Spark, fuel, compression, but no start!! By mossy173 (Read 17,305 times)

[rob63]

I had a Peugeot Ludix (2 stroke) 50cc a few years ago which had a rather annoying problem. When I held the plug against the head it would show a nice bright spark, but when I popped it back in the engine it wouldn`t spark at all. I used a spark plug tester to check it whilst it was in the engine, like one of these.

[mossy173]

Well, that's very interesting. How could that happen, and what was your solution?

In other news, I was sent some literature by the guy who sold me the BBK that quoted a 0.01" valve gap. Tried that this morning, no help.


Here's a picture of the engine bay and the hoses, as Alleyoop requested:

[Alleyoop]

Looking really hard at everything I noticed IT may be a simple fix I HOPE. But the BOOT CAP does NOT LOOK like it is ALL the way DOWN and on the PLUG(which would create a NO SPARK).
So give that BOOT CAP a good hard push down and see if it CLICKS onto the plug. The way it is THE SPARK PLUG MUST STICK OUT QUITE A BIT which on most heads they are well inside the hole of the head.
Alleyoop

[mossy173]

I think that might just be the design of the head. When I seat the boot I feel three good clicks, and it feels the same as when I seat it on the plug outside the head to test for spark.

[Alleyoop]

DAMN!! thought I had it, but now lets check the length of the hole, it could be the plug is not protruding into the chamber and is short.

FIRST put a mark on the plug where is sticks out even with the head.
Then Take a piece of wire and put a 90 degree bend on the end and stick it in the hole and move it to the side so it hooks inside the chamber and then mark the wire where it hits the head on the outside. Then take the wire put it along side the plug and see if the marks match up.

If the wire mark is above the mark on the plug the plug is to short and is grounding out on the head.
Alleyoop

[Alleyoop]

Another thing it could be is that the head is defective and they did not thread the spark plug hole all the way through which would not allow the plug to reach down far enough. Also some have found that a head was by the plug hole was to thick and the plug again would not reach the chamber.

But it is obvious that the Spark Plugs Spark is NOT sparking inside the Chamber. Maybe you can also try grabbing the BOOT CAP while you crank it and try moving it around.
Alleyoop

[mossy173]

Well, a few things have happened. I tried wiggling the boot, nothing. I tried measuring how far into the cylinder the plug was projecting--it seems that the threads in the head are the same length as the threads on the plug, and the end of the plug is projecting into the cylinder.

I was about to pull off the cylinder and start installing the old one when I decided I'd try shooting starter fluid or gas straight into the intake port. I removed the intake manifold and shot some starter fluid straight in, and although I didn't hear a "bang" or anything of that sort, the engine sped up to probably 1.5x the speed the starter motor can spin it. I was able to get this to happen with gasoline from a squirt bottle, too.

I also noticed something else--I'd lean on the starter and squirt a bunch of gasoline into the intake port, and the engine would go slow-fast-slow as (I guess) it went from flooded to ok to lean. so that might have been some weak firing, but at no point was it strong enough to keep the engine going without the starter.

Might it be fuel then, after all? Maybe the compression is low despite the rough test I've tried? SHould I get a compression gauge?

[rob63]

Nov 15, 2012 10:31:55 GMT -6 mossy173 said:

Well, that's very interesting. How could that happen, and what was your solution?

Never found one unfortunately, did a load of testing with a multimeter and swapped a load of parts over but I gave up in the end and broke it for spares, it was a bit tatty anyway. I must`ve spent about 15 hours on the thing and became convinced that the spark pixie had been around during the night.

Should I get a compression gauge?

Always worth having one in your tool box, takes the doubt away.
Just a thought but it might be worth popping the airbox off and poking your finger inside the carb venturi to manually lift the vacuum slide, then crank the engine to see if it makes any difference ?

[Alleyoop]

You said you had tested for compression with your finger. Now if you did test it properly and IT BLEW YOU FINGER OFF THE HOLE then it has enough compression for it to run. If you just felt some pressure then YES it is very low in compression.

But to insure it does have good compression you will need a compression gauge, and the compression shoud be no less than 120, 150+ is really good.
Alleyoop

Alleyoop

[mossy173]

I think I've got a lead. I just realized a couple things--

1. My spark plug tests are done with the plug out of the cylinder, and because I don't have a second plug, there is no compression
2. The starter motor can spin the magneto much more quickly without compression, giving what appears to be a strong spark
3. When starting, everything is much slower, the spark will be much weaker
4. This normally would be compensated for by the battery at low engine speeds
4a. the battery is mounted (and grounded) in the frame which acquires electrical continuity with the engine through the swing bearings and the shock absorber

5. I disconnected the engine from the frame because of clearance issues and never reconnected it.

Right now the spark is running solely on the magneto, which at the low engine speeds during starting, would not be able to produce sufficient spark unaided by the battery. I suspect that when I bolt the scooter back together I'll get it to start.

What do you think? Is that a "that'd do it"?

*edit* wait a minute, no...if the engine doesn't have a ground to the battery, how could the starter possibly be spinning? damn, that must not be it...

[rob63]

Well the main earth will be through the lead from frame to engine, and as the motor mount has rubber bushes in it there shouldn`t be an earth through the motor mount....unless I`m missing something ? Might pay to pop the mount in anyway, if it fired up you don`t want the motor wobbling around all over the place.
If you suspect a weak spark because of cranking, how about trying to kickstart it ?

[Alleyoop]

The battery has nothing to do with the SPARK PLUG, as a matter of fact you can take the battery out and start it using the KICKSTARTER(UNLESS THE CDI IS A DC CDI then yes the Battery Feeds the CDI with 12Vs to feed the Coil). Also if the COIL is a TCI there again you need a battery to feed the COIL.

Also in your PICTURE I can see the GROUND WIRE from the bolt on the Valve COVER and it appears to go to the FRAME. NOW if the Ground wire at the frame has paint under it will NOT MAKE A GOOD GROUND. So make sure the paint under the Ground Wire is metal and not painted if painted SCRAP the paint off down to metal.
Alleyoop

[mossy173]

BIG NEWS.

It runs, but very reluctantly. I had to bring out the 500A portable jump-starter from the car and connect that to the battery before the starter could kick the thing fast enough to start, but it started. The thing is, it won't idle. I took some videos, maybe they can make things clearer:




And a second start:



What do you think? How can I make it idle?

[Alleyoop]

Mossy,
Let us see how the spark plug is fireing it may be that it is fireing but has a weak spark or fireing every now and then, In that case it may be the COIL PICKUP. Because in the Video when you were cranking it that is more than enough for any scoot to start.
Alleyoop

[mossy173]

alright, here it is. I apologize for the nearly useless video quality, but perhaps my description will help flesh it out.

I wanted to see what the spark was like with compression in the cylinder, so I left one spark plug in the hole and the other I stuck in the boot and laid on the valve cover. The plug showed a rapid blue spark without any discernible misses. I tried this with the plugs contacting in series, and there was still a spark in the gap of the visible spark plug.