Project Quad - Will not start

[Sangiyok]

Hi All,

I hope this is the right area to post looking for help.

I've got an old Yamaha Big Bear that I'm trying to get running as a bit of a project between jobs, but I've run into a problem that my novice mechanical skills cannot get around.

• Compression test on the engine looks good.
• I am getting spark from the lead when I press the start button. (and the fire suggests there's spark from the plug)
• Various bits and pieces are spinning (behind recoil starter for example).

So things are not looking bad, but while I don't have the new carby in yet, I wanted to test with a little spray of Engine Start (like I've seen on all the Youtubes :p) and instead of sucking the spray in the intake manifold and burbling to life, it seems to backfire (explosively) and catch on fire.

I know you can do damage if you go crazy with the Engine start, but this happens with even a tiny spray.

What should I look for that might be the cause of this, anyone?

 

Thanks so much

 

[Mech]

Check the valve clearances. The inlet. It might be slightly too tight.

Try to check the exhaust isn't blocked. Maybe disconnect it at the head.

I'm presuming it did use to go and got parked up in a running state, and  hasn't been rebuilt

[MarkinAR]

Check the timing if valves aren't the problem.  Sounds like timing is 180 off.

[Cj Winds]

Mark, can you explain how timing could be off 180 deg?

[MarkinAR]

On a 4 stroke engine the piston gets to TDC twice in a cycle. It may be that the timing was set on the exhaust TDC and not compression TDC. Have to make sure the marks on the cam are correct and make sure both valves are closed at TDC. 

[Cj Winds]

So you are talking about the mechanical timing of the valves? 

[MarkinAR]

If you were to assume that timing could be set when the piston is at the top, it is possible to set timing on the exhaust stroke and not true TDC which is the compression stroke. This, it's 180 out. I've done that years ago. Both valves have to be closed and the piston has to be at the top to be at true TDC. 

[Cj Winds]

What timing are you talking about? Engines have ignition timing and valve timing. Most of these types of atvs have a pickup coil on the crank that tells the cdi when to fire a wasted spark ignition.

[Mech]

Not all spark systems are wasted spark. Some only fire every second rotation.

Some cdi systems that run off the crank, that is get their low voltage off the stator, need more than one rotation before they will fire. They have to see several low voltage pulses before the trigger coil will fire them. The cam might need turning 180 degrees so the valves are closed on that second rotation of the crank.

MarkinAr sounds like he's familiar with these yamahas..  I'm not, but I'd check what he's suggesting.

[Cookie]

I don’t like starting fluid to diagnose or make conclusions about an engine. Get the new carb or clean out the one you got and then see what’s happening. I’d only run it with the air filter on, if possible. When cranking it over, feel for air blowing out the exhaust to be sure there’s no mouse nest in the muffler. Also, when spraying the starting fluid, open the throttle wide open to get the mist past the plunger/butter fly valve. 

[Cj Winds]

First I say "most of these types of atv's are wasted spark", and then Mech comes back with " not all spark systems are wasted spark. Weird.

Of course not all spark systems are wasted spark, but I am guessing this one is. If the atv only has a pickup coil on the crank then how would it know what stroke it was on?

[Mech]

You make a good point there about opening the throttle wide open to lift the slide Cookie. If it's right down the ether isn't going to get in there.

And I agree, starting fluid isn't a good test really. It's full of oil or kero or something and can foul the plug. A teaspoon of fuel down the sparkplug hole is better.

Stale fuel can also foul sparkplugs. Have you got fresh fuel CJ ?

[Mech]

"Of course not all spark systems are wasted spark, but I am guessing this one is. If the atv only has a pickup coil on the crank then how would it know what stroke it was on? "..

Don't guess, be sure.

If you  want to know how the electronics involved work read up about "voltage doubler", and digital logic "flip-flop".

The stator sends out pulses of alternating power, they get "stacked" to charge a capacitor to a higher voltage, then the trigger coil triggers a discharge of the charged up capacitor. On a two stroke they fire every revolution, and on some four strokes that's fine too and they do run a wasted spark. Some engines though, because of a combination of valve overlap and ignition advance, don't like that, so they count the low voltage pulses as they charge the capacitor, and don't  let the trigger coil do it's thing till more than one full revolution has happened after the last firing, then it triggers at the next trigger signal.

Your backfiring with the engine start spray is likely because of what Cookie was saying..

 

Edited February 13, 2022 by Mech