Pop pop on exhaust side

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Chadderbox70

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Hi, just trying to fine tune a 360 with about 10.5 compression. Iron J heads 2.02 . Medium cam lunati 703.

I get a pop pop at random in the exhaust.

I have the timing set around 18 degrees intial.
It doesn't seem to matter where I advance or retard it too. I get a random pop pop normally I run around 12-15 initial and then another 18 or so all in. Mechanical billet msd distributor and 6a box. Msd blaster coil.

I wonder if it doesn't like my autolite 65 plugs if I should step down. ?

I lean out or enriched the carb and I still get the same problems

I have checked all my spark plug wires

I'm running out of ideas , my plugs look good still if hasn't run much or hit the street yet

Thanks
 
I have super thick gaskets and locking bolts. The exhaust is super tight. All brand new.

It's like a misfire or unburned fuel

Nothing backfires through the carb
 
The exhaust gases are an extinguisher, there should be no unburned fuel nor oxygen in there.
When the exhaust leaves the valve, it exits into high heat hell.
Hence the question how can it pop without oxygen, it must be sucking in somewhere.
I often get someone to plug the tailpipes with a lotta shop towels, cuz it's gonna get hot, then fire it up and quickly, (remembering the friend with hot hands), walk around front of car listening for leaks.
That's just where I'd look first.
Good luck.
 
I would double check the timing, and check the balancer to make sure the outer ring hasn't slipped, this could give you a false reading on the initial and total timing.
 
In reverse order of importance IMO
I don't know about the later heads but with the earlier 273/318/340, no exhaust gasket is needed. Flat to flat.
On the exhaust itself or with headers, I prefer thin so the clamping is more even and less likely to distort the flanges, BUT many headers in particular are made with a rough raised bead - and those need a thick gasket.

Exhaust size. Big tail pipes, big tubing, short exhausts, allow air up into the system at low rpm and decel. It used to be pretty common to hear pops in the exhaust of sports cars as they decelerated during races. Excess fuel, no vacuum advance, short fairly open exhausts all contributed.

Initial timing seems to be ballpark correct - as always depending on rpm. I don't think that's the cause. With no vacuum advance, the mechanical can come in quick. Since we don't know the specific conditions when the popping occurs, hard to comment further.

Fuel. This you may be able to clean up. If the other posts about the 703 are correct, the idle should have decent vacuum for a cam with that lift and duration. I suggest a thorough mapping out of the carb. Start at the begining.
Fuel levels.
Then take it off and check the primary transfer slot exposure.
Note how many turns of the idle speed screw to get to that exposure. On an older Holley. the primaries should have a .020 to .040".
With strong idle vacuum, slightly less exposure will be needed depending on the idle feed restriction and idle air bleed Holley used.
If you have the drill bits, measure the idle feed restriction and air bleeds.
Count how many turns out for each of the idle mix screws.

My thinking is that the fuel level may be a bit high in one of the bowls, or the transfer slot exposure is a little bit too much, and as a result there is excess fuel once the engine is cruising above idle speed.

Leaning out the off idle and increase the idle mix to get the idle where it is now. That would be the approach if the problem is excess fuel. A little higher in the throttle opening and it might be necessary to mess with the air bleeds but thats getting deeper than may be neccessary.
 
What type of fuel pump? Verify correct fuel pressure at the carb.
My brother-in-law’s Maverick developed a pop in the exhaust and we tracked it down to a failing fuel pressure regulator. He was running an electric Holley pump.
 
Exhaust valve not closing 100 percent
Broken valve spring
 
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What my vaccum guage shows with timing set around 18 initial and my carb leaned as per Holley and idling around 850rpm

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PXL_20240228_182309482.jpg
 
At idle and lower rpm it happens

Just a pic to show you what the engine looks like. I know it's incomplete and the linkages aren't on in the pic.

It's got a factory mechanical fuel pump

I had a machine shop do the heads and they have brand new comp valve springs. I cc them and there was no leaks
 
It's running pig rich so it has to have excess unburned fuel in the exhaust.
 
I can tell you that your idle timing is too much, and the sooty plugs are the proof.
The only thing keeping that engine running is the multi-strike box.
I can run that cam at 5degrees, and cure that sooty problem right up.

Here's how it works;
you buzz the timing up, and the rpm goes up with it. So you close the throttle with the speed screw. Now the engine runs out of idle fuel from the transfers, so you richen up the idle mixture screws and Voila, you get a nice idle.
The problem is
As soon as you open the throttle, and the transfers come on line, the AFR goes rich, and the plugs get sooty, and it's rich all the time

Here's the cure;
Reset your mixture screws to 1/2 turn out. leave them there.
set your idle timing to not more than 12*, and leave it there.
set your idle speed to, in the window of 550>650, in gear, wherever it idles best.
Put the trans into N/P and make sure the idle speed has not increased more than ~100 rpm. If it does, then the engine is making too much Idle-power.
The cure is less idle rpm, or less Idle timing.
If the in-gear idle was at the top of the window, try 50 rpm less.
If the idle-rpm was already on the low side, then take out 2 degrees of timing, and reset the idle-speed.

>Your engine will make the most sub-stall rpm power, with the idle-timing maxed out. This could take up to around 30/35 stinking degrees. But you cannot drive it like that on the street. The car will be very difficult to drive at low rpm.
Your engine can be adjusted to idle just fine down to 5* advance, maybe even less.
>Your engine does not car about how many inches of vacuum it idles at. As to vacuum, the only things that are important are that the number is stable, as in not drifting up and down, and that it appears to be more or less glued in one place.
>Your engine does not care what the idle-timing is, anywhere in between those two numbers. The only thing that changes when a 220ish cam is used, is the idle/off-idle power. So you have full freedom to set the timing anywhere in that window, until it soots up the plugs or becomes hard to drive or bangs the transmission when you put it into gear.
>Your engine, the first time it cares about timing, is at stall rpm. And the second time it cares is after about 3400rpm.
To make the throttle opening exactly right, for that cam, if you care to know, is;
Take the carb off, and flip it upside down. Then with the speedscrew, set the throttle opening to reveal the transfer slots open to about exactly square, but not less than. After this, do not move that speedscrew. Now, flip the carb upright and set the mixture screws to 1/2 turn out. If your carb has a 4-corner idle system, close the secondaries up tight but not sticking, and close the secondary mixture screws; your #703 cam does neither need nor want that system to be operational. If you need to relax your link-rod to make this happen then do it. Reset all your accelerator pump parking spots as may be required. Reinstall the carb, fill it up and start the car.
DON'T TOUCH ANYTHING
Let the engine warm up, then retard the timing, without looking at the timing marks. Use the timing to get the rpm down to 750, then put the trans in gear. if the rpm drops more than 100 or the transmission bangs, then readjust the timing for 700 rpm in neutral and try it again. and so on.
If the rpm falls to less than 550 in gear, STOP retarding the timing.
If the idle-timing falls to less than 10 degrees, Stop there.
Go check your WET fuel level;
If it is at spec or higher, reduce it about a mm or 1/16 and restart the test.
If it is below spec at least 1mm, leave it there.


The above assumes that ;
1) the PCV is correctly plumbed and functioning correctly
2) that the oil-level in the pan is below the crank
3) that the fuel level is stable
4) that all the air that the engine is getting, it all came past the transfer slots.
5) that engine coolant level is at least 180* and is stable
6) that the convertor is 100% ok.
7) that the fan is not disrupting the bowl vent.
8) that fuelTank is properly vented
9) if you have a vapor canister, that it is functioning properly
10) if you have electric fans, and your engine coolant temp is jumping back and forth 30 or 40 or more degrees, I can't help you.

BTW
the condition of those plugs make them junk.
Get some Champion coppers in a heat-range of 12. I forget the number; maybe RJ12YC/something like that, with a 5/8 hex for header clearance. but 12s.
 
Awesome advice. I'm gonna try those suggestions for sure. I'll try to get video of what's happening
 
Chadderbox70 said:
What my vaccum guage shows with timing set around 18 initial and my carb leaned as per Holley and idling around 850rpm

View attachment 1716217468

View attachment 1716217469

View attachment 1716217470
Click to expand...

I don't have any idea what Holley is saying these days. The first steps are the ones I already provided and come in part from their former VP of Engineering Mike Urich. Fuel level first. Transfer slot check is needed because we can see its way too rich for conditions. That's also in his book. For trimming the idle fuel air mix, turn in the screws until a slight drop in rpm or vac, then increase toward rich 1/8 turn. The reason going toward lean is better is that perfromance drops off quickly when going to too lean. Then turning back richer is needed to better handle the load when going into gear.

Timing. 18* at 850 rpm should be about right for that cam and engine. You can try a a couple degrees less or more but that's not the immediate issue. When you try more or less initial, then readjust the idle mix if needed. Finally test by shifting into gear if its an automatic. The least drop in rpm (or vac) is the strongest tune.

9" Hg at 850 rpm is IMO low if that is in neutral (park) and warmed up. At least the way people talked about that cam. I was expecting stronger but that's OK. Lets just see what we (or really you) can do to improve on that a little. That should just come naturally as the AFR gets cleaned up.

Thank you for the photo. That is certainly not an old school 4777. LOL. What is it? And does it have 4 corner idle?
So now I'll backpedal a little. Even more important now to check transfer slots. But now I'll encourage you to measure the air bleeds and if you have the small drill bits, check the idle feed restrictions. Once the blocks are visible, check the "emuslion holes" too and any kill bleeds. Take pictures and ask questions if you need.


Chadderbox70 said:
At idle and lower rpm it happens
Click to expand...
Thank you. That fits into the scenario described. Issue is with the low speed circuit (aka idle circuit).
Chadderbox70 said:
Just a pic to show you what the engine looks like.
Click to expand...
Very nice, and also helpful seeing what you have.
 
The idle port is restricted by the idle mix screw. Those let us do some fine adjustment to the idle mix, but they are only part of the story.
The transfer slots also feed fuel at idle. The portion above the throttle blade also bleeds air into the mix, speeding the fuel along.
The more the primary throttles are open at idle, the less restriction to fuel going coming out the transfer slots below the blades.
1709769977148.png


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You can check amount of slot exposure below the blades at idle with feeler gage or drill bits.
While you're at it, write down how many 1/4 turns in of the idle speed screw from just touching gets .020, 030, and .040. Then you'll always know how far open the throttles are.

Idle circuit is a misnomer. Low throttle circuit is a better way to describe it.
As the engne comes off idle, the portion of the slot under the blades becomes more open and the air bleed less. Fuel continues to flow with increasing throttle until the main circuits fully take over.
So idle, decel, off-idle, and cruising on flat up to 50 to 60 mph.
 
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I had a spark plug wire that would occasionally get partially burned through. Started popping randomly as well. New plug wire and problem was fixed. Sometimes it is the easy stuff that causes problems.
 
Idle timing at 18* is not too high & not causing the problem. Complete rubbish. Plus OP says he has adv & ret timing, problem still there. Millions of cars of that era had 20+* of timing at idle.
I would first check the p/up gap in the dist. Make sure it is about 0.010". If gap is too big, or reluctor touches p/up, no spark on THAT cyl; unburned gas into exh system, gets ignited in pipe, pop.....Remove the dist & check each each cyl for gap...or any looseness of the internal dist parts.
Next check & measure the resistance of the plug leads, with the lead still plugged into the cap. Check the rotor & inside of cap for any signs of arcing.
I would replace the spark plugs with NGKs in a 5 heat range. A plug that is too cold will also cause your problem.
Could also be as others have mentioned: carb related, broken v/spring, valve not seating because of incorrect lifter pre-load.....or wiped lobe.
 
I agree that 18* degrees is not too high.
The point is that to get the idlespeed DOWN
with that much timing,
is gonna take shutting the throttles. And then the transfers get shut down,
and then the mixture screws get cranked open, and then the instant you step on the pedal, the entire low speed circuit goes rich;
and there is no good reason mess your carburation up like this , NONE.

That cam will idle at 5 measely degrees, or at 25. that is not the point!
the point is how easy this problem is to fix. That engine could be popless in the time it takes to swap out those plugs, and reset the T-slot sync.
If only the plugs are swapped, and no other changes, it will just start all over again.
If the carb is synced up, and those plugs left in there, then the car will need to go for a clean out run, to get the plugs to clean off, and if it's not done right, they'll just glaze up, overheat and maybe slough off insulator material, which can destroy the rings or the entire engine.
So I mean, how about you guys consider the big picture, that there is only one reason to run 18*, and is that so you guys can have something to flap your gums about.
 
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