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Engine misfire Hi, I got this misfire problem when I ride more than 40km/hr or high rpm [CB125E don't have tachometer]. Have used new spark plug and thoroughly cleaned the carbie with carbie cleaner and air compressor. Bike has only 980km on the clock. Have adjust the fuel/air mixture screw but still no luck [carbie only have 1 adjustable screw other than idle adjuster]. A friend told me that it could be the coil and another said could be the valve clearance. I'm a bit reluctant to do anything on these 2 possibilities considering the bike has only done 980km. Thank you in advance for your reply.

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  • Motorcycles Master
  • 40,813 Answers

If it is under warranty , take it in for a warranty check
other than that it could be a problem ignition control module , breaking down at the high rpms
carby problems get worse with rpms and the idle /mixture screw is inoperable at high rpms
there is a possibility that it could be valve clearance for that cylinder but I would concentrate on the coil and ignition system area first

Posted on Apr 19, 2017

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Posted on Jan 02, 2017

  • 1 Answer

SOURCE: hesitation problems 2003 yamaha wr 450 f

sounds like a jetting issue. change the main jets starting from the largest one you own and work your way down.

Posted on Dec 14, 2008

SOURCE: idle speed problems

Hi Walker14,

You have to sync thecarbs and also you will have to reset the air-fuel mixture screws.
One of the carburators should be factory adjusted, I dont wn a kawasaki but I own a YAMAHA SECA XJ750R and thro it I am sharing my experince.
Once you have located the factory preadjusted carburator you can thn set the others to its specs. I use the feeler gauge to do for my bikes carburators and it works fine.
With regards to the mixture screw..... get the all in tightened but slowly aas to not damage the pin end of it!.
Start the bike and get it to normal running temp and thn set the screws 2 and half turns out...you may need to set your idling speed before you begin this setting of the mixture screws, you prolly will hveto go higher on revs or vicecersa ....onc eyou are at the mixture screws and adjusting them , you will surely note the differenc ein sound of he bike an its idling ..
Hope this will help.


Knight.

Posted on Feb 02, 2009

  • 1 Answer

SOURCE: looking for the air fuel mixture screws on carb any help?

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Posted on Apr 09, 2009

  • 3567 Answers

SOURCE: Yamaha DT 125 1986 - High Revs when idling

There have an air leak between the carb and the cylinder and/or the head and cylinder. Old hard / cracked rubber fittings and O rings are usually the problem. When extra air gets into the system it leans the fuel mix and the high idle is the result. It also means the engine runs hotter. Also, cracked or broken reeds can be a source.
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Posted on Aug 10, 2009

  • 3 Answers

SOURCE: Where is the idle screw, air/fuel adjustment screw on a VT750Cd

It just need to be perfect keep trying different air flows

Posted on Aug 18, 2009

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Factory settings for a walbro 997 on my remote control baja


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Mar 14, 2017 | Miscellaneous

1 Answer

Backfires after running and stops, must shut off to stop the backfireing. I have replaced the fuel filter, and checked the gas cap vent,what else could cause the backfire?


For a more specific answer, you should post the applicable elements of Make/Model/Year/Engine Model/Transmission Model/Deck Size, and hours on meter if you have one. A short description of the events preceding the beginning of the problem is also helpful.

Several possibilities for small engine backfire:
  1. Needs new plugs. Start with new plugs.
  2. Shear pin under flywheel is slightly damaged throwing off the timing. Remove flywheel examine shear pin for any damage. Just go ahead and replace it; if it even looks nicked or sliced.
  3. Coil, or spark plug wire, is going bad. Inside the Coil are two windings of wire and no moving parts. With shorted (melted together inside) windings you can still get a spark, but it will be weak. With open (burnt/broken) windings, you get no spark at all. There are also electronic components like diodes which may be failing. Coils are subjected to heat and vibration and do fail on occasion. Another coil consideration is the spark plug wire. Back in the day, we would replace all our spark plug wires when we gave our car a tune up; because spark plug wires do fail. The only way to replace the spark plug wire on a mower is to replace the Ignition Coil.

    Note: Coils may not fail completely at first. They can be complicit in the situation where a mower starts and runs for a while.They may cause backfire. As it heats up the coil parts expand and cause the engine to shut down. It does not re-start until the machine has cooled enough to allow the coil to cool. This can be 30 minute to several hours; perhaps it starts again the next day only to repeat the failure.

    Short of a well running engine, with no special equipment, the only field-test for a coil and the spark plug wire output is to visually assess the color of the spark at the end of the spark plug. Alternatively, you may use a spark tester. A spark tester gauges the strength of spark by making it jump a bigger gap. This test calls for a gap in the range of 4.2 mm (0.166 in). It's important to note that setting a test gap beyond 5.0 mm (0.200) could damage the ignition components. Briggs and Stratton manuals, for the newer engines, specifically state that you should not test for spark with the spark plug out. I do not know why this is. They specify you should use a spark tester.

    B&S notwithstanding (at your own risk), with a new plug attached to the plug wire hold the threaded part against a ground point or wrap some bare copper wire around the threads and fasten the wire to the engine frame or other good ground. Make your shop or garage dark so you can see the spark easily, then crank the engine. The flywheel must spin rapidly (at least 350 RPM). The strength of the spark is revealed in the color. A red or yellow spark is weak and probably will not spark in the cylinder. A blue or white spark is strong and has enough voltage to fight across the spark plug gap even under pressure within the cylinder.
    Note: a good strong spark also makes a strong snapping noise as it fires.
  4. Needs a valve adjustment or a valve job. This is high on the list of the most common reasons for backfire. Start with adjustment.
  5. The other thing that causes backfire is fuel mixture too lean. On a small engine this is not usually an adjustment problem. It may be the fuel system/carburetor is dirty or partially clogged. It may also be an air leak in the gaskets of the intake manifold. It might be low fuel pressure from a bad pump or clogged fuel lines.The electronic fuel shut off valve may be malfunctioning. If nothing else solves the backfire, check fuel flow through the pump and lines, remove the carburetor and clean or rebuild it. This is the least likely of the options so save it for last.
Here is a link to a great essay on magnetos and spark plugs.
Village Science Magneto Spark Plugs

For Twin Cylinder Engines
A power balance, or cylinder balance, test should be run on a twin cylinder engine. If compression is good and cylinder leakage is minimal (in other words if the engine is not worn out, doesn't have too many hours), but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel delivery or ignition problem (fouled spark plug or bad plug wire).

If an engine is hard starting, runs rough, misses or lacks power, a cylinder balance test will determine if both cylinders are operating to their full potential. You will need an engine tachometer, a spark tester, and an insulated screwdriver.
  1. Using the spark tester, with the plugs in, start engine and verify spark is equally good on both cylinders.
  2. Remove spark tester.
  3. Connect engine tachometer
  4. Run engine at top no load speed. Note rpm.
  5. Ground one cylinder spark plug with screwdriver. Note rpm.
  6. Ground other cylinder with screwdriver. Note rpm.
The difference in RPM between the cylinders should be less than 75 rpm. If the difference between the two cylinders is greater than 75 rpm, the cylinder with the least rpm loss is the weakest and is a potential problem.

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1 Answer

Engine keeps backfiring under load motor is an enduro xl 13.5 hp


Need Make/Model/Year/Engine Model/Deck Size, and hours on meter if you have one. A short description of the events preceding the beginning of the problem is also helpful.

Several possibilities:
1. Needs a valve adjustment or a valve job. Start with adjustment.
2. Needs new plugs. Start with new plugs.
3. Shear pin under flywheel is slightly damaged throwing off the timing. Remove flywheel examine shear pin for any damage. Just go ahead and replace it; if it even looks nicked or sliced.
4. Coil, or spark plug wire, is going bad. Very hard to test a coil for weakness without test equipment that the average person doesn't have. Check the first 3 problems, first. If it still backfires, then replace coil.
5. The other thing that causes backfire is fuel mixture too lean. On a small engine this is not usually an adjustment problem. It may be the fuel system/carburetor is dirty or partially clogged. It may also be an air leak in the gaskets of the intake manifold. If nothing else solves the backfire, remove the carburetor and clean or rebuild it. This is the least likely of the options so save it for last.

If compression is good and cylinder leakage is minimal, but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel delivery or ignition problem (fouled spark plug or bad plug wire). A power balance, or cylinder balance, test should be run on a twin cylinder engine.

If an engine is hard starting, runs rough, misses or lacks power, run a cylinder balance test to determine if both cylinders are operating to their full potential. You will need an engine tachometer, a spark tester, and an insulated screwdriver.

1. Using the spark tester, with the plugs in, start engine and verify spark is equally good on both cylinders. Remove spark tester.
2. Connect engine tachometer.
3. Run engine at top no load speed. Note rpm.
4. Ground one cylinder spark plug with screwdriver. Note rpm.
5. Ground other cylinder with screwdriver. Note rpm.

The difference in RPM between the cylinders should be less than 75 rpm. If the difference between the two cylinders is greater than 75 rpm, the cylinder with the least rpm loss is the weakest and is a potential problem.

Note: If the spark plugs are new this could be an indication of a weak coil. Of course it could also be compression (rings), or valves.

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