Check this out and give opinions!

Is this lean or rich? Pops and crackles a little when I back off and it also pops flames every now and then so I've assumed running rich. Goes incredibly well! It's a 09 yz250f and it was only idling for a minute or two at the most when I pulled up. Haven't touched anything on the bike since I got it about a year ago and I've put about 15-20 hours on it. Should I be concerned??

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Pops can mean lean pilot or bad seal in the head pipe, usually at the head, but sometimes at the mid pipe. Glowing Ti head pipe is not unusual. The flames are unspent fuel mixing with oxygen in a hot leaky head pipe. First thing, pull the exhaust, replace the gasket between head pipe and head. My guess is you will be done after that. If you still pop, which I doubt, it's lean, not rich.

You should do the exhaust first, but you can try turning the fuel screw out 1/4 turn at a time to richen the pilot circuit. Next step is to go up a step on the pilot. All easy stuff.

If you haven't checked valve clearances since you got the bike, do so. Also easy. Not worried if you are a little tight on the exhaust (hope you are just in spec) but if you are loose on the exhaust (out of spec) that's carbon build up and you'll need to pull the head and clean it up.

I figured glowing so much might be from being rich and it's throwing so much flame out instead of a small efficient combustion on a correctly set up carby. Just concerned coz I've been riding it for a while like this and didn't want to touch it since its going extremely well.

It can be counterintuitive. I agree don't touch the carb yet. Fix the exhaust first.

Remember, leaner is hotter. If you Google decel pop or lean pop you will learn a lot.

All normal. Do not let your bike sit there idling. I must be moving to get the airflow needed to keep it cool.

 

Beware, if you mess with the pilot jet/fuel screw (which are intended only for idle) to resolve a lean decel condition (caused because you race bike does not use a ACV), you may find hot starting much more difficult.

 

Check your exhaust for leaks, replace the packing.

That's the stainless fmf mega bomb by the way. It gets that red relatively quickly when idling like in a minute the very front glows but a fee minutes and it can spread a fair amount. That pic was after it sat for about a minute idling when I was out riding still sound normalish? Like I understand of course they glow but this just seemed a little more than average.

Starts perfect when hot also. When cold just a 3 slow kicks with choke and one good kick and she's goin.

I'm sure this isn't the right place for this and yea I've researched but there weren't any pics and my 06 one didn't glow as much as this one from memory which is why I'm a little concerned. Also figured if it was lean it probably would be dead by now lol.

All normal. Do not let your bike sit there idling. I must be moving to get the airflow needed to keep it cool.

 

Beware, if you mess with the pilot jet/fuel screw (which are intended only for idle) to resolve a lean decel condition (caused because you race bike does not use a ACV), you may find hot starting much more difficult.

 

Check your exhaust for leaks, replace the packing.

Hmm now I'm a little concerned. It is a fairly rapid popping on deceleration which from what I just read is lean =/ the damage is probably already done then huh.

In every likelihood you have done no damage. Will was kind enough to jump in, and he will check me if I am wrong. His point about the fuel screw is important to give you a heads up on potential consequences so you don't get frustrated later. Keep track of any adjustments you make to anything so you can go back if necessary.

Now, you have two seals in your exhaust system and both have to work. Your header will be much hotter if they don't because unspent fuel (oxygen and gas) are being burned in the header. The head pipe gasket is like $6 and the midpipe gasket is closer to $20. I suspect since you have an aftermarket head pipe that it wasn't properly sealed in the first place. Fix it and your problem will go away. I had an 06 KXF that glowed normally after sitting for a minute, but before I sealed the system properly it popped like crazy and you could read by the glow of the pipe. While your exhaust is off, re pack it. Your packing is probably fried.

If you are truly lean, a new pilot is like $5 and takes 5 minutes to swap out. Cross that bridge later.

You cannot hurt a modern 4S by being lean. At worse, the bike simply will not be able to run or be very low on power.

 

You need to take a step back, forget the old wives tales, and perhaps read some jetting books.

 

You select the pilot and adjust the fuel screw for a perfect hot idle.

You select and set the needle for perfect steady state, under load running. Of course tested at various thrott;le positions to determne where on the needle, if any, a change is needed.

You select the main to permit WOT runs.

You then set up the AP to prevent bogging.

 

 

You own a race bike,. Not a bike meant for riding around the yard or putting on trails. It has no 'feature' to control popping on decel. Racers do not coast., They go flat out then hard on the brakes.

 

You also have a 'race pipe' (if you have the stocker, put it back on) that makes decel popping worse.

  • skweggnogskwash,

  • as a neophyte let me try to help you...I have only been riding 2 years and last year the same thing happened to me (i installed a powerbomb header) and i freaked out....I can assure you that a glowing header is nothing to worry about, also as far far as popping on decel that too is somewhat normal...as william said it is a RACE bike, it is not meant to coast or put put around, throttle should be on or you should be braking...NO COASTING.Also do yourself a favor and install an adjustable fuel screw and then google  "hanging idle yz250f" and "popping on decel yz250f" that should answer alot of your questions, but again glowing header and slight popping on decel are normal. Also, use TT as a resource, i can not tell you how much I have learned from this site, it is amazing. Last but not least try to read posts from KJ, William1 and sunruh they know their shite.

Thanks for the information! Was a little concerned because of the old wives tales! It goes incredibly well which Is why I thought it was setup fairly well it's just that because of the extent of the glowing I was a bit concerned.

You cannot hurt a modern 4S by being lean. At worse, the bike simply will not be able to run or be very low on power.

 

You need to take a step back, forget the old wives tales, and perhaps read some jetting books.

 

You select the pilot and adjust the fuel screw for a perfect hot idle.

You select and set the needle for perfect steady state, under load running. Of course tested at various thrott;le positions to determne where on the needle, if any, a change is needed.

You select the main to permit WOT runs.

You then set up the AP to prevent bogging.

 

 

You own a race bike,. Not a bike meant for riding around the yard or putting on trails. It has no 'feature' to control popping on decel. Racers do not coast., They go flat out then hard on the brakes.

 

You also have a 'race pipe' (if you have the stocker, put it back on) that makes decel popping worse.

 

Help me out here, Will, because I am unclear about what the wives tales are, but in case it has something to do with sealing the pipe I will ask if a recent case-study last week makes logical sense to you. 

 

2011 KX250F went through a complete motor rebuild.  No carb on this guy.  Ran like a top before rebuild, no changes to jetting cause there aren't any jets.  Took it out to play and the thing popped like firecrackers on decal, which had never happened before.  To us, given all of our experiences at tracks and bikes of our own is not all that common, even if "normal" to hear decel popping.

 

In addition to this new and annoying popping, the header pipe got so hot that it melted the right rad shroud to the point where plastic was dripping onto the header pipe.  That's pretty hot.  This shroud has been in place for years, and never melted before . . . so, I figured we had a problem.

 

My theory is that the problem was a lazy mechanic failing to remove a copper crush gasket and just layering another one on top of it, which prevented a tight seal of the head pipe to the head, and that subsequent correction of that problem resulted in zero popping and no more melting shroud.

 

Also, OP may not have a basis for comparison, but we did.  Bike ran like crap compared to it's usual explosive self.  Now, I did re-cut the exhaust valve seats combination with re-fitting the pipe, but I don't think that was the problem.  

 

Am I off base here or what?  Maybe I just completely missed your point.  I think if OP seals that pipe, he will be a happier camper.  So will his neighbors, lol.

You probably hit the nail on the head with the doubled up exhaust gasket. It probably also moved the header just close enough to the shroud. After a pipe is installed, a close eye should be given concerning how the pipe 'lays'. When I istall a exhaust, it is down with all bolts loose. Midpipe bolt is tightened first, then the header and muffler gently moved to ensure they are at the point where they are 'comfortable'. Then I tighten the header and the muffler. A lot of people crank down the header, then the muffler, tightening from front to back. This can tweak the header to head fit.

 

A bike that suddenly (either slowly over time, after a crash or right after a 'freshening') it is due to cam timing (rare) vacuum leak (rare) exhaust leak (very common) and/or exhaust packing (very common).

 

The 'wives tale' I as refering to earlier were the ones regarding being lean and the engine always being ruined. That is a hang over fro the air cooled 2S days of the 70's and before. To get max Hp., you had to run on the edge of lean. Go over the edge and a piston with a hole in it was the result. Because of the lack of power valves and reeds, fouling was a issue so a tendency was to try to use a hot plug to burn off the accumulated spooge. It was a fine line. But with the davent of reeds, power valves and water cooling, the engines (2s) became much more forgiving. Now the issue with them is improper warm ups and poor premix leading to seizing. With a 4S engine, too lean will simply not run. Sure, flat out Bonneville runs too lean will cook an engine but we are working with MX bikes and not road racers or LSP ones.

 

A 4S at idle uses so little fuel that the difference between too lean to run and too rich to start is about the amount of vapor on your breath after a drink of water. Look at the difference in hole diameters of a 42 and a 45 pilot. Remember that a fuel screw total adjustment range is only about that amount of difference. If you could spit gasoline, each goober would run the engine for 15 seconds or more. Spit on an exhaust, and a spot the size of a dime will be cooled, certainly not a 8" length of pipe.

 

So what I see when people complain of glowing headers is because they just installed a thin walled aftermarket one, finished up at sundown, let the bike just sit and idle, then are shocked to see it glow red. Exhaust gases at the exhaust valve can be 1,600 degrees. A thin wall pipe transfers this heat readily. If you took a temp reading of a stock system just before the muffler and then on the same bike, same point with an aftermarket, you'll find the after market is cooler because the thinwall pipe sheded the heat.

 

As far as decel popping, people remove perfectly good (although probably heavy) OEM systems because 'they need the power'. The lack of muffler 'unmasks' existing popping. They make the mistake of doubling the header gaskets (like you mentioned), having none at all (the original is stuck to the stock header) or the fit is poor and it just leaks. I have also seen a lot where the mid pipe fit is really poor (usually on slip ons). Anytime an exhaust is removed, the gasket in the head should be inspected. After inastall, the sytem checked for leaks (wet towel over the end of the muffler, listen for hissing leaks.) Aftermarket systems need regular repacking (few owners ever do this) and the system checked for leaks anytime the bike is dropped.

Valve adjustment time! Fixing that popping on deceleration. My kx250f did the same thing, not as red but I never really let it sit and get that hot.

You probably hit the nail on the head with the doubled up exhaust gasket. It probably also moved the header just close enough to the shroud. After a pipe is installed, a close eye should be given concerning how the pipe 'lays'. When I istall a exhaust, it is down with all bolts loose. Midpipe bolt is tightened first, then the header and muffler gently moved to ensure they are at the point where they are 'comfortable'. Then I tighten the header and the muffler. A lot of people crank down the header, then the muffler, tightening from front to back. This can tweak the header to head fit.

 

A bike that suddenly (either slowly over time, after a crash or right after a 'freshening') it is due to cam timing (rare) vacuum leak (rare) exhaust leak (very common) and/or exhaust packing (very common).

 

The 'wives tale' I as refering to earlier were the ones regarding being lean and the engine always being ruined. That is a hang over fro the air cooled 2S days of the 70's and before. To get max Hp., you had to run on the edge of lean. Go over the edge and a piston with a hole in it was the result. Because of the lack of power valves and reeds, fouling was a issue so a tendency was to try to use a hot plug to burn off the accumulated spooge. It was a fine line. But with the davent of reeds, power valves and water cooling, the engines (2s) became much more forgiving. Now the issue with them is improper warm ups and poor premix leading to seizing. With a 4S engine, too lean will simply not run. Sure, flat out Bonneville runs too lean will cook an engine but we are working with MX bikes and not road racers or LSP ones.

 

A 4S at idle uses so little fuel that the difference between too lean to run and too rich to start is about the amount of vapor on your breath after a drink of water. Look at the difference in hole diameters of a 42 and a 45 pilot. Remember that a fuel screw total adjustment range is only about that amount of difference. If you could spit gasoline, each goober would run the engine for 15 seconds or more. Spit on an exhaust, and a spot the size of a dime will be cooled, certainly not a 8" length of pipe.

 

So what I see when people complain of glowing headers is because they just installed a thin walled aftermarket one, finished up at sundown, let the bike just sit and idle, then are shocked to see it glow red. Exhaust gases at the exhaust valve can be 1,600 degrees. A thin wall pipe transfers this heat readily. If you took a temp reading of a stock system just before the muffler and then on the same bike, same point with an aftermarket, you'll find the after market is cooler because the thinwall pipe sheded the heat.

 

As far as decel popping, people remove perfectly good (although probably heavy) OEM systems because 'they need the power'. The lack of muffler 'unmasks' existing popping. They make the mistake of doubling the header gaskets (like you mentioned), having none at all (the original is stuck to the stock header) or the fit is poor and it just leaks. I have also seen a lot where the mid pipe fit is really poor (usually on slip ons). Anytime an exhaust is removed, the gasket in the head should be inspected. After inastall, the sytem checked for leaks (wet towel over the end of the muffler, listen for hissing leaks.) Aftermarket systems need regular repacking (few owners ever do this) and the system checked for leaks anytime the bike is dropped.

 

Got you.  Always appreciate when you take time out to teach, Will.  Thank you.

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