.....I'm getting a little more popping from the tail pipes than I'd like, not that I mind but I don't want to wreck my pipes. Why did this happen since the last time I drove the car ? If you have any thoughts of answers please let me know ! My car is a totally stock 1960 Cadillac Coupe De Ville with a stock 390 mill. Thanks, 60'
stock 60 caddys shouldn't pop when you let off the gas. my first thought is you got a hole in your pipes or muffler. that's what my motorhome does and it's got a muffler that blew a hole in itself a while back. those caddy 390's rock... had a 62 several years back. ever since then when someone says thier car rides like a Cadillac I laugh. nothing glides down the road as smooth as a 5000 lb Caddy.
Sounds like the timing is too far advanced, but not sure why that would change? Reaaalllly thinkin out loud here, but perhaps the advance is not retarding as it should? Poor vac*** would affect the timing, but should actually retard it.
what kind of exhaust do ya have? i have straight pipes on my '60 and they cackle pretty good when ya let off the gas.....
Running a bit rich added to a small exhaust leak can cause popping on decel. As an added note, a carbuerator dashpot helps to eliminate a sudden snapping shut of the throttle plate. On decel closed throttle is the point of highest engine vacuum which in turn can draw more fuel than needed through the carb. Excess fuel in a hot exhaust will cause popping and back fires....
I was thinking too far advanced, but it would be too far retarded, wouldn't it? Its been a long day, but Im gonna second the vacuum leak. Or it could just still be a Cadillac
She's running the stock manifolds. SAY WHAT ? Just a Cadillac !! It's like riding on the sofa when I'm p***ing cars on the freeway ! I'm thinking vacuum leak somewhere ! Thanks guys, 60'
If your vacuum advance is connected to manifold vacuum, the timing will advance when you let off the throttle. Generally, popping in the tailpipes is caused by a leak in the exhaust sytem ahead of the muffler.
Leaky/worn exh.valve? oh, and when people ask me if my Chrysler rides like a Cadillac, I usually say "I sure hope not!"
Just messin with you and the 5 guys that said "my Cadillac does the same thing!". I have earned the right, my moms car when I was a kid (early 80s) was a 67 Caddy convertible. Wish she still had it!
Not to bump an old thread, but thought I'd chime in in case somebody finds this looking for their problem like I did in the future...the timing thing could very well be the cause. I go off on a bit of a tangent as I was on the hunt for a solution for another problem when I accidentally solved another. I had the same issue as the OP, with a solid exhaust system with Flowmasters (PO's work), with the exception of a small exhaust leak near the collector or exhaust manifold. After reading this thread, I was gonna have my neighbor look at it (owns exhaust shop) to figure where it was coming from exactly. Now, I noticed some stumbling off idle and loping when it shouldn't have been during low acceleration, low speeds. When you're giving it just enough to cruise at 35-40mph or less in 4th gear. It was fine the first week or two I'd been driving it, but changed after I cleaned the engine off. I thought oh great, I messed up the vacuum by disturbing something somewhere... After getting out the dwell meter and setting the points properly (always fun!), and checking the vacuum with the gauge and carb cleaner as you do, my vacuum was solid. I also replaced the plugs, which were mostly tanned evenly, despite being old. Although a couple of the old Autolites in it were gapped oddly, like they'd been put in straight out of the box. Some were gapped as low as .20! A piston didn't smack it, either. I also guesstimated the timing by just a tad to make it idle smoother after the points had been adjusted right. I say guessitmated, because a timing light is worthless when the timing marks have been surface rusted over. Couldn't clean the harmonic balancer the right way to get the marks to show up, I was pissed anyway...what the hell, already running rough. Only changed it a tiny bit, hooked up the vacuum, and tested how it ran. Against better judgement, I also made the carb just a little bit richer because this stumbling was driving me nuts and I was trying everything to get rid of it. Was gonna replace the fuel filter too, but forgot it. Obviously wasn't the filter then. Still gonna replace it, however. Anyway, after all of this, it got rid of my stumbling. Not sure exactly what particular adjustment got rid of it, because it still persisted even after I'd changed the plugs, adjusted the points, checked the vacuum, timed it...the carb was the last thing I did. A few minor adjustments, figuring the problem was still there, I sat down with it in neutral and gave it some gentle off idle acceleration...holy ****, is it gone? Is it finally ****ing gone?! I was losing patience that day. So I took it for a short run down the road, it didn't stumble or lope, and I noticed something else...when I let off, no popping or rumbling. Perhaps the points were just worn, and when adjusted by the PO (if they were at all), affected the timing, and it cascaded from there. Maybe it was the carb, I'm not sure. It did seem tuned a bit lean, but why would it have started running poorly after a couple weeks? Maybe old gas, maybe new gas, maybe this, maybe that... Will probably need some more tuning just to make sure of everything...need to figure out how to find my timing marks again (bump starting and checking pistons for TDC, what fun), and maybe make sure I didn't tune the carb too badly, but it seems for now the popping has gone away, even with the minor exhaust leak still there. Still gonna get that sealed up...watch; after that gets fixed, I'll have to re-time or re-tune something else again. Ah well...runs smooth right now. No pings, knocks, stumbles, or pops. That'll do.
Exhaust leak is probably going to be your number one cause. I've had computer controlled throttle body trucks pop like that when dropped in 2nd going down hills, and they've all had goofy exhaust systems on them. The one I'm running now I can't get the clamps to seal the converter in, and I'm too lazy to try to weld it up.
The Jalopy Journal
