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Fuel Starvation under hard throttle???

Guest Anonymous

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Guest Anonymous

Ok, so I finaly just figured out what was wrong with my DCOE's today (i'm at school so I can't work on it much), and i got it fixed and running. I took it out on the street to see how it was running, and it did fine, but when I would mash the pedal down, the engine would somewhat sputter a bit, and then it would accelerate. It did this like it wasn't getting enough gas at first. I don't know what is causing this. I'm using one of the older style mechanical fuel pumps, but I heard it should work fine with these, so I don't think it's the problem. What could it be that causes this? Just not tuned yet, or could I have some wrong internals in the carbs? Here is what is in them on a pretty much stock engine besides header and exhaust.

Choke 30

starter jet 100 F5

Pump jet 40

Intake discharge valve 50

idling jets 50 f8

emulsion tube F11

main jet 115

Air corrector jet 190

Aux venturi 45

Hope you can help, i'm eager to drive my car for real.




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Guest Anonymous


Not knowing anything else about the state of your engine, gotta ask some questions first.


is the ignition system up to spec (distributor springs/weights not sticking, providing consistent correct advance, maximum advance set correctly, static timing set correctly, etc.)? Is the engine healthy (valves, compression, proper flow from the pump, no clogs in the lines, etc.)? Check all the basic stuff first before trying to troubleshoot the carburetors.

That said, that jetting looks *close* to book spec for a stock engine (although I'm willing to bet it could benefit from a step larger mains and larger pumps w/the same or larger pump exhausts, and possibly f16 emulsions, as well as 32 or 33 mm chokes, idles in the 55 range, with f/air ratings tuned to suit progression). As that last sentence reads, there are some things that I'd change, but it's all dependent on what the engine's *doing* -

this is why jetting suggestions are starting points. They'll often even get you pretty close. Optimizing, however, is another thing, and requires extensive street time and a sensitive ass, a dyno, an oxygen sensor welded into your downpipe, or any combination of the above.

But first, take a look at how it's reacting under all other conditions, assessing the whole picture:

Constant-throttle cruise? Off what kind of throttle position does it stumble (1/2, 2/3, closed/idle, etc.)? How's it react in the upper rev ranges (5000 and above)? How's power when you slowly feed in the throttle from any constant-throttle position in the 2000-to-4000 rpm range? When you do it quickly? Anywhere else it stumbles, or seems down on power?

What's the engine doing RIGHT at this point? Any point where it behaves itself? What kind of mileage are you getting? What do the plugs read on full-throttle and constant-cruise cuts (crude, usually inaccurate, and subject to lots of other things/plug heat ranges/etc, but a starting point)

Floats set? Clogs in a filter or the in-line screens in the carburetors themselves? Proper fuel pressure not overpowering the needle and seat (3.5psi max?) Have the carbs been rebuilt - i.e., are all the passages clear of crap, the needle/seat or grose jet working, floats not leaking, throttle shafts not leaking or sticking, butterflies aligned in their bores as pairs and synced? Venturis tight in their bores? Fuel enrichment/choke devices not leaking or stuck open? Air filter or no air filter (if so, is it overly clogged?)? Velocity stacks or no?

There's more, but that's just the basic stuff to check first.

Remember - and this is a generalization, and only applies when things aren't *excessively* rich or lean - if all else is healthy, then bog is usually rich and sputter/stumble is usually lean. Overly one way or the other (which you shouldn't be, based on that jetting) causes any number of the above, based on conditions.

More basic ideas:

-Idles control basic idle mix (screws control *volume* of fuel) and more importantly, progression off closed throttle to about 1/4 open.

-Pump jets control any kind of quick-change throttle position on up to about 3/4 throttle, and add a small amount of top-end enrichment. Pump exhausts control pump shot fine-tuning (and duration to some extent, larger=essentially leaner).

-Airs are 3/4 throttle and up correction, usually 5000 rpm and up, though this varies based on airflow, how rich or lean the mains are, etc.

-Mains are constant-throttle above 1/2 opening or so, and to some extent affect acceleration. A small extent.

-Emulsions control WHEN the mains come in relative to the idle/progression circuit.

Everything I just wrote is usually constant, and *always* dependent on each other. Any number of different combinations can be used to gain up to probably 95% or so of the same result. Read the books (Des Hammill's is the most accurate, Pat Braden's, almost totally useless - but Braden goes into more theory that's useful for the beginner. Hammill's correct about jetting procedures and tuning, Braden is not (you should read his Alfa Romeo "Bible" with an Alfa tech sitting over your shoulder - it's so full of errors and poor procedures it shouldn't be published, according to most of the Alfa techs I know.)

Also check the Weber Factory tuning guides/setup manuals, Pierce Manifolds carries these. These are the MOST helpful, obviously, but the most difficult to get.

And yes, the mechanical fuel pumps work fine. Webers require fuel *volume*, not fuel *pressure* - which the mechanical pump easily delivers.

When tuning, remember: DCOEs react to manifold pressure and throttle position, NOT rpm - though the other two are obviously dependent on that, the relationship isn't always constant between the three.

HTH. Let us know what you turn up.



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Guest Anonymous

Ok, I read your post a couple hours ago, lets see if I can answer some stuff. I didn't drive it too much as it is unregistered and uninsured right now, so I didn't want a $$$$$$ Fine for that because of speeding. If I slowly eased into the accelerator and than gave it more, it seemed to be fine. It was mostly just pushing the pedal down pretty hard that caused the stutter at first. I think at higher speed at maybe 3500 rpm it was somewhat sputtering though.

Then engine has about 170,000 miles on it, puffs a little smoke on decel from a hill. It drove really well with the 38/38 I had on it before, was very fast.

The carbs probably weren't synched that well, my car is pretty noisy, so the tube method isn't too good, and the synch tool I got didn't fit so I couldn't use that. I just went by engine movement, if it was wobbling too much I turned the screw until it was rather still. I rebuilt the carbs, took them all apart and cleaned them before they were put on the car.




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