Hotter Plug. What are the trade offs?

[Kenneth S (222)]

Hi Tech Heads,
My mate and I have been having a wrestle with this 74 CZ 250 trail bike. Out of 3 ride days so far it has only started once. Lucky he didn't have access to a box of matches last time!
We took it to a local bike shop and the guy put a hotter plug in. The thing started right away and has been starting in the first 5 kicks every time since and seems more responsive in the lower rev range.
I have never heard of this before. Perhaps the guy we bought the bike from had the wrong plug in it from the beginning, we haven' been able to locate the exact manual for this model yet.
Is this common practice?
What is the down side?
What should we be looking out for?

[chrisdespo]

using hotter or colder plugs is a bit like fine tuneing your jets if you get my drift. the plug are not so different they just disapate the heat slower or faster, if you look up inside the plug you will see that the porcelin insulator comes in contact with the plug body at differant depths the further in the longer it takes to get rid of the heat that makes it a hot plug . if your just a bit rich (to much fuel) a range hotter in the plug may save you doing to much jet swapping , like if you had tuned your bike at sea level say at frankston and you whent and raced at say the old Marysville track your bike would be rich if only by a little change to a warmer plug and you would find that you had the coulor a bit better , but nothing is better than having the right jets for the right altitude or weather conditions hence why we have practice so you can get the mixture correct.

[bazza]

chris is correct.   If he gets jetting spot on,timing spot on and starting is normal you could go back to normal heat range. Even Av gas may make start easier than crappy fuel from local gas station.

[Lozza]

http://www.youtube.com/watch?v=LbR75wq9nmM&feature=player_embedded#!

[Kenneth S (222)]

OK, Thanks guys, that helps a lot. A lot of stuff I did not know.

[Viper666]

All before are correct but if a bike is jetted to standard without too many mods & you need a hotter plug (Smaller number) then this implies weak spark/coils failing. A hotter plug is a cheap fix but I would bet that it is only a temparay one.
Viper666

[Viper666]

Forgot to add I have seen quite a few engines with holes burnt through the piston because of the use of hotter plugs to fix a weak spark issue, wrong jetting or low compression.

[Lozza]

All before are correct but if a bike is jetted to standard without too many mods & you need a hotter plug (Smaller number) then this implies weak spark/coils failing. A hotter plug is a cheap fix but I would bet that it is only a temparay one.
Viper666

+1

[Kenneth S (222)]

Apart from lining the tank, changing to fresh Caltex fuel, replacing the petcock, thoroughly dismantling and cleaning the carb, we've gone even one hotter and the colour of the plug is almost perfect now but it continues to be a bugger to start when cold. When it does go it goes well though and the smoking has cleared up as well.

What is the secret to starting it cold when it hasn't got a choke?

[Montynut]

Kenneth
It sounds to me like you have weak spark at low (kick) speed. There are several possiblities. If it is electronic then weak magnets in the flywheel could be a cause or stator coils that are on the way out with low resistence between turns. If points check condenser or replace and check points for bad connections or burnt contact surfaces. Could also be magneto coils on the way out the same as in an electronic stator. If it has a modern after market system many need to be at a higher speed than the older ones before they trigger. Sure sign of this is if it will not kick start but starts instantly when push started.

Buy far the most common cause of weak spark at low engine speed is a high resistance earth circuit.

Remove the points backing plate and clean the contact between it and the engine same for stator/magneto backing plate. When I say clean I mean remove all oil/grease and make a good clean metal to metal contact.

Check that engine mounts are clean, rust free, paint free between engine and frame on at least one but preferrably more mount points and make sure all are tight.

Remove and clean back to bare metal between the coil and frame mount.

Replace any connectors that even look the least bit suspect and put a new plug cap (non resistor type) on the plug lead after checking it for high resistance.

Even run a temporary wire from the stator/magneto backing plate to the coil mount to provide a perfect earth return for the ingition system and try. This will tell you if you have an earth problem very quickly.

The amount of energy and the voltage present at kick over speed is very low so it is easily affected by any high resistance in the circuit that should not be there.

[Kenneth S (222)]

Thanks Greg, that's excellent info. I'll PM you to confirm the best order to tackle all possibilities

[tony27]

From your comment about no choke I'm guessing you're still using the standard Jikov carb, even holding down the tickler on top long enough for fuel to come out the overflow I always found starting to be hit & miss. A mikuni helped out big time