Ignition timing

[redmandan]

So i'm probably about to fire the engine for the first time tomorrow following a cam chain replacement and the deepest engine work I have ever attempted. Today I checked the valve timing as I have had the cams out (all just within spec) and after a few beers this evening something hit me:


Given that the crankshaft turns 720 degrees in one full rotation of the engine, when I re-installed the cams at TDC for piston 1, how do I know it's at TDC for piston 1 and not piston 3? Then I remembered that the HT leads are divided between 1+3 and 2+4. Does this mean that 1 and 3 spark at the same time regardless of whether the piston is in compression or power stroke? If so then it only matters that the cam chain dots lined up with the markings on the engine whilst the crankshaft is at TDC as the spark plugs spark twice per 720 degree rotation.


Or I could be wrong and tomorrow it will blow up.

[red98]

If ive read that right , your right , and if this is your first time at a top end rebuild you worked  that out well ....well done 


worst thing that can happen , If you have the timing out the piston can hit an open valve , bend It or damage the piston , ive done lots of rebuilds over the years and always check and double check all timing marks and before hitting the starter button turn the engine over by hand first just to make sure.....iam sure you'll be fine and will keep an eye on your posts tomorrow.........

[redmandan]

Thanks mate! I think my logic is sound I just wanted some confirmation.

I've had the head off a Yanmar marine diesel engine before but its easier when you're going to go +-10mph on a sedate river jaunt vs 70mph+ on 2 wheels!




I realize that smashing a piston into an open valve is probably the worst thing to do so I made sure to turn the engine over many times by hand before I buttoned everything back up. I had to anyway to check the valve timing.


This was all because I hated the tin can rattle of my top end and the most likely diagnosis was a worn cam chain.

[limax2]

I don’t want to cloud the issue because it sounds like you have done it right. However it is no. 1 and 4 that spark together from the same coil, also 2 and 3 spark together.  Pistons 1 and 4 are at TDC together and at the same time 2 and 3 are at the bottom. Since you have set the cams with no. 1 at TDC you will be fine. However if you connect the spark plug leads as I think you describe with 1 and 3 from the same coil it will only run on two cylinders. Of course I might have misunderstood your description!

[redmandan]

Limax2 you are absolutely right. My post on ignition timing was incorrect 1 and 4 fire whilst 2 and 3 are at the bottom of their stroke. This ensures an even balance in forces as the engine fires.

So basically my question is, do the plugs fire twice within a full 720 degree rotation of the engine? And I think the answer is yes.

[Fazerider]

Yes, you're correct, each plug fires every 360º. So one of the two plugs in each pair will be firing on an exhaust stroke.
The hot gas in that cylinder is ionised and at low pressure so conducts easily. It effectively grounds that end of the coil making the big voltage available at the other end (where you need it).

[redmandan]

Fazerider you da man. If the stars align tomorrow and the 8 month pregnant wife and 2 year old kid are asleep at the same time I'll be able to re-assemble the beast and hopefully fire it up!


In the event of a success or failure, beers will be drunk.

[Bretty]

As everyone else said. You're right, cylinders spark on exhaust stroke and compression stroke, obviously only compression stroke fires as there is fuel.
Check all the dots and the 'T' on the crankshaft line up perfectly!! If any of them are one tooth out, the engine will run ok, but isn't quite right. I remember doing mine and it wasn't quite perfect. And it bugged me so I took it all apart again until it was spot on. It will and should be spot on!

[red98]

"wasted spark system"   to give it its proper name      ..........looking forward to some beers tonight , for the right reason 

[Bretty]

It makes you (well me, coz I'm weird) wonder if you would have a stronger spark and double the spark plug life if it only fired on the cylinder that needed it. It is handy though for helping to diagnose misfires.

[red98]

Mmmmmmmmmmm...no update ?

[redmandan]

Haven't had it running yet, sorry! Today I had a few minutes spare and I've got the carbs back on. As soon as I reconnected the airbox I realised I needed to connect the fuel cables before locating the carbs back in their place.

I think I can get them on with a bit of fiddling, but it requires the toddler and pregnant wife to be asleep at the same time and the wife doesn't give priority to bikes.

[red98]

I think you'll struggle trying to get the cables on with the carbs fitted , if you do manage it let us know how you did it   


yep , got to put family first , gets easier the older they are , kids not the mrs   

[redmandan]

You were right, I had to take the carbs back out again to get the throttle cables back on again. My new Chinese inlet rubbers are a little tighter than my old ones so this was a bit of a pain. But once that was on I had enough time to get everything else back on. Made a short video documenting the process for anyone who's interested:

[His Dudeness]

well done that man get yourself a morgan carb tune and give the carbs a balance and it'll be purring like new. they're not cheap but they're a good investment and you'll have it for life

[red98]

Yep.well done fella.......needle on the rev counter is dead steady , ill be surprised if the carbs are that far out.......now go get yourself a beer ...well deserved   

[psyfi_1]

hi guys my bike is firing only on two cyclinders 1 and 2 and the other two are only warm...... i have noticed my coils are set 1-2 3-4 so first coil is firing 1-2 and second coil is firing 3-4 i guess this is not right and that is why its only firing on two cylinders....... should i re-seat leads to 1-4 2-3???????

[BBROWN1664]

hi guys my bike is firing only on two cyclinders 1 and 2 and the other two are only warm...... i have noticed my coils are set 1-2 3-4 so first coil is firing 1-2 and second coil is firing 3-4 i guess this is not right and that is why its only firing on two cylinders....... should i re-seat leads to 1-4 2-3???????

yes