cutting out at 3/4 and it's not the jetting???

[44oEX]

hi

I have a 04 450R and noticed that it's cutting out at 3/4 trottle, it won't let me go past that...I figured it was just jetting so I did a plug chop and the plug it a perfect nice brown. I will try to play with the needle tonight anyway just to see what it will do, but if it's not the jetting what else could it be?

I have the normal pipe, filter, etc... all the normal mods

thanks

[bwamos]

So, you actually cut the threads off of your plug and checked the mixture band at the base of the insulator? That's a plug chop. Looking at the color of the plug in general isn't accurate.

If you're bogging cutting out beyond 3/4 throttle.. how did you get a plug chop for that range?

Have you tried increasing your mainjet size? 3/4 throttle+ is 90% the main jet. If it's not that you have something goofy going on.

If your airbox lid is off.. put it on.. see if that helps. Usually equivalent of going up 3-4 main jet sizes. If not.. get a main jet 2 sizes larger, and one 4 sizes larger. Try them both.

I'm not trying to be harsh or condecending at all. Forgive me for sounding like I am. There's a lot of misinformation out there on what a plug chop is.. which is not your fault.

[rooster300ex]

Ok lets get this straight. What is a plug chop?, and what is it when you put a new plug in and run it at a certain throttle posistion and pull in the clutch and shut it off at the sametime.

[bwamos]

Copied from GPRacer2500's post a short while back:


The above needs to be reiterated because it's important and not known by many (thanks TravEX). You've got to look WAY down inside the plug at the base of the ceramic insulator. It can be quite hard (sometimes impossible with the naked eye) to get a good look at this area without cutting the threads away, like this:

http://www.strappe.com/plug_section_16.gif

Or this (linked because it's a big image): Plug w/ threads removed (http://www.locastros.com/plug_148_main.jpg)

An alternative is to use plug reading tools which are a lot like the thing your doctor uses to look inside your ear:

http://content.answers.com/main/content/wp/en/thumb/7/7d/180px-2spark_plug_viewers.jpg

http://www.pro-system.com/graphics/spk1.jpg

Forget about plug reading charts that have pictures like these:

http://www.theultralightplace.com/images/Plgnorm1_small.jpghttp://www.theultralightplace.com/images/Plgoilf1_small.jpghttp://www.theultralightplace.com/images/Plgcrbn1_small.jpghttp://www.nightrider.com/biketech/images/oil.gif

That might be useful for figuring out why your lawn mower won't run but when you're trying to decide between a 150 or a 160 main jet, those guides are useless. Most of the "common knowledge" about reading spark plugs is for commercial mechanics trying to diagnose engine problems--NOT performance enthusiasts trying to fine tune their carburation.

If your engine is missing badly, you pull the plug out, and a quick glance reveals it's covered in sooty deposites--well, obviously the jetting is too rich. You don't need to cut up a plug to tell that. Frankly, anyone who is considering doing plug chops to fine tune their jetting shouldn't need to look at the plug at all to tell if the jetting is THAT far off. Engine sound and seat of the pants feel will make large swings in jetting blatantly obvious. When one starts testing different jetting to fine tune a carburator, that's when it becomes important to know how to read plugs from a performance tuning perspective.

Just to be clear, I'm not saying that pulling a plug to take a somewhat causual looksy doesn't provide some information. What that gives you is a very rough idea if the jetting is even close. This can be helpful for jetting beginners that are trying to figure out why their engine runs like crap. But don't think that pulling out an old plug and taking a look at it's general condition is really telling you very much--it isn't.

Different parts of a plug gives clues as to what might be happening inside your engine (correct/incorrect plug heat range, abnormal combustion, ignition timing). But if you're looking at your plug to evaluate air/fuel ratios you've got to look in the right place.

Lastly, to address the original question: it only takes a few seconds for a test plug to begin showing meaningful data (that is, after the engine has been warmed on a non-test plug and everything is set to begin the actual plug chop). Often it's helpful to make two or three WOT passes of 5-10 seconds each. That will help to make a more clearly defined ring on the plug for easier reading.

Hope that info helps!

Oh, and 6 months ago, I didn't know any different either. ;)

[44oEX]

ok well by a plug chop I meant just cut the engine, I did have it WFO but it was only revving out to around 3/4 of the RPM's and cutting out badly...the I just killed the motor, keeping the trottle pinned.

as for what part of the plug I was looking like, I was looking at the porcelaine part of the plug, I didn't know about looking deep down in the plug, but I never had anyproblems jetting this way, I know the tip it self is not a good indicator for jetting but the porcelaine is ( or was, I think anyway ) I know normaly when I can get the jetting to make the plug brown ( in the RPM range i'm working on ) the engine works perfect in this range.

if you think it's the main jet and not the needle, i'll play with that to, and maybe a little with the needle to see what I can get out of it.

thanks

[adinocr7]

I'm dealing with the same problem. I cured the low end bog and installed a dyno jet needle and main (running #132) and a #55 leak jet. The thing is sick up until I go past 3/4 throttle. I tried several different main jets. No matter what the thing will not wind out past 3/4 throttle. I'm at a loss.

[44oEX]

well I was up to a 215 main, and dropping it down. i'm down to 160 right now, and it's getting pretty close...a 155 or 150 would probably do the trick, but I don't have any here.

[adinocr7]

44oEX: I appreciate the advice. Last night I ordered a full HMF Exhaust and two new UNI High Flow Air Filters. I cut out part of the airbox (the top section with the snorkel) to work with the HMF I'm getting in.

The exhaust comes with the appropriate jetting. I also have on hand the following jets (standard) 165, 170, 180, 190, 195. I'm keeping the Dyno Jet Needle installed (it does what its supposed to do . . . 1/4 - 3/4 is sick!) but I'm going back to the standard (Honda) Main Jets so I can just drop the bowl and pop them in and out with a socket. The Dyno Jets can only be removed with a flat head screwdriver . . . royal in the @@s when experimenting with jets.

Once I get the exhaust I'll experiment with the mains. I'm hoping that all will be good once thats taken care of. Really fussy machine man. My KFX 400 had a Outerwears cover, full Yoshi Exhaust and UNI Filter. I used a 170 in the winter and a 160 in the summer. It ran like a top every time . . . no tuning what so ever.

I just can't wait to get this machine right because I know its not even close to making the power it should be.

[44oEX]

yeah same with me..I just want to get the power this thing should have

[boricua]

ive read on here in several posts, that jetting is more throttle position rather than rpms.

if this is true, then its sounds like you have another issue other than jetting.

also, you said cutting out, like backfiring or you feel the machine holding back?

maybe your hitting the limiter a lot sooner than you think you should, especially after the mods....

[GPracer2500]

Originally posted by boricua
...jetting is more throttle position rather than rpms...

In a sense, it's ALL throttle position dependent and NOT AT ALL rpm dependent. The only caveat to that is that in some rpm/gear combinations a misfire might not show up clearly while in another configuration it's obvious. But the function of each carb circuit (pilot, needle, main, etc.) is directly linked to throttle postion not what gear you're in and not what rpm you're at.

[wilkin250r]

Originally posted by GPracer2500
In a sense, it's ALL throttle position dependent and NOT AT ALL rpm dependent. The only caveat to that is that in some rpm/gear combinations a misfire might not show up clearly while in another configuration it's obvious. But the function of each carb circuit (pilot, needle, main, etc.) is directly linked to throttle postion not what gear you're in and not what rpm you're at.

Your posts are always spot-on, so it's almost a compliment when I say that this is one of the few times that you'll be corrected. ;)

What you say is great in theory, but reality doesn't work that way. The Venturi effect that pulls air from your bowl isn't a linear relationship. The best way to explain is to compare it to wind resistance. When dealing with wind resistance, twice the speed equals four times the wind resistance, and ten times the speed equals 100 times the wind resistance.

The Venturi effect isn't a direct square like wind resistance, but it's certainly not linear. More air will result in increasingly more fuel. Obviously, at any given throttle position, the higher the RPM, the greater the airspeed through the carb.

If you were to examine an air-fuel meter on a dyno run at a FIXED throttle position, you would see a lean mixture at low RPMs (when airspeed is low), and a richer mixture at high RPMs when airspeed increases. I would have to do a lot of research and calculation to come up with specific relationship, but I could probably ballpark to say that 4 times the airspeed results in about 5 times the fuel.

Now, when tuning your carb, there is nothing you can do about this relationship except be aware of it.


And, more than anything, I just wanted to be able to correct you, because I don't think I'll get many opportunities, and occasionally I like to gloat. :blah:

[GPracer2500]

Wilkin: Touche....

....And as much as I'd like to leave it at that, I can't help but add that the phenomenon you so accurately described is the reason carbs have main air jets. Hardly anyone is even aware that they exist--let alone use them in tuning. Which is fine--you'd really have to be on a mission to smooth out an unruly A/F line to delve into that. Most FCRs have user replacable main air jets (YFZ FCRs being a notable exception) while some other carbs have fixed orifice main air jets (400EX, 04/05 450r). It's the main air jet's job to correct the richening of the mixture as rpm climbs. On some carb/engine combinations it's more effective than others. For example, Dynojet feels that the 04/05 450r main jet should be plugged completely; presumably because they've found they're able to achieve a flatter A/F curve throughout the RPM range with that part of the main circuit disabled [shrug]. Dynojet says, "...The entry and exit tapers of our main jets differ from those of other companies. Due to this, the fuel flow changes as a function of velocity through the carburetor venturi. This means that two jets of equal orifice size will have a different fuel flow curve relative to intake air speed..." If not for Dynojets tapered jet design I'm guessing a plugged main air jet would not half the same affect.

Good eye Wilkin. It's when the devilish details are examined that our knowledge base grows. :)





ETA: I didn't mention the stuff about the main air jet to try and discount the importance of what you mentioned. The main air jet's abilities to counteract velocity changes in the venturi isn't perfect. If it was we'd all enjoy nearly flat A/F curves with little effort. Sometimes you can get them really flat but I see more meandering lines than flat ones and venturi affect has a lot to do with that.

[44oEX]

haha I learned that in university last year :D ..I feel so special now lol.

anyway, it was the jetting, but you guys where going the wrong way, I was WAY to rich...not lean. I have it running pretty darn good right now with a 150 in there and pilot is a 52 I think ( didn't play with that today lol )

so now the test will be at the first turn in the race tomorrow to see if it really did help it or not, I can get it to hit the limiter now, before it was going to 3/4 and just sputtering and missing.

thanks for the help