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Do main jets affect popping back on 308?

Discussion in 'Technical Q&A' started by Ted, Jun 18, 2004.

  1. Ted

    Ted Rookie

    Nov 3, 2003
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    Garland, Texas
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    Ted Gage
    Thanks for the excellent threads on 308 carb jetting and synchronization. My 308 has been in storage for 12 years, now back to life. Needless to say, it wouldn't idle at all. Strangely, the idle jets were 45's. I'm up to 57's (1.60 times increase in jet area) and I have 60's to try (1.78 times increase in jet area). I'm now achieving excellent idle but still sporatic popping back at 2500 RPM. I've gone up 15% in mains and air correction to compensate for today's fuels (140,210), but I'm wondering if this is enough considering the required change in idle jets.

    Are the main jets contributing to popping back?

    Thanks to all.
     
  2. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
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    Ted
    Can you provide some more data -- when you say, "popping back", what's popping -- through carbs (spitting with fuel) or out the muffler? Idle or 2500?

    If idle/spitting up through carbs, it's generally caused by two things: incorrect set up (float heights, sync'ing etc) or leaness. From your description of your jets, assuming stock cams and venturis, there is no way you are lean at idle, so...

    If idle is fine, but it is spitting through the carbs at 2500 and its is localized to this rev range, check your emulsion tubes versus specs. You may be trying to correct an e-tube issue with idles and mains.

    Your jetting combination sounds rich.

    Popping through the muffler is generally a sign of a rich mixture.

    When stock, my 77 (which had lumpier cams than a 78) ran 135 mains, 55 idles, F36 e-tubes and 220 air correctors.
    Philip
     
  3. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
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    FWIW, my 1975 is using .60, 135, and 210. Stock emulsion tube (I forget which it is). This gives smooth even running, a hint of popping up thru the carbs during the first few seconds of cold start, and a constant A/F at full throttle of 13.5:1.
     
  4. cavallo_nero

    cavallo_nero Formula 3

    Nov 3, 2003
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    By tinkering with the idle mixture screws on my 78 euro carb GTS, i can alter the popping or backfiring, but i have yet to get rid of it all together (but at the expence of performance, i can eliminate the backfiring). Increasing the idle jets cured most of the backfiring tho. ONE IMPORTANT NOTE ON MAIN AND AIR CORRECTION JETS, everything i have read on jetting (and i have done extensive jetting at dyno and on my own), is that if you INCREASE main jet size, you DECREASE air correction jet by 3 times the ammount. like, going from 135 main and 200 A/C, to 140 main but a 185 A/C, this will really help the top end power and rev (gets the velocities up in the emulsion tube well). so, i would try a 185 A/C jet with the 140 main and not a 210 A/C, if you are unhappy with your bottom end power, then split the difference with the A/C jet. At 6500 feet above sea level here where the air is thin and weighs less, i am running 140 mains with 175 A/C and 60 idle jets with a F24 emulsion tube, and 34 mm venturies. i have a custom dual exhaust, electromotive ignition, krank vents, and hi comp pistons, and of course euro cams. My peak HP at the rear wheels was 180 HP, which does not compensate for altitude which they say is around 24%, and driveline losses are about 18% i believe. so, to compute total power, i believe i need a starting number, then deduct 24% then 18% to get to 180. from what i can calculate,my power at sea level from the engine is roughly 295 HP.
     
  5. 208 GT4

    208 GT4 Formula 3

    Dec 27, 2003
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    First I'd like to say that I'm no expert on Carbs, though my '75 GT4 has them, so please take my thoughts under advisement.

    I've got a bit of a rough running problem, so have been reading about possible cures. I presume your car was running OK prior to going off the road? If so the jet setup it has, used to work once upon a time?

    I have read that if the floats in the carbs aren't floating then you get overfueling and "popping back" (their words & yours). This is the sort of thing that can happen if a car has been off the road for a long time...i.e. the floats have cracked or otherwise leaked?
     
  6. cavallo_nero

    cavallo_nero Formula 3

    Nov 3, 2003
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    good point GT4, i guess after years of sitting, i would replace plugs and wires also. and drain the gas out of the tanks.
     
  7. 208 GT4

    208 GT4 Formula 3

    Dec 27, 2003
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    Yes, that's a good idea as it's a pretty cheap thing to do. I've ordered new leads (my plugs are <1 yr old) as a precaution to sort my problem. If draining the tanks is a problem, I have had considerable success (on a Porsche that had been sitting for 2 yrs without starting) with adding a water scavenger to the fuel, along with a couple of gallons of new fuel. It started first time on that mixture having replaced a blocked fuel pump.

    P.S. there are screws in the underside of the distributor cap to remove the leads, which I found out about thanks to a member on here.
     
  8. ham308

    ham308 Formula Junior

    Nov 3, 2003
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    Is this the correct direction? I remember reading you need smaller jets for modern fuels. Which way should you go? or is there no real difference with modern fuel.
     
  9. chrismorse

    chrismorse Formula 3

    Feb 16, 2004
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    chris morse
    Ted,

    One thing you did not mention was your exhaust. Is it stock? Reducing the exhaust restriction by installing a tubi or less restrictive exhaust pretty much requires a jet change. My car ran fairly well untill i took off the 100 pound thermal reactor muffler and installed a flowmaster. I got a lot of popping through the carbs at idle and a lot of popping and banging through the exhaust on over run - a lot. Changing to 140, 200, and 60's solved that problem allmost entirely and it feels noticably stronger.
    After 12 years of sitting, I wonder if some of the gas turned to caca inside the carb passages??? Maybe try some snake oil carb cleaner
    HTH
    chris
     
  10. Ted

    Ted Rookie

    Nov 3, 2003
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    Garland, Texas
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    Ted Gage
    I'll try to respond to each of the questions and provide more detail.

    Yes, the "popping back" is through the carbs. I've looked closely, and every throat will occasionally pop. It's not limited to any specific cylinder.

    The carbs have been throughly cleaned and rebuilt with new kits from Pierce. I've set the float levels at the recommended 48 mm to the deck with no gasket. Removing the top covers, the fuel levels appear to be correct.

    Back in 1980, when I fiirst bought the car, it ran and idled well with the original jetting. But in those days you could buy 103 octane. I seem to recall that I was beginning to experience some popping back when the car was put in storage in 1990.

    The restoration began as follows: The engine was pulled, and the heads rebuilt with stainless valves. No more sodium-filled exhaust valves. The bottom end was measured and met factory spec. All new hoses, gaskets, belts, bearings, and seals were installed. The exhaust system exhibits less backpressure than stock.

    The fuel tanks were flushed with Chemtool recirculated through the system for 48 hours. They were then drained, new hoses installed, and filled with Premium. So the fuel system should be clean.

    All spark wires and plugs are new as is a new Electromotive Tech II, used only for ignition at this time. The system appears to be solid.

    When first started, the only way to keep the engine running was to pull choke. One also had to advance the idle speed screws significantly to keep the engine alive. Since going to 57's, the engine will idle smoothly, the idle speed screws are engaged less than 1/4 turn after contact, and the engine vacuum is up to about 12 Inches. Each increase in jets size resulted in improvement. On Monday, I'll install the 60's and see what happens.

    The emulsion tubes are stock.

    Cavallo, that's very interesting information about going the other way for air correction. The "rule of thumb" from Weber is +60, i.e., the air correction jets should be 200 if the mains are 140. My initial main and air were 130,200 which is +70, so I stuck with that when I installed the larger mains. That's how I ended up with 140,210. I'll try smaller air correction jets.

    So far, it appears that the mains installed are the same as many have homed in on. I'll report back with results for the 60's.
     
  11. cavallo_nero

    cavallo_nero Formula 3

    Nov 3, 2003
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    Ted, Got this off teh Inglese induction site about webers. about increasing main jets and corredponding decreasing a/c jets. hope this helps, although it probable wont afect the popping. my car does it and i would like to eventually take care of the backfiring. i have a set of 185 A/C if you would like to try them......

    from the site:...................
    Tuning the main circuit for maximum power is something that can be done by a series of road tests and a handful of jets. The simple rule of thumb for jetting Weber carburetors is, if you want to implement a change over the entire rpm range, you play with the main jet. If you want to change the way the car feels at the high end, that's where the air corrector comes in. Also, you should keep in mind that the air corrector is a finer adjustment that the main jet. Example: One step upward in the main jet (richer) equals about the same as three steps down on the air (less air: richer). A change of air corrector would be appropriate; for instance, if the engine pulls strong to 5,000 rpm and then goes flat. This would mean she's going lean on you up top; drop the air corrector three sizes or so, and you'll probably be able to buzz that engine right up to 7,000 rpm. If the motor feels sour all the way up, go one or two sizes heavier on the mains only. No magic! So, tell me, what's so hard about jetting these Webers?

    from http://www.inglese.com/tech.htm
     
  12. Ted

    Ted Rookie

    Nov 3, 2003
    17
    Garland, Texas
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    Ted Gage
    60's were the answer. Arter installing this size idle jets, "popping" moved from the intake to the exhaust. A few tweaks of the air mixture screws and the engine performs over range with no problems.

    The air correction jets were changed back to 200's (smallest I have). If power flattens out before 7000 RPM, we'll order some 190's and see what happens. Thanks Cavallo.

    Final jetting:

    Idle 60
    Main 140
    Air 200
    Venturi 32
     
  13. chrismorse

    chrismorse Formula 3

    Feb 16, 2004
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    Ted,

    Am running the same jets with K&N & Flowmaster set up. Runs strongly but still have a bit of trouble getting one of the idle screws to make any diference. sounds like a carb rebuild is in the offing.

    Life is good,
    best,
    chris
     
  14. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
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    Hans E. Hansen
    I had a similar problem. I had a lean hole, and the idle mix screw didn't seem to do anything. This took a few six-packs to figure out, but it turned out to be a defective idle jet. The jet looks perfect and isn't plugged, but replacing it fixed all.
     
  15. chrismorse

    chrismorse Formula 3

    Feb 16, 2004
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    chris morse
    Thanks for the 12 pack inspired wisdom, think i'll swap the offending jet with a known working one - if it works, I owe you a six pack. not that i would'nt mind doing 4 carb overhauls, it's just that i have a few mechanical porjects currently in the air, so to speak.

    thanks,
    chris
     
  16. bill308

    bill308 Formula 3
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    May 13, 2001
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    One thing I found out is that you must have air flow to a barrel in order to adjust the mixture. Idle mixture is best set at the lowest practicle idle speed. If a particular barrel isn't flowing enough air, mixture adjustment for that cylinder is not possible. Bump up the idle speed for the cylinder in question and see if it makes a difference.
     
  17. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
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    Umm.... This isn't right. The gas (actually, it's a bit of a gas/air emulsion) that goes thru the mixture screw is influenced TOTALLY by vacuum, not air flow. Just look at the position of the idle mix orifice to see this.

    However, reflecting on the above post, the carbs obviously must be in some sort of reasonable balance/sync.
     
  18. bill308

    bill308 Formula 3
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    Hans,
    If you put an air flow meter on the carbs of a good idling 308, at about 1000 rpm, you should read about 3.5-4 kg/h. This is not a lot of air flow but is is significant. This air, which slips past the throttle plates (they are not supposed to be perfectly sealed against the bore), mixes with the emulsified fuel from the idle circuit to support combustion. Emulsified fuel mixes much more easily with the air flow in the barrel than does raw fuel and emulsified fuel is easier to meter. Only a tiny bit of air is used to emulsify the fuel via an air bleed in close proximity to the idle jet. If you have the opportunity to look at the idle diagram in the Haynes Weber manual for DCNF's, the air flow slipping by the throttle plates is clearly shown.

    The bottom line is that without air flow past the throttle plates, even at idle, there will not be enough air to burn the emulsified fuel. Without flow past the throttle plates, changes to the mixture screw will have no effect. The cylinder will not fire.
     
  19. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
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    I guess I mistakenly read your message as saying that airflow *determines* fuel flow from the mixture screw orifice. Airflow has no 'drawing power' (ie, venturi action) here, as it does 'higher up' in the carb. Throttle plates can be completely shut and you will still have fuel flowing from the idle mix orifice.

    But, obviously, I agree that w/o air, you can't have combustion. So, therefore, you can't adjust the mixture. :)

    Your reply basically says what I did: The carbs must be sync'd/balanced to adjust mixture. Otherwise, you won't have proper amount of air for decent combustion.
     
  20. bill308

    bill308 Formula 3
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    Hans,

    Let me see if I can go into a little more detail here and get it right.

    A venturi works by accelerating the air flow to a higher velocity. This higher velocity results in lower air pressure (bernoulli effect-conservation of energy). Leakage past the edge of the throttle plate results in a localized increase in the air velocity and a consequent reduction in air pressure adjacent to the idle feed hole. This is why the idle feed and progression holes are placed where they are. If the throttle is closed too far, emulsified fuel will likely be drawn into the bore but without enough air, the cyclinder will not fire. This pressure drop caused by the air flow past the throttle valve plate aids in drawing the emulsified fuel from the idle feed hole. As the throttle valve is slowly opened farther, the progression holes are uncovered and the air flow past all the orifices is accelerated to sustain and increase the draw of emulsified fuel until such time as the throttle plate has moved enough so that flow past the progression holes slows, and finally stops, and then the mains take over. The mains take over at typically a low vacuum condition so air velocity of the idle/progression holes will be slower due to air boundary layer build up along the wall of the bore. When the throttle is opened rapidly at low rpm and under load (yes even accelerating the engine in neutral is a load), the engine must rely upon the pump jets squirting a stream of raw fuel to augment running as the air flow past the idle/progression holes will be relatively slow because of the large area uncovered by the throttle plate and the relatively low rpm of the engine.

    Ther's a lot of physics happening here. It's amazing that the damn things work at all.
     
  21. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
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    I don't need a tutorial. I've probably written too many as it is.

    The fact is that there isn't anywhere enough airflow in the vicinity of the idle mix orifice to assert any sort of bernoulli effect. A whisper of air just isn't going to get the job done. But below the throttle plate there is a bunch of vacuum that will suck the fuel out, regardless of airflow. It's for this very reason that there is a progression circuit, because if small airflows would generate enough of a bernoulli effect, then the main circuit would be all that is needed.

    Altho not mentioned anywhere, I seriously doubt that there is any realistic emulsification of the fuel that ends up going thru the idle mix orifice. If you look at the diagram, you will see that there will need to be a reasonable amount of fuel flow to be able to create a situation where air will flow into the system. I'm sure this is the case when the progression holes come into play, but at the very low flow rate of the idle orifice only, I don't think this is happening. This is conjecture; I'd love to be able to have a small camera in the various places to prove it. However, a partial test that I've done is to block the air orifice supplying the air for emulsion and checking exhaust gases. No change at idle.
     
  22. bill308

    bill308 Formula 3
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    OK Hans, after further thought, I guess I was reaching a bit with the Bernoulli effect having a dominant role in drawing emulsified fuel from the idle and progression holes. Thanks for challenging that idea. The important thing is that you recognized the idea that without sufficient airflow past the throttle plates, it is not possible to adjust the mixture. The other thing is that the flow of air past the edges of the throttle plate is not a steady state flow but one that has some pulse characteristics, as you probably already know. It was the increase in air velocity associated with the vacuum pulses that steered me toward offering the bernoulli effect. Sorry about that.
     

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