--In my experience, popping in the exhaust under closed throttle deceleration is usually caused by a combination and interplay of the following 4 factors/errors;... 1) air entering the header pipe,between the head and the collector, and 2) too much unburned fuel in the exhaust, and 3) late ignition timing, and 4) a too far open curb idle adjustment.
It usually begins with late timing.To compensate for the late timing, a guy might just crank up the idle speed. Then when the engine returns to idle from some higher rpm, a large amount of fuel gets pulled into and through the engine because of the exposed transfer port and ends up burning in the header, where the leaky pipe provides the air. Then you get the popping.
This is all avoidable with correctly setting the primary butterfly, to sync up the transfer port, and controlling the idle speed with timing and occasionally cracking the secondaries.Big cams like timing.
Your combo needs 3 things right now; More initial timing, a transfer port sync-up and elimination of the combustible mixture in the header.
The following discussion assumes either your dizzy is not equipped with a vacuum advance can, or you have defeated it and blocked the sparkport.
-It starts with giving it more initial timing and syncing the transfer port. Once this is done the third may take care of itself.There are 2 ways to do this; the best way and the easy way;
1)The best way, I know of to do this is to remove and drain the carb. Flip it over. Make sure the choke mech is disengaged.Now study the transfer slots,on the engine side of the butterflys.What you want to see is the slot form a little square, beneath the plate. Use the curb idle screw to set it. Next move to the secondary cracking screw. Set the screw in such a way that the secondaries are completely closed but not sticking closed; that is to say, just barely open.Ok, now reinstall the carb.And finally set your fuel mixture screws. They generally have an operating range of 1.5 turns out +/- 1 t. See where yours are.
2)The easy way; If you are sharp, back out the curb idle screw a bit until the engine wants to stall, then crank in a bit of timing to bring the idle speed up.You will need to do these two steps more or less simultaneously. Now gently open the butterfly by hand to be sure the transfer-ports are still working.Do this as many times as is needed, until the engine stumbles or hesitates,when gently opening the butterflys.Then crank the butterflys back open about 1/2 turn. This method is somewhat tricky, but gets you there with less mess or fuss, and if you cant get it , you can still go back to method 1.
-Now that the t-ports are synced we can move on.
-Next; if you have a Holley,lets set the float level.For all others we will assume its right---for now.
-Next lets fine tune the mixture screws. With the engine warmed up, put the idle speed up using the fast idle cam, to something like 2200rpm.If your carb doesnt have a choke you will have to use the curb idle screw. Just count the turns so you can return to the current setting. Ok, so were at 2200 ish rpm.Adjust the mix screws for best rich quality.If they dont seem to work, put them back to 1.5turns and reduce the rpm by 200 and repeat. When the engine responds, return to the previously set curb idle that you memorized.If nothing seems to make a difference, set them to 1.5 , and we will return here later.If the idle speed now is reasonable and the quality is good,we are done.From this point on, do not monkey with the curb idle screw.But if the idle speed/quality is not stable and the engine seems unhappy; we have 3 options; More timing,Cracking the secondaries, and fuel level
-Next;Lets fine tune the initial timing.As previous posters here have said, every combo is different.However, some generalizations can be made.1) the bigger the cam, the more initial. 2)If the starter kicks back, its too much.3) if when you put it in gear it wants to die, she probably wants more. OK. I think some initial numbers were thrown out there. I dont recall you mentioning your cam size, so lets just do this; Put the light on it and see where we are at. If it falls between 14 an 24 we are close.Write the number down so that if we have to, we can return to it later. If you have a vacuum gauge hook it up now.By hand, increase the rpm until the vacuum gauge settles down and gives you a maximum number, then return to idle.This is your target vacuum.Obviously changing your timing alone wont get you that number,For now, its just a target to remember.Now if your initial is on the low side of the range(14)crank in some initial and observe the vacuum reading.Keep doing that until A)the vacuum peaks, or B)the engine complains, or C) you hit the estimated max(24), or D) the idle speed gets excessive.Badaboom thats done.OH, not quite.Now you can go back and revisit the mixture screws if they didnt previously respond.
At this point the engine should be idling pretty nicely. Theres a couple of more things to do yet before we can drive it.Idle speed,Accelerator pump adjustment,Total timing, Rate of advance
-Idle speed; If the idle speed is too fast, you will have to reduce the initial timing.As a last resort for setting the idle speed if it is too fast, you can turn the speed screw out a bit. Keep track of how far you move it in case you need to return to it.If you encounter a hesitation after the adjustment,you will have to put it back. If its too slow, you will have to increase the secondary opening. And again, as a last resort you can turn the speed screw in a bit.In every case the mixture screws may require readjustment.
-Total timing;The total, all in, timing has to be limited to what the engine will accept at WOT without detonating, on the fresh fuel you are using. Numbers have been tossed out.Ive never had a BB, so cant give you a max-power number.I have heard it may be between 34 and 38 with iron,open chamber heads.With good fuel and on the street, 32* should be a safe starting point.So,Lets see where youre at.With the engine cooled off some, start it up with the timing lite at the ready.Rev it up to 3000 and read the timing.Increase to 3400, read the timing. Increase to 3800, read the timing. Repeat until the timing no longer increases, or the pistons come through the side,lol.No, No just kidding.If youve done this before, theres no need to do the indexing, just rev it up til it reaches a max number.This usually occurs before she gets to 4000. Now if this number is more than say 36*,It will have to be limited.Figure out how it needs to be done in your dist, and getrdone.
-Next will be roadtest, and finally rate of advance.
-So for the roadtest; We are at this point only concerned with full-load WOT/ no detonation, and assumes the carburetion is at least close, and the cooling system is running a decent temp. So use 2nd gear. If you hear rattle at any time, back off, increase rpm by 200 and retest. So, cruise it on up to 3600 or so and roll into WOT, listening for the tell tale rattle.With 3.55s and an auto this will be about 55 to 60 mph, so; Pick your test highway carefully.If it hasnt rattled by 4800, it likely wont and test can be concluded. If it does rattle, and then quits, take note of at what rpm. If it doesnt quit, back off the dizzy 2* and retest, until it quits.If you experience zero rattle, crank in 2 * and retest. Repeat until it rattles, the back up 2*. its probably best to not exceed 36* for now. So now its back to the pits. After a cool down period, Put the T-lite on and lets see where its at. Rev it up and read the number.This will be your max timing spec. Write it down.Finally revisit the base timing. If it has moved too far from the previously optomized number, its back to the bench with the dizzy for modding.
-Finally, the rate of advance. New roadtest. This time we are not concerned with full load WOT, but rather part-throttle roll-ons. So 1st gear will be fine. Now remember the Max timing, and the base timing now are fixed. Were not going to mess with those any more. So, from stopped, gently roll into it.No rattle is good. From stop, this time a little harder, roll on it.No rattle is good. Repeat until it rattles.If it rattles at any time, you are going to have to slow the rate of advance down. This is done by use of those pesky springs in the dizzy.You will need, depending on the dizzy, to change them out for stiffer ones, or bend a spring anchor. Then retest. Repeat as often as necessary to eliminate the rattle.Then check for rattle on roll-ons at other rpms and load settings. Change/ adjust the springs as may be necessary.
-And double finally, you will have to prove your jetting is correct.
-So there you have it.You should be able to get it pretty close in about 4 to 6 hours depending on your skill level. With practice, in chassis tuning gets faster. Most of the time is spent inside the oem dizzy. There are aftermarket types that are more tune-able.
- Now, I should tell you, Ive been at this one-finger typing, half asleep, for quite a while. So if anyone spots errors or omissions, feel free to jump in.
Ok, gotta go to work.
Im baaack.
- Now remember that target vacuum peak, we recorded earlier. Heres how that number gets to mean something. When the vacuum peaks, this is an indication that the engine is starting to become more efficient.Before this rpm, the pistons were sending some ingested air back into the intake, due to the late-closing intake valve. Now when the engine vacuum peaks its partly/mostly because the rpm has risen to a point where there is no longer time for this reversion to happen. The point is this, for best fuel economy, your cruise rpm should be at/above this number.
-And heres my take on the vacuum advance system; If your building a track car, you probably dont need one. ALL other street type cars will benefit from having a well tuned system.You will enjoy more mpgs, smoother/quieter cruising, a cooler running engine, better part-throttle response and quite possibly more part throttle torque. I cant imagine running without one.
-If you are anything at all like me, you will be fine tuning your timing for many tankfuls to optomize it. A fully adjustable vacuum advance distributor will make tuning a lot more enjoyable.I know you just bought a new dist, but it may not be the one for you.If this seems rather daunting, be advised that there are companies out there who build timing boxes that have either adjustable or selectable preset, timing curves built into them.Pricey tho.
-Now, lets see how THIS plays out.lol. All the best to you.
