Kaboom backfire on startup

[Guest]

Hey guys. It's been awhile since I've been on here. Had to make a new account. 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. Any help or input would be greatly appreciated. Thanks in advance.

[hjfisk]

Did you mistakenly switch plug wires?

[Swim_fan02]

Did you mistakenly switch plug wires?

No I did not. Each cylinder has their own ignition coil which can only be plugged into the harness one way.

[terrylee]

Does it make a difference if you put petcock on prime and try to run bike that way? Check to be sure you're getting good spark? Check connections to signal generator and ignitior, clean or replace if look bad. Might try removing plug caps (they unscrew) cut off end of wire about 1/4 inch or so to get clean end and screw caps back on. Hate to say it but one of my 850s did the same thing till I rebuilt carbs. and installed new O-rings inside. Now runs great. terrylee

[Swim_fan02]

Does it make a difference if you put petcock on prime and try to run bike that way? Check to be sure you're getting good spark? Check connections to signal generator and ignitior, clean or replace if look bad. Might try removing plug caps (they unscrew) cut off end of wire about 1/4 inch or so to get clean end and screw caps back on. Hate to say it but one of my 850s did the same thing till I rebuilt carbs. and installed new O-rings inside. Now runs great. terrylee

No it does not make a difference if it is on prime or on. They both are getting good spark. Plugs and wires are fresh and new. And as I stated before the carbs were totally rebuilt with new gaskets and seals. The whole 9 yards.

[tom203]

"....they don't seem wet"

after some cranking with "choke" on, there should be signs of fuel on plugs. In your carb cleaning, did you clean out the "choke" circuit- feed hole in bowl, tube to throat, etc. ?
Did you remove camshafts or signal generator plate?

[Swim_fan02]

"....they don't seem wet"

after some cranking with "choke" on, there should be signs of fuel on plugs. In your carb cleaning, did you clean out the "choke" circuit- feed hole in bowl, tube to throat, etc. ?
Did you remove camshafts or signal generator plate?

They are a little wet. Not sopping wet like you should expect from backfiring. And yes I cleaned out the whole carb including the choke circuit. I did not touch the camshaft at all. Not exactly sure where the signal generator is. Unless you are referring to the ignitor. Yes I removed the whole plate that the r/r, turn signal relay, starter solenoid, and ignitor are fastened to. I did not removed them individually. Left them secured and just removed the whole plate so I could paint the frame.

[GSX1000E]

Did you make certain that the plate still had a good ground after you painted? Have you tried adding another ground wire from that plate to the battery or frame ground?

[Swim_fan02]

Did you make certain that the plate still had a good ground after you painted? Have you tried adding another ground wire from that plate to the battery or frame ground?

Funny you say that because that's what I was thinking so I made a ground jumper from the plate to the battery.

[Swim_fan02]

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?

[tom203]

In your first post you said you had spark to both sides, so it seems ignitor is working, BUT is it firing the correct plug at correct time?

[Swim_fan02]

In your first post you said you had spark to both sides, so it seems ignitor is working, BUT is it firing the correct plug at correct time?

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

[tom203]

Any chance you had ignition coils off for painting and reinstalled them opposite?

[Swim_fan02]

Any chance you had ignition coils off for painting and reinstalled them opposite?

Yes I did take them off when I painted. And no I did not put them on opposite.

[pete]

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:

While holding the advance plate with the R side point up, slip the ignition rotor over the shaft. The lobe with the embedded magnets must face to the right when the R side on the advance assembly is pointing up. Spread the weights and push the rotor down until it engages the weights. Ensure the rotor rotates freely on the shaft as the advance weights move out and back. (adapted from Dynatek installation insructions)

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: