Kaboom backfire on startup

[Swim_fan02]

Sorry... saw this thread yesterday and didn't have enough time to formulate a proper reply!

Anyway, there is one other way your ignition timing can be off relating to the signal generator.

The rotor on the end of the crankshaft mounted to the advance plate can be installed 180 degrees out on the 450's, and I believe the symptoms are as you describe.

If you look closely at the rotor, you will see it has two lobes on it, but only one has embedded magnets.

The correct alignment for the rotor on the advance plate is like this:



Make sure the shaft is lubed with a little engine oil before installing the rotor and ensure the wafer thin washer is underneath the rotor as well. I thought I had a photo of all the advance assembly bits in pieces but I can't find it at present...

This is the best photo I have to try to help you visualise this:

I'll take a look at it but I never removed that. Is there somehow a way it can get out of whack on its own? The bike ran last summer. This winter all I did was unbolt the motor from the frame and take it out. I didn't do anything internally to it.

[pete]

Ok so I got the multimeter out today and checked some things. I'm getting 11.3v and 12.2v and around 4 ohms at the coils. Ignitor is getting 12.2v. The signal generator seemed to be a pita. I was getting 64ohms on only one pair of the wires. Kept coming up infinity on the other. Not sure if I was testing it right but I basically unplugged it from the main harness and tested at the connector. Kinda thinking the signal generator could be my culprit. Any thoughts?

That definitely doesn't sound right!

Looking at the wiring for that, you should see 4 wires in total. One is unrelated and goes to the oil pressure switch (green/yellow I think?).

The other three are for the signal generator.

One (black or black/white?) is common to both and the other two are the other side of each pickup coil.

Measure from the common to each one of those and each one individually should give you 60 - 80 ohms if the pickup coils are serviceable.

That's what I'm trying to figure out. Would the signal generator have anything to do with that?

As above, but what I say here is if you're getting spark on both sides, then the pickup coils should be serviceable otherwise you wouldn't get spark on at least one side.

I'll take a look at it but I never removed that. Is there somehow a way it can get out of whack on its own? The bike ran last summer. This winter all I did was unbolt the motor from the frame and take it out. I didn't do anything internally to it.

Typically no it can't although I have seen someone post that both of the tabs on the advance arms that lock into the rotor and make it advance were sheered off.

In that case, yes he experienced the same symptoms because the rotor was able to spin freely on the shaft and get out of time.

Grab a hold of the rotor and turn it by hand. If it turns and springs back only, then that is the correct behaviour. If you turn it and it keeps turning, that's bad and will likely be causing your issue.

In addition to all the above, have you done the ignitor tests as outlined in the Clymer manual?

[Mekanix]

A quick check you can do yourself.

1 put the right cylinder at top dead center on the firing stroke.

Take the plugs out and lay them on the engine.

2 look at the rotor and figure out which lobe has the magnet in it. ( usually the one with the two holes in it. )

3 turn it forwards and let it spring back. You should see a spark, If not you might have to move the crank forward or backwards a bit to get it so that the rotor is lined up just right so that the magnet springs past the pickup and makes a spark on that side.

It could be making the other side spark and that's what we are trying to find out.

Another way is to just turn the crank forward a bit to get the rotor out of the way and use a small magnet or magnetic screw driver the same way.
All you have to do is wave it past the pickup and it should spark.

Hopefully it will fire that side. If not. Try the other pickup and see what happen's.



You said 11v+ at the coils and ignitor. What about while you are cranking ? what is the ignitor getting ?

I'm thinking maybe you don't have enough power when its cranking under load with the plugs in and its only once in a while that you get a nice jolt that gives you a spark or it could be the momentum of the engine carrying it over when you let off the button and it has enough power again to fire but its just backfiring instead of making it all the way around.

A bad ignitor or bad ground for the ignitor will do this as well.

[Swim_fan02]

I will definitely check those out. I haven't been able to tinker with it the past couple days. Hoping to get to it tomorrow. This is the advice I've been looking for. Thanks guys. Will post back my results once I get to it.