Yamaha Badger - Hard to Start


My son has a 4 stroke Yamaha Badger that I am having a hard time starting. It has electric start and turns over with no problems, so it is not a starter or battery issue. If I roll it and pop it into gear it starts right up.

Is this a CDI issue? Can I test the CDI? I can fix this myself, but I lack the expertise to diagnose the problem. Any help would be greatly appreciated. Thanks, Rob {Rob W., ME 2000 Yamaha Badger (YFM80M)}


I don’t think the problem is with your CDI. When the CDI goes bad you can usually start it but the idle is all over the place & it just does not run right.

It is more likely that you have a loss of compression. When you pop (push or bump) start the Badger the piston is traveling fast enough to compensate for a loss of compression that prevents you from starting the bike normally.

The three main causes of compression loss are worn or broken piston rings, loose spark plug and improperly adjusted valves.

The first thing to do is check the spark plug. Is it seated correctly? If not, tighten it and try to start the quad. If this solves your problem, you are done. Nothing is ever that easy for me, so let us assume that the plug is fine and move on.

Now you need to check for compression. Do not just remove the spark plug and put you thumb over the hole. The puff of air you feel is not sufficient to tell you anything. You need a compression gauge. If you do not already own one, they are pretty cheap and readily available at your local auto parts retailer or try one of the retailers’ links below.

We use a Motion Pro compression testing kit. This may be a bit of an investment for your own personal needs.

You can also purchase a new or used compression tester on eBay. Just remember that you need one with metric threads small enough to fit your spark plug hole. Your spark plug is a C7HSA, I believe this will be a 14mm thread diameter. You can also look for a tester with a tapered end that you can just push into the hole, not thread in.

Good compression is 175 psi, plus or minus 10 percent, with the throttle held wide open. If your gauge shows 100 psi or less, there is your problem.

Next, check your valve clearances with a feeler gauge. Yamaha recommends that valves be checked every six months. Remember: always check valve clearances when the engine is cold and the piston is at TDC. The intake valve should be .10 to .15mm {.004 to .006in}. The exhaust valve should be .15 to .20mm {.006 to .008in}.

There are many causes of compression loss, but the fact that you can push start the quad and ride it, rules out most of them. If your compression is low and the valves are good, the most likely cause is the piston rings. Make sure that you order a top end gasket set with your rings. Damaged gaskets will also cause a compression loss.

Yamaha Badger CDI

In the unlikely event that your compression is good… Here are the CDI testing values you asked for. For the Yamaha Badger, the service manual lists Ω {Ohms} resistance values at 68° Fahrenheit {20° Celsius} so do not test it outside when it is freezing or if the bike has been sitting in the sun all day in the summer.

Using a Multimeter or Volt Meter set to Ohms, poke the white with blue tracer and white with red tracer wires. A properly functioning CDI will give you a value between 264 and 396 Ω.

For assistance with abbreviations or terms on this page,
please visit the Need to Know and Basic Tools pages.

Other Possible Causes of Compression Loss

  • Loose cylinder or head
  • Broken head gasket
  • Worn, damaged or seized piston
  • Seized cam or crank shaft
  • Improperly installed piston ring
  • Seized piston ring
  • Improper valve timing
  • Broken valve spring

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