Jetting lean, should I be nervous?

[RoostRider]

A freind of mine just bought a '99 KDX 200 (how dare you suggest I influenced him?). It is mostly stock with an FMF pipe and silencer.

I had some jets lying around from when I jetted my bike...

main- 152, 158 and 160
pilot- 48 and 42

I told him what I run and seems to work really well for me. main-155 and piolt- 45.

Obviously, those jets are in my bike so he can't use them, so he put in the 152 and 42. We have yet to chop test the bike and won't have that chance before we go riding next.

Here's where I get iffy. I thought my bike was acting just a touch rich (but I don't mind being just a little rich as it saves on the motor), so i felt confident that the jets he picked were o.k.. But now I went and checked on the FMF website and it recomends 155-160 main and 45-48 pilot.

I 'thought' that the 152 and 42 were recommended by some other site (can't remember which one though.. is there another site that has this calculation feature?)... But now FMF has me nervous.

Should I have him change out to the jets that are richer than mine until we can do a chop test? 158-48. Or do you think this is a good place to start and just keep an eye (ear) on it and take it easy if it seems lean?

 

[RoostRider]

oh, we're at 500' or so in elevation too, and the colder weather is coming soon. (he doesn't want to be changing jets with the weather)

 

[mnnthbx]

Every bike no doubt is different, so what I run should mean nothing to you. That being said, with the FMF woods pipe and Turbine II, I run a 42, 152.

 

[canyncarvr]

Which pipe..it matters. The -30 (rev) takes a considerably richer jet-set than the -35 (torque).

Just because FmF says different I wouldn't worry about it. They can't even keep the names of their kdx pipes straight...what would they know about jetting one?

You didn't say what he took out.

A good place to start is with CDave's recommendation for that bike. His site is linked in the 'Every kdx rider should read...' thread stickied to the top of this forum.

...and that's not 48/158.

If you haven't read through the information posted there several times you certainly should.

 

[John Harris]

Every bike is different--that said you should be fine with the 42 pilot, but I would go with a 155 or larger main jet. Not many have seized at idle, but wide open is usually where problems occur. A little spooze will not ruin you until you get it checked out. Afterall, the mains are very easy to change. Cheers John

 

[CaptainObvious]

You can't blow-up the motor with a lean pilot jet or high-clip position on the needle. You'll just have bad throttle response.

The main is a different story. A lean main will usually reveal itself after closing the throttle after a prolonged use of WOT. That is one reason why you need to clutch and kill the engine during jetting changes.

...but I don't mind being just a little rich as it saves on the motor...

Actually, it hurts the motor. The excess spooge clings to the powervalves and hardens. This can lead to a host of problems including frozen powervalves, broken actuation gears or a clogged pipe.

You're doing the right thing by helping your friend in jetting the bike.

 

[Braahp]

Its always better to be to the rich side. Being just a tad lean could cost major $$$. With Fall/Winter approaching there is going to be more oxygen in the air making the bike just a little leaner anyways.

 

[RoostRider]

...and that's not 48/158.

If you haven't read through the information posted there several times you certainly should.

I get this to mean that he shouldn't be running whatever jets I have left over?... lol.... This was just to get him out of the 'super rich' jetting that he was running (160-48)

Thank you all for the help. I have read that article several times, but sometimes I get lost on the net and can't remember where something was. I read his recommendations and he suggests the 152/155 - 42/45 combo, So I think we'll leave it there and take a richer main with us (they are very easy to change).

Oh, and my comment about richer being safer was more along the lines of the fact that I think it is safer to err on the side of rich and keep an eye on the spooge than to err on the side of lean and be buying parts.

Thanks again.

 

[canyncarvr]

Well, not exactly.

I guess 48/158 is better than stock..but not by much. If you are going to change stuff, why not get close the first time? A set (pilot/main) will cost less that $10 so it's not a huge deal. Going to the trouble to change from oem (48/160) to 48/158 won't accomplish much if anything.

I'm not trying to be catty, evasive or unhelpful. There is just too much to guess on when it comes to jetting-by-thread.

I've ridden my bike @500'el. and a 152 would have had me getting hauled back on a trailer in short order. Granted, that was in sand..and that is not your average trail-riding situation. Heck...I put a hole in the piston with a 164! I happen to think the plug came apart..but it could be argued the hole was burnt in, too.


As mnnt said (so what I run should mean nothing to you) I'm currently running 38/152/EEK-4/TV-7 with a -30 pipe. Run that at 500'el and you probably won't be running long.

And, yes...too rich isn't doing you any favors at all.

42/152 will probably be ok if you're dinking around. I'd be concerned if there is any screaming WOT going on.

Taking a look at the plug a few times would be good. Color isn't going to tell you anything in this case, but insulator texture and general appearance might. You don't want to see a pock-marked insulator or certainly any shiny bits stuck on the plug. You could have an oily mess of a plug (the dinking around part on an old plug noted above) and STILL see bits of your piston (the shiny part above) on the same plug if you do a lot of WOT with a big load at close to sea level.

There are only something like five circuits in the carb...and they all interact to some extent with each other. A poor main choice will effect the idle of your bike. A poor pilot isn't likely going to cause any engine failure...just crappy riding with poor throttle response.

You didn't mention the pipe...again. If it's a -30 I wouldn't advise less than a 155 until you check it out. If it's a -35 a 152 will probably be ok.

DO have some spare plugs on hand. Lubricate the threads of any new plug (never-seize is good!).

How did you come up with your pilot choice? Hi-idle is only a starting point. Response is the correct way to do it. From a slow speed in 2nd gear under load a quick wick of the throttle to about 1/2 should get you nothing but GO. No hesitation. Try a 42..maybe even a 40. Tweak your airscrew no more than 1/8 turn and a time..run your bike for a bit before you determing that setting is/is not working.

Have fun!

 

[RoostRider]

I got'cha... how I determined the jetting for his bike was by using what I run on my bike as a starting point. (He has the same model bike. I know that doesn't mean it's all the same, but it is likely a good starting point as I know his is running way rich and mine is only slightly rich)

I run the 155-45, which left a leaner set (probably the leasest I would ever want).. the 152-42...

My bike is a little rich on the pilot (the ariscrew is out about 2 for best response), and a little rich on the main (but I like to hammer on my bike like an MX'er).

I picked up a 155 main to throw in his bike and I think we'll run with the 42 pilot (they didn't have the 45) to start with, as we will likely not have the oportunity to chop test before our next ride.

We will bring the other mains with, cause they are easy to change on the trail, but you know how that goes. Who wants to stop and tinker with jets when everyone else is revving thier motors.

We should be all good. Thanks for the help.