Backfire out of carb?

[CrustyOreo]

Ok so I've cleaned and replaced the gaskets in my B4s carbs.
When I try to start it, all I get is a backfire out of the carbs. It never seems to fire through the exhaust.

What should I check next?

[greg737]

a backfire that goes out through the carb is usually a sign of improperly set ignition timing, because it indicates that a sparkplug fired while an intake valve was still open. 

in the case of your Bandit there's not much that could cause the timing to change from the factory settings. 

the only thing I can imagine happening to a Bandit to cause something like this to happen would be if the control wires from the ignitor box somehow got connected to the opposite coils.  this would have the effect of changing the timing by 180 degrees.

that's why I asked you (a few days ago) if it were possible that one of the following had happened: A. the coils or just the high tension coil wires somehow got physically swapped (don't know if this last item is even physically possible), or B. the coil control wires in the bike's wiring harness somehow got swapped out, left for right.

according to the manual the left coil is controlled by a White wire and its high tension wires go to cylinders #1 and #4.  the right coil is controlled by a Black wire that has a Yellow tracer line on it and the right coil's high tension wires go to cylinders #2 and #3.

[CrustyOreo]

Just double checked. All looks well on the wiring part.

[andrewsw]

I've seen exactly this symptom with mis-wired coils and greg737 has described. better recheck those sparkplug wires.

A

[CrustyOreo]

So I went through the carbs again and double checked the wiring. Same result. Back fire through the carbs 
I think a compression test is in order.

[El Gringo]

I can't see how changing the carb o-rings will have made it lose compression.

Was it running well/reasonably before you changed them?

Have you checked the plugs themselves?

It does sound like plug caps on the wrong cylinders

[CrustyOreo]

I've never had it running.

[El Gringo]

Ah, in which case i'd go for

Fresh plugs,

Check the spark on each one,

Check leads are on the correct cylinders,

Check valve clearances

Check top end timing, eg are cams on the chain in the correct position, unlikely but you never know

[CrustyOreo]

PLugs are fresh. Each of them have spark.

Correct leads on correct cylinders.

[El Gringo]

Clearances and top end i would suggest

Also when it's back firing is it ignited mixture and flame coming back through the carbs or wet unburnt mixture and is it on all 4 carb intakes?

I presume that you're running it with the air box off for now?

Edit to add: Also check for vacuum leaks round the carb rubbers and also on the vacuum take off stubs

Cheers

Chris

[greg737]

I wonder if the problem isn't related to the Ignitor Box.

The Denso ignitor box on these Bandits is very delicate.  Over the lifespan of this motorcycle people were ruining them all the time by accidentally inducing an over-voltage in the bike's electrical system (it usually happened when somebody was trying to start a bike that had a dead battery by jumping it off of a running car).  The ignitor box apparently doesn't have any over-voltage protection built into it so the capacitors blow up/burn up.

When one of these not very mechanically skilled owners realizes that the ignitor box isn't working any more they go out an purchase the first "Bandit" ignitor box they can find or even a GSX-R400 ignitor box without a thought to the fact that there were several different ignition system configurations during the Bandit and GSX-R.

Correct me if I'm wrong here: Your Bandit has never run since you got it, right?  You have been working to get its carbs and ignition system working and you've made some progress but it has always backfired through the carbs.  That's the closest to running you've gotten, right?

[CrustyOreo]

Correct. It has no run since I've bought it and the back fire out the carbs is the closest.

[greg737]

you should to do a bit of research on your Bandit to determine which CDI box you need.

Here's a datasheet that forum member T6nis showed me:



As you can see this datasheet indicates there are at least 2 different CDI boxes out there for the Bandit 400.  The datasheet shows that during the production lifespan of the Bandit the ignition system sub-contractor, Denso Corporation of Japan, changed the configuration of the signal generator wheel (the arrangement of the "teeth" on the wheel are different).  Also, if you look at the wiring pin-out on the plugs you'll see that at some point in Bandit production Suzuki reversed the inputs from the signal generator (pins #7 and #8).

To further complicate things I've discovered that my 1993 U.S. import model Bandit 400 has 7 wires leading to the CDI from the wiring harness instead of 6 wires.  Every wiring diagram in both the Bandit 400 Service Manual and the Bandit 400 Service Manual Supplement only shows 6 wires going to the CDI.  My CDI has 2 plugs from the wiring harness, one plug has 4 wires in it and the other plug has 3 wires in it.

[greg737]

About a month ago I sent my CDI box to T6nis. 

Because I'm modifying my Bandit with a fuel injection system that will also control ignition, I won't be needing the OEM CDI.  So I gave it to T6nis. 

When he got the CDI in hand and tried it in his Bandit he was initially disappointed because it didn't work.  Then he checked the wiring at the connector plugs from the bike's wiring harness and discovered that pin #7 and #8 were reversed (he had a black wire in the plug going into a brown wire on the CDI and a brown wire in the plug going into a black wire on the CDI).  As soon as he sorted this mis-wired situation out the engine ran just fine.

[andrewsw]

To further complicate things I've discovered that my 1993 U.S. import model Bandit 400 has 7 wires leading to the CDI from the wiring harness instead of 6 wires. 

There is some speculation that the 7th wire controls a limiter that is engaged in 5th and 6th gear. Some machines apparently have a variable resistance that comes from the gear selection and controls this. My 7th wire on my '93 was just open when I got it. I've grounded it through a resistor based on various anecdotal bits of data floating around with the idea that the proper resistance will force it to be unlimited at all times.

I've never seen any definitive information on whether that is true...