What causes it to miss and backfire on one cylinder?

SOURCE: 46 cc Sears/Poulan chainsaw will not start

Four things I can think of:
1) There is a short in your spark plug wire. When you bend it to connect the plug in the machine it allows the exposed cable to contact the engine block. Examine the wire all the way to the coil.

2) Flywheel key is sheared ( you obviously know how to take off the flywheel since you checked the seals).

3) Flywheel air gap is not set correctly. Loosen the coil. Put the cover of a notebook or the cardboard piece of a cigarette pack between the magnetic portion of the flywheel and the coil. Tighten the coil down. This will get you the proper gap.

4) Your cylinder/piston or rings are scrubbed out. Even though it passed a compression test, on rare occasions it will loose compression only when it fires. So do the following:

Remove your exhaust manifold. Look into the cylinder with a flash light. You are looking for scratches. Anything larger then a light fingernail is definite proof you need a new cylinder/piston assy.

If it passes that test do the following:

Pour mixed fuel straight into your cylinder until 1/4 full. Slowly pull your saw over while looking into the cylinder. If you see bubbles after the ring/rings go by then you need a new cylinder/piston assy. On rare occasions if no scratches are present on the cylinder wall or piston, you may be able to change just the rings.

First if you can get a inline spark tester so you can see if your getting spark with the plug installed.

SOURCE: yamaha fazer is miss firing

I have had the same problem. It is not the spark plugs - although it is always a ggod idea to change them to eliminater them as a potential issue.
The Spark plug holder can be a problem - in my case the wires leading from the ignition lead into the holder - had started to work loose- a common problem apparently. Solved by changing the holders.

SOURCE: 1979 suski 750,bike runs on 2 left side cylinders,

Hi and welcome to FixYa,

With the indulgence of craigjamesbt...

A few additional ideas to consider:

• it would be an electrical/ignition issue if 1 & 4 or 2 & 3 will not fire but not 3 & 4;
• almost always, raw fuel on exhaust indicates a flooded engine. A flooded engine would most likely be a carb issue;
• best way to determine if it is an engine concern is to do a compression test;
• a way to test the right carb pair is to deliberately starve them of fuel by temporarily plugging/blocking the inlet port/seat with rubber similar in appearance as to those used for glue gun, not installing the needle and float. Assemble the carb and install. Start the bike and use/spray carb cleaner on the right side carb. Note if 3 & 4 will fire even intermittently.

SOURCE: misfire forward cylinder

First you'll need to get a jet kit from West coast star. Then you'll need to jet the carbs to run equally. This may take some trial and error. Another thing that worked on my bike was the upgraded coil kit they sell. It's almost double the voltage of the stock coils and comes with better insulated wires for a more uniform resistance. I haven't had this problem since. It'll make the cylinders fire evenly and save you from those pestiferous fouled plugs. I hope this helps.

Arnie Burke

• 7339 Answers
• Posted on Sep 04, 2009

SOURCE: Hi I have a 1989 Yamaha Virago the rear cylinder

better look at carb-there r 5 diff jets but its probly the mid range one-be careful here because rong ajust can hurt engine-u hav a clymer manual? library has many books on mc maintenance-sayonara