Exhaust popping 330GT

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330GT_London
Posts: 17
Joined: Mon Nov 02, 2020 12:39 pm

Exhaust popping 330GT

Post by 330GT_London »

Hi everyone, hope you can help with some advice for loud popping in the exhaust system.

I have tried tuning the car many times, but the exhaust still pops relatively loud at decelerations. Sounds like it is somewhere under the car or more towards the rear.
Tried making it leaner and richer, but can't get the popping away either way.

Some background information:
I got my 330GT about 2 years ago and had it for a full professional services / light restoration at Ferrari specialist as it had been stored away for a long time . This included:new exhaust system, timing adjustment, rebuilt carbs, new fuel lines and tank, new ignition leads, distributor renewed, new roller rockers, re-set valve clearance, compression test amongst other things. Everything needed to get it running nicely. The first year I didn't have any problems with popping from exhaust and it was driving nicely.

The popping started a little last year and I also noticed more black spitting from the exhaust when starting in the morning ( the floor is white, so very easy to see). The popping is now getting louder. It doesn't happen continuously, but every now and then when I drive in around town. Feels like it is more likely on deceleration.

New tasks just completed:
New Distributor Cap
New Distributor contact points
Flushed and cleaned carb jets
Tuned ( many times )
New spark plugs ( the old ones where a mix of some dark and some ok: NGK BP6ES gapped at .030)
I tried now NGK BP5ES gapped at .022 ( hotter plugs as I drive a lot in traffic to see if this is better)

Still pops !

Driving information:
I drive a lot in the city because I am based there, I know this is not ideal as the car is driven a lot a low speed and traffic. Now and then I take it on the motorway and it drives well at high speed without the popping. Still, I do need it to drive well in traffic as I live in the middle of London.

Appreciate anyones advice and experience with similar issues.
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330GT_sparkplugs.jpg
330GT_engine.jpg
Jumprun
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Re: Exhaust popping 330GT

Post by Jumprun »

You may be describing a symptom of an exhaust leak.
When you say "popping, do you mean a backfire type sound? If so, you may hear it toward the back of the car but it may be originating at a junction closer to the engine.
On deceleration there is more unburned fuel getting into the exhaust manifold which can ignite from air getting sucked in at a leaking junction. I know it sounds strange, but it does happen. Early air cooled Porsches are notorious for this issue, and other carbureted highly tuned engines are susceptible also.

I would check the triangular copper gasket where the exhaust manifold connects to the exhaust pipe system.

Tom in SoCal.
Jumprun
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Re: Exhaust popping 330GT

Post by Jumprun »

Nice engine compartment photo, that is an uncommon view from straight above.
330GT_London
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Re: Exhaust popping 330GT

Post by 330GT_London »

UPDATE:
Just went for a ride. The popping is one loud pop ( like a shoot gun ), happens the moment I leave the gas pedal ( decelerate ). Somewhere in the exhaust system toward rear or middle.
chris
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Re: Exhaust popping 330GT

Post by chris »

Like Jumprun said, one reason (if ignition and mixture are right) could very well be unburnt fuel being ignited in the exhaust. When you go off the accelerator unburnt fuel in form of a wall film is sucked off the walls through negative pressure and may ignite if mixed with oxygen coming from an exhaust leak. Try to seal off the exhaust with the engine running by having someone push rags on the tips. In your case it would better be two persons. Having the car on a lift look for leaks where exhaust gases are pushed out. Hope I described it in an understandable way english not being my mother tongue...
Jimmyr
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Re: Exhaust popping 330GT

Post by Jimmyr »

Exhaust leaks are a common source, ignition may be another. Use your timing light and check the advance curve on the distributors at idle and 42AM on all cylinders, 1-6, 7-12. there may be something not right with the points or advance weights. If you may recall, cars at the drag strip using all the advance they can for HP, alway pop loud when they let off the gas. Thus firing a little in the exhaust cycle, and they are over rich too.
John Vardanian
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Location: San Francisco Area

Re: Exhaust popping 330GT

Post by John Vardanian »

Sharing a similar experience... you seem to be describing detonations inside the exhaust pipes, which can stem from raw fuel igniting inside the pipes. If a cylinder is not firing, unburnt fuel can pour into the pipes then ignite by the other cylinders, creating gunshot-like pops. In my case, while screwing in a sparkplug, I had managed to pinch the electrodes together.

john
PF Coupe
330GT_London
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Re: Exhaust popping 330GT

Post by 330GT_London »

UPDATE 2:
RPM down when going off into first gear from standstill.

If i go very gently into fist gear with low rev - it's fine.
If I try to go into fist gear with the rev a bit high, the RPM drops quickly as it goes into gear / starts moving.

So high rev / fast start into fist gear, drops the RPM very low as it engages. The car still moves, but it is loosing power / RPM as the clutch pedal goes up, so it becomes a bit of a hesitant slow start. It then drives fine.
With clutch pedal depressed and pressing gas pedal the RPM goes up as it should: it's when pushing the gas pedal and lifting the clutch it happens.

I was wondering if maybe this could be related to the popping, something with ignition timing or points ? Or is this tuning issue or carbs ? Or something else ?

Will double check for leaks in the exhaust system.

Let me know what you guys think, appreciate the comments !!
330Jim
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Re: Exhaust popping 330GT

Post by 330Jim »

Another suggestion for a potential cause . Looking at your plugs 11 looks lean while 12 looks rich. They should be similar since they are feed by the same carb barrel. Try spraying some carb cleaner where the intake manifold meets the head while the engine is running. If the running changes it indicates an air leak that will lean out that cylinder. However a lean cylinder typically only makes a popping noise that sounds like it comes from the engine compartment.
In general you are running rich looking at your plugs. I wonder if a rich mixture with an exhaust leak as others have said creates a combustible mixture in the exhaust and the lean cylinder popping provides the ignition source.

If you do not have a manifold leak, I would recommend a compression test. There may be something else wrong that is causing the lean mixture on 11.

Cheers Jim
330GT_London
Posts: 17
Joined: Mon Nov 02, 2020 12:39 pm

Re: Exhaust popping 330GT

Post by 330GT_London »

SOLVED !!

Dual distributor points professionally timed by Ferrari specialist on the phasing machine.

So I had the suspicion that something was not right with the timing of the distributors as it was also running a bit sluggish / bad acceleration. Jimmy also mentioned this as a possible problem. I first changed all the 4 points for new ones and tried to time it manually, but the manual timing is very hard to get perfect. I did some research here on the forum and realised that it was strongly recommended to get this done on the machine ( Tom mentioned this somewhere ). I then sent the distributors off to a specialist and got them timed perfectly.
Installed the distributors back in to the exact same position and put in fresh spark plugs ( NGK BP6ES at gap .025 ). Tuned the carbs again.
I did also install new distributor cap and arm, looked a bit worn ( just to make sure everything was fresh ).

The car now runs perfectly, even better than before ( i think ) and the popping has disappeared. Didn't drive it that much yet, but feels really good and responds well.

It shows that the timing of those dual distributors points really makes a difference. Seems like mine gradually got worse and worse.

Will post updates if anything changes.

Thanks for all the comments and advice !!!
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330GT_ditributor.jpg
330Jim
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Re: Exhaust popping 330GT

Post by 330Jim »

Congratulations on solving the issue.

Cheers Jim
AEB
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Re: Exhaust popping 330GT

Post by AEB »

I too am experiencing the dreaded popping of the exhaust. It used to happen only when cold and on start up idling however now it seems to be prolonged. I intend to change the spark plugs as a starting point. I am using Autolite AR 51 plugs with a .025 gap. The previous owner had the distributors professionally gone over and put on a machine for timing by an experienced Ferrari shop. I have the paper work for this. My car does have Pertronix electronic ignition installed by the previous owner. How often do the distributors have to be serviced? I purchased the car 4 years ago, sn 7091 1965 "interim" car. I have put about 3500 miles on the car since I bought it. Thank you in advance for your input.
Art
1965 330 GT 2+2 Series 1 Interim car sn 7091
chris
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Re: Exhaust popping 330GT

Post by chris »

Taking into account that yours have been serviced three years ago and that you run no points because of the Petronix I would start with checking timing and the things mentioned in the thread above. If in doubt about the distributors it is easy to just take off the distributors and check them on the distributor bench. Just remember to mark the finger positions and do not disturb engine while they are off. If the advance curve is out of tune they need to be dismantled and serviced. After 3 years and I guess limited use I would not expect the advance mechanism to have serious wear. Tom has much more experience and for sure will comment.

BR
Christopher
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tyang
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Re: Exhaust popping 330GT

Post by tyang »

Hi Art,

WIth Pertronix, it's not as easy to put it on a distributor machine to check without an adapter to supply voltage to the unit. Pertroniz eliminates the issue with phasing the two sets of points in the distributor, so you would only be checking advance on a distributor machine. I would check timing on the engine first to make sure something didn't go out of adjustment. You should be able to see AF10 at idle and AM42 at 3K rpm or higher. I try to mark the timing marks with different colored paint so you can quickly see which ones are for PM1/6 or PM7/12. If I'm speaking another language, I can explain in better detail!

Tom
'63 330 America #5053
chris
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Re: Exhaust popping 330GT

Post by chris »

Hi Tom,

I use a simple distributor machine from ingenioes.com.
I bought an electronic adapter for 89€ to run also distributors with pertronix systems. It teally works well and I can recommend it.

Chris
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