See-thru tuning spark plugs

[John Vardanian]

Thanks Dyke. So, we reached the conclusion that the Colortune is of no use for the earlier Ferraris. The Youtube demos I saw were on SU carburetted engines, it seemed to work like magic there. I'll put mine on the shelf for future use elsewhere.

john

[Steve Meltzer]

Darn shame it's not as reliable or consistent as we'd like. As an ex-semipro chemist, it's so appealing. s

[330Jim]

John, I do not believe your car is running right. The fact that you can turn your idle mixture screws all the way in tells me you are getting fuel from someplace else. You can get it running well in this situation, but when you get it running right you'll find it is a lot better than you expected. As DWR46 points out something is not right, just like DWR46 I typically find the throttle plates are too open and you are getting fuel from the progressive circuit. Instead of float bowl levels I often find the timing is to retarded. However there can be a lot of reasons. I find it best to make sure the ignition system is completely sorted out first before messing with the carbs.

Cheers Jim

[Steve Meltzer]

While I didn't have this exact problem, I had a somewhat similar circumstance with one of the DCZ carbs on my GTC. You could take the idle speed screw out of the carb and the idle wouldn't change one RPM! (I didn't fool much with the idle mixture, 'cause I could never get that far in the tuning algorithm.) Turns out that the throttle plate didn't seal the bore, no matter how much you fooled with the sector gears or throttle shaft, so the three little progression holes were exposed to engine vacuum, all the time. Many $$$ to Mike Pierce and that problem is now solved. Now, idles and adjusts, as it did back in the day. steve

[John Vardanian]

Jim, I could turn one mixture screw in all the way to the bottom, got blue flame and the engine still idled, albeit not so smoothly. But if I tried getting a blue flame on another carburetor as well, the engine stalled. The V12 can take a lot before it says uncle.

As for the plates sealing, I bench tested my carburettors last summer, with my homemade device driven by a shop-vac. The plates seated fine then. Maybe I should retry the bench testing again.

john

[330Jim]

Hi John,

Am I correct in assuming that by "plates" you mean the butterflies? If so I have no doubt they sealed just fine. However for the engine to run properly they have to be open a small amount. With Webers and Holleys and other similar carb designs the amount the butterflies are open is important. If they are open to much then they will allow the carb to start pulling fuel from the progressive or transition circuit. If that happens the engine will run with the idle mixture screw completely closed. This makes the idle mixture very insensitive to adjustments and therefore very difficult to tune up properly. There are several factors that can cause this, but lately I have seen a number of cars were the ignition timing was too retarded. Retarded timing results in a slow idle speed, to compensate the idle speed is increased to the desired speed. When that is done the progressive or transition circuit is exposed by the butterfly causing fuel to be pulled in by that means. To fix it I prefer to advance the timing. In some cases (i.e. very custom engines) you have to drill holes in the butterflies to allow them to close down more. Stombergs and SU's do not have a progressive / transition circuit so this is never an issue on those types of carbs.

In regards to color tunes I have also found them to be sensitive to valve guide clearance too. Worn guides will allow a small about of oil consumption, minimal blue smoke while driving and non visible at idle. In those cases, setting the fuel mixture a little on the rich side tends to result in the best idle. Personally I have found the color tune to be useful on some engines, and not so useful on others. I usually just tune for the best idle speed.

Please take my comments with a grain of salt. I am just sharing my experiences. Tuning a V12 with the car in your own garage can be tricky, giving advise on how to tune one with out seeing it over an email, is much harder.

Cheers Jim

[John Vardanian]

Thanks Jim, I am puzzled. The engine idles and revs up fine. I have checked my timing, it’s right on and each distributor is (or should be) dialed in as well in duel, advance and points-separation. I did the complete test about 500 miles ago on the Sun machine.
Regarding the plates, can you explain “for the engine to run properly they have to be open a small amount.”? I thought by the virtue of the fact that the engine idles smoothly somewhere south of 1000 RPM and all throats suck equally means the plates are slightly open. Maybe I am still missing something.

My routine for checking the carbs is put six suck meters on the six throats and go about playing with the carbs just as described in Ridgley’s 1970’s write-up. It has always worked.

Maybe it’s time to rebuild the carburetors. It will be a good winter project.

john

[330Jim]

John,

I am getting into the nuances of carb tuning that are a little deeper than is typically discussed in most guides. I would not rebuild the carbs based on my comments; frankly I doubt there is anything wrong with your carbs. I would suggest studying how these carbs work. In particular I recommend studying the idle mixture circuit and the progression / transition circuit. If the butterflies are too far open (in the idle position) they will allow fuel to flow from the progression circuit, if that happens it will be difficult to adjust your idle mixture. Your comments about your idle mixture screw suggest this in occurring on your car. If you pull a carb off you should be able to see a little bit (small in your case) of the progressive circuit exposed. As we stated there are many things that can cause this. I have found in many cases the timing is retarded. If that is the case, there is typically a significant improvement in the way the car runs when it is corrected.

The required timing at idle is a function of the cam timing valve overlap and compression ratio. Cams with more overlap or engines with lower compression ratios will require more advance. Typically you need approximately 12 degrees at 800 RPM. But this can vary based on the engine characteristics; some cars like a Miura require as much as 22 degrees other cars as little as ~8 degrees.

Cheers Jim

[John Vardanian]

Thanks Jim, re your comment “if you pull a carb off you should be able to see a little bit of the progressive circuit exposed”… is it safe to say that if you had the idle too high, like 1000-1100 then it’s possible that fuel can flow from the progression circuit? Mavbe that’s my problem, my idle is a bit too high.

john

[330Jim]

I my experience your idle speed by itself is not enough to cause it to pull fuel from the progressive circuit, but it does contribute a bit. Keep in mind I have not seen your car so everything may be fine. I commonly find this issue on DCO3’s used on early Jags. I’ve also seen it on a Miura, a Jeep with a Holley… etc, so I tend to look out for it. Getting this right, sorts out a lot of off idle drivability issues.

Try an easy experiment. Advance the timing at idle 4 to 5 degrees (both distributors). If your idle speed increases significantly, try readjusting the idle speed to ~900 RPM and tuning the idle mixture screws. I bet you will find the idle mixture screws a lot more responsive and easier to tune by ear. If all that work out as described, take it for a drive.

Cheers Jim

[afwrench]

Hi Jim you seem to know quite a bit about the fine tuning of these cars. To further pick your brain;if one dials in 5 more degrees advance at idle is there a danger of to much spark lead when full advance comes on at higher revs.Great discussion BTW. Best regards,Mike

[330Jim]

Hi Mike,

Your correct there is a risk, the ideal advance give you a peak cylinder pressure 10 to 15 degrees after top dead center. Some engine have to much compression and have the have the timing retarded from ideal to avoid detonation. Other engines will have a compression ratio that is low enough that it is possible to advance the timing to much before you run into detonation. In these cases, to much advance will reduce the engines performance potential. The only way to determine to the correct advance levels is to dyno test the engine or careful acceleration performance testing (dyno is better).

Almost all engines tend to like a little more advance at idle. I asked John to do the experiment so there is a good chance he would get the feel for adjusting the idle mixture when the butterfly was in correct position. This will give him an idea of how the carbs should behave (unless he has fuel leaking in from another source). I also asked him to do it because of the number cars I have encounter that are too retarded for fear of detonation. I suggested 4 to 5 degrees to be a little conservative, I have seen several cars that were around 10 degrees retarded. In those cases the performance improvement was eye opening (and yet the car felt pretty good before the change). The only way to find out is to try it and see what happens.

In regards to detonation (typically from too much advance), a little bit that is encountered in street driving where you lift off the throttle when it occurs has very little risk of doing any damage. It is the sustained detonation over a period of time that will destroy pistons and sometimes cylinder heads.

Cheers Jim