Loss of power under acceleration - puzzled

[Nut124]

Sorry about the long post. I'm reaching out for help after a string of strange loss of power incidents.

About half dozen times now, most recently today, my -78 Fiat has lost power, where it will not accelerate, but will cruise at 40mph on the idle/progression circuit. When this happens, the tailpipe sound is abnormal when I give it throttle, kind of like the lean spot between idle and main ckts in IDFs. Usually, idle and low speed running seems normal, up to about 3000rpm.

The car has cams, header, HC and dual 44IDFs. It has been running awesome other than these incidents.

Initially, I thought this was perhaps fuel delivery related, stuck float or something. I do have a fuel pressure gauge at the carbs and it has always shown normal pressure.

Today, I was on my way home and floored it on a Hwy entrance ramp, good power, got up to about 75mph under full throttle and lost power. Engine would not pull at above 3000rpm and sounded bad if I tried. Had to limp to the next exit at 45-50mph.

Previously, this happened last nite; was coming home at nite, spirited back road driving. Suddenly, no power at above 3000rpm. Started cruising home at 40mph and then, suddenly, after 2-3miles of running on the idle circuit, power was restored and it ran good again until the incident today.

After the incident today, I pulled the plugs. #2 was totally black. All others were light brown. I replaced #2 plug assuming it was fouled and expected it to run OK. It did not. Still no power, running on 2 or 3 cyls. ??

Now I'm wondering if this could be a bad plug wire? What confuses me is that it seems to correlate with the carb main circuit. However, maybe it just correlates with asking for power. Cruising at 40mph requires very little power.

It is very strange. On Wednesday this week I was at a race track and did three 20min track sessions with hot laps, plus drove there and back with no issues. I did experience this same loss of power before the track session as well.

I'm puzzled. Please send any suggestions, ideas, questions.

I did do a crank compression test just now. All 4 cyls 180-190psig. Good there.

[tima01864]

My experience with IDF's is that the jets can easily clog with a speck of dirt.

[18Fiatsandcounting]

When this happens, the tailpipe sound is abnormal when I give it throttle, kind of like the lean spot between idle and main ckts in IDFs.

I've experienced this sound before, but it was with a "normal" carb setup like 32/36DFEV or the like. Sort of a "hollow" sound in the exhaust, like air was passing through the engine but weak or intermittent combustion. Your blackened plugs would seem to confirm this.

I'm leaning towards a weak spark under certain conditions. Carbs don't usually cut in and out, but the quality of the spark can depend on load, rpm, mixture, etc. Sounds like the spark for 1 or 2 cylinders might be cutting out at times. Could be plugs, wires, weak coil, etc, as I'm sure you know.

Try decreasing the spark gaps (0.023" or so) and see if things improve. Look also at the plug wires for places where the insulation is wearing through. I had a plug wire once that had worn through the insulation on the bottom (where I couldn't see it) and it would spark to the cam cover rather than the plug under certain conditions. I only found the issue by revving the engine in complete darkness (in the garage) and looking very closely.

-Bryan

[Nut124]

Thanks for the advice.

It's starting to look like there have been multiple problems;

1. Initially, i think there were some incidents where a float would get stuck open after WOT and flood, foul plugs. Weber spec for float drop is 32mm, which would put the float in contact with the bowl and the eTube posts. I was told to set drop to 23mm instead by a fellow on an other forum. He claims Weber had re-issued the correction at some point, but I have not heard about it nor seen in print.
2. Several occasions of dirt in the idle circuit. This could have caused plug fouling. I need to replace the 20yr old air filters and replace the fuel filter just in front of the carbs.

3. Deteriorating coil. Starting to look like the coil maybe the culprit. Right now, the carbs and the fuel system look fine. Idles and cruises perfect, but will not accelerate at all above 3000rpm. Does this all the time now. Have a new coil to try tonite. A fellow at an other forum said that 9 out of 10 carburetor problems are ignition problems. Another fellow said that a failing coil can act as a rev limiter.

I will post a report after I try the new coil.

[18Fiatsandcounting]

Thanks for the update, Nut, and I'll be interested in hearing what you find out. I have to admit that, in all these years with all these Fiats, I have never replaced an ignition coil. My current '69 and '71 both have the original coils. Perhaps I should think about replacements, although they seem to work just fine. Maybe the early originals were better quality?

-Bryan

[Nut124]

My experience with IDF's is that the jets can easily clog with a speck of dirt.

Should have taken your advice earlier.

After completely rebuilding my ignition system, it's starting to look like dirt in a carb.

I used a small tube and a brake bleeder vacuum system to suction fuel from the carbs via the eTube well into a container. Repeated multiple times, refilling the carb bowls in between. One carb was totally clean.

The forward IDF had multiple chunks or flakes of lead (?) debris and some apparent plastic debris. These were all too large to fit thru the main jet. I suppose they could get sucked up by the jet at WOT causing the loss of power and then often fall back down when the car sits idle only to repeat the same the next day.

Lead plugs are used during carb mfg to close some drilled passages in the carb body I hear. Due to rain showers, could only test for about 25min today but ran w/o problems for the entire time.

The good thing is, I have a new ignition system now. Rebuilt the old dizzy. New coil and pickup, cap, rotor and wires. The dizzy oil seal had crumbled, 45yr old. There was oil inside the dizzy.