76 280z backfiring on startup

[austinnelson117]

I have no idea what the problem is, and im stumped. I have a 1976 280z, carburated with a holley carb.

my engine bay

The cars my DD. I replaced the oil pump maybe 500 miles ago. The timing was off, but I followed the FSM and set it TDC, and put the dist. to 11:25. After that, it fired right up and drove GREAT for a couple weeks. Then the seal on my water pump blew and i unloaded all my antifreeze on the highway and had to get towed 25 miles back to my home to fix it. The car drove great for about 3 days. I parked it last night, after driving normally, and this morning when I went to start it, it started for about 2 seconds, and died (which is always does when it's cold, once i crank it again and choke it a bit it runs fine.)

Now when I go to start it, it'll crank for about 4 seconds then just a HUGE bang from the exhaust (straight piped). I can't get it to manually start by dropping the clutch, or using the starter to force the crank. It just wants to crank, bang, crank bang. I have not messed with anything mechanically since it ran fine. That's why im baffled.

Video:

Once it backfires, I don't continue cranking anymore. I just stop to avoid damaging anything.

What do I do!?

Edited September 13, 2012 by austinnelson117

[Wade Nelson]

I'd pull you plugs and make sure you're not hydrolocked. That'll make a really loud bang when you crank. And it won't start.

Pull the plugs, crank it, and see if it cranks normally. Have someone else crank it for you and see if any fluid gets ejected from any of the plug holes If you have a blown head gasket that would explain the hydrolocking AND the blown water pump seal.

If it's truly JUST an exhaust backfire, then I'd look for an air leak @ the exhaust manifold gasket or downtube.

[austinnelson117]

We just changed all the plugs - and each plug was out and we cranked it. Engine cranked fine, sounded just like every other day. Nothing spewed out. The original plugs looked fine too, no oil or anything. just normal wear and tear. It will backfire on startup for about 2 mins of trying, then after it'll just crank and crank and crank. After it sits for a while, and I go to start it again, itll repeat this process. I have no idea what's going on.

[Wade Nelson]

Sounds to me like it has jumped time -- timing chain has jumped one or more teeth on the sprocket.

Have you done a cranking compression test? Or checked vacuum while cranking?

[austinnelson117]

No, I don't know how to do / have the tools to do that with. My plan was to tow it to a shop in the morning and have them take a look. I'm stumped and need a car.

[austinnelson117]

Also, what I also tried doing was backing the car in the street, putting it in first, cranking the motor and using the starter to get the car going. While in gear, I let the starter continue to turn crank, hoping this would somehow start the car. Which it didn't. I know it's not the best thing for the starter - but I at least know nothings locked up, or broken internally as everything seemed to run smooth, and no cranking, or scraping noises.

[Wade Nelson]

Imagine one of your exhaust valves being stuck open WHEN the spark plug fires. The fuel/air mix not just in the cylinder, but also in the exhaust manifold is getting ignited. That's what your backfire is. It could be a broken distributor gear or bent valve, jumped timing, blown head gasket (allowing fire from one cylinder into the next one) etc.

Compression / vacuum testing will point you in the right direction to look.

The fact that you recently replaced your oil pump SUGGESTS to me something wrong with your distributor drive, like a broken tooth, whatever.

I don't think your repairs are going the be cheap.

[Wade Nelson]

How about posting a followup of what your mechanic found?

[austinnelson117]

My mechanic is myself, I do all my own work. That way I know its done right. The exhaust was open after the headers and I remember driving and hearing the exhaust popping a lot. Long and behold it backfired and knocked the timing off. Reset everything tdc, distributor wise, etc and.car runs great again.

[Wade Nelson]

I don't see how backfiring could knock the timing off. Please elaborate.

(I intend to sell this response to Click and Clack, so....be careful!)

[austinnelson117]

Not sure what exactally. All I know is I had trouble starting it for the longest time. Eventually it just quit. (The other day). When I put the engine tdc and opened the dizzy, when it was supposed to be on cylinder 1 it was on five. I also opened the timing cover and watched the chain and it was very tight and seemed good. I bought another one in plans on changing it but thank god I didn't have to. After resetting everything it cranked faster and smoother than normal and seems to run so much better now.