ColourTune spark plug to troubleshoot ignition

[911T70]

Well I took my time and, following the advice received here, finally managed to tune my Zeniths the best they've ever been! The trick was to set them between rich and lean at idle AND to get consistent firing. I really took my time and waited a few seconds between each quarter-turn. The car runs surprisingly well, with no back-firing at all, not even when decelerating going down a hill. Thanks so much for the great advice - it's going to be a great summer! Wish I could buy you all a beer!

[fizeus]

It's quite normal a little backfire on deceleration (especially in 3200-2700 rpm range)... If you still have the enrichment circuit try to close the main screw of that circuit.... Anyway take the deceleleration poppings as a characteristic of your engine.
Have a beer to this forum and enjoy your Porsche driving 😀😀

[911T70]

Originally Posted by fizeus

It's quite normal a little backfire on deceleration (especially in 3200-2700 rpm range)... If you still have the enrichment circuit try to close the main screw of that circuit.... Anyway take the deceleleration poppings as a characteristic of your engine.
Have a beer to this forum and enjoy your Porsche driving 😀😀

I took the enrichment circuits off a couple of years ago, knowing that it rarely does what it's supposed to. Much prefer the cleaner look!

The car has never run better - more power without any hesitation or flat spots in the rev range. I took the plugs out yesterday to have a look and two were sooty (rich) and one was too lean, with the ceramic insulator pure white. I'll try to tune their carbs better.

I'm very surprised that the car no longer backfires a bit on deceleration - it always did in the past and, like you said, I took it as a trait of the engine. Does it indicate that the carbs are too rich/lean?

[fizeus]

Really hard to read spark plugs with modern fuels... Also it could be a temporary fouling of the sparks...(unmeaningful). With leaded fuels it was general consensus to do the plug chop to correctly read the plugs...
Anyway you didn't mention what about compression... Did you ever made a compression test on your engine? Have checked ignition timing and dwell, is your valve clearance ok? I ask you this because this can also be a wearing problem. (you probably will live better accepting it instead of top rebuilding the engine).

My impression is that you have one cylinder too lean and you hear it popping because that cylinder goes to a 'surge lean' condition on deceleration.
Simply try to enrich the cylinder with whitish plug and have a ride.... AFTER this test, try to slightly lean the richer cylinders.
Conversely you shoud buy an O2 sensor to know the popping reason, again if I can give you a suggestion without being rude...
Keep in mind you have a carburated engine and with the gunson you already have a very strong carburation... Live with it... When it pops on deceleration it's because your engine is asking you to continue to push on the gas pedal.... That sound is so sexy to my hears...
You are on a unique car and she has a unique sound... Do not try to make it sound like a modern car. :-)

[911T70]

Originally Posted by fizeus

Really hard to read spark plugs with modern fuels... Also it could be a temporary fouling of the sparks...(unmeaningful). With leaded fuels it was general consensus to do the plug chop to correctly read the plugs...
Anyway you didn't mention what about compression... Did you ever made a compression test on your engine? Have checked ignition timing and dwell, is your valve clearance ok? I ask you this because this can also be a wearing problem. (you probably will live better accepting it instead of top rebuilding the engine).

My impression is that you have one cylinder too lean and you hear it popping because that cylinder goes to a 'surge lean' condition on deceleration.
Simply try to enrich the cylinder with whitish plug and have a ride.... AFTER this test, try to slightly lean the richer cylinders.
Conversely you shoud buy an O2 sensor to know the popping reason, again if I can give you a suggestion without being rude...
Keep in mind you have a carburated engine and with the gunson you already have a very strong carburation... Live with it... When it pops on deceleration it's because your engine is asking you to continue to push on the gas pedal.... That sound is so sexy to my hears...
You are on a unique car and she has a unique sound... Do not try to make it sound like a modern car. :-)

A compression test was done during a PPI and the numbers were 'good'. After all of the problems that I've had to resolve with this car, I don't really trust these numbers but I'm quite happy with the way the engine performs for now.

I probably didn't express myself clearly, but the car now doesn't pop nor backfire at all! I was wondering if this might be a symptom of improperly tuned carbs, whether they're supposed to backfire during deceleration? I am planning on getting an O2 sensor eventually to monitor the AFR at all times but for now I'm trying to master the ColourTunes! I'll check the carbs whose plugs are not perfect to see if I can optimize the tuning.

I don't think you're being rude at all - in fact, I really appreciate your advice. I think it's so cool to be getting advice from a fellow Porsche owner from Italy!

[1QuickS]

Backfiring during deceleration is typically a rich mixture being ignited in the muffler or exhaust system.

Try adjusting side-to side air flow balance using STE air flow meter at 2000 RPM and then mixture strength. Have a helper hold throttles open or use an adjustable link to replace fixed link from manifold @ #3 cylinder to throttle cross bar; this replicates actual throttle application...do not grasp throttle cross bar to hold throttles open.

When released, throttles should revert to idle conditions with all six throttles pulling equal air. If this is not the situation then throttle cross bar must be checked for having design geometry. I provide a procedure on my web site for this.

Zeniths were designed to allow fuel to enter carbs during deceleration to reduce NoX emissions associated with lean mixtures in this operational situation.

Be sure gaskets at exhaust flanges are sealing well, as mentioned previously a rich mixture will backfire during deceleration or with the introduction of air into the rich mixture otherwise referred to as "air leak".

Your engine & exhaust look especially nice & sanitary! I especially like the transmission jack you have for installing the motor/gearbox. I had such a device & simply "loved it" for that task!!!!!!!!!

An O2 sensor is good for verifying engine performance while driving..."Seat of the pants" meets digital technology; one confirms the other. But, just because the O2 says one thing the engine may want something other than optimum mixture. Use seat-of-pants to get what you need for mid-range drive-ability & O2 sensor to assure you do not burn the engine up with a too lean mixture during sustained high RPM antics.

A colortune also provides a visual display of mixture response to opening the throttles. You may be surprised the mixture is not constant during progression...

[911T70]

Originally Posted by 1QuickS

Backfiring during deceleration is typically a rich mixture being ignited in the muffler or exhaust system.

Try adjusting side-to side air flow balance using STE air flow meter at 2000 RPM and then mixture strength. Have a helper hold throttles open or use an adjustable link to replace fixed link from manifold @ #3 cylinder to throttle cross bar; this replicates actual throttle application...do not grasp throttle cross bar to hold throttles open.

When released, throttles should revert to idle conditions with all six throttles pulling equal air. If this is not the situation then throttle cross bar must be checked for having design geometry. I provide a procedure on my web site for this.

Zeniths were designed to allow fuel to enter carbs during deceleration to reduce NoX emissions associated with lean mixtures in this operational situation.

Be sure gaskets at exhaust flanges are sealing well, as mentioned previously a rich mixture will backfire during deceleration or with the introduction of air into the rich mixture otherwise referred to as "air leak".

Your engine & exhaust look especially nice & sanitary! I especially like the transmission jack you have for installing the motor/gearbox. I had such a device & simply "loved it" for that task!!!!!!!!!

An O2 sensor is good for verifying engine performance while driving..."Seat of the pants" meets digital technology; one confirms the other. But, just because the O2 says one thing the engine may want something other than optimum mixture. Use seat-of-pants to get what you need for mid-range drive-ability & O2 sensor to assure you do not burn the engine up with a too lean mixture during sustained high RPM antics.

A colortune also provides a visual display of mixture response to opening the throttles. You may be surprised the mixture is not constant during progression...

Thanks for that information, Paul. So the fact that there is no backfiring at all on deceleration is a good sign! Still, as an exercise, I will try adjusting side-to side air flow balance using STE air flow meter at 2000 RPM. I will find the procedure on your website, with which I am quite familiar, having spent a lot of time there. It's thanks to you that I am starting to understand how my carbs work and that I am using ColorTunes today.

Nobody noticed that the top of the driver's side carb is installed front-to-back in the photo! I had a terrible time with my carbs and couldn't understand why the float bowls emptied themselves down the throats every time I turned the engine off. It's thanks to some of your posts that I realized that the idle bleed jets were blocked because that bank wasn't properly assembled.

Thanks for the compliments on my engine's neatness - it's a result of habits developed during a 25+ year career of working in optics/electronics research labs. I'm very happy with my engine removal and installation process - made some time-lapse videos documenting it:
https://www.youtube.com/watch?v=SxAcLwzAJ3w

https://www.youtube.com/watch?v=eAZm6SyTS1U

[rbogh901]

Originally Posted by 911t70

thanks for the compliments on my engine's neatness - it's a result of habits developed during a 25+ year career of working in optics/electronics research labs. I'm very happy with my engine removal and installation process - made some time-lapse videos documenting it:
https://www.youtube.com/watch?v=sxaclwzaj3w

https://www.youtube.com/watch?v=eazm6syts1u

very nice. Thanks for posting!