1986 460 with stock 4180 4 bbl carb with vacuum actuated secondaries. The carb was rebuilt a couple years ago and has been working great.

I recently removed all the Air pumps and associated hoses and plumbing. It is a clean and complete delete.

I also replaced the timing chain (no changes to the cam timing since the one I replaced was already set straight up), water pump and distributor.

I had to replace the distributor because when I pulled the stock one out I discovered the housing was broken! Probably was like that for 40k miles. Unbelievable!

Anyway, thing idles and runs better than before but during the shake down I experienced a pretty loud backfire which seemed to come from the motor as opposed to the exhaust...

It happened as I was pulling a steep hill on the freeway at around 3,100 RPM, I pushed open the throttle and the secondaries were just coming on when suddenly Ka-bam! Backfire! Startled the heck out of me!

Fortunately, since the idle and idle speed hasn't changed I don't think I blew out the power valve (maybe because the throttle was wide open) but of course now I'm wondering why it would backfire.

Base timing is set at 8* BTDC and I don't think it should have any vacuum advance at WOT, but I do wonder if the new distributor has a different advance curve, or perhaps has more mechanical advance, than the original. Please correct me if I'm wrong here but IIRC heavy trucks are supposed to have stiffer advance springs and less overall mechanical advance than cars.

I will also note that I pushed into the secondaries a couple other times; on the flat, and and also at lower RPM on a lesser hill, and it was fine.

Any ideas you guys might have on this would be greatly appreciated!

 

You said it didn't back fire through the exhaust. Do you mean it spit back through the aircleaner?

 

In my limited understanding, a backfire is usually caused by one of two things: A very lean air/fuel condition which causes the charge to still be burning when the intake valve opens again, or a cross-fire that causes a cylinder to be fired at the wrong time.

Thinkg of the fuel situation, was the backfire the first time into the secondaries? And then they worked? If so I'm wondering if the float was hung and you didn't have fuel in the bowl. Then maybe the backfire jarred it enough to swing loose and the bowl filled. But, if the backfire was after using the secondaries then I'm at a loss.

As for the timing being a problem, I don't think so. Typically too much timing will cause pinging or detonation. But having pulled and replaced the dizzy it might be possible for the rotor to be pointing partially at the next or last cylinder and the spark might have found that it was easier to jump to that one rather than the correct one. (A cylinder under load has high internal pressure and the spark has a hard time jumping across the plug's gap.) Or, you might have rerouted the wiring causing those for adjacent cylinders in the firing order to run parallel to each other and a spark was induced on the wrong one.

I lean to the idea of the rotor pointing to the wrong cylinder since you had the dizzy out. Maybe you got it back in a tooth off?

 

Good morning Dave and Gary!

Gary, with the timing cover off the timing mark on the balancer was verified good and the timing was set to 8* BTC with a light. It idles fine and pulls right up to 3,500 RPM on the primary no problem, and would probably do more but I don't care to flog it any harder. Before it pretty much quit around 3,200 RPM and I'm thinking the new dizzy advance profile has something to do with that.

I did push into the secondaries a couple times before the backfire incident. Both times it was briefly, around 2,900 RPM, on level ground / less of a hill, and no problem.

I will say however, the secondaries seem to come on a little later / slower than they did before the air pump delete.

Dave, I'm assuming the backfire was on the intake side because it sounded like it came from inside the doghouse as opposed to out the tail pipe. Operating conditions at the time were 3,200 RPM, 3rd gear, and pulling a steep grade.

Last time I put gas in it was 2/22 but of course that's no guarantee the fuel in the secondary bowl was fresh even though I'd been into the secondaries a couple times prior.

 

Given what you've said about already having been into the secondaries successfully then I'd bet you have good fuel in them. And, if the secondaries ran out of fuel I would have expected it to bog rather than backfire. So I'm guessing it isn't carburetion.

Back on the dizzy, you can have the right timing and still be off a tooth on the gear. But if so the rotor won't be pointing directly at the cylinder to be fired. That means it has a chance of firing the next or previous cylinder in the firing order as the advance moves the rotor even more.

 

Im sorry but there is no such thing as having proper timing and being a tooth out. Ignition timing is right or it isn't. When you cannot achieve proper timing due to the distributor hitting something, that's when being a tooth out matters.
Just like it doesn't really matter which terminal is used as #1 other than the convenience of being able to use the one marked as #1 on the cap.