ColourTune spark plug to troubleshoot ignition
 

My '70 911T is running rough this spring and I suspect the ignition. While tuning the carbs using a ColourTune spark plug, I was surprised to see flashes of light (running too rich in this case) at a variable frequency! I calculated that there should be a spark at 1.4 Hz at 1000 RPM but, as you can see on the video linked below, there seems to be missing sparks. I looked for ColourTune videos on YouTube and saw some that show the same behaviour. Shouldn't there be a flash at every second revolution?

https://youtu.be/kimvO4NXewg

Thanks!

Good question. I always thought maybe it would just be too hard to see all of the flashes and so didn't expect to be able to count them and correlate with RPMs. In any case, you are correct, needs more blue, lean it out a bit.

At idle, 1000 RPM, each cylinder should fire at close to 1.5 Hz, which is easy to see. Have you used ColorTunes and observed the same behaviour, where sparks seem to be missing?

And yes, that cylinder was leaned out!

My Observations
 

I've used the ColorTune to adjust my PMO carbs.

My spark is intermittent (just like your video) when I lean out the idle mixture screw a bit too much.

When adjusting the mixture screws with the ColorTure - I normally adjust until its starting to lean (intermittent white), slowly adjust / open the screw until it starts to go rich (initial / obvious yellowing) and then set the screw at the mid-point in between (blue).

I've found with this process the engine ends up a little rich overall, but runs nice and smooth - much smoother than I've ever been able to get it tuning by ear using the lean drop method.

Good luck.

you are making this waaaay too complicated.

follow the advice above.
I the mixture is off enough for a cylinder the may or may not fire.

great tool. I bought one for my CIS 930.

Thanks for the advice, but I'm troubleshooting the ignition and trying to determine if sparks are occurring like they should. The car runs rough, feeling like it's not firing consistently. Maybe the ColorTune isn't the right tool for this.

I'd still like to understand why we don't see a steady pulse of light though, especially when running rich (intermittent when too lean according to Gordo2)!?

Ignition vs Fuel
 

Based on your video - it looks like you have that carb's mixture setting lean, causing that cylinder to fire intermittently.

When i'm using my ColorTune - it's pretty obvious when the carb mixture setting is too rich. The cylinder tends to fire in a regular, timed / consistent pattern when rich - but the flame appears as a lazy yellow as opposed to a sharp, consistent on / off blue or white as I lean the carb / cylinder out; ultimately progressing to an intermittent flame when adjusted too lean (just like your video).

Next time I break out my ColorTune I will try to get a video. Good on you for videoing the the shot you posted - it can be a challenge just getting a mirror positioned in the right spot to see what's going on, let alone doing so while trying to get a camera at the right angle / position to capture the image... You don't by chance have 3 hands?

If your ignition is in relatively good condition (dizzy cap and rotor are in good shape) - I can't imagine what could cause it spark intermittently as per what you are seeing. As such, I tend to think the issue is associated with the fuel delivery / carb.

Gordo

Too rich & too lean will cause a cylinder to not fire. If lean then I have found that the spark will jump someplace else than at the tip of the spark plug, such as between end of the cable and the inside of the Colortune if insulation is not good (old plug wires or dirty porcelain).

When using Colortune, be sure you can definitely adjust mixture to rich (orange) and then cause it to go blue. I screw mixture screw 1/4 turn per adjustment & then wait for a slow five count to allow settling.

If you have air leaks around the throttle shaft then you will have enough erratic air coming in through the bearing/shaft interface that the mixture will go lean & then back again. These air leaks typically begin to appear for Webers with mileage around 90k miles.

Thanks a lot for that information! I'll try tuning the carbs again soon following your advice and hopefully I'll get them in a state where the color looks good AND firing is consistent. My distributor/rotor/wires/plugs are all new but my CDI is the original Bosch, which is why I'm worried about the ignition. I can't believe that circuitry still works!

Getting the mirror and the camera at the right angle was not easy, but at least I didn't get shocked like I did a couple of times last year. I'm always terrified when I'm positioning the mirror around the plug cable; I should look for a plastic one!

My Zeniths were re-built recently so I don't think I have air-leaks, but this possibility is good to know and I'll keep it in mind.

Thanks again!

If you suspect an ignition problem you HAVE to use a timing light.
Hook it near the spark plug and point it to engine pulley... Timing marks will guide you to the diagnosis.
Do not forget that spark are provabilistic events so do not worry to much if you see some erratic behavior......
Also you can use timing light in conjunction with the colortune... If you see missing sparks on the colortune and not to the timing light try changing cables.
Here is my cent

I use a mechanic's mirror to view the spark; the kind with a mirror on a stick with a swivel to connect them. I tossed the plastic tube thingy. The mirror I like is 1" wide by 2" long. I bought mine through McMaster-Carr, the world's BEST source for quality tools & sundries! Part number 1017T17.

Zeniths tend to maintain throttle shaft/bearing fitment quality longer than the Webers do. However, "rebuilding" does not address the issue I describe. "Rebuilding" typically is cleaning & gasket replacement only. Throttle shaft/bearing fitment is a much more invasive procedure.

I should definitely get a larger mirror, preferably non-conductive! I tried using an inspection camera (borescope) but the high voltage messes up its electronics and it shuts off. Too bad, because it would make visualizing the plug so much easier.

I wrote this a while back. . . enjoy

Adventures with the Gunson Colortune on an SWB 911.

I use a round mirror about 1.5 inches in dia. i don't use the mirror that came with it.
i of course have the long adapter on the color tune(CT) but i put heat shrink over it to keep it together. i need to go back and glue it then heat shrink it all.
i lost the washer that goes on it so i don't use one and cant tell any difference.

i loosen it the plastic tool then use the part that the plug wire goes on to remove it.

make sure you get a good snap when you connect the plug wire. i only had spark jumping issues when i did not have a good connection.

Thank you! I hadn't thought of using my iPhone to see the plug. I'm surprised the electrical field from the high voltage doesn't affect it. I'd be worried that it might damage it or wipe its memory.

You mentioned Bosch Distributor Lube in your write-up. I've been looking for it - do you know where it can be purchased? Seems to be NLA everywhere I've looked?

I use the Klein spark plug tool through which the long extension can extend. I added a layer of electrical tape to the nut adapter so that it stays jammed in the Klein tool when I remove it after threading in the ColorTune plug. I had a terrible time with the washer that came with it, loosing it a few times inside the cavity, so I use a washer from an old plug, which stays captive so it never falls off. I hated using the ColorTune at first but now that I've used it a few times, I've improved my technique and it's not so bad. It's certainly worth the effort and it's paid for itself just with the improved fuel economy!

Some of my hard-earned Colortune tricks:

*I never used the washer; not needed for low RPM operation like idling.
*A magnetic wand is handy to retrieve the Colortune if it falls out of the socket.
*I cut an internal chamfer in the end of the extension lead to maximize the chance to mate it with the threaded end on the Colortune.
*Like 911T70 stated: The Klein spark plug tool is handy if you have one.

no washer. lost it second time I used it. I think it is in #6. no need to tighten it very tight either. I just use the extension for the CT to tighten and then remove once I break it free with the plastic tool.

one day I will glue the extension permanently to the CT.

Yes. There is no way I could come close to the accuracy listening the way I tried to in the past. The visual cues are so much more precise and get things within a quarter turn reliably where waiting for an RPM change seems to leave so much margin for error.

Listening to the Colortune praise is a little humorous to me. I was touting its use when I was beginning my carburetor remanufacturing business and was roundly dismissed for using "old technology" in lieu of O2 sensors. O2 sensors are fine for analyzing on-the-road mixtures during transition and main circuit operation but idle mixtures are the domain of the Colortune.

Don't forget that idle air flow balance must be achieved before final mixture adjustments are performed.

When I test run completed carburetor projects I use a Colortune in each cylinder of my test engine which is a luxury since I do not need to shuffle them to tune individual cylinder mixtures. I do still rely on Lean Best for mixture adjustment and use the Colortunes to provide supplementary information. As a disclosure: My carburetor test engine is a one liter, three cylinder Geo Metro so engine performance during Lean Best tuning is more obvious than on a six cylinder.

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!

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?


http://forums.pelicanparts.com/uploa...1526329514.jpg

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!

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

very nice. Thanks for posting!