Dual IDF Tuning Quandary

[justrhines]

Hi all,

Been working on a dual IDF (actually, EMPI HPMX) carb setup for my otherwise stock 1975 (1756 cc). Got everything together and running (jets and venturis per the IDF tuning thread and MiraFiori), and have the idle tuned up, but cannot get the engine to take a load. Namely, opening the throttle in first usually gets the motor to die out and often backfire out the carbs. If I feather the clutch ever so slowly and get the car past the "dieout point", I can accelerate and hold redline RPM in 5th. Have varied the accelerator pump throw and drilled out a spare set of jets to see if I was running too lean... to no avail.

Was wondering if anyone else who has done the conversion had any wisdom to share.

FYI.... main jets 125, venturis 32mm, idle jet .55, air corrector 210.

Thanks,
Justin

[seabeelt]

Sounds like it might be lean on the idle jets. Try going up a size or two

[Nut124]

Justin, what did you do with the ign vacuum advance?

When going to duals, the ign vacuum can become a problem. W/O the vac advance, there is insufficient advance at 1500-2500rpm and this can cause spitting back thru the carbs.

I have the vacuum line hooked up to #1 intake runner. However, the vacuum pulsates wildly and you need to implement a damper circuit or the pulsation rattles the vac diaphragm.

The other way to compensate is to modify the dizzy to provide less total mech advance so you can set idle and low speed advance higher than normal, to 20 some deg or so and not exceed 36-37 at top end. In this case, you would not connect the vac advance line.

[SteinOnkel]

Spitting back out through carbs indicates a timing or balancing problem, not mixture. Check valve lash and ignition timing.

Also, invest in a wideband.

They are also most likely unbalanced. An unbalanced engine with four individual runners will never run properly.

A good dual carb setup (there's two mutually exclusive things...) will have four ports with constant vacuum on them and four bleeder screws. Bench-synch your throttle blades, disconnect throttle linkage, bring engine to operating temp then connect four vacuum gauges to the ports while engine is running. Get them as close together as you can. Bring down highest vacuum reading to others (you can only bleed off vacuum).

No pickup is usually due to incorrect idle mixture (too lean) or faulty accelerator pump(s).

[18Fiatsandcounting]

I'm no expert on dual carbs, but my first thought was the accelerator pump as previously suggested. When the throttle plates open, the air rushes in and it takes a moment for the fuel to "catch up", so the mixture goes way lean. The accelerator pump is meant to correct that. My second thought was the idle jet size, as if it's too small, the mixture will go too lean when you're moving off of idle to the point at which the main jets kick in (around 1500 to 2000 rpm depending on engine and carb).

However, if one of your IDFs is running lean compared to the other, the cylinders fed by those carbs might idle OK but stumble or spit back through the carb when you progress from idle to main jets. Really lean mixtures burn slowly, and so they might still be burning in the engine when the intake valve opens for the next cycle, hence the "spitting back through the carb" (also known as backfiring through the carb).

I don't know how easy this would be to check, but if you could see whether the spitting back is coming from just one carb or from both, that info would be helpful.

-Bryan

[justrhines]

Hi All,

Thanks for the advice! Been inactive for a bit with school, but ended up finding out the issue.... bad spark! Prior plugs had been fouled beyond workability and I wasn't getting a strong enough spark to light off the extra fuel. New plugs and running great!

Maybe next time I'll do the easy stuff first...

Justin

[Nut124]

Justin, glad you figured it out.

I'm learning as well that with IDFs, it is easy to foul the plugs. When adjusting idle mixture, if the engine does not respond to idle screw adjustment (running on 3 cyls) in a particular cylinder, suspect a fouled plug and replace it. Often the engine does run OK at load when the mix leans out.

Alternatively, if the idle speed is too far open, the progression circuit over shadows the idle mix screw and the screw seems not to respond.

I never had to deal with this much until I went to the IDfs. But I do recall, back in the -80's as a young kid, I served as a spare hand in a rallye racing crew truck for an Ford Escort Cosworth RS2000 driven by my cousin. He complained about fouled plugs after every stage and we were replacing them constantly.

Check your plugs after some driving. With the IDFs, a lot of the driving around town takes place on the idle and progression circuits. Pic below is of my plugs showing #2 and #3 quite rich.