Spark Plug Gapping And Other Info

[90GTVert]

I mostly gap my plugs to 0.026-0.028" for the Minarelli or 139QMB. Should be about there out of the box, but check. 0.030" is the most gap I recall using. Large gaps may make a larger spark, but can be too much for the ignition system, especially at high RPM. Setting the gap around 1/2 the recommended spec can help determine if the ignition system is causing problems. I usually set them down to around 0.014" for troubleshooting. If misfiring suddenly goes away with a small gap, most likely your ignition system is not up to par and you need to find the culprit.

Not checking the gap on cars was mentioned. I would suggest checking the gap on any plug you install, for car or scooter. Most of the time they're right, but stuff gets dropped or something is missed along the line here or there and it doesn't take long to check.

You can use the tapered or wire-style gap checkers or even feeler gauges. Wire or feeler gauges are probably most accurate and leave you less likely to damage thin electrodes like iridium plugs use.

I use the hole in a cheap round tapered style gap checker to open up the gap if it's too small.




Put the plug in there so the tip of the ground electrode is on one side of the hole and the other side of the hole rests against the base or threaded area of the plug as shown below.






Then pry gently and open the gap as needed.




You can get away with opening a gap by prying between the center electrode and ground strap on a conventional plug, but it's not a good idea and it can break or damage some plugs. Don't even think about gapping an iridium plug that way in a pinch. Tools like the one pictured above are on the counter at most parts stores for something like $1 last time I looked.


If the gap needs to be closed you can tap the ground electrode on a hard surface somewhat gently.

[Deleted]

I am now reading these threads because of the 1986 Yamaha Razz I acquired. Thought I would chime in here with some info about spark plugs that many never consider.

1.. ALWAYS check the plug cap on single ground electrode plugs, because you have no way of knowing if someone dropped that plug and bent the electrode.

2.. ALWAYS thread chase the cylinder head before installing new plug(s), carbon build up can and will prevent proper installation and can damage the head. Not to mention that a plug that is not seated completely "DECREASES" the compression ratio and reduces power

3.. REMEMBER to blow out the carbon from the cylinder with compressed air before installation

4.. ALWAYS use a tiny amount of Aluminum Anti-seize on the plug (first couple threads near ground electrode) to keep it from seizing to the head. This especially applies if you use Bosch plugs as they have no anti-seize coating on the threads

5.. NEVER wire brush the threads of an old plug and reuse w/o adding anti-seize

6.. ALWAYS use a torque wrench to install the plug. Over or under tightening can change the heat range of the plug.

NOTE: Using NGK plugs is a good idea on more modern engines, however IF you have a scooter like this Razz that uses standard 14mm plugs, you would be better served to use Champion or Bosch. btw.. Champion has more spark plugs patents than any other company and they have licensed their tech to AC, NGK, Autolite, and some others

Tech Tip: If you do not have a spark plug thread chaser, you can make one by using an old spark plug and a thin cutoff wheel to slice three deep grooves into the vertical threads from the ground electrode up