Question about 2005 Suzuki RM-Z 450

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Backfire My bike has constant backfiring through the exhaust while riding down the road and also I hear a spitting noise somewhere in the engine. When it makes the spitting noise, you can feel it hesitate slightly. I changed the spark plugs and battery, and poured sea foam and b-12 in the gas.

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  • rickb1709 Dec 11, 2008

    My bike is fuel injected and I experience similar problems. It will run fine initially, but then I suddenly lose power. The engine seems to run too rich because I can actually smell the gas and when I stop gas will drip from my exhuast. When this occurs I get a F1 red indicator on my tank. Sometimes if I turn the bike off and re-start it, it resets and runs fine. I am not sure what power command module is installed on the bike, it was installed prior to me buying.



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Well, without physically hearing the noise it's a bit of a guessing game. However based on your description and past experience (all of which is from my past life as an auto tech of 23 years) it sounds to me like it may be a fuel problem, probably lean; generally a spark related misfire will occur more under load than steady state and a rich fuel condition will typically lead to bogging, while a lean condition is likely to cause misfires. I don't know if your bike is fuel injected or carbureted but judging by the vintage I'd guess it was carbureted. I'll assume that the engine runs fine under full load acceleration. You may have a carburetion problem (do you have single or multi carbs?) possibly a weak fuel pump although that would tend to show up more under heavy load than steady cruise or even over advanced timing causing detonation. I'm not sure of the specific systems on your bike so I can't offer any advice on how to check timing or examine the carb(s), but I can offer general procedures that may be helpful. Assuming again that you have carbs and a manual choke system, you could try putting a little choke on when the problem occurs to see if the problem goes away, if so this would prove the theory that the engine is running lean. One other thing I would do is to look at the plugs that came out of it (hopefully you saved and numbered them) and then pull the new ones back out and examine and compare them. Perhaps you could take some good pics and post them for us to look at and offer opinions on their condition and cause? Can you duplicate the problem in neutral? Is it only occuring at one particular road or engine speed or any steady speed? Does ambient temperature have any effect?

Posted on Nov 20, 2008


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Make/Model/Year/Engine size, these are helpful for a more precise answer.
However, you are describing a backfire problem that occurs when the engine is placed under a load.

The major contributors to backfire under a load (and in general) are:
1.Bad fuel
2.Weak Spark
3.Poor Timing
4.Poor seal due to valve considerations (adjustment and wear).

So here are the fixes:
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  2. Weak spark may be due to coil problems or bad coil gap. Check coil gap.
  3. Change fuel to the highest octane you can buy. Old fuel looses octane and does not burn as hot. Do not store fuel for more than 3 months.
  4. If your engine has a flywheel key, it may be damaged. This will throw off the timing.
  5. Check the valve adjustment. A sticky, bent, or worn valve can cause backfire. Worst case you may have to do a valve job (have the valves and seats reground).
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backfiring means you have an air leak or a lean mixture. Look for leaks at the header pipes or perhaps a crack in the exhaust.
Cold air getting into the exhaust is usually what causes the backfire noise.
You may have a lean engine running situation but the bike would have other issues besides the backfire.
Let me know if you are having any other issues or if it just the backfire.

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