Kaboom backfire on startup

[Swim_fan02]

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.

 

[Guest]

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.

 

[Guest]

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:

 

[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.

 

[Guest]

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?

 

[Guest]

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.