A lot of the time on these forums the topic of timing is thrown around:
“My bike won’t start?” – “check your timing”.
“How do I detune my YZ250’s hit?” – “retard your timing”
“I’ve got detonation problems!” – “retard your timing”.
“I’m looking for more performance and am willing to run race gas” – “advance your timing”.
“I’ve just bought an aftermarket crank” – “make sure you check your timing as mine was 6 degrees retarded”
When should I test/change my timing, common reasons:
- To make sure it is set correctly because its easy to do
- You bought an aftermarket crank (set it back to zero degrees or otherwise)
- De-tune the hit when you get on the pipe (1 or 2 degrees retarded)
- To combat detonation problems (1 or 2 degrees retarded)
- You're playing with your squish/shaved down your cylinder head (only because this can cause detonation problems)
- Looking for minor performance gains and are willing to run race gas (1 degree advanced +) note: I am no expert on this, you should do your own research before considering advancing your timing.
- You can also use the process to check if your crank rod and main bearings are in spec. In theory, if there is any delay/free play in movement of the dial gauge when you rotate the flywheel then one of those is out is out of spec.
In this thread I intend to
- Explain how to put together a tool to check your timing using a dial gauge of FeeBay and an old spark plug
- Brief pictorial guide on how to test your timing
- What timing is
Build the tool
As you can see, its simply a dial gauge and an old spark plug I’ve chopped up and put on it. Works perfectly!
What you’ll need:
1. A dial gauge. I got mine for about $15 of FeeBay. No doubt its Chinese. Some people will tell you to stay away from Chinese and get a better quality one, that’s up to you.
2. An old spark plug.
3. A grinder or dremel to grind the metal lip off the spark plug ceramic. Pictures of this below.
4. A hammer and screwdriver.
5. A drill
Steps: (pictures of each below)
1. Grind the metal lip off the spark plug. This is what holds the ceramic part in.
2. Grind off the arm on the bottom of the spark plug.
3. Place the plug into something such that you can knock out the ceramic with the screwdriver and hammer.
4. You’ve now got your thread!
The next steps vary depending on the dial gauge you bought.
5. On mine I needed to drill out the hole a bit as it was too narrow to fit my dial gauge in. See picture.
6. I also needed to shorten it slightly as my dial gauge was not long enough. See picture.
edit: somewhere in this process you should consider adding a vent hole such that the compression when the piston is at TDC does not affect your reading. See Mark6299's post
Clarification: Shortened it slightly as described above in step 6
How to test your timing
Note, the Yamaha service manual has a section on how to test and set your timing to zero degrees. I highly recommend you refer to this, it’s better than my guide below.
Steps: (pictures of each below)
1. Remove your fuel tank. Drain your radiator fluid and then remove the hose that attaches to your cylinder head.
2. Screw in your new tool!.
3. Remove your flywheel cover.
4. Rotate your flywheel counter-clockwise, and while doing this pay attention to your dial gauge.
5. At a certain point your piston will reach Top Dead Center (TDC). At this point your dial gauge will stop and start turning the other way. Set this point on your dial gauge to zero degrees by unscrewing the set screw and rotating the face.
6. On a 2000 YZ125 my timing needs to be set 0.02”/0.52mm below TDC. Refer to your service manual for your number (I think YZ250s are 0.007"?). To do this rotate the flywheel clockwise (ie the other way to the way you were turning it before) until it gets to 0.02”/0.52mm (depending on your dial gauge). Note my bike was very slightly advanced.
7. Look at the notch on your flywheel. Is it perfectly aligned with the mark on your stator? If not, loosen the stator screws and align them.
8. You have now set your timing to zero degrees!
Yes I know my bike is pretty dirty, but I’ve spent heaps of $$$$$ time and effort on it to make sure it runs great!
And yes the engine mount is broken lol
What timing is:
The Wikipedia article below explains it better than I can.
At its simplest – timing refers to at what point the spark plug fires. So by setting my timing to 0.02”/0.52mm Below TDC, that means that when the piston is 0.02”/0.52mm below the top most point of its cycle, the spark plug fires.
What a minute, wouldn’t it make more sense for the spark plug to fire when it is at absolute TDC? No – to quote Wikipedia: “The need for timing the spark is because fuel does not completely burn the instant the spark fires, the combustion gasses take a period of time to expand”
Changing your timing such that the spark plug fires further from TDC (ie advance the timing) or closer to TDC (ie retard the timing) has various implications. I won’t discuss those here, nor am I an expert on them all.
Note that aftermarket cranks are notorious for being set several degrees retarded from the factory and so you should always test your timing if you buy an aftermarket crank.
Mostly, the TT community. I've learnt so so much on here and saved so many $$$ by learning how to do things myself.
Carver's original YZ250 timing thread
Pete's bike blog
Mark6299's post regarding a vent hole
Edited by tim512, 19 December 2011 - 07:14 AM.