Momentary hesitation at 3600 rpm - Alfa Romeo Bulletin Board & Forums
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post #1 of 15 (permalink) Old 07-07-2019, 11:05 AM Thread Starter
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Momentary hesitation at 3600 rpm

The 40 DCOE 27 Weber carburetors (Italian) on my stock 1600 underwent a refresh 4 years (and 18.5K miles) ago. It has been running great ever since: steady idle, smooth acceleration, plenty of power. The only issue is an occasional, momentary hesitation (<1 sec.) under power on the highway within the range of 3600 to 3800 rpm. This might occur 6 times over 100 miles, and always while holding a steady speed on level or slightly uphill grades, and rather unpredictably. I can't recall if it's ever occurred when accelerating or climbing moderately steep grades. Any ideas as to what might cause this symptom? Here are some specs on the engine:

123/ALFA-4-R electronic distributor (standard set up)
Bosch blue ignition coil
NGK BP5ES spark plugs (nicely brown color at the electrodes)
stock mechanical FISPA fuel pump and regulator/filter

Carb set up:

New brass floats
Chokes: primary 30, secondary 4.5
Main jets 120
Air correction 180
Emulsion tubes F-16
Idle jet 65F5 (standard)
Needle and seat 175
Accelerator pump jets #35
Accelerator pump bypass jets (2 only) 60

Thanks for your input.

Daniel Romero
'67 Duetto (white w/red)
'02 BMW 330 ci (silver w/black)
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post #2 of 15 (permalink) Old 07-07-2019, 01:54 PM
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65F5 is unusual. I would expect 45F8 or similar. 65 is a massive fuel orifice. BP5ES is an unusually hot plug. BP7ES or BP6ES are more normal. But I don't see a link between these and your symptoms.

Ed Prytherch
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post #3 of 15 (permalink) Old 07-07-2019, 02:33 PM
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50F11 idle jet is a worthwhile try on a fairly standard 1600 w/ twin webers.
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post #4 of 15 (permalink) Old 07-07-2019, 02:49 PM Thread Starter
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I did try BP6ES pugs in the past, resulting in a rather sooty residue at the electrodes. Bumped up to BP5ES with a more satisfactory result. Could having to go with a hotter plug be due to a too rich mixture in the combustion chamber? I currently get approx. 23 mi/gal with 91 octane non-oxygenated gas. I know very little about changing carburetor components to know what changes to make with the various jets. Would changing the idle jets from 65F5 to 50F11 entail any other changes, other than idle adjustments and synchronization?

Daniel Romero
'67 Duetto (white w/red)
'02 BMW 330 ci (silver w/black)
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post #5 of 15 (permalink) Old 07-07-2019, 04:32 PM
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synchronization unlikely; idle adjustments probable

If you need BP5 rather than BP6 you likely are too rich at idle right now.
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post #6 of 15 (permalink) Old 07-07-2019, 05:53 PM
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65 F 5 sounds like a cold start jet. Probably have a 50 F11, or 50 F 8 idle jet. Hesitation or miss could also be an ignition problem. My 10 yr old coil just died. My giulietta had similar problems. Also, today's gas with ethanol will rust out your tank in a heart beat and clog the fuel system. Check for rust in the fuel filter.
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post #7 of 15 (permalink) Old 07-07-2019, 06:59 PM
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It is a cold start jet. Mechanical fuel pump? FRB11 regulator? Diaphragm in the regulator might have stiffened up from todays fuel. Check fuel pressure. F-16 E tubes are very forgiving of correct a/f emulsion with variable fuel levels in the float bowls, and a reason I and others like them, but gradually falling bowl levels until the 175 N & S fully open might be a fuel pressure issue. Just a guess.
Ed, his idle is probably 45 or 50F8 which should be fine.
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post #8 of 15 (permalink) Old 07-09-2019, 06:11 AM
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“occur 6 times over 100 miles, and always while holding a steady speed”

That’s roughly once every 15 minutes. That really doesn’t seem like a carb issue.

Reminds me of a problem I had with a Pertronix unit (in an old Jag) where RFI caused the Pertronix to malfunction and cut off ignition after about 7-10min of driving. A quick reset (ignition off/on) would resolve the problem. It only took a second to flip the ignition key off and back to on. But it got annoying when in traffic or steep mountain roads.

I wonder if you have a similar problem, but the 123 is smart enough to reset itself (hence the 1 sec power loss)?

My solution was to change my plug wires to a spiral wound suppression type. RFI could also be caused by a bad insulator or anything which allows some of the high voltage to ground NOT via the spark plug (arcing). If you think this might be the case, check the distributor cap for contamination and your wires and ‘boots’. Sometimes looking at your engine at night (rev it in neutral) makes it easier to see arcing.

John F.
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post #9 of 15 (permalink) Old 07-09-2019, 07:29 AM
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Hi there, try turning your headlights on next time, could be a 123 hiccup. See the item on the right side column here about after driving for a few kms.

Cheers,

Carson, 4 Alfa's, 9 Cars, 4 Motorcycles
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post #10 of 15 (permalink) Old 07-09-2019, 09:50 AM Thread Starter
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Thanks to everyone for their insight and advice. I agree that the rather sudden and short hesitation at higher rpms makes an electrical system the most likely source, so I will focus my diagnosis on the electrical system. I changed the coil about 4 years ago, which doesn't seem all that long for a coil to become faulty. But I guess age doesn't always correlate with equipment failures. The high test leads were new about 9 years ago; I'll double check the rubber boot positions and all connections for the distributor to the spark plugs, grounding wires, etc. I like the idea of checking for sparks to ground in the dark, something I can look for tonight. I always run with my headlights turned on for safety.

Everyone's comments about the cold start jets were also quite helpful. I will look into changing over to 50F11 or similar sometime soon. I was always curious as to why I need the hotter BP5ES plugs to keep from fouling; the richer mix due to the idle jet setup seems like a plausible explanation. I'm driving my Spider to the AROC Convention next week, so any carburetor changes may have to wait (I hope!) until my return to Minneapolis.

Daniel Romero
'67 Duetto (white w/red)
'02 BMW 330 ci (silver w/black)

Last edited by AlfaEmitter; 07-09-2019 at 09:52 AM. Reason: headlights on comment
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post #11 of 15 (permalink) Old 07-09-2019, 09:53 AM
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Ah, maybe need a voltage spike suppressor (a diode) to keep spikes out of the 123.

John F.
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post #12 of 15 (permalink) Old 07-26-2019, 01:00 PM Thread Starter
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The momentary hesitation phenomenon continued during the first 2 days driving to the AROC convention. While checking the oil on day 3, I did notice that the hot wire from the ignition coil to the 123 ALFA-4-R electronic distributor was resting lightly against the dipstick tube/block. On a hunch (could an electrical short through the insulation in a hot engine be responsible?), I tried a quick fix to isolate the wire by tethering it to lower radiator hose with a loop of electrical tape. Voila! No more random hesitation for the entirety of the convention. During my 980 mi return to Minneapolis, the hesitation reoccurred a handful of times, so my Ďrepairí was not absolute. I suspect a faulty ignition coil or a loose connection may ultimately be the source of the problem. Iím currently on a non-ALFA excursion until early August; a more permanent repair will have to wait. In any event, Italian electrons continue to move in mysterious ways!

Daniel Romero
'67 Duetto (white w/red)
'02 BMW 330 ci (silver w/black)
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post #13 of 15 (permalink) Old 07-27-2019, 08:42 AM
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Marconi did it in Italy when he invented radio. It's been downhill for the rest of us since. Pesky electrons....


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post #14 of 15 (permalink) Old 07-27-2019, 12:10 PM
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Had the same thing happen to me yesterday in my Euro spec 77 Alfetta. Tooling along and for a split second the hesitation, not like a fueling issue, just no power for a split second, then all was fine, I remember thinking this isn’t good, happened once before, just very random and not repeatable. Odd

Cars ruined my life! 😱
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post #15 of 15 (permalink) Old 07-28-2019, 03:53 AM
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My SPICA Spider was doing that in a sweeping G-induced turn. It turned out to be a clue to change the fuel filter. Don't know the physics at play but it was a fuel feed issue not an ignition issue. My rule of thumb is if you haven't been futzing with wires and ignition before the problem occurred then it's 99% not the ignition. A neighbor of mine changed the plugs on a Chevy and it ran like crap..Come to find out one plug wire was not locked in place on the plug. Ignition is pretty easy to check too..so are loose wires and dirty connections and primary grounds.
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