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What causes the popping on de-acceleration?

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I can not seem to get this popping out of my '05 CRF450 and no one at the track could tell me if it was lean/ rich and which circuit it was.

Any help would be appreciated.

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A little popping is ok, how bad is it? If it is more severe, it usually means you are lean somewhere.

What jetting do you currently have, elevation and temp?

Try to pay attention to what you are doing before it happens. Are you hard on the gas, slow trail riding, etc? That helps determining where you are lean.

Try searching this forum. Lots of jetting suggestions for the '05.

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Well, it pretty much pops all of the time.

I changed nthe fuel screw settings, the pilot, the needle jet as well as the needle clip position, and the main jet multiple times. I feel like I should be way to rich w/ my settings but the bike is pulling real hard w/ great throttle response across the board. Regardless of which changes I have made to the jetting, it has seemed to make a minimal difference in the popping.

The bike is an all stock '05 CRF450 w/ the backfire screen removed.

Pilot up to a 45 from the stock 42. (hey it starts now!)

needle jet is the red JD needle in the second from the bottom clip position. (tried lots of stuff here w/ no perceptible change in the popping) Maybe I should go back to the blue needle?

180 main!! (Tried the 175 and I had no top end power, tried the 178 and I think I had the best power, now have the 180 in and it still pops)

All over the place w/ the fuel screw and there has been no change in popping.

The only change that has seemed to help is the big main jet but it still pops quite a bit.

Any suggestions on where to go? I really am starting to suspect that going back to the blue needle jet w/ the 178 main is the place to go. James, what do you think?

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Sun,

Popping on deceleration is one of those tricky things. Most people say it is from a lean condition (if the sound is coming from the exhaust) but it can be a rich condition (if the sound comes from the carberator) Popping in the carberator sounds more like a cough.

The 05' CRF450 seems to pop through the exhaust, so I would go with the lean theory. The lean condition can be caused by one of two things -not enough gas in the fuel air mixture or too much air. I know this sounds redundant, but how the fuel air mixture got too lean, IE too much air, not enough gas, is important to understand before you can fix the problem.

If you are to lean on the fuel side, it is easy - identify where the lean condition (popping) is occurring and add more gas to the mix through jetting. These are simplified and approximate - Fuel Screw - 0-1/8, Pilot - 0-1/4, Needle - 1/8-3/4 and main - wide open.

The second way a lean condition occurs is too much air. This usually occurs as an air leak. Intake side or exhaust. An Intake leak obviously creates a lean condition, but most people forget exhaust. The lean condition from an exhaust leak is created when fresh air is pulled into the exhuast pipe from the vacuum created on deceleration. This fresh air combines with unspent gas in the exhuast pipe and a secondary burn is created (popping). This usually is most noticeable on deceleration. This is were it gets tricky.

The only theory that I can come up with as to why the 05' CRF450 with a rich main (opposite of what you might expect) causes popping is if you have a rich main it will leave too much unspent gas in the exhaust pipe, especially if you accelerate hard and chop the throttle close. Those unspent gasses can ignite and cause popping, which mimics a lean condition. In theory, going down from a main that is too rich will leave less unspent gas in the exhaust and stop the popping. It worked for me.

You are exactly were I was a month ago. Check here http://www.thumpertalk.com/forum/showthread.php?t=218595&parentpage=3 All I can tell you is I went from a 178 main to a 175 and then to a 172 before the popping got down to a tollerable level. I didn't notice a loss on my top end. I have numerous friends who run a White Bros Pro Exhaust and they run a 158 main with no loss on top end. (I think a 158 is too lean)

The main is the easiest thing to change. Try a 172 and let me know what happens.

Not to confuse things, but improper valve clearences (IE duration) can cause popping too. But that is for another discussion and post.

Kevin

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improper valve clearences (IE duration) can cause popping too

Too tight or too loose?

I have bad popping on decel from steady half throttle with my 04' WR450 w/ 13.5:1 piston. I've gone from a 42 to a 58 PJ & fuel screw out 2 turns with only a small improvement. I've taken apart the carb several times (Including the security screws that hold the float basin on) and blown carb cleaner and compressed air through all the circuits.

So, all the things that can cause popping on decel:

- Pilot circuit too lean:

58 pilot makes little to no difference, fuel screw all the way out.

- Slide plate upside down:

Square edge is down ... that's correct.

- Vacuum leak on intake boot.

It's tight, and I've sprayed it with 4D-40 while running and it doesn't change the rpms.

- Exhuast vacuum leak. Possible. How do I check that? The doughnut is tight.

- improper valve clearence

New head & they were set by the dealer who rebuilt it for me. Possibly off, but it starts good.

- Bad fuel

Drained all the race gas and put in fresh 92 pump.

- Main too rich.

178 main. It runs great 1/8th to WOT though. I might try backing the main off a bit. (Good tip, thanks)

- PAJ too big.

It's still stock. Could go smaller.

- Hot start plug.

I had a problem with the plastic threads stipping and replaced it with a new one. Previously I think it was running with the hot start partially out. I may have gotten a minor amount of dust in though the hot start hole, but I've since cleaned it excessivly with carb cleaner. Could the oring not be sealing?

The popping and overheating are the only signs it's lean, and it only happens on decel after closing the throttle. Otherwise the bike rips.

Any other suggestions of things I should look at?

- Ryan

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Don't get overly excited guys. More than likely it's just a little bit lean on the pilot circuit. Refer to the sticky at the top of the forum to set the pilot circuit. A little popping is perfectly fine. The '06 YZF450's are notoriously lean from the factory and need to be richened up a lot. That is bad popping, a little crack during engine braking is perfectly normal.

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A little popping is normal yeah, but we're talking an Orville Redenbacher bakeoff going on under my seat whenever I let off the gas.

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- Exhuast vacuum leak. Possible. How do I check that? The doughnut is tight.

I took alook at this again and it wasn't tight. I didn't transfer the doughnut when I put the new head back together. It looked like alot of exhaust was leaking around the end of the headpipe.

- Ryan

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