I've fought with this problem since I first fired off the new 496.

When I start her up, she either fires up beautifully, or backfires against the starter. It seems to start up fine when it's cold but once it gets warmed up, the backfire is a pretty regular occurance. Once it's running, it's perfect. No pops, bangs, or anything that shouldn't be, just tons of neck-snapping torque.

It's already destroyed the drive in two starters. I got a new drive and put it in so I'm ready to get this backfire resolved so it won't tear up another one.

I finished building this engine over a year ago, but after fighting with this (and many other issues while building the car) for many weeks with no decent results, I decided to take a break from the whole car and get back into motorcycles. There's only about 450 miles on the engine.

I'm ready to get all the issues resolved so the Chevelle will be ready for my wedding next month so I'm hoping you guys can help me out.

I've only got the stock HEI (with an MSD coil) in it right now, so I'm going to install an MSD 6AL and put in larger power and ground wiring (I've got some pretty puny lines coming off the battery right now). Hopefully, that will be the last of my backfires and trashed starters. Anyone have this happen to them or have any opinions to throw out?

Thanks in advance guys,
---Chris

What's your initial timing set at? Too much initial advance can make it kick back against the starter, and could also cause a backfire through the intake.

chris

If you've got inadequate cables in the starting circuit, you create a severe voltage drop when the starter slams home. If it's bad enough, the module in the HEI will fail to trigger the coil, or there will be a delay and you get a momentary lack of spark. The 6AL is not going to fix anything.

Unfortuantely, I really don't know. I have an adjustable pointer on the timing cover and didn't tighten it down enough when I first started the engine. It vibrated loose and lost TDC, so I've just been kind of running it by ear.

---Chris

TJC, what do you recommend? Bigger wires?

---Chris

Originally posted by FireRescueFL:
Unfortuantely, I really don't know. I have an adjustable pointer on the timing cover and didn't tighten it down enough when I first started the engine. It vibrated loose and lost TDC, so I've just been kind of running it by ear.

---Chris could definetly be the problem, timing adjusted by ear can be WAYYYY to much.

For your battery cables, you need a minimum of 00 (double zero) gauge battery cable, on both the positive and ground. Your HEI needs a 10 gauge wire from the ignition switch.

We had a stock HEI on our L78 and it did the same thing a couple of times on start up then died completely. Just about the loudest backfire than I have ever heard. Sure drew unwanted attention. Went back to a stock GM points ignition with Accel points and it works just as well.

You really need to get the timing figured out. Period. Shoot, you could be at 30 degrees initial!
Also, ditto on the 00 batt cables. A must with a big block. And make sure the battery is bad ass too.
I had a weak setup once, and even with correct timing, it blew the starter clean off the car. Ruined the flywheel as well. Cost me 2 weeks in the bracket wars......
Ron

I agree with the timing issue. FIND TDC again, tighten up the pointer, plus make a mark on the damper (you could use a white marker or some "white-out"). Once you know where 0* is...you can work from there.

I figure anywhere from 12*-16* initial would get you in the ballpark of solving the problem. Remember to disconnect and plug your vacuum advance while sorting all of this out. I wouldn't even worry about plugging it back up till you got this issued solved. However, you WILL want to plug it back up once things check out ok (it's for mileage and can make a HEAP of a difference).

I had a friend swap intake gaskets for me one weekend while I was out of town. I HAD to have the car running Monday morning, but of course the intake gaskets blew just before I was heading out for the weekend.

Anyway, I get back from the weekend and start the car up for the first time. It kicks, snorts, backfires and generally gives me fits. I retarded the timing, by ear, a smidge to get it to smooth out as I had no timing light on me at the time.

We head out for auto tech school and car gets about 5 mpg LITERALLY. I burned a solid half tank of gas in 40 miles if that tells you anything. :eek: Anyway, the first chance I get at school, I pull the timing light out and see what it's on. Sumbitch....34* initial or so. And that's with me retarding it back a bit. My friend probably had it close to 40* and I'll never know how I didn't blow something up that way, but I guess I got lucky. Yowza. I dialed it back to about 16* and the car was a different animal. Ran, started and shut down smooth. Gas mileage returned.

Moral of the story is that timing by ear can cause fits (even tho old-timers will swear by it) and there is no substitution for setting with a light.

We're all curious as to how it turns out, so keep us posted.

Chris,
If it starts ok sometimes, then other times it wont, I wonder if the advance mechanism in the HEI is sticking and not coming back to the retarded position when you shut of the engine ? HEI's are pretty well known to wear out the advance wieght bushings. Also, you did not mention if you are running vacuum advance or not, and where you have it hooked up to. Should be running it and have it hooked up to manifold vacuum.

Thanks for the replies guys. I'm heading out to get a better set of wires and a piston stop this morning. Once I get everything done, I'll let you all know how it's going.

Keith, you hit the nail on the head. It really is the loudest backfire I've ever heard... and it definitely draws attention, just not the right kind.

BTW, the battery is an Optima red top. The distributor is a GM vaccum advance model plugged into the intake manifold.

BillK, how do I go about checking the HEI for proper function? Needless to say, ignition is definitely not my forte.

---Chris

could definetly be the problem, timing adjusted by ear can be WAYYYY to much.
Amen, graemlins/thumbsup.gif I've seen too many people try do set timing by ear because it didn't sound right.

Do the TDC like everyone said.

Couple other points to throw out there.

HEI's are good distributors unless the bushings are worn out, then they'll bounce like a rubber ball at RPM and then wear out the springs.

Is the motor even making enough vacumm for that advance to work? Perf. Dist. I thought used to say to set the Init. timing with the V.A. plugged? Any takers on that one?

After many V.A. problems whan my car was on the street I opted for a Mech. Adv. set up which I think when correctly tuned can give less headaches than the Vac. Adv. setup JMHO.

Is it possible that your new motor/starter combo alignment is off? My buddies 67(only changing the bad block on a rebuild) ate starters(aftermarket) and flexplates like crazy. Never truly found the cause but we put a new dowl in the back of the crank, double checked everything, put a new flexplate on it, checked for square,etc. and never had a problem again.
graemlins/hurray.gif

If the timing is off none of the last statement should be used unless it does it after the timing is right
smile.gif

Rob

Good news and bad news.

First the bad: I went down to the local speed shop and they don't have a piston stop. They put one on order for me and it should be here next week. graemlins/beers.gif

The good news is I got a set of 00-gauge battery wires and got them in. After that, I took a chance and just turned the distributor clockwise a bit and tried running it. It fired up and it ran pretty well, but it diesels when I shut it off. Actually, it starts making a clanking sound at around 550-600 rpm and below and then diesels when I shut it off. What gives with that? Idle screw to far open? I've got it set where it will still idle in gear (between 1000-1100rpm), but when it's in neutral, it runs at about 1300-1350rpm.

But, the really good news is that is doesn't backfire anymore! Now I've just got to get that noise and dieseling under control. Hopefully when I can time it properly, everything will just fall in line. :rolleyes:

---Chris

You may have turned it back just a bit too far and that's why its doing that. Unless you were just way out there and now your getting closer.

Until you can get a TDC check and a timing light on it you'll never truly know.

Sometimes they'll diesel when the plugs are way too hot.

Rob