cylinders 1 & 2 not happy, good spark & compression - Alfa Romeo Bulletin Board & Forums
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post #1 of 17 (permalink) Old 06-16-2019, 05:12 PM Thread Starter
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cylinders 1 & 2 not happy, good spark & compression

Hey All,

Happy Father's Day!

Challenging problem here. First, '74 Spider 2000 Veloce w/Weber carbs. I've just replaced most of the fuel lines, tank filter, new fuel pump, and tank. All seem pretty good. This issue started just before I changed all the above out, but thought it was related to a torn fuel line (since replaced).

Other details: Fuel is getting to the webers, spark is good, compression is 150-160 across the board.

The car is waaay down on power. When idling, and the spark plug wires are removed from either cylinder #1 or #2 the engine is not impacted (no change in idle). Cylinder 3 & 4 nearly stop the motor when the wire is removed. So, cylinders 1 & 2 are not making any power and are fed by the same DCOE40 carb.

When the air filter is removed, fuel splashes/sprays out of the venturi. The backside of the filter was soaked.

Is it too simple to say the weber is delivering too rich of a mixture for proper combustion? Thoughts?
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post #2 of 17 (permalink) Old 06-16-2019, 06:24 PM
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Any chance the firing order is incorrect?
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post #3 of 17 (permalink) Old 06-16-2019, 07:19 PM
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I’d also check the float level in both carbs.

I had a freshly overhauled 40DCOM delivered with a tiny leak in its float. Over weeks it filled, leading to dramatic over-fueling of cylinders 3 and 4. As it happened so slowly, I didn’t catch it in time, and it washed all the lube off of those two cylinders, wiping out them and those two pistons.

You can do a quick level check by dropping a piece of clear 1/8” fuel line down into an emulsion well after running the electric pump long enough to fill the float chamber.

May we assume you are using a low-pressure fuel pump rather than the electric pump that served the Spica system?
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Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...
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post #4 of 17 (permalink) Old 06-17-2019, 01:16 AM
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check the rubber carb mounts too, they split

62 LANCIA FLAMINIA GT 2500 PROJECT (now sold) 64 SPRINT GT (Gone)
68 1750 ROUND TAIL SPIDER 70 1750 GTV 62 101 GIULIA SPRINT
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post #5 of 17 (permalink) Old 06-17-2019, 07:16 AM Thread Starter
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It possible though unlikely. I'll check tonight as this is an easy one to rule out.
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post #6 of 17 (permalink) Old 06-17-2019, 07:25 AM Thread Starter
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I'll also check that tonight. I've ditched the old box fuel pump in favor of a Carter unit. Will over pressuring of the carbs lead to the emulsion well overflowing?
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post #7 of 17 (permalink) Old 06-17-2019, 08:10 AM
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The fuel pressure should be 3 psi +/- 0.5. Flooding is dependent upon the pressure and the size of the needle valves. 2.75 should be big enough for a stock or moderately tuned 2L engine. Smaller needle valves are more tolerant of over pressure. But if only one carb is flooding then the float may be incorrectly adjusted or even punctured.

Ed Prytherch
79 Spider
76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #8 of 17 (permalink) Old 06-17-2019, 09:06 AM
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Quote:
Originally Posted by alfaronto View Post
I'll also check that tonight. I've ditched the old box fuel pump in favor of a Carter unit. Will over pressuring of the carbs lead to the emulsion well overflowing?
As noted, there is a spec for float-type carburetors. They don't need much pressure to work correctly, primarily just needing adequate flow to feed the engine. Too much pressure will push the needle off the seat and overfill your carbs, resulting in a flow into the barrels that is quite a bit more than the engine wants.

The Weber-specified method of setting the float height requires removal of the top and manually measuring a certain clearance. A quicker, and possibly more reliable way, is to remove the round jet cover, screw out an emulsion stack, turn on the pump to fill the carbs, and measure the height of the fuel level down in the well. There's a neat trick using a clear lucite rod, but I generally just drop a piece of clear, small diameter fuel line into the hole, place my finger over the top, and pull out the resulting column of gas. You must draw a line on the fuel hose which you use to line up with the upper surface of the aluminum at the emulsion hole, and another line 25mm below the first. Insert the hose to the top line, cover with finger, and pull it out. The fuel level should be pretty close to the 25mm line.

Needless to say, if you remove an E-stack, turn on the pump, and fuel rises to the top, or near the top, of the e-stack hole, you've found your problem.
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Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...
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post #9 of 17 (permalink) Old 06-18-2019, 02:28 PM Thread Starter
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sadly the firing order is bang on

Gonna swing by the local Alfa guy here in TO and see if there is a quick diagnostic path.
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post #10 of 17 (permalink) Old 06-18-2019, 04:27 PM
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I had similar worries this week with my 1600 GTV with DCOE40s. No #4 cyl just didn’t seem to be pulling its weight, with no rev drop when plug wire pulled, tho car still pulled like a train.
Richening the idle screw for that cylinder restored a consistent rev drop.
(Now, my amateur brain is wondering if I am compensating for another problem altogether, but I can’t think what, as spark and compression were good)
What this says to me is that the differential in fuel delivery meant 3 cylinders were carrying the 4th. In your case, it’s two cyl carrying the other two. What’s left, if jets are clean and everything else seems ok, is carb balance, which I can’t see mentioned above. Maybe your rear carb is opening butterflies well before the front, and the rear carb is carrying the idle load. This may also explain the down on power thing.
There are a few tips here on the BB on checking consistent butterfly openings thru the progression holes, which is probably the most accurate way to do it...you just need good eyesight.
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post #11 of 17 (permalink) Old 06-18-2019, 05:32 PM
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What kind of fuel pressure regulator are you using and what pressure is it set to?

Ed Prytherch
79 Spider
76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #12 of 17 (permalink) Old 06-20-2019, 04:35 PM
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Quote:
Originally Posted by alfaparticle View Post
What kind of fuel pressure regulator are you using and what pressure is it set to?
I second this, I had a very very similar issue to you and turned out that my fuel pump was delivering too much fuel. AlfaParticle showed me the proper fuel pressure regulator to get and that ended up saving my tail.

Assuming the spark plugs are new and properly gapped, I also second checking the float level and maybe even cleaning the idle jets.
I feel like its safe to say the problem will be carb related since its the two cylinders served by the same carb, though I could be wrong (wouldn't be the first time).

When you say fuel comes out of the venturi's, is it dripping out or shooting? Leak or a sneeze ??

Silver 1974 Spider- Weber 40DCOE151 carbs, BWA Wheels, 80K original miles 2014 Blue Honda Accord, automatic, 4cyl
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post #13 of 17 (permalink) Old 06-20-2019, 06:29 PM
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Originally Posted by Ranz View Post
Whatís left, if jets are clean and everything else seems ok, is carb balance, which I canít see mentioned above. Maybe your rear carb is opening butterflies well before the front, and the rear carb is carrying the idle load. This may also explain the down on power thing.

There are a few tips here on the BB on checking consistent butterfly openings thru the progression holes, which is probably the most accurate way to do it...you just need good eyesight.
I did the dance of the progression holes for a few years with two torches and then built myself a DIY manometer from two Aldi salad dressing bottles. Freakin amazing result.

TLDR: The accuracy and tolerances on that balance screw is far too fine using the progression holes, use a manometer.


Question: is the emulsion stack mentioned above just the great big long jet/tube things? Iím still working through a rough idle (have balanced the carbs, fitted 123Ignition... could be an air leak)
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post #14 of 17 (permalink) Old 06-21-2019, 09:19 AM
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Start with the simple things.
Fuel level, and carb sync.
BTW- in general, if you suspect air leaks: as the engine is idling, squirt carb cleaner at the suspected area. If the idle is affected, you located a/the problem.
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post #15 of 17 (permalink) Old 06-21-2019, 09:31 AM
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One can get the carb synch only so far off. If I understand the OP’s original post, he’s essentially lost the contribution of two cylinders. Even dramatically mis-synched carbs will make power once you get away from the lousy idle.

Most indicated sources of problem

Firing order swapped
Distributor cap failed (crack or carbon track)
Failed carb fuel supply or float level control.
Twin dead mice plugging intake tracts.

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...
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