Motorcycle Forum banner

1 - 2 of 2 Posts

Premium Member
8,960 Posts
Discussion Starter #1
Eye asked about my use of hot and cold plugs. The short story.
Once upon a time bikes were mostly 350 and 500 singles, and they formed the main racing classes. They had a single carb and a magneto with a style of points. A primary chain to the gearbox and a cover on some that leaked oil.

For racing, the factories produced some exotic four strokes like the Honda six and the Guzzi V8. But the real hit and break through, for the average rider, was the Yamaha 2 stroke 250. They made as much power as a 500 Norton, and would win the 250 and 350 class at the IOM.

For us guys raised on 650 four stroke twins, these things were a bit different.
We had to be scientific type chemists to mix the oil and gas. We used a dial gauge on the piston crown to get TDC and the number of millimeter fractions BTDC for timing. To start we put in number 7 or number 8 NGK plugs. With zero throttle while pushing in second we would just ease the throttle open until they started to crackle. Run for about 5 minutes, then switch to 10, 11 or 12 plugs. Run it on the track at WOT and hit the kill switch. This is where the real science started. We needed to know if the plug heat range was correct, if the oil mixture and the fuel mixture was correct. Another item was the gear ratio.
The old way: I had been to C R Axtell shop in LA, and made hand drawn notes of how to read a plug. There is a lot more to it when you hear it from a pro. Prior to that I thought I knew what I was doing. I was a dummy.
The new way: NGK has a guide on line. It is easy to find.
Same thing with the fuel mixture. There is a guide on line.

A cold plug, number 12 say, is for an engine that will be run hot, as in WOT at the track, on a hot day. A 10 perhaps for an overcast day.
A hot plug, number 7, is for a cold motor to get it fired.
The NGK site has more detail on this.

If your street bike calls for a fairly hot plug, then it is dissipating heat quite well away from the combustion area.

The older two strokes would seize fairly often, but the pistons got better and they became quite reliable. The worst I owned was a Kawasaki triple. Kevin Cameron has explained the problem in one of the major magazines.

We needed to do this warm up plug changing act with an eye on the clock, for the start of the next race. On the line the engine was kept running around 3000 revs, there was no idle adjustment. When the flag dude appeared, around 8000, when the flag went up 10 to 12000, and as the flag came down WOT and the clutch was slipped out slower than anything ever imagined. A bit too quickly and the engine would bog and be very hard to recover. Meanwhile the herd was gone. 100 meters of clutch slipping would do it.

If everything was correct we would hit 12000 at the end of the main straight.
The best in the day on these things, was Steve Baker from Bellingham. He went on to become the first world champion from the US.

Unkle Krusty*

8,606 Posts
I also raced a lot of different bikes, classes and engine types, both 2 and 4 strokes and here's my take on what you said above Krusty, and obviously just my very humble opinion as usual::biggrin:

Normally in an air cooled engine a 7 plug as in NJK7 heat range, works perfectly if the bike is jetted correctly and not held wide open all the time.

An 8 plug in my experience is normally too "Cold" for the street and makes starting difficult but is probably better for Wide open throttle.

In 1974, I bought a brand new 1974 JAWA CZ 250 Motocross bike in a crate and after doing all of the PDI and setup work, took it out to the track to break it in. I absolutely couldn't get it to start no matter what I did. It was flooded all of the time. Later I discovered that the break-in plug from the factory was a 9 or 10 and the world famous JIKOV carb (Jack-off carb) was setup very rich. So home I went.

Got home and put a new NGK7 in and swapped out a relatively new Mikuni 32mm unit, kicked it once and it started right up and ran perfect for the 3 years that I raced it.

Now in all honesty, all of my racing experience was with air cooled bikes so all bets are off with the water cooled ones as far as jetting and heat ranges go.

I can read a plug easier than I can read my Kindle;)

1 - 2 of 2 Posts