Carbs synced!

Discussion in 'Technical Q&A' started by Mike328, Jan 31, 2004.

  1. Mike328

    Mike328 F1 Rookie
    Rossa Subscribed

    Oct 19, 2002
    Boulder, CO
    Full Name:
    Spent part of today fine tuning the new rebuilt carbs... (See other threads for more info, trials, and tribulations).

    Bottom line: Car is running WELL and STRONG. I took it out for a drive tonight and am pleased. Not a single backfire, not a single hesitation pulls strong all the way up to redline. Still not as smooth as I'd at the transition to mains, but I think I may have a bit of fine tuning to do.

    Some notes.

    I was successful in using the new sync method based on NOT using the Throttle Stop Screws / Idle Speed Screws to adjust the idle.

    In order to do this, I went (for better or for worse) against the conventional wisdom with the air balance (aka air bleed) screws. Instead of one air being always closed, and the other to match, I actually used these to achieve the idle speed I was looking for.

    Bottom line: I targeted 3.5 kg/hr air flow for all carburetors, and used the air balance screws to achieve this. With the appropriate idle mixtures set, this gave an extremely smooth idle with no backfiring or other subtle popping. Idle was right at 1000, which is about 1100 rpm indicated.

    NOTE that I can really only get away with doing this BECAUSE I was able to set all throttle stop screws to just "barely" touch the throttle stops. Because I've had the carbs off, I know that this position results in the first progression hole being covered, which is exactly what the carbs require for smooth transitions. Granted, I'm not quite there in terms of the transition; I think I need to confirm that the right hand carbs are in fact set appropriately (with the "horizontal" linkage).

    A few things I can right away tell have been cured, either due to the carb rebuid (+ new gaskets) or due to

    1. My popping on overrun is less than it was; barely noticeable.

    2. My idle is very smooth, with no backfiring. I never had backfiring problems really, but as I got a better "ear" for the way my engine ran, I noticed very subtle, muted backfire/popping like noises coming every five seconds or so from the engien when idling. Gone.

    3. Since I last had the carbs tuned by a mechanic, my car developed a soft, medium, or loud backfire when it would be shut off. Loudness of backfire was directly related to current Engine Gas Temperatures (for which I have a gauge) at the time of shutoff. This, so far, is now gone.

    Also, note in previous threads I thought I had an airleak, since my RPMs would rise even AFTER the throttle was "Closed." This is now gone. Everything has come together correctly. This problem was related to the symptom of 7 kg/hr flow rate.

    THE PROBLEM was PRECISELY that I in fact had my throttle stop screws adjusted WAY WAY in, such that on each bank the throttle was pretty significantly open. This was from the early tuning stages, when I was trying to get an idle. To keep the idle correct, I ended up having WAY WAY lean idle mixture screws. This was especially evident when I changed my spark plugs last night.

    Shocked at this realization (suggested by the spark plugs), I quickly got some more confidence with the mixture screws and just backed them out more than I had, noticing the RPM change. Most of them needed to be backed out significantly. Of course, the RPMs began to rise, which was correct because I then was able to back out the erroneously-high throttle stop screws.

    I still have the following problem though:
    I did a quick check of the air flow rate on the FRONT bank carbs vs. the REAR bank carbs at 1500 RPM. The front bank was between 1 and 2 kg/hr HIGHER than the rear bank. (They're all synced +/- 1/8 kg/hr (yep, one eighth!) at 1000 RPM. The entire rear bank was around 8 to 9, the front bank 7 to 8.

    I've triple checked very thoroughly all of my throttle linkage, and confirmed that both main carb levers move at PRECISELY the same time, within essentially no tolerance. I confirm this in the "feel" of the accelerator pedal as well, in that there is no "step" in the resistance of the pedal like there was at one point before I got the linkage in shape.

    Is 2 kg/hr difference between the front and rear banks something to be concerned with? I think it is. I think the problem lies in the basic position of the throttle plates, and using air bleed screws to set the idle. I'm thinking that, I need to CUT/CLOSE the air bleed screws on the rear bank universally, while just SLIGHLY increasing its throttle adjustment screw to be a bit more open. Problem is, I might be messing up the progression sequence on the rear carbs if I do this...
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  3. ferrarifixer

    ferrarifixer F1 Veteran

    Jul 22, 2003
    Full Name:
    Phil Hughes
    I'd re-check the butterflies are fitted properly to the spindle. They are adjustable on oversize holes, and it's very easy to fit them off centre.

    With a carb off the engine and in your hand, they should seal almost perfectly, and snap fully closed without rubbing the body of the carb.

    If they're not centred perfectly, just loosen the little spindle screws and snap the shaft open and closed a few times until they centralise themselves properly.

    Once this is verified, the flow of each bank should be very very close, so check for slack in the linkages, and always pull it from the cable on the 5-8 cam cover and NOT the knuckle near the 1-4 cover.
  4. Peter

    Peter F1 Veteran
    Rossa Subscribed Owner

    Dec 21, 2000
    B.C., Canada
    Congrats dude! I'm glad you got it running smoothly.

    I've certainly learned alot from these discussions, thanks!
  5. thecarreaper

    thecarreaper F1 World Champ
    Silver Subscribed

    Sep 30, 2003
    mike , thank you for sharing this all with us, i too feel i have learned a great deal of ferrari specific info from your threads. i know it must have taken you considerable time to present this in such a nice way for it to be preserved for all time!
  6. Mike328

    Mike328 F1 Rookie
    Rossa Subscribed

    Oct 19, 2002
    Boulder, CO
    Full Name:
    Thanks for the kind words, guys. I like to think I try to contribute to the community just as much as it contributes to me!
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  8. ham308

    ham308 Formula Junior

    Nov 3, 2003
    NE Switzerland
    Full Name:
    Richard Ham
    Mike, can I ask you a question. If anbody knows about progression holes and throttle valves, it's gotta be you :)

    Symptoms are - the car's out of gear and as I gently press the throttle it inreases in speed slowly up to say 1500 rpm and then quickly goes up to over 2000 rpm. It won't easily hold 1800 rpm. It's not a problem, more acedemic really, but does this tell you anything about the progression holes and the position of the butterfly valves?

    Did you find anything like this when you were setting yours up?

  9. jw6513

    jw6513 Karting

    Nov 24, 2003
    Full Name:
    Jim Whitney
    Sounds like things are working out well for you Mike. Did you get a chance to see how the rebuild affects performance in slow tight corners. I'd be interested in knowing. Good luck, and thanks for the information. Jim
  10. Mike328

    Mike328 F1 Rookie
    Rossa Subscribed

    Oct 19, 2002
    Boulder, CO
    Full Name:
    Richard, though I've been through the ordeal I am still a novice. The difference is, coming from things from both a scientist's and an engineer's perspective, I aim to understand the inner workings and reasons for the carbs--both the why in the design and the how in the function. I really am still very much forming my understanding and opinions on these things.

    Just because I've been vocal lately with the carbs by no means is indicative that I'm an expert. I'd say I'm gaining competence!

    Perhaps somebody else might be able to chime in to help you with your problem.

    I can say that I did not experience the problem you're having. If I had to take a guess, I'd be a bit worried about the respective air flow balance of each of the carbs in relation to each other, as well as the relative throttle plates for each carb in relation to each other.

    Your best bet is to get at the carbs and check each barrel's respective air flows, both at IDLE _AND_ at whatever higher RPM you can maintain. These are very important starting metrics.

    One thing I learned: The carbs are complex, and so is their interaction. If their out of sync relatively, or have bad air flows in an absolute sense, this causes all sorts of strange problems. I had symptoms of an air leak--nope. Throttle plates just weren't closing. As I got things synced up, _AND_ got their flow rates in line, all the strange problems disappeared.

    Get into the carbs, get some metrics with their airflow at idle and 2k RPM. That'll be a starting point.

  11. Mike328

    Mike328 F1 Rookie
    Rossa Subscribed

    Oct 19, 2002
    Boulder, CO
    Full Name:
    Yep, I drove it 20mi to a super bowl party tonight. I've had some time to test, but not too much time with the slow speed stuff.

    1. In general, I've found that the problem is still there--I notice it from time to time. However, I feel strongly that it is NOT as bad. It's not blatant. In fact, not once in my driving today was I "distracted" by it, even though I was aware of it once or twice.

    2. I've concluded my idle circuit is still quite lean. It is my hypothesis that these turns are somehow causing a sharp leanness in the idle circuit which can be exacerbated if your idle circuit is already lean. This is really me just speculating.

    I'll know more once I do a "Take Two" on the carb sync--do all the fine adjustments with what I've learned. I think the problem might be "tunable."

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  13. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    Roseburg, OR
    Full Name:
    Hans E. Hansen
    Mike: Re - airflow at fast idle. I'd adjust the fore-aft linkage to achieve equal airflow at fast idle. There is a certain amount of give/flex in the system. It doesn't take much - perhaps 1/4 on one of the linkage rods. This small adjustment shouldn't significantly affect your idle flow rates.

    Although, if you're generally happy with the way it runs, you might want to just leave well enough alone!

    Ham308: My car exihibits this. I'm reasonably confident that it's the distr timing advance starting to kick in. Probable sequence of events: Rev to 1500rpm ---> distr advances slightly ---> increased advance raises engine speed ---> distributor advances even more. This sequence seems to stabilize at around 2000rpm.
  14. Spasso

    Spasso F1 World Champ

    Feb 16, 2003
    Land of Slugs & Moss
    Full Name:
    Han Solo
    Congrats Mike,
    I'll have my chance next winter to do the same. Very helpful information.


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