C3 4ME

I did a search in the archives and what I found is that a popping out of the exhaust on decel is generally caused by a lean condition. Can timing cause the same? How about a rich condition, can that cause it? My car will burn your eyes if you stand behind it in the garage, so I'm fairly sure it's running rich. Lars I intend to send you the carb once I get my tax refund for the once over.

lars

Popping out the exhaust on decel is almost always a lean condition that occurs when the throttle is slammed closed at elevated engine rpm. What happens is that the cylinder mixture goes momentarily lean, you get a lean misfire on a few cylinders, and this misfire dumps unburned air/fuel into the exhaust where it then eventually ignites (explodes). It is possible to have an over-rich mixture causing burning in the exhaust, but this over-rich mixture will seldom cause an actual misfire, so the air/fuel is burning as it's leaving the cylinder and not "exploding" in the exhaust. You'll see this type of rich condition on some road race cars where you see the flames coming out the exhaust on deceleration. But with the flame, there is no "popping" with the rich condition - the popping occurs with lean misfire.

Incorrect timing advance can contribute to the problem: A lean mixture burns slower than a rich mixture, so you have to have adequate ignition timing to fully burn the lean mixture. If you're lean with retarded timing, the popping is increased.

If you're smelling "rich" exhaust, chances are you have a timing issue - rich exhaust causes high CO. CO is odorless. What you can smell is high Hydrocarbon (HC) content. High HC is caused by timing.

C3 4ME

Thanks Lars. I know I need to check the timing. I also with my tax refund intend to buy a dial back timing light and make sure that's where it's supposed to be, before doing anything about the carb. I forgot to mention that my rear bumper also gets sooty. I've had a person following me back from Carlisle last year keep an eye out and he said my car was not smoking at all

cardo0

Do have your vac adv set up?

What is your vac can vac at full vac adv?

At what vac does your vac can lose adv?

What is your initle mech adv?

What is your adv at idle without the vac can connected and what is your idle vac reading?

What is your adv at ildle with the vac can connected?

cardo0

C3 4ME

the only question I can answer is that the vacumn advance is hooked up. Don't have a timing light, but will get one soon to get this straightened out.

cardo0

U will need a vacuum gauge to measure the eng vac at idle. And a Mighty-Vac hand pump (sold for bleeding brakes) to test the vac can.

Good luck,
cardo0

C3 4ME

I do have the vacumn gauge, but no vacumn pump. It only runs about 12 in. of vacumn, which I hope is attributed to the noticeable cam.

cardo0

Mighty-Vac's really aren't very expensive but maybe u can borrow 1 from someone. But it's the easiest way to test the vac can.

Only 12" vac is fairly low and i bet u have trouble with your headlights and power brakes. What size cam is it?

What is the rpm and mech adv at idle with the vac adv disconnected?
And take another idle vac measurement with the vac adv disconnected from the vac can hose. U may need to average the reading for needle bounce.

cardo0

C3 4ME

I wish I knew anything about hte cam, but it was in the car when I bought it and the seller couldn't find the paperwork on it. My headlights and power brakes work fine though. In fact the headlights arent' even slow to go up or down. I'm going to have to get back on this one in a couple of weeks when I have the $$ to buy a timing light and the vacumn pump. I'll post back then.

lars

Just get the timing light - you can skip the vacuum pump for the purposes of what you're doing. Just get your total timing set up to 36 degrees (drop me a note for the timing paper if you need it) and hook up your vacuum advance to manifold vacuum. You can check the total vacuum advance just with the timing light. On decel, any vacuum advance is going to be pegged out, so the exact specs of the unit are irrelevant to your popping issue as long as you have vacuum advance in the 16-degree range. Once timing is set up right, any popping on decel is related to a lean condition, regardless of what vacuum advance unit you are using.

Why don't you just back your idle mixture screws out 1 or 2 turns and see if the popping stops or reduces...?

V8FastCars@msn.com
Lars

fauxrs2

How about a popping backfire type sound on shifts.. but not when decellerating - particularly pronounced when cold

lars

It's the same thing - on shifts, you're letting off the gas pedal (closing the throttle) and the engine rpm is decelerating with closed throttles. If you get a few pops out the exhaust, you're running lean.

C3 4ME

Lars, my Q jet mixture screws are something I've never seen before. They are not slotted like every other Q jet I've ever seen. The look like a double "D", and I"m not sure what tool I could use to turn them. I've heard of q jets with caps over the mixture screws, but I thought that was only on computer carbs and mine is definitely not a computer carb. Lars could you please send me your timing paper to Petes81z@yahoo.com.

lars

I don't know what year car or carb you have, but late 70's Q-Jets have mixture screws without screw slots. You go to NAPA and buy the mixture screw adjustment tool for about $10. All Q-Jets after 1975 had plastic anti-tamper caps on the mixture screws.

For the timing paper, just drop me an e-mail request to:
V8FastCars@msn.com
That way I can "reply" back to you with the paper attached.
Lars

C3 4ME

I finally got the timing light today. It's sitting on 14 degrees initial but nowhere near 36 degrees total even at 3000 rpm. Will get a set of springs and weights per Lars's paper and go from there. One question though (consider this timing for dummies). When I disconnected the vacumn advance the car wouldn't run at all. I had to turn the idle way up to get it to stay running. Is this normal?

coinwasher

Give me a call sometime 357-6996 (or email coinwasher@aol.com) maybe you can bring it over (rt 33 and 295 area) and we could mess with it. Sounds like the distributor could use a little attention.
Dan

C3 4ME

Dan thanks, I may take you up on that offer. I'm going to play with it some per Lar's paper and see what I can do first, and If it doesn't help I'll give you a call.

BarryK

sounds like the vacuum advance to connected to full manifold vacuum (that is good) so when you disconnect it the idle will drop. Just increase the idle speed to the normal speed via the idle adjustment screw and than adjust your total timing so that it's all in somewhere in the 2800-3000 RPM range. Than reconnect your vacuum advance - when you do that your idle speed will jump up again so now you just need to back the idle speed back down to normal.
On my '65 whenever I disconnect the vacuum advance I always have to very quickly turn up the idle speed before she will stall because the idle drops so much and that particular motor needs a higher idle speed just to stay running because of the cam that is in it - it's will not stay running at anything below 750rpm (factory 30-30 cam).

coinwasher

Man, I love 30-30 cams, that brings back some mustang crushing memories. We used to have Mustangs for lunch, Cougars for dinner, and an occasional Super Bee for midnight snack

C3 4ME

Thanks Barry, that confirms what I was thinking. I'm using your sticky post and Lar's timing paper as my guide, and hope to get this one worked out on my own. Mine has a pretty noticeable cam in it also. It wouldn't idle at all with the vacumn advance disconnected, so I had to crank the screw up to get it to hold an idle. Hope to pick up a set of distributor springs tomorrow and get back on this one.