Spark plug wont fire in cylinder

[TheNecromancer13]

So my spark plug fires just fine when I hold it against something, but when I try to get it to fire in the cylinder it wont fire no matter what I do to it. Any ideas? The engine is a grubee skyhawk raw racer 66cc and I assembled it last night and now can't get it to start cause of this.

 

[Davezilla]

These can be tough to start the very first time, but will start easy after this.

Since you know you have spark, next thing you need to check for is fuel... is it making it to the carb? and if so, is it going thru the carb? The reason I'm asking is because these tanks are notorious for having junk inside them that can clog up float valves and the main jet, this will stop an engine from running or make one die on you for no reason when cruising etc. Fuel filters are highly recommended for these because of this problem.

Other than fuel it needs compression, but you'll feel it if it's got good compression... Only thing to check here is that the head is bolted down good, a loose bolt can bleed off enough compression to prevent it from starting.

Once you know you got fuel, spark, and compression it should start and run.

 

[Davezilla]

I know a few other problems that'll cause them not to start like the magnet being on backward, it'll spark, but not at the right time... we'll go into that if everything else looks good.

 

[Ludwig II]

Have you tried a plug from a different production batch/lot? sometimes they fail in use but look ok outside the engine.

 

[2door]

How do you know the plug isn't firing after it's installed? Are you just assuming because the engine doesn't start?

Fuel, air and ignition are required for the engine to run. How have you confirmed fuel flow to the carburetor from the tank? Is the choke open? How are you trying to start it? Are you using the choke. How about the primer button, (if the carb is an NT)?

Give us some details and we can offer more suggestions.

Tom

 

[TheNecromancer13]

Fixed it. Turns out the stock spring in the spark plug boot was a piece of @#!%. After replacing the spark plug (the original was terrible) I switched the spring with another one I had lying around and now it runs fine. Plus I adjusted the air fuel mixture.

 

[2door]

The kit supplied spark plug boot is the weakest link in the chain. The first thing every builder of a motorized bicycle should do is throw that plastic thing in the trash and replace it with a good quality automotive boot and a spark plug with the top cap installed.
That kit boot is probably responsible for more trouble than any other facet of this hobby.

Tom

 

[Slogger]

The kit supplied spark plug boot is the weakest link in the chain. The first thing every builder of a motorized bicycle should do is throw that plastic thing in the trash and replace it with a good quality automotive boot and a spark plug with the top cap installed.
That kit boot is probably responsible for more trouble than any other facet of this hobby.

Tom

Listen to this man.
He speaketh the truth, he do!
I ran into the same thing, right off the bat.

 

[TheNecromancer13]

So how can I replace the boot if its attached to the cdi? Can I just cut off the wire, strip it, and solder it to the new boot or something? Or do I need a whole new cdi, and if so, where do I get one?

 

[maniac57]

The wire simply unscrews from the CDI box. Some newer models are glued, but it's still fairly easy to dig it out and screw in a new wire.

 

[2door]

The wire simply unscrews from the CDI box. Some newer models are glued, but it's still fairly easy to dig it out and screw in a new wire.

Just be sure you don't break the little threaded piece that protrudes from the base of the socket where the wire screws in when you're digging out the wire.

Unless you specifically purchase spark plug wire with a metal core (conductor) you're going to get 'resistance wire' from most auto parts stores. It doesn't have a metal core but a thread wrapped in insulation. You have to be sure the core is securely threaded into that little steel screw in the CDI.

Actually it is an often repeated myth that the kit plug wire is junk. In fact it is better wire than the resistance wire because it has a metal, usually copper, conductor. The spark plug and the spark plug boot are junk. But the wire, unless it is too short for your application is good. Don't believe me? take a resistance reading of a wire from the parts store and the same lenght of kit plug wire and tell me what you see. The higher the resistance the lower the voltage to the plug.

Tom

 

[Slogger]

Hey, this is a good chance to mention this-
That added resistance keeps the spark plug wire from acting as an antenna, transmitting interference across a lot of the EM spectrum. It shows up as static on AM radios and snow on tv sets. It can also interfere with those speedo computers and whatnot. While it does cost a bit of spark, it has its advantages.

 

[2door]

Hey, this is a good chance to mention this-
That added resistance keeps the spark plug wire from acting as an antenna, transmitting interference across a lot of the EM spectrum. It shows up as static on AM radios and snow on tv sets. It can also interfere with those speedo computers and whatnot. While it does cost a bit of spark, it has its advantages.

Resistance wire, or as it used to be known, TVRS, was made for just that purpose. It reduced or eliminated radio interference. In today's cars with all the electronic gadgetry it is a must. RF interference would certainly mess with a computer.

The difference is the power of the ignition system. Cars have much more, higher voltage, than our little bicycle engines so any resistance will impact how much spark is available at the plug. Not saying an engine won't run using resistance wire. It will just run better if you don't use it. Also the chances of getting a good electrical connection at the CDI and plug boot are better with metallic core wire.
Just something to keep in mind when you're looking for performance.


Tom