understanding of the jetting process.  I also have some general questions.

 

1)  Is there a guide out there that goes into a little more detail about Carbs and how to jet them?  Most of  the ones I see are very generic and they say (increase jet as this changes or decrease jet as this changes) Im looking for one that says why you are doing it and what is happening.  Also looking for one that explains Needles and why riding style may dictate what needle/jet combo works better.  I don't even mind buying a book as long as its helpful and useful.

 

2) Do you change/tinker with jets if you know you are going to a new area?  I live and ride currently in the 4300' elevation range.  Local shop jetted the bike to the 5000'-7500' range for the 60F-80F temp range, for now I am pretending that this will be perfect for the sake of understanding..  If I am planning on going into the Idaho High country, say in the 8000-9000+ foot range do I need to Re jet for this? 

Similar, I want to make a trip down to Colorado and spend an extended weekend riding near Ouray, some of the roads there are peaking 11,000-13,000 feet  How do you jet or plan to ride this?  It would be pretty difficult for me to try and pretune the bike for the trip so do you just follow the manual, re jet it, then make notes how the bike ran so that in the future you can dial it in better?

I am enjoying riding low in the deserts but want to be able to run in the mountains again.  Some of our elevation changes can be so drastic I want to make sure I am doing this correct for the sake of the bikes engine.  Where I know I change elevation frequently and often would one of these aftermarket carbs be a wise investment or would I benefit more personally spending time learning how to jet a carb first?

 

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Most owners manual do cover jetting pretty good. Keep in mind too, fuel/.oil ratio affects jetting as well.

Do not look at altitude and temps seperately. Look at air density. That combines temp, humidity and air pressure.

As temps go down, air becomes more dense. As air pressure goes up, air becomes more dense, as humidity drops, air becomes more dense. In some cases, one change offsets another and the air density does not change.

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The way I learned to jet was you start with your idle and pilot and then work your way up from there, adjusting the mid range and then top end. Learning to read spark plugs will help greatly. Usually you can tell if your jetting is off, if its lean the bike will seem to have lingering RPMs or want to just snap as soon as you crack the throttle, rich will feel really boggy and it'll spooge. Theres a learning curve. I'm by no means an expert. The thing with smart carbs, they will get your jetting in the ball park, just good enough to run ok. If you want to get it just right and crisp you gotta learn to tune a conventional carb. I like the PWK, pretty simple and responsive to adjustments and tuning.

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This diagram helped me. It helps to know the various parts of your carb too. Look at part diagrams and breakdowns of your specific carb, they all have the parts laid out and their specific names. jet-chart.jpeg

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25 minutes ago, William1 said:

Most owners manual do cover jetting pretty good. Keep in mind too, fuel/.oil ratio affects jetting as well.

Do not look at altitude and temps seperately. Look at air density. That combines temp, humidity and air pressure.

As temps go down, air becomes more dense. As air pressure goes up, air becomes more dense, as humidity drops, air becomes more dense. In some cases, one change offsets another and the air density does not change.

So you would look at density at the given day/s you are comparing and use that value instead of temp and altitude?  Potentially a lower or higher elevation may not matter depending on the others?

11 minutes ago, ASP1227 said:

The way I learned to jet was you start with your idle and pilot and then work your way up from there, adjusting the mid range and then top end. Learning to read spark plugs will help greatly. Usually you can tell if your jetting is off, if its lean the bike will seem to have lingering RPMs or want to just snap as soon as you crack the throttle, rich will feel really boggy and it'll spooge. Theres a learning curve. I'm by no means an expert. The thing with smart carbs, they will get your jetting in the ball park, just good enough to run ok. If you want to get it just right and crisp you gotta learn to tune a conventional carb. I like the PWK, pretty simple and responsive to adjustments and tuning.

Thank you I noticed that it seems people either love the smart carb or don't really care for it so that is why I figured I would save the money and play around with tuning my carb on my own and learning.  If after a year or so I am tired of dealing with it I may explore the smart carb some more.

8 minutes ago, ASP1227 said:

This diagram helped me. It helps to know the various parts of your carb too. Look at part diagrams and breakdowns of your specific carb, they all have the parts laid out and their specific names. jet-chart.jpeg

Ahh Yes little things like this are what I learn best from.  They help me visualize what is happening and why. 

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Ahh Yes little things like this are what I learn best from.  They help me visualize what is happening and why. 
Yeah at the end of the day we are trying to adjust for air density. Usually you're going to hear people talk about altitude and temp though. You'll see guys say at x amount of altitude I'm running this jetting. Usually in the owner's manual they will tell you sort of how to adjust your carb based on that. Temperature from what I've seen will affect your jetting but not to a huge extent. On my bike I ran the same jetting on 90 degree days in the summer and 50 degree days in the winter with very little difference. All I did was adjust my air screw a 1/4 turn, that's about it. In winter your bike will run a hair leaner due to cold dense air. Lots of jetting is trial and error, getting to know what you like at certain altitudes, and getting really good at swapping brass on the go lol. You really need to be comfortable with your carb. Funny story, I had my carb flooding one day at the track, dudes next to me thought I was done for the day at that point. I pulled the carb off, cleaned the needle valve, and had the bike running in 10 mins. They probably thought I was nuts having it all spread out across the tail gate of my truck . It worked though. Also, write down settings so you have them saved. For example at sea level you may like a certain setup but at 10k feet you may like a different setup. Itll save time when your preparing the bike for a ride.

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So re consulting my owner manual and it does show a bit more detail about jetting.  I read through it this weekend but pages were sticking and I wasnt watching page numbers so I missed like half of the engine tuning section ha.

 

I was watching the dirt bike channel guys video on jetting and it doesn't look terrible on these bikes.  Can be done without removing the carb if needed.  

 

On the photo of the "lean" spark plug what is causing it to get dirty? Is it physically getting hot and discoloring? I've only ever seen optimal and fouled plugs from friends just dumping oil in the gas and never really measuring ha.

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@flyfisher117

google Spanky’s jetting guide

this is how I learnt to jet 

really easy to understand 

i always start from the bottom 

so pilot jet and airscrew

Also research about the Suzuki needles range 

pick a premix ratio and oil and keep that consistent then jet around that

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2 hours ago, kxrob said:

@flyfisher117

google Spanky’s jetting guide

this is how I learnt to jet 

really easy to understand 

i always start from the bottom 

so pilot jet and airscrew

Also research about the Suzuki needles range 

pick a premix ratio and oil and keep that consistent then jet around that

I will do that! 

 

Planning on just running the Motorex 2T oil at the KTM recommendation for now.  I figure that will reduce one unknown or variable while I learn.

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So re consulting my owner manual and it does show a bit more detail about jetting.  I read through it this weekend but pages were sticking and I wasnt watching page numbers so I missed like half of the engine tuning section ha.
 
I was watching the dirt bike channel guys video on jetting and it doesn't look terrible on these bikes.  Can be done without removing the carb if needed.  
 
On the photo of the "lean" spark plug what is causing it to get dirty? Is it physically getting hot and discoloring? I've only ever seen optimal and fouled plugs from friends just dumping oil in the gas and never really measuring ha.
I think it gets hot and there isn't enough fuel there to get a nice chocolate brown colored electrode, it's usually white or grey or really light brown when lean. You'll feel it with your bike. An extreme case would be what happened with my YZ400, full throttle run away. You haven't lived till you've had a 400 cc 2 stroke run away wide open, nothing short of cutting off fuel supply with stop that, it was pretty crazy. But that will only happen usually if you run your bike conpletely out of fuel, or jet it REALLY REALLY lean, of if theres an air leak somewhere, but yours is new so that shouldn't be an issue.

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