Sucking sound when I turn the key off

[skyhawk04kilo]

I've just finished restoring a '78 Special and have put about 50 miles on it so far (very exciting!!)

Anyway, I've got a bunch of little issues but the only one I'm totally stumped on is this: I'll shut down the bike with the kill switch and everything's normal. But when I turn the key off, there's a loud popping / sucking sound coming from the engine area, but I'm not sure exactly where. The only conclusion I have come up with is for some reason, turning the key off fires the coil one last time and lights off whatever mixture is in the cylinders. Is this normal? Is it something completely different from what I think it is? I have a Pamco ignition with the big black single coil.

 

[TeeCat]

Congrats on your success so far!

What sort of breather setup do you have? My immediate thought is that you may be hearing air/oil mist/vapor being exhaled through the breather as you shut the bike down. It's normal to exhaust a bit of that from the head... that's what the breather is for. I think this would be more likely to be heard, though, if you are venting to atmosphere (not recirculating through your air box for reburning); the earlier bikes did this, but I'm not sure about yours.

TC

 

[skyhawk04kilo]

I don't have an airbox, the breather just empties to the atmosphere. Is that going to turn the bike into a big mess after a while?

I don't think that's the problem, though. It has something to do with the key. I'll hit the killswitch, wait 30 seconds and turn the key and it'll make the noise. It's caused by turning the key off.

 

[pyroman]

It has to be a spark jump burning un-ignited mixture left in the cyliner? I guess. Or, possibly a rip in the space-time continuum.......

 

[lassenc]

I have the same thing with the PAMCO. Guess is unburned mixture getting burned off as the spark jumps when current is cut off.

 

[TeeCat]

Now, that's weird! Guess it kind of makes sense, though, though!

By the way, you might want to affix a mini-cone filter to the end of your breather hose. It will eliminate/collect any mess from the oil vapor.

TC

 

[skyhawk04kilo]

Thanks for the tip, Teecat. Will be ordering a cone filter ASAP as the bike is very clean and I'd like to keep it that way. Here are a couple pics, and yes I know they are crappy angles. There was a car in the way at the time.

This popping doesn't bother me, I just want to make sure that it's normal and not the result of a failed/failing component that's going to leave me stranded or ruin other components.

 

[TeeCat]

Hey, VERY nice!

On the cone filter, just get a little tiny breather-type one such as this (about 3/4 down the page - mine is blue, though it does not really show because I have it low on the bike). They have them at Advance Auto, etc..

TC

 

[XSLeo]

What I have done on the breather is use a short section of the stock hose, about 2 inches. Plug this on the breather. Get a #80190 Power Brake Check Valve from the HELP! section At the parts store.
One side of the valve fits in the short section of hose on the breather. I had some 3/8 rubber hose, I put about 2 feet of this 3/8 hose on the small end of the valve and ran it down between the carbs, back and down behind the engine.
The valve will just let air out of the breather and not let any back in. This creates a small vaccum inside the engine. This helps keep oil leaks to a minimum. They say it helps the piston rings seal better, giving a bit more power.
On the poof, when you shut off the key as mentioned, just one last firing of the coil, The poof is the burning off of fuel left in the cylinder. My Harley does the same thing.
I don't think you will hear it as much with a stock air box. A less restrictive air intake won't baffle the sound as much, kinda like the muffler on the exhaust.
Leo