Ignitor or signal generator problem???

[Guest]

I know I'm posting this again but it seems like I keep getting the same responses. I have checked, double checked, and triple checked all of the obvious things. Well I will get right to it. I just got done rebuilding the 81 gs 450l. Gave her some new paint, polished her up nice and replaced some old rusted parts. Got everything back and assembled and now she won't start. When I crank it over it will backfire a little bit and then after a few times BOOM. Big backfire. I know you are going to tell me to clean the carbs check fuel delivery spark and all that. The carbs are spotless. Cleaned them inside and out. The are practically brand new. Floats are within spec'd height. Stock airbox and it has new rubber boots. New air filter. Plugs are new and gapped to spec. I'm getting spark on both sides. Both cylinders have good compression. When I rebuilt her I didn't tear into the motor at all. It was a cosmetic rebuild. It ran last summer before I tore it down. I had some spare ignition coils laying around so I swapped them thinking it was that to no avail. Same thing. Still backfiring and not starting. Checked plugs and they don't seem wet. I don't think the timing is off as I didn't touch anything to do with that. So I'm starting to lean towards the ignitor. 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?Any help or input would be greatly appreciated. Thanks in advance.

[tom203]

It's like deja vu again! It makes little sense to start another thread on same problem.

you said.... "I don't think the timing is off " in this thread, so why not check just to be sure. you removed some stuff to paint, so doublecheck the position of that stuff that relates to timing, i.e coils and signal plate. Some gas, compression and spark at about right time should work

[Swim_fan02]

It's like deja vu again! It makes little sense to start another thread on same problem.

you said.... "I don't think the timing is off " in this thread, so why not check just to be sure. you removed some stuff to paint, so doublecheck the position of that stuff that relates to timing, i.e coils and signal plate. Some gas, compression and spark at about right time should work

The coils are good. Timing is spot on. How do I test the signal generator and ignitor to ensure those are functioning properly?

[tom203]

It's sparking so coils and pickups are working, but it seems timing is NOT spot on, did you follow pete's advice and check?

[Swim_fan02]

It's sparking so coils and pickups are working, but it seems timing is NOT spot on, did you follow pete's advice and check?

I did follow Pete's advice and check it. Everything to so with the timing was on. I'm totally stumped here. I can't figure this out and it's getting pretty frustrating. Is it possible for the signal plate to have rotated 180 on its own?

[tom203]

Take a close up pic of your signal generator plate (as mounted) and post it- pete or some other 450 owner will compare it to their setup. The setup on mine is different.

[Swim_fan02]

Take a close up pic of your signal generator plate (as mounted) and post it- pete or some other 450 owner will compare it to their setup. The setup on mine is different.

I will as soon as I get home from work.

[tkent02]

Backfires out the intake, or backfires out the exhaust? If it's really loud it's probably the exhaust? Just one side or both?

Rotate the engine, the other signal generator will have the 64 ohms. That's how it works.

Opens and closes as the engine turns like a set of points.

[Swim_fan02]

Backfires out the intake, or backfires out the exhaust? If it's really loud it's probably the exhaust? Just one side or both?

Rotate the engine, the other signal generator will have the 64 ohms. That's how it works.

Opens and closes as the engine turns like a set of points.

It back fires out of the exhaust. It sounds like I'm racking off a few slugs out of my 12 ga shotgun.

[tkent02]

OK, somehow it is sparking when it can't fire in the cylinder, after a while enough unburnt fuel blows out into the exhaust for a spark to ignite the fuel there. BOOM!! It is likely sparking while the exhaust valve is open, which is roughly 180 out from normal. Cylinder one is firing when cylinder 2 should be, or vice versa. Somehow you changed the timing. Crossed wires somewhere or something like that.

[Swim_fan02]

OK, somehow it is sparking when it can't fire in the cylinder, after a while enough unburnt fuel blows out into the exhaust for a spark to ignite the fuel there. BOOM!! It is likely sparking while the exhaust valve is open, which is roughly 180 out from normal. Cylinder one is firing when cylinder 2 should be, or vice versa. Somehow you changed the timing. Crossed wires somewhere or something like that.

Correct me if I'm wrong but on the 2 cylinder 450 shouldn't both cylinders fire at the same time?

[tkent02]

Not surehow the 450 does it, never owned one. Are the two pistons going up and down together or are they one up and one down? Either way, no they fire at different times. Just as a test, swap spark plug wires? MAybe some PO did something funny…..

[tkent02]

OK I just looked at a crankshaft on Ebay, one piston is up while the other is down. They cannot be fired at the same time. That is why they have two coils instead of one coil and a wasted spark system.

[Swim_fan02]

Not surehow the 450 does it, never owned one. Are the two pistons going up and down together or are they one up and one down? Either way, no they fire at different times. Just as a test, swap spark plug wires? MAybe some PO did something funny…..

I'll check that when I get home from work.

[eil]

(Edit: nevermind me, I missed the two most recent posts)