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Mon Nov 09, 2009 11:38 am

So, it's a rare 70+ degree sunny November day here in Vermont. I take the bus out to enjoy the weather and run some errands. I stop into a glass shop to see if they can cut a driver's side mirror since my OG glass fell out and broke the other day.

As I pull out of the glass shop parking lot, BANG! It sounded like an M-80 went off inside the bus. The engine is still running and no warning lights come on. I immediately pull over and shut it down. Grab the fire extinguisher fearing the worse and run to the back. Nothing looks amiss, everything looks on the surface just as it should. I decided since it sounded fine when I pulled over I would give it a try. It starts right up. Go back to the rear of the vehicle. Everything sounds and looks OK and I drive it home the 20 miles without incident. Checked it out in the driveway again and I can find nothing wrong.

Every time I begin to feel comfortable around this bus it does something that scares me. My right ear was ringing from the concussion for several minutes.

Any ideas?

Mon Nov 09, 2009 11:43 am

Sounds like a backfire.

Does the timing advance and retard smoothly?

Mon Nov 09, 2009 11:45 am

I guess I have never heard a backfire like that.

Here's where my real ignorance shows since I don't know exactly what you mean or what I do to test it. Sorry. :oops:

Mon Nov 09, 2009 11:49 am

He is talking about the vacuum advance on your distributor...it advances at higher rpm's. You could start by posting your dizzy numbers to figure out what your running. If this only happened once and is not a regular occurrence then i wouldn't be too worried.

Mon Nov 09, 2009 11:50 am

aopisa wrote: I guess I have never heard a backfire like that.

Here's where my real ignorance shows since I don't know exactly what you mean or what I do to test it. Sorry. :oops:

You mean you have never seen the movie "Uncle Buck"? Gunshot backfires. :lol:

He is speaking of your distributor, and if it functions properly. Grab your Bentley Book and check the timing on your engine. If you cannot do it yourself, have your mechanic check it out.

Mon Nov 09, 2009 11:54 am

peaceful warrior wrote:
You mean you have never seen the movie "Uncle Buck"? Gunshot backfires. :lol:

Man,I love that movie! :D

Mon Nov 09, 2009 12:00 pm

Ahhh... forgot about Uncle Buck. Seem to remember a few old Little Rascals episodes where backfiring autos figured prominently too.

The distributor is pretty well nestled in there. Do I have to remove it to find the number?

I think I can borrow a timing light (until I buy one and a tach/dwell )and figure it out with the help of my Bentley.

Mon Nov 09, 2009 12:06 pm

aopisa wrote: The distributor is pretty well nestled in there. Do I have to remove it to find the number?
No, you just need a little mirror to look around the distributor to find the number. You need to know what distributor your running, so that you can properly time it according the Bentley.

Mon Nov 09, 2009 12:13 pm

Usually a backfire in there will blow the 3 way fitting for the power brake booster off the air plenum or will case the little door inside the AFM unit to not move.

If either of those things happen, your bus will not start.

Hopefully you have the stock distributor unit in there, which would be a # 021-905-205P or a 022-905-205S. Confirm with a flashlight and a mirror.

Here is what you are going to do, first set the dwell anglefor the points to about 47º +/- 3º or roughly about 0.016-0.018" of point gap.

Then hook up the timing light and rev the engine up and down a few times. There should not be a lot of "timing light scatter", in that you want to see a nice smooth advance and retard curve with changes in RPMs.

Then unhook the vacuum hose and let it just lay there. Rev the engine until all of the centrifical advance the distributor has to off is all in and turn the distributor until that mark is about 28º BTDC on your timing scale. Do it a few times to be sure. Although the RPM really doesn't matter, it will be at about 3500-3800 RPMs so the engvine will be pretty revved up.

Turn off the engine. Tighten the distributor clamp. Put the vacuum hose back on. Start engine and see how close your are to ~7.5º BTDC at correct idle speed. Don't change it, just make a mental note about it. Anytime you adjust the points, you have to check the timing with the light.

That is why you need to own both a timing light and a dwell/tachometer. I would suggest buying a dwell tachometer that has a volt ohm meter function to it. It will be very useful down the road.

You will be adjusting the idle speed to the correct 850-900 RPMs using the "big screw" on the throttle body.

Mon Nov 09, 2009 12:17 pm

ah yes...I remember the "oh shit, what was that?!?" phase... I'd pull over and crawl all ove the bus looking for a disaster too. :shock:

Timing is critical and could have been the problem. After you read up on timing your distributor come back here and post any questions on the procedure you might have. We can answer them before you start.

If you do this yourself you'll have less "oh shits" and when you do you'll know what they are...

Frank

Mon Nov 09, 2009 12:21 pm

Are you sure the bang even came from your bus?

Maybe it came from another source.

Have you checked for bullet holes?

Mon Nov 09, 2009 12:24 pm

Did the engine crank a long time before it started? If the muffler is full of gas it can make a noise like you describe (backfire). You have to be pouring the gas to it with no ignition for that to happen.

So, if there was some fault in the ignition and you turned the engine over a lot, then the ignition started working again, BANG!

I don't like this explanation because it does not really fit the symptoms you describe. But it's all I can think of right off the bat.

Mon Nov 09, 2009 12:24 pm

Randy in Maine wrote: Then unhook the vacuum hose and let it just lay there.

Shouldn't he plug it with a pencil or a golf tee or sumthin'??

Mon Nov 09, 2009 12:24 pm

Patrick199 wrote: Have you checked for bullet holes?
Wow... :lol:

Mon Nov 09, 2009 12:28 pm

Or maybe he's driving around with a big dead bird on the roof. :D

Mon Nov 09, 2009 12:32 pm

Traveling Writer wrote: Randy in Maine wrote: Then unhook the vacuum hose and let it just lay there.

Shouldn't he plug it with a pencil or a golf tee or sumthin'??

Not needed for what we are doing here, Mr. T. Writer. Very observant though. 8)

Although all of us bus guys do keep a couple of golf Ts in our toolbox for just that purpose. :D

Mon Nov 09, 2009 12:43 pm

Randy

Thanks for all the information. I am a really inexperienced noob so your very helpful and detailed procedure kind of looks like this to me:

昨天以广州话为代表在安徽南部地区使用的人使用汉语作为母语和, 汉字在另方面

..but I'm working on it.

Frank-

It was more like an involuntary F#%&!!!

Patrick-

I live in rural VT/upstate NY so definitely not a drive by shooting. It is deer season however and my big green bus could be mistaken......

I'll check the luggage rack for a mallard.

Rusty wrote:
Quote: Did the engine crank a long time before it started?

No it has started like a champ all day even at 30 degrees this morning.

Mon Nov 09, 2009 3:18 pm

aopisa wrote: Randy

Thanks for all the information. I am a really inexperienced noob so your very helpful and detailed procedure kind of looks like this to me:

昨天以广州话为代表在安徽南部地区使用的人使用汉语作为母语和, 汉字在另方面

Ok, we all get that you're a noob. If you keep writing stuff like the above, people are going to less inclined to keep helping out.

Sometimes instructions from very experienced people are tough, but as long as you're willing to read and try things out, you'll start to get it.

But don't keep telling people that you're too much of a noob to get it. Otherwise, then you're going to have a rough time taking care of that bus.

Mon Nov 09, 2009 3:23 pm

Is it possible that a spark plug partially popped out? That makes a very big bang! I wonder if the bus would run very well at all if a plug PARTIALLY unseated-

Mon Nov 09, 2009 3:32 pm

Quote: Ok, we all get that you're a noob. If you keep writing stuff like the above, people are going to less inclined to keep helping out.

Sometimes instructions from very experienced people are tough, but as long as you're willing to read and try things out, you'll start to get it.

But don't keep telling people that you're too much of a noob to get it. Otherwise, then you're going to have a rough time taking care of that bus.

Got it thanks. It's just that I feel pretty self conscious in the presence of so many people that know so much and I am very grateful for their help. I guess I understand a lot of this conceptually (book smart) but when standing in front of the engine it doesn't make a lot of sense to me. Most of my free time is spent reading and trying to learn things from this forum.

I think it's comments like yours that make me feel more of an outsider and less inclined to ask for help.

I will keep quiet about my lack of knowledge and keep bookmarking things until it starts to gel for me.

Thanks.