Topic: Shotgun blast when ignition turned off

[wjustinleigh]

I recently rebuilt my 71 cb500 head, got everything put back and am having some trouble getting it going.  I have 120-130 psi in each cylinder so that looks good, have spark and can tell gas is getting to the chamber but despite fluttering I can't get it to catch.  I can't get a timing gun on it since I can't get it to idle so maybe that's an issue.  It's been apart for six months, and I was only planning to have it apart for a month so I hadn't drained and refreshed the gas so that may be an issue too.  The weird part is after attempting to start it for a while when I turn the key off it gives a shotgun blast backfire.  I'll stop attempting to start it, wait a minute, then click the ignition to the off position and bam the neighborhood is under attack.

I can't think why the ignition would have anything to do with unspent fuel in the exhaust finding a hot spot.  Any thoughts?

[flybox1]

Cam installed correctly? (180deg out?)

[ekpent]

As for timing you can do a 'static time' with a simple test light and the engine not running but ignition on. It will get it very close and you can eliminate that possible cause.

[rb550four]

Static time with a test light? You have to be at least close, what did you use to determine spark timing?
after it's fired, when you  turned the key on, that should have burned the fuel in the combustion chamber up. Are you introducing more fuel or how clogged up are the carbs?when it's burned or out of fuel there's nothing to give a blast, it has to be coming from somewhere.

[ANDY W]

so what u saying is that the engine wont start and then after u done cranking u shut the ign switch to the off position and it backfires out of the exhaust?

[wjustinleigh]

Good points, thanks!  This was the second go around on the cam due to improper cam timing so I was careful to have it on the T 1-4 mark with the notch flush to the surface so I feel pretty good about the cam timing.  It has dyna coils and ignition, so it's possible that the ignition wires are flipped but I was pretty careful with that first time around.  I'll verify tonight.

Good point on the static timing, forgot about it this time, will do that tonight.  In the past it was always close enough to get an idle and adjust with the gun.

Carbs are very clean, completely disassembled, cleaned (twice).  Feel very good about the clean carbs with functional jets.  I'm not introducing more fuel anywhere, and even if I was I don't know why it would care about the position of the key.  Maybe that's just coincidence but the timing was exactly when the key was turned and replicated three times. 

I think the fuel has to be passing through unburned, due to the attempts to start it probably had some flooding which would pass the gas through.  The whole engine is still cold though, I don't know how it could be hot enough to backfire.

ANDY W - that is exactly correct.

[ANDY W]

is it backfiring when u crank it? or only when the key is switched off?

[wjustinleigh]

Only when the key is switched off.  When cranking it sounds good, just doesn't quite catch.

[ANDY W]

id start by checking for power to the ignition coils when the ignition switch is on and off. sounds like ur getting power to the coils with ur ign switch is in the off position not the on position, causing a spark plug to fire and ignite the unburnt fuel from cranking with no spark

[wjustinleigh]

I can tell I'm getting power to the coils when switched on - I can hold the spark plug against the head and see the spark.  I can also hear it trying to start, which I wouldn't get without a spark.  I'll put the tester on the coil to see if I can see anything odd when flipping the ignition on/off.  I was thinking there might be some grounding issue that was creating a spark somewhere when moving to the off position.

[stewonepot]

Hi try swapping the points wires around blue yellow.
maybe!
regards Stew

[MCRider]

Hi try swapping the points wires around blue yellow.
maybe!
regards Stew

+1   

[wjustinleigh]

I was going to trace them back to make sure they were backwards before I tried it, does it damage the engine to try starting it with the coils reversed?

[MCRider]

I was going to trace them back to make sure they were backwards before I tried it, does it damage the engine to try starting it with the coils reversed?

No.  PMs

[ekpent]

Static time with a test light? You have to be at least close, what did you use to determine spark timing?
after it's fired, when you  turned the key on, that should have burned the fuel in the combustion chamber up. Are you introducing more fuel or how clogged up are the carbs?when it's burned or out of fuel there's nothing to give a blast, it has to be coming from somewhere.

I just use one of these for a simple static timing light, can use a meter also.   There have been some other very good suggestions so far.

[HondaMan]

If you have a Dyna S ignition and Dyna's 3-ohm coils, you have a bigger problem... If you have their 5-ohm coils, it should start, but the battery is probably low. The backfire is unburned fuel that is being lighted by the coil when the power is switched off (which makes it spark) while the intake valve is open on a cylinder. If the battery is low, especially with the 3-ohm coils, this can happen.

[PeWe]

Mixed up yellow and blue wires from ignition to coils?
I had similar problem when I had mounted the ignition rotor 180 degrees wrong. Switch yellow and blue confirmed it when it worked. I corrected rotor and connected yellow-yellow and blue-blue.

[stewonepot]

Cheers Guys
I think you are right on the wiring.
Stew

[scottly]

The backfire is unburned fuel that is being lighted by the coil when the power is switched off (which makes it spark) while the intake valve is open on a cylinder.

This is a characteristic of some electronic ignition systems; they may generate a spark either when switched on or off. My bike almost started once, simply by switching the Gerex ignition on!  

[Bodi]

It's not just with electronic ignition, a points setup will spark whichever coil is powered when you turn the key off. The problem here is that the engine isn't firing normally but he obviously has fuel.
Reversed points leads should still fire the points - but at the wrong time near BDC: that usually gives you plenty of carb backfires when it sparks before the end of the intake stroke.
I would check that it's sparking at all - plug held against engine case - and also confirm that the static timing is close. Is this a new DYNA installation?

[wjustinleigh]

Dyna installation - ignition and coils.  It's not a new installation though, I had it on there before I took the head apart.  Checked static timing, it's good.  Plugs have spark (test that against the head first).  Tried with ignition leads switched and didn't get any engine fluttering at all.  I found the adjusting screws on the carbs were only out about 3/4 turn so I put them to 1 3/4 (book says 1 1/2 - 2) but no change.

Since the screws were in so far it probably wasn't getting much air, I'm thinking it flooded it.  Maybe when I stopped for a bit it disapated then when I switched the key off it would backfire.  New plan is to pull the plugs, re-test compression and spark, replace any questionable plugs, then try it fresh with the new carb screw settings.

[Duanob]

I'm with Fly I would 2X check the cam position. If it's one or two degrees it will want to start but not easily. You can get some popping and backfiring.