Yet another jetting question!

[MotoBK]

2001 KDX 220: 42 pilot, 145 main, Clip in 3rd position from top.

I recently did a wiesco Top End, and Boyesen Power Reeds. I now have bogging(I Think) from 3/4 to Full. It just hits a bog wall and the engine will not rev any more. If I turn the air screw from 1.5 turns to 2.5 then the top end comes back....and it runs strong again. I bought a 142 main, and threw that in it seemed to make things better at 1.5 turns....but I was told that is a bit to lean. I know the process to figure jetting by starting with the pilot...then working to the needle...then the main. I tried droping the needle to the fourth position....and that made things worse as well. I just wanted to know if this sound straight forward to anyone...as far as the way my motor is responding. I cannot tell the difference between a too rich bog, and a to lean bog. Any advice is appreciated.

 

[julien_d]

Bogging rich is not a bog at all but more of a sputter/breaking up. A bog is lean, where it's not choppy but the motor just kinda goes buuuuhhhhhhhhhhhhh.

FWIW I just finished a new top end on my 200 and it wouldn't run for crap. I ended up going from 42/155 to 40/150 to get it running right again.

Best bet would be doing a plug chop on your current 142 MJ. If the plug doesn't read lean at WOT on your new MJ then that's fine. I have best luck choosing pilot and needle position by throttle response under load. If it's breaking up in the mid range your overall setting on pilot/needle is too rich. If you get a bog then it's too lean. Believe it or not the pilot and needle have much more affect overall than the main jet does.

 

[sr5bidder]

going leaner in the winter just don't seem right
I have never tried it.... but how about going to a hotter plug

I really have never used hotter or colder plugs and want to find info on this tuning tool and how it is properly applied

 

[julien_d]

A hotter plug is rarely the answer. Unless the plug is building up deposits you won't get any real benefit going hotter, just a motor that runs a bit hotter overall.

 

[Rich Rohrich]

I really have never used hotter or colder plugs and want to find info on this tuning tool and how it is properly applied

Plug heat range isn't a tuning tool. It's a matter of getting the plug to run at a temperature that is hot enough to stay clean of deposits that can cause fouling while maintaining a temperature low enough to keep the plug from becoming a source of ignition before the normal firing point ( pre-ignition) . The engine parameters influence the plug heat range required not the other way around.

The OEMs choose a plug heat range based on how long, in seconds, the engine will be expected to operate at full load, full throttle. They are thinking worst case with these tests.

A "cold" spark plug is one with an insulator nose shape that conducts heat more readily. Because it can transfer heat from the nose to the large mass of aluminum in the head quicker it can resist full load heat build-up longer. Eventually given enough time at full load it will eventually reach a heat saturation point where it will act like a glow plug and fire the mixture without a spark.

Motorcycle OEMs tend to err on the cold side with their recommendations for two-stroke plugs as a safety measure, but a spark plug that is too cold will foul easily and not be able to reach high enough temperature to clean off the deposits that cause short-circuiting of the spark and eventually complete fouling.

Hotter plugs work well when the loads tend to be low or the engine spends a lot of time at part throttle and this is followed by relatively short bursts of WOT throttle.

What this all adds up to for out purposes is the heat build-up and subsequent running temperature of a spark plug is largely determined by engine load, spark timing, jetting, and cylinder head heat dissipation.

The important thing to take away from all this is, hotter plugs to NOT raise the engine temperature on their own, and they do NOT add heat to the engine. That's the thing that seems to be the point of greatest misunderstanding in all this.

 

[sr5bidder]

hey thanks Rich I found your post from 1999 that you have just copied and pasted, but have also read something from Gordon Jennings: "there's a 125cc motocross machine residing in our shop which runs a lot stronger and cleaner since it has been retimed for less advance, jetted leaner, and been given a hotter spark plug."

you say if it doesn't foul forget about it while in the same artical Gordon speaks of fuel film on the eletrode and the insulator tip being hot enough not to allow any soot build up..
forgive me I am just venturing into spark plug wisdom...hell I ran the same plug that I bought my kdx with for 1-1/2 yrs rebuilt ithe engine and started it up with the same plug and broke the engine in and the only reason I changed it was to check on the general health of my jetting with a new plug

 

[sr5bidder]

A hotter plug is rarely the answer. Unless the plug is building up deposits you won't get any real benefit going hotter, just a motor that runs a bit hotter overall.

from what I have read spark plug heat range has no effect on engine temps, but can cause preignition and hole in pistons if too hot of plug is used

 

[julien_d]

Nice. Good information there. I agree with absolutely everything right up to this;

The important thing to take away from all this is, hotter plugs to NOT raise the engine temperature on their own, and they do NOT add heat to the engine. That's the thing that seems to be the point of greatest misunderstanding in all this.

I can't wrap my head around the possibility of this being true. Given one particular engine set up as the standard, you have plug A and plug B. Plug B has a higher operating temperature in our current engine setup than does plug A. So naturally plug B introduces heat into the engine that plug A does not. There's nowhere else for that heat energy to go. It doesn't just disappear or get canceled out. Whether or not this would be enough temperature increase to notice, I do not know. My understanding is that it's pretty minute. I do know that if you continue going hotter, you get pre-ignition, and then you definitely get enough heat to notice a problem.


Definitely not trying to start an argument, but I would definitely like to expand on this one. It's interesting stuff!

 

[sr5bidder]

it is ..isn't it!

the whole thing that brought this up is the 2nd post in a few days that has said that leaner jetting was required to get a fresh top end to perform..and in colder weather??

.my thought is a new top end has better compression than what was performing well before the rebuid so the new top end should be able to compress the mix to a higher pressure and burn it better..........I'm lost mine did great on the new top end and same jets and the reports are JD going down from 155 to 150 and motobk being at 145 going down to 142....but thats a 220 so I dont know...

I am at 45/2nd from top/155 boysen 607 and fmf woods pipe I guess I should be glad my bike didn't require any messing around with jets....I do admit I droped the needle after the rebuild, though I could have tried the needle before but never did.

 

[sr5bidder]

oh JD the insulator on the hotter plug stays hotter not the spark.. from my understanding so far its to keep build up around the electrode from happening

 

[Rich Rohrich]

Given one particular engine set up as the standard, you have plug A and plug B. Plug B has a higher operating temperature in our current engine setup than does plug A. So naturally plug B introduces heat into the engine that plug A does not.

Nope. Plug B just transfers the available heat from the plug nose to the steel plug body more slowly. Same heat energy in the engine, same heat introduced to the plug nose, but just a less efficient cooling path to the available heat sinks the steel plug body and the cylinder head.

If you were to remove the steel plug washer install a thin fiber washer in it's place you'd see the plug act like it was a hotter plug. No change in energy, just a less efficient path for the heat to travel along.

Thermodynamics aside, the moral of the story is:
- plugs need to be hot enough to keep the nose fairly clean
- load, and time under load have the biggest influence on plug heat range required
- If your riding stays the same, and your bike is jetted correctly, the weather has no influence on the plug heat range required.

If you pull plugs once in a while you'll see they will give you a very clear indication of whether or not they are running at the right temps.

Evidence of a grainy insulator or a glazed look points to an excess of heat. The ceramic will look like a glazed donut with a weird shine if it's running too hot. If it's on the edge of being too hot you'll see small spots where the glazing is just starting to appear, usually close to the center electrode.

If the insulator surface looks similar to a new plug with just a very slight coloring (two-strokes only) then it's running at a safe temperature.

If it's too cold you'll see too much color and a build up of deposits on the ceramic.

hey thanks Rich I found your post from 1999 that you have just copied and pasted, but have also read something from Gordon Jennings: "there's a 125cc motocross machine residing in our shop which runs a lot stronger and cleaner since it has been retimed for less advance, jetted leaner, and been given a hotter spark plug."

you say if it doesn't foul forget about it while in the same artical Gordon speaks of fuel film on the eletrode and the insulator tip being hot enough not to allow any soot build up..

I have preached about running plugs hot enough to keep the insulator clean for ages. You have to keep in mind that most people are convinced they should see a plug color that looks very dark (too cold with too many deposits) so trying to convince them to do otherwise scares some people. No one ever broke an engine because they ran too cold a plug, but they sure have hell have slowed a few down. ;)

Long ago, Dr. Theodore VonKarman wrote:

"When you are talking to technically illiterate people you must resort to the plausible falsehood instead of the difficult truth."

The sad fact is, lots of riders don't want to understand or don't want to be bothered, so the advice to not mess with it if the plugs don't foul is a safe approach for them.

Gordon Jenning's took some flack over the years for his views on plugs but in my opinion he was dead on accurate. I think he took the correct approach by explaining things in detail and trusting people to be sharp enough to do the right thing with the information rather than just glossing over things with a "plausible falsehood" like most magazine guys do.

I try to do the same as GJ when I can. I'm convinced the more riders know the better the chance of them making good decisions and having more fun along the way as a result.

As you get deeper into the specifics of tuning you'll find that the chocolate colored plugs that so many strive for are a just safety crutch for disinterested (or technically ignorant) tuners. They are a long way from a proper setup.

 

[julien_d]

Thermodynamics aside, the moral of the story is:
- plugs need to be hot enough to keep the nose fairly clean
- load, and time under load have the biggest influence on plug heat range required
- If your riding stays the same, and your bike is jetted correctly, the weather has no influence on the plug heat range required.

Quite clear, thanks!

.my thought is a new top end has better compression than what was performing well before the rebuid so the new top end should be able to compress the mix to a higher pressure and burn it better..........I'm lost mine did great on the new top end and same jets and the reports are JD going down from 155 to 150 and motobk being at 145 going down to 142....but thats a 220 so I dont know...

yeah, that was my thought as well. This would of course be dependent on the bike having been jetting while compression was at the lower end of usable range, or even less than spec. There was about a 30psi difference in compression from when I had previously jetted my bike to this time around. The wiseco piston I installed also had a smaller window in the intake skirt than my pro-x piston. I'm not sure what affect that has on the jetting requirements, if any.

The 220 also runs considerably leaner on the jetting than a 200 does, so 140 may not be that far off depending on elevation and temp. My guess is it had more to do with the pilot and needle than the MJ though. I'd like to see the OP go back to the 142 MJ, and keep the smaller pilot jet, then play with needle position and airscrew setting.

 

[whenfoxforks-ruled]

Other than the plug temp, I got some backwards jetting attempt out of this thread? There are some who swear by starting with the main, but? How is it the air screw out more is making it run better? Use that air screw to adjust in your pilot first! Do not be afraid of leaning in winter. Every engine is different. Even that fact has been blown out of the spooge, I know a guy who can port 2 cylinders to run the same! Guess who? And guess who he collaborates with for a bonus? I love the kdx guys, going for making their bike run akin to an mx bike, THAT I know a little about. I can take a guys bike that thinks the plug is too cold, and when I ride it, its fine, whats the answer? Vintage Bob

 

[sr5bidder]

I think I'll give a hotter plug a try pretty soon, but I have to remove the one in there now and see if it's getting deposits like my old one had ...you know the dark color....

To the OP sorry this has gone so far off in left field.

 

[Rich Rohrich]

I can take a guys bike that thinks the plug is too cold, and when I ride it, its fine, whats the answer? Vintage Bob

How hard the bike is ridden has a major influence on the plug requirement as well as the jetting, but it seems to go largely ignored by most riders.

In the Plug reading 101 thread you can see this visually by looking at

- Post 13 - Colder plug 9 series with a fast rider
- Post 15 - Hotter plug 8 series but much slower rider
- Post 34 - Detailed explanation of the specifics

http://www.dirtrider.net/forums3/showthread.php?t=92818&page=2&pp=10

The rider and the track conditions will determine the plug running temperature to a huge degree. If you take a perfect setup for dry fireroad type riding and ride the next weekend in tacky mud you'll most likely increase the load to a significant degree. When the load goes up so does the plug temp. Whether or not it changes enough to warrant a change in plug heat range is where your testing and experience comes into play.

The important point is, checking the plug once in a while when conditions and riding type changes will teach you a whole lot about what is happening inside the engine. Who knows, some people might even find this part of the riding experience to be fun. ;)

 

[MotoBK]

Running Much Better Now!!

Last night I went trough Canadian Dave's step by step carb setup guide to check if you have the correct pilot jet. The pilot was good. I have a little less than two turns. I put the needle clip in the second from top position for the 1/4 to mid range. I then took that bike out to a local spot this morning. The throttle was perfect up until upper mid range to WOT. I rode for about an 30 minutes like this, and It do not get better. Now knowing that the bike was completely warm, and the issue was still present. I decided to put the 142 Main back in.......OMG I love my KDX 220 now!! The top was there with more than enough for my beginner needs!! Hill climbs were literally " A Blast!!" the throttle was crispy almost throughout...a little midrange burble just ever so slightly. Thank all of you for your help and advice....the bike finally feels really good. I will try to do a plug chop next weekend....but if 142 is the next jet down...then is there really a point to it? I guess just to make sure I am not over lean. I'm a newbie to motorcycles, and I am sure that I will have more questions from time to time. Thanks!! MBK

 

[whenfoxforks-ruled]

Even seat of pants jetting, 1 up or 1 down should be VERY noticeable. JUST as long as you are through all the other variables(like there really is another mechanical issue somewhere, regardless of how not-manly it is that you may have "missed" something?) That is the biggest gray area to get through, mis-information. Jetting is easy! THANKS RICH! Ride it in the power-band for ever, Vintage Bob