Backfire!

[TedW]

So today I drove my truck 6 miles to the boatyard and left it there for 5 hours while we wasted the afternoon floating on Casco Bay.

When I turned the key to go home it sounded like a rifle shot - my ears rang for about ten minutes. Then I turned the key again and it started right up, no problem. We drove without incident and everything looks normal under the hood.

I have a 2.25 / Weber 1 bbl / Ducellier w/Petronix / K&N / SpongeBob air freshener

So, what happened? Why? Was any damage done?

[Les Parker]

Did the vehicle deisel when you switched it off, before enjoying a swim?
With that loverly Ducelier dist. you might have a problem with the vacuum unit. Probably fuel built up in the exhaust silencer (especially if LH exit) and ignited when you stared it up. I've known the silencers to explode at the seams under similar circumstances.

[TedW]

No, Les, It didn't diesel at switchoff.

Did the vehicle deisel when you switched it off, before enjoying a swim?

I used to have a dieseling problem, which YOU fixed by selling me a solenoid for my Weber. Lesson, here, boys and girls - Les is The Man.

Yes, it is a LH exit.

So, Les, why would fuel be building up in the silencer? What can I do about it?

[Les Parker]

Ted,

What octane fuel are you using? Also the dist. (Ducelier) maybe getting towards the end of its life, the vacuum units on them do not seem to last as long as the Lucas units.
As to why there is a build up of un-burnt fuel, have you tuned the carb or is it running a little rich? If running slightly rich, some of this can reside in the silencer till it finally ignites with booming effect.

Please advise.

[TedW]

Ted,

What octane fuel are you using? Also the dist. (Ducelier) maybe getting towards the end of its life, the vacuum units on them do not seem to last as long as the Lucas units.

The Ducellier is new- about 2-3k miles on it.

I use highest octane gas available; early on I was under the (apparently) mistaken impression that high-test was required for our lumps, and have run it ever since. It has always been in the tank when I've had the truck tuned, so I've hesitated to run anything else.

How else can I check for rich-running? Look at the plugs, and hope for that golden-brown coating?

The truck purrs like a kitten otherwise.

[mechman]

I've had this problem crop up before as well. What I've found is that the advance plate in the Ducellier tends to stick a bit if it's not lubricated properly or the distributor is worn. there are a couple test to try:

Test 1:

Warm up your engine, take your truck for a spin. After you get back, shut it off and pop the distributor cap. Locate the pin where the rod that comes out of the vacuum biscuit attaches to the advance plate. This actuating rod pulls into the canister when vacuum is applied, advancing your spark. Gently push on this pin, away from the vacuum canister and thus retarding your spark, and note how much the advance plate backs off.

Test 2:

Pull the vacuum hose off of the carb and apply some human-supplied vacuum while watching that advance plate. Note how much it advances, then note how far it returns after you release the suction... uhh, vacuum. Repeat that a few times, and note any changes. Lube the advance plate and run the tests again.

I'm willing to bet that you'll find that on Test 1, you'll be able to nudge the advance plate back at least 5 degrees. On Test 2 I'll bet you'll find that the advance plate is not returning to base timing every time you release the vacuum. BUT I'll bet it will work a LOT better after it's lubed.

Mech

PS - Even though you could just lube the plate and go, do the tests first. It's really a good idea to fully diagnose the problem before you throw a repair at it - you may have another issue going on, too. But in my experience I've found that a backfire before cranking is usually a sticking advance plate in the distributor. YMMV.

[TedW]

Mechman: Thanks! Now, a dumb question for you:

BUT I'll bet it will work a LOT better after it's lubed.

What do I lube it with?

[mechman]

I like to use white lithium spray grease with the wand attached to the nozzle, but sparingly - aim for the points of contact. Move the advance plate with your finger while spraying it, to get the grease into the contact areas. Some folks prefer WD-40, but I find that gets dirty fairly quickly and is messy. Avoid spraying your points!!! Also avoid any electrical connections.

If your advance plate is still not returning to base timing after lubing it, carefully detach the vacuum advance rod from the plate then check the plate to see how well it moves. Sometimes the rod hangs it up, but sometimes the return springs on the Ducelliers get a little weak and can't pull back against the diaphragm in the canister. If so, your repair might get a bit more involved.

Mech