32/36 DFEV - no fuel thru venturi

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zonker
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32/36 DFEV - no fuel thru venturi

Post by zonker » Sat Jul 06, 2019 11:10 pm

I have a bit of a dilemma with the carb I've installed on my '71.

Only the idle and choke circuit and accelerator pump is working, but no fuel is being atomized thru the venturi when you open the throttle. It doesn't seem to be a float level or fuel problem, the bowl will fill without issue. It just won't make the transition from bowl to discharge tubes in the venturi bores.

Besides something more apparent like clogged jets, does anyone have other thoughts on where to look for repair? Not sure if this is a power valve issue or something else. Thanks in advance.
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Re: 32/36 DFEV - no fuel thru venturi

Post by fiatfactory » Sun Jul 07, 2019 7:42 am

You haven't put the Aux. Venturi's in backwards, have you?

they have a spring clip to one side. and an open hole on the other for the fuel to pass thru... put them in backwards and no fuel comes out.

SteveC
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Re: 32/36 DFEV - no fuel thru venturi

Post by zonker » Sun Jul 07, 2019 9:23 am

I did slide the venturi out, it was in the correct way although it did not have a clip - it fits loosely. Think a missing tension clip could be the culprit? I'm going to pilfer another carb for it's clip, remove the jets and emulsion tubes, then blow out the passages and see what that gets me.
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Re: 32/36 DFEV - no fuel thru venturi

Post by fiatfactory » Sun Jul 07, 2019 9:53 am

if it's a loose fit, then there is no way the low pressure generated by the increase in speed in the flow of air over the aux venturi discharge (by Bernoulli's principle in fluid dynamics but air behaves pretty much the same) will pull fuel up from the jet well, so yes this could be the culprit.

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Re: 32/36 DFEV - no fuel thru venturi

Post by bartigue » Sun Jul 07, 2019 2:24 pm

Is it a Weber or a copy? I've had a few copies without the little integrated clip. They work fine - but aren't loose either. They press fit rather stiffly into the cavity made for them. I would take out the venturis and, with the engine off, pump the gas a few times to see if fuel spills out of the orifices. If so then they aren't blocked. Check the venturis (blow out with carb cleaner) and reinstall.
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Re: 32/36 DFEV - no fuel thru venturi

Post by zonker » Sun Jul 07, 2019 3:35 pm

Well... it was a valiant attempt. Found some clips, removed the carb top and reset the float to spec (38.5mm), Then I pulled out the venturis and added the clips. Better fit. Then removed the jets and emulsion tubes, as well as idle bleed jets and blew them all out. She idles fine but the carb will lean out and the engine will start backfiring if I try to articulate the throttle past the idle. I can gradually feather it open to 1300rpm and then is leans out and starts it's pyrotechnic show. I'm stumped.
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Re: 32/36 DFEV - no fuel thru venturi

Post by 18Fiatsandcounting » Sun Jul 07, 2019 4:18 pm

Do you have a vacuum gauge that you could connect to the intake manifold? At idle the engine should be pulling between 15 and 18 in. of mercury, give or take, and it should be fairly steady. As you slowly open the throttle, the vacuum should drop but recover. If the vacuum drops at part throttle to only a few inches or mercury or less, you won't have enough vacuum to engage the main venturi circuits. If that's the case, the problem could be an air leak that is appearing under part throttle.

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Re: 32/36 DFEV - no fuel thru venturi

Post by zonker » Sun Jul 07, 2019 4:50 pm

Good point, I'll need to check the vacuum readings with a gauge.

I did block off the brake booster hose because there is a slight vacuum leak there, but it was a minor leak, only affected the idle about 25 rpm or so. The car does act odd - the longer it runs, the lower the idle speed goes until it plateaus at 720 rpm or so (that's when it's adjusted to 850 rpm right after it falls off the choke fast idle cam).

Also, how about downsizing the emulsion jets and see if that makes a higher vacuum pull on the fuel thru the venturis? OEM 28/36 carb had 195, original 32/36DFEV has 165, and I have a 160 in it's place right now. I have a set of 120's from an 1800 32ADHA2 carb - Maybe I'll try those?

Perhaps cam timing? The cams line up just dandy to the tdc marks but maybe i'm off a tooth relative to the crank? I did the screwdriver in the sparkplug hole trick and I'm pretty sure I nailed it. But it does act odd so... I guess there's that.

UPDATE: Double checked cam timing and all is good - checked with timing cover installed and the marker on the crank lines up with the lowest mark on the timing cover when the cam gears are all lined up.

Installed a vacuum gauge and when the car is on fast idle cam or right after it falls off, the idle speed is normal (fast idle - 1300 rpm, idle 850 rpm) and vacuum is in the 15-18 range. BUT when the motor warms up and the cooling fan kicks on, the car's idle drops to 720 and the vacuum is in the 12-15 range.

Something else weird I noticed - the ignition timing at idle speed is set at zero degrees, and when I point it at the intake or exhaust cam, or crank all are showing at their respective TDC marks.

It stands to reason then, that as you increase rpms. the ignition centrifugal advance will kick in and one should see the timing marks start to advance on the crank pulley... but it's doing the opposite. The timing marks start showing up retarded (in a clockwise direction) from zero as you apply rpm! How is that even possible? FYI, I'm running a 81 Spider Marelli electronic ignition unit (not dual points where I could possibly understand that phenomenon).

WTF. This is driving me nuts.
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Re: 32/36 DFEV - no fuel thru venturi

Post by 18Fiatsandcounting » Sun Jul 07, 2019 7:53 pm

I agree, that seems very odd. I'm not familiar with your electronic ignition, but does your system have a centrifugal advance and a vacuum retard? If the retard was working but the advance wasn't, that's about the only way I could explain what you describe. Either that or your engine is running backwards, which I consider at this point to be "unlikely". If you put the car in reverse and it moves forward, though, you have your answer...! :shock:

How about just adjusting the idle to 850 when the temperature is at the point when the cooling fan is on? I try to adjust idle speeds and idle mixtures after the engine is totally warmed up, perhaps 20 minutes.

I wouldn't change emulsion tubes, as that's not likely to be the issue. Still sounds to me like there might be an intake air leak and an issue with ignition timing.

-Bryan
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Re: 32/36 DFEV - no fuel thru venturi

Post by fiatfactory » Sun Jul 07, 2019 8:33 pm

Cool, I got the aux vent issue correct by the sound of it.

Vacuum gauge is a good tool, if you're pulling 12-15 inches of vacuum then you should be OK. Idle dropping a little when the fan kicks in, sort of normal. I agree with adjusting the idle speed up a little when the engine is warmed

Weird timing, perhaps check the dist cap is the correct one for the ignition distributor. I've seen the wrong cap on a car do weird things, when the post and rotor don't line up too well and the spark jumps forwards / backwards at the point of ignition. Disconnect / clamp off the vacuum advance line when setting your idle / baseline timing. Check the advancing of the distributor with it off and see if that makes a difference.

SteveC
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