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Newbe jetting help...

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Posted by: reepicheep---------------------

I've done pretty extensive research, and have a good range of guesses to start with. I'm looking for help dialing in my tune now.

It's a 95 KDX-200 with Boysen 607's and an FMF Gnarly Torque pipe, with factory silencer. Running in Ohio and Kentucky, which is maybe 900 feet above sea level. It has the Eric Gorr 225 "more better everywhere" port, not broken in yet. I am running 32:1 mix now, but after break in would prefer 40:1 or even 50:1 if I can do so safely.

The bike came to me a mess, but the jetting was stock, 160 main and 48 slow jet, clip 3rd from bottom. That was before the 225 port and with stock exhaust.

My first guess (based as much on what jets my dealer stocked as it was on what I wanted) was 152 main and 42 slow jet, clip 3rd from bottom (down is richer). It started hard, but pulled OK up top, with stutters when whacking the throttle. That was with stock expansion chamber and silencer. It felt similiar though perhaps a little worse after the FMF Gnarly Woods pipe went on.

So I went back to stock (160 main, 48 slow, clip 3rd from bottom). Top end was worse (I think) and low end was possibly better. It started first kick, idled ok for about 60 seconds with choke on before stuttering, and choke off it would dial in to a nice idle with the idle screw.

(On all of the above, I start with the air fuel mixture screw at the factory recommended 1 1/4.)

So I left the slow jet alone (still a 48), lowered the clip one position (two up from the bottom), and put in a 155 main. I expected this to richen up the low end just a bit, and bring back the top end.

Well, I can't really get on it because it was late and I don't want to annoy the neighbors, but it was definately a step backwards and not in the way I expected. First kick it starts, then revs like crazy. Lean, right? But I richened it up. So how did that happen? Tweaking the idle screw had no effect at all... and it ran about the same with the air fuel mixture screw in all positions.

I violated my own cardinal rule of "only screw up one thing at a time" and removed some slop from the throttle cable... so I think that was my first problem. I don't think the slide was completely down, so I am guesing that is why the idle adjustment did nothing, and why it was racing.

So I have one question now... this is really framing the thread for my whole "dial in the tune" process to get help (and help others) from people that actually know what they are doing.

So first question... does "rev like crazy when first started and cold with no throttle" mean lean slow jets? Or rich slow jets? Or "something else screwed up". I don't have any intake leaks, but I am pretty sure I had just a little tension on the slide from the throttle cables.

Input is welcome, but I'll have more data after work tomorrow when I can actually let the thing warm up and give it the "up and down the cul-de-sac how many gears will it wheelie in" test...

BTW, the simplest advice I have seen so far is where Jeff Fredette says simply 45-48 pilot, and 155-160 main. Sounds pretty reasonable.


Posted by: Joburble---------------------

I know just a bit less than nothing about jetting, however FMF racing have a jetting center here http://www.fmfracing.com/TechSupport/JettingCenter May be worth a look as you are using an FMF pipe.


Posted by: _JOE_---------------------

The fast idle will come from too much air, be it from a lean pilot circuit, hanging up throttle slide, lack of fuel supply or air leak between carb and cylinder. I would bet it has to do with the cable adjustment.

That being said, after the throttle slide is verified to be closed I would double check the connections of the carb to the engine and make sure the air screw is at least 1 1/2 turns out.

You should stop making a bunch of changes at the same time. The pilot can be set as soon as you get it to start and run well enough to get it warmed up. Once you get that roughed in you can do some full throttle runs and get the main sorted. Seat of the pants dyno results will get you get you really close here. Then tune the needle for best mid range response/pull. As long as you take it a step at a time and keep track of the results it won't take long to get it tuned great and will get you familiar with what the bike should run and sound like with an accurate tune. Soon you'll know from the sound and feel what adjustments might help compensate for temperature, humidity or altitude.

Here's a link to good guide for jetting......http://www.dirtrider.net/forums3/sh...315#post1340315


Posted by: whenfoxforks-ruled---------------------

Once you get the jetting sorted out, you intend on lowering the oil ratio and dumping more gas in??????? Read Joe's jetting link some more. Vintage Bob


Posted by: reepicheep---------------------

Thanks, that is all helpful info. I'll start working on it this week, time permitting.

I'm still debating on the mix ratio. I had originally decided just to stick with 32:1, but having only put about 2 gallons through the rebuilt motor, it's already starting to make a mess. Of course, it's not jetted right anyway.

Looks like the FMF guidance (nice stuff! Thanks for the link!) sticks with 32:1 as well. So I ought to just stick to that and be done with it, though 40:1 is still tempting as both cheaper and cleaner.

So less oil = richer mixture huh? Makes sense, I'm sure the gas burns a lot better then the oil.


Posted by: _JOE_---------------------

Premix ratio shouldn't effect spooge much all, at least between 32 and 40 to 1. This is the premix ratio not the air/fuel ratio which is what we're referring to as far as carburetion. It's common for people to refer to both as rich or lean, a rich premix ratio has a smaller number like 20:1 while 50:1 would be leaner. I prefer to say thinner. When tuning the carb rich means more fuel and less air. As you cut the premix ratio there is a larger volume of fuel in each quantity of premix being drawn into the engine. Many people attempt to bandaid poor jetting(plug fouling/spooging) with a thinner premix only aggrivating the already rich jetting. I would recommend 32:1 from the get-go and stick with it. Jet it right and you'll have a strong performing, long lasting engine.


Posted by: reepicheep---------------------

Cool! That wasn't hard at all.

Start with the low speed jet, which is 1/3 throttle, and tune that. I had a 48 in there, clip 2nd from the bottom (richer). Air fuel screw 1 turn out (in is richer, out = is leaner) Warm it up and do 1/3 throttle pulls.

Then turn fuel air screw out 1.5 turns (out = leaner). Ran better at 1/3 throttle.

Then turn fuel air screw out 2 turns (out = leaner). Ran better at 1/3 throttle.

At the same time I tried full throttle pulls (I know you should only do one thing, but if you look at the Boyesen charts, the clip and slow jet and air screw supposedly have no effect at WOT). I had a 155 main in there, and it was a choppy pull at WOT. I knew that before, I had a 152 in there, which did well with the stock expansion chamber, but was running choppy with the FMF Gnarly Woods pipe. So that made me believe the main was now lean (as a result of the new better pulling exhaust).

So I pull the carb, and move the clip to the middle position (moving it up one notch, moving up is getting leaner). Leave the 48 slow jet.

I also put the stock (160) main back in.

Restart, re-warm up, and it was great!

1 turn out on the fuel air screw, 1/3 throttle, ran pretty good.

1.5 turns out on the fuel air screw (leaner), 1/3 throttle, ran even better.

2 turns out on the air fuel screw (leaner), 1/3 throttle, pulled a little worse. Sweet! Looks like that is dialed in.

WOT pulls hard.

So here is where we are... I think I have the low speed all dialed in. Not surprisingly, it is right where Jeff Fredette, Eric Gorr, and FMF said it would be .

For the main jet... 142 to 155 (going richer) pulled harder. 155 to 160 (going richer) pulled harder. My guess is that 160 is about perfect, but to know for sure I would have to put a 165 in there and feel it get worse... there aren't any other nice hacks like for the low speed jet where I can use the fuel air screw to tell what side of perfect I am on.

A 165 main would be richer then anything else I have heard of anyone running though. It's richer then the high end of what Fredette, Eric Gorr, and FMF recommend. So that seems insanely unlikely. So I'm guessing I should just leave it at a 160 and enjoy it (it runs great until it gets too high an RPM and gets off the pipe, then it gets choppy).

So that's my experience. It wasn't that bad. The Boyesen guide that comes with the 607's was extremely helpful.

Now to get an actual full day ride on this thing... the bike is getting dialed in about the same time my knee surgery is being declared "fully healed and ready to be re-injured "


Posted by: reepicheep---------------------

Oh, and for the "bottom line summary" for the archives to help others searching...

* 1995 KDX-200.

* Altitude 900 feet above sea level, 60 degrees ambient.

* Eric Gorr "more better everywhere) nickisil 225 big bore kit and porting. Weisco piston.

* FMF Gnarly Gold Series Woods expansion chamber.

* Stock silencer.

* 32:1 fuel mix (non synthetic decent quality 2 stroke oil).

* Stock needle. Stock slide.

* Clip in middle (3 from top, 3 from bottom).

* 160 main.

* 48 low speed.

* Air Fuel 1.5 to 1.75 turns out.

* Boysen 607 reeds.

* Airbox lid intact, snorkel removed.

* Foam oiled filter.

(Geesh. Lots of variables there... no wonder no two bikes are the same.)


Posted by: whenfoxforks-ruled---------------------

Mind the colder weather and richen it up when need be. Do not concern yourself if your main is so big, try it, especially if you are out below 30 degrees. Adjust the needle the same way, go all the way down then back up, get to know the difference. Does JD jetting offer needles for your bike?. Vintage Bob


Posted by: _JOE_---------------------

Absolutely try the next size up. Get two or three bigger while you're there. Worth a shot, at least you'll know it's as it gets.


Posted by: reepicheep---------------------

Update!

Well, I took the rebuilt KDX for the first long woods ride yesterday (where I was not chasing a 9 year old, and I could ride harder). Temps were in the 60's, a great day for riding.

Significant jetting changes as a result. I would ride a few laps (maybe 10 miles of technical trail), then make an adjustment.

Lesson #1... don't trust changes you make in your driveway. I thought I had let the bike properly warm up, and thought I was riding far enough to tell a difference, but my low speed jet was *way* off.

So yesterday, I ran a lap with the 48 I had in there, and it was the same. Bogging at partial throttle openings. Swap to the 42 (because it was the only other one I had with me), and the thing ripped again. Still just a little bog at very low RPM's, but a much wider powerband, and the thing is a total wheelie machine now, even at partial throttle openings.

(I was having a blast jumping off the top of dirt moguls climbing up hills... the KDX just leaps off of them if you keep it in the power band).

So the lessons...

1) Don't trust samples from short data runs.
2) I don't think my "play with the air fuel screw" is a reliable way to tell if you are on the rich or lean side of the problem. Just swap the jets and make another run.
3) Don't get suckered into thinking "the slow jet controls low RPM, the fast jet controls high RPM". I keep slipping back into that thinking, and when I got the fast jet closer, I fooled myself into thinking the slow jet was better. The way it works is "slow jet controls 1/4 to 1/3 throttle opening behavior" and "fast jet controls wide open throttle" regardless of RPM (which I am sure isn't completely accurate either, but it's a more useful oversimplification ).

Wide open throttle feels pretty solid, up until I get over some magic RPM line and the engine starts breaking up. I need to get a tach on there to see if thats just as fast as my motor will go, or if thats a jetting issue. But for now the 160 feels pretty good, though I may try a 155.

The 42 slow is running pretty well, but that seems scary lean to me. So for peace of mind, I'll probably drop in a 45 (which I have had trouble finding in stock locally) and see if it goes better.

(the real lesson here is that if you want it jetted right, you really have to go find somebody with a dyno and a 4 gas analyzer... and that anything I am going to do on my own will probably get it jetted better over time, but never perfect...)


Posted by: Rich Rohrich---------------------

Quote:
Originally Posted by reepicheep

The 42 slow is running pretty well, but that seems scary lean to me. So for peace of mind, I'll probably drop in a 45 (which I have had trouble finding in stock locally) and see if it goes better.


You can't hurt the engine from running too lean a pilot jet, and in almost all cases that will be true of the needle position as well as long as the needle jet has enough area to feed the main at WOT, but that's easy to verify. For the type of riding conditions most will encounter the worst that comes from being too lean on the pilot is a hesitation/bog. If you get way too lean on the needle it will be obvious from the performance most of the time. Worst case on the needle posiiton being too lean is possibly some spark knock in low speed, high load situations where the throttle is open past 1/2 and maybe a throttle run on coming down from high speeds.

No one ever broke an engine from running too lean on the pilot. Lean is good in the low speed portion of the fuel curve.

Quote:
Originally Posted by reepicheep
(the real lesson here is that if you want it jetted right, you really have to go find somebody with a dyno and a 4 gas analyzer...


It's rare that jetting done on a dyno will translate correctly to a real world riding situation. Jetting is about matching the fuel curve to a load sequence, and duplicating real world loads on a Dynojet type setup is nearly impossible. You can get it in the ballpark, but razor sharp jetting takes work in the real world.


Posted by: reepicheep---------------------

Thanks Rich, great information.



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