Carb problems i'm guessing


13 replies to this topic
  • macca1810

Posted May 22, 2008 - 06:56 AM

#1

Hey i'm new to this i just purchased a yz426f 2001 model after riding my mates bike loved the power but my bike was fairly cheap so has a few minor problems which annoy someone like me who aims to get the bike perfect. The throttle is sticking and i'm guessing it might be a dirty carb cause it seems like its not in the cable. I guess i'll have to wait for the manual to arrive before i attempt to pull it down and clean it. Also its got an aftermarket pipe which i'm also guessing has made the jetting wrong as the bike back fires Anything i should know about these bikes? Thanks Jack

  • matt4x4

Posted May 22, 2008 - 07:03 AM

#2

Usually it is in the cables - the return mechanism has a very strong spring.
Go to here and get your manual instead of waiting:
http://www.yamahaown...ook.com.au/?r=0

After market pipe will lean your bike out, adjust the fuel screw and/or get a larger pilot jet, and likely main jet or raise the needle by dropping the clip one notch

  • grayracer513

Posted May 22, 2008 - 08:25 AM

#3

After market pipe will lean your bike out, adjust the fuel screw and/or get a larger pilot jet, and likely main jet or raise the needle by dropping the clip one notch

That is not a fact in every case, and in some instances, it's the exact opposite of what happens. A change in pipes does not even dictate that any change in jetting will be required, and in a great number of cases, the same jetting that worked with the stock exhaust works just fine with an aftermarket pipe. Where the core size of the muffler is increased significantly, the result can be that the engine runs excessively rich with the new pipe.

Generalized axioms such as this lead to trouble when people assume they are true, rather than simply basing their tuning work on what the engine tells them it needs.

  • matt4x4

Posted May 22, 2008 - 10:26 AM

#4

In some instances, yes, in most instances, you add a performance pipe you'll be running lean due to the fact that you are allowing more air through the system because of less restriction in the pipes, as soon as you do that, you need to add more fuel to compensate, thus going with larger jets.
9/10 times when I see someone type they added an aftermarket pipe and their bike started to backfire (as the OP stated), it IS running too lean.

  • grayracer513

Posted May 22, 2008 - 11:10 AM

#5

When more air flows through the system, more fuel is automatically drawn through the carb, since that is how they operate. In most cases where one adds an aftermarket pipe and ends up with a decel backfire, an air leak in the new exhaust is at fault. Looking more closely at the original post, you will see that the came with the pipe that is on it, so the pipe change logic does not follow.

The fact is that most YZF's were and are delivered with stock jetting that makes them lean. Those who add a pipe first and rejet later naturally find some improvement in going richer, then incorrectly reason that it needed to be jetted because of the pipe. Those who jet it to run correctly with the stock exhaust usually find that only very minor tweaking of the needle setting is required when they add another pipe.

Additionally, the myth of the need to run rich is further perpetuated by people who find them selves driven to suppress all traces of decel backfire, no matter how slight, in spite of the fact that it is an entirely normal thing for a YZF426/450 to do, and in moderation is an indication of correct pilot circuit jetting. Also, the obsession with eliminating any bog when the throttle is snapped open from an idle, even though this is never actually done while riding, produces bikes that nearly drown in their own fuel.

The majority of riders who jet their own bikes are, in my experience, running jetting that is excessively rich, to the point of being counterproductive much of the time, and only get away with it because four-strokes tolerate it so well. If your bike is anywhere nearly correctly jetted now, I could bump your main jet to the next size, and I'll bet you a beer you'd never know the difference.

  • BASSic

Posted May 22, 2008 - 12:08 PM

#6

The majority of riders who jet their own bikes are, in my experience, running jetting that is excessively rich, to the point of being counterproductive much of the time, and only get away with it because four-strokes tolerate it so well. If your bike is anywhere nearly correctly jetted now, I could bump your main jet to the next size, and I'll bet you a beer you'd never know the difference.



If most people don't notice, then how can you be certain that you're jetted correctly?

  • grayracer513

Posted May 22, 2008 - 12:58 PM

#7

If most people don't notice, then how can you be certain that you're jetted correctly?

Spark plug reading used to be teh simplest way, and still can be used, but it's much more difficult with unleaded fuel. The process is to make run at whatever throttle range you are checking (like full throttle for the main jet, etc.), then chop the throttle, pull the clutch, and kill the engine. Once stopped, you pull the plug and check the color. With lead, the insulator would be a light to medium tan color, but with unleaded, the insulator will come out bone white except for a very narrow band of color at the extreme base of the insulator, down in the plug. Reading this is tricky and less than accurate.

Gas analyzers are the ultimate, but off of a dyno, they're hard to use, and they're expensive. So, you experiment. Go richer in stages from a starting point until performance suffers from it, then leaner, then take your best guess in between. If you have a measured course to run it on where variables like traction can be limited and/or another bike to run against as a constant, that will help.

Here's Eddie's comments on the subject:

http://www.thumperta...699#post2881699

Idle jetting is a little simpler:

http://www.thumperta...ad.php?t=327405

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • matt4x4

Posted May 23, 2008 - 05:10 AM

#8

Greyracer - I understand all that, but if a bike backfires too much on decelleration, it's usually pointing to the pilot jet, in which case, you have to adjust the fuel screw out some, then, if you're 2.5+ turns out, you have to go to the next size pilot jet and start over with the adjustments - I pretty much stated that above.
Either way (afermarket pipe leaning the bike out or not) it's where he should be starting once he's certain the carb is clean.

  • grayracer513

Posted May 23, 2008 - 06:41 AM

#9

... if a bike backfires too much on decelleration, it's usually pointing to the pilot jet, in which case, you have to adjust the fuel screw out some, then, if you're 2.5+ turns out, you have to go to the next size pilot jet and start over with the adjustments - ...

And if the backfire is caused by an air leak in the exhaust, you will end up with a bike that has a #52 pilot, sluggish off-idle performance, fouls plugs, dies on a throttle chop (idle down, worse with auto clutches), and still backfires. The exhaust has to be addressed first and eliminated as a cause if decel backfire is the concern. Idle circuit performance should be based only on how the engine runs, using the method Eddie laid out in the second link I posted, whether it backfires or not when you're done.

  • matt4x4

Posted May 23, 2008 - 07:20 AM

#10

Sounds like you just want to argue - I know you know a lot of stuff, but you sound pretty condescending with your posts.
Realistically - if you go from a 42 pilot, to a 45, to a 48, etc, I think you'd figure out after moving up 2 sizes that the pilot is not the problem...common sense tells you that.
Almost every bike I've worked on it's been carb related, there have been a VERY small percentage of bikes where an exhaust leak was the problem - I tend to troubleshoot from the common problems first, tends to save time, since it's easy and cheap to swap a pilot to the next size, then that's what I do, if that doesn't fix it, THEN I'll look elsewhere. This just works for me and is only meant as advice, take it or leave it.
I see far too many posters telling people to rebuild this and that and completely overhaul their bikes when all they needed to do was change the dirty oil out.

Like you, I only try to provide people help that will actually work without draining their bank account in the process.

  • macca1810

Posted May 24, 2008 - 04:13 AM

#11

Hey fellas i did some mucking around today and found that i could get the bike to run sweet with no gutless spots and drop back to a lower idle instantaneously when off the throttle if i have the hot start (red choke knob) out while running and riding it. I'm not a mechanic but from that i can draw that it isn't getting enough fuel as i think a choke restricts air slightly making it suck more fuel. I guess a new jetting mix could help so i'll buy one of those jetting kits and have another play and see what i can get out of it Thanks for the help fellas

  • grayracer513

Posted May 24, 2008 - 07:33 AM

#12

You've reached the wrong conclusion on a couple of points. The hot start is not a choke, and the "choke" on the FCR carb is not a choke in the true sense, like that in a car carb, either. The FCR cold start knob opens a circuit that allows additional fuel (and some air) into the intake stream to enrich the mixture for cold starting.

The Hot Start, though, is a simple air bleed that pipes air from above the slide into the intake, and if it runs better with that open, it clearly indicates that the bike is too rich.

I forget, did you ever check the plate on the back of the slide? It needs to be installed with the rounded edge up, not down, as many people will do by mistake.

Just another point of curiosity, is your red hot start plunger above the black choke knob, where it belongs? I wonder if they can be switched.

  • macca1810

Posted May 24, 2008 - 06:29 PM

#13

Yeah well like i think i said earlier i'm a newbie on these thumpers from the two strokes and am only drawing on some knowledge of other motors. I haven't pulled anything apart yet cause i don't have a manual yet and can't download for whatever reason those internet ones.
Try and diagnose for me grayracer:
-Red start knob is below and sits further out from carb
-Runs near perfect with red knob out and playing with mixture screw
-With red knob in it will stick to a high idle when decelled
-Has an aftermarket pipe
There were some other points but i can't remember them at the moment
Thanks for clearing up some of my points

  • grayracer513

Posted May 24, 2008 - 08:04 PM

#14

First be sure your air filter is clean.

Do you know what kind of pipe the bike has on it? Does it have a section at the back that has a small stack of plates in it, and the exhaust comes out from between them?

It would be a good idea to check the valve clearance be sure you don't have an intake valve too tight, but your problem is apt to be in the carb.

Look at the back of the throttle slide. Their is a plate there that should look like # 11 in the illustration here. If it is inverted, or has a crack, or a piece of the bottom edge missing, it could be your problem.

On the manual, did you try this site?:

http://www.yamaha-mo...ice/manuals.jsp

I get errors from the Australian site on the 2000 model. Let me know if that works.

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