SPICA - Adjust mixture after new Bosch fuel pump? - Alfa Romeo Bulletin Board & Forums
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post #1 of 10 (permalink) Old 05-06-2018, 06:47 PM Thread Starter
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SPICA - Adjust mixture after new Bosch fuel pump?

The fuel pump on my '74 Spider finally gave up the ghost this past week. I can't verify it's age but it was a SPICA/Bosch two port version, so might have given 44 years of loyal service. Unreliable Italian cars indeed!!

The reason I replaced it was that the fuel pressure light was coming on and staying on for 30 seconds or so before going out prior to start. Otherwise the car was running fine. I replaced it with a new Bosch pump from Centerline. The install went fine and now the fuel pressure light blinks momentarily on start-up like normal. However, the car seems to be running rich after the swap. The idle is sort of 'loping' and it seems to stumble a bit at lower RPM's.

Based on my (admittedly less than expert) understanding of the SPICA system, even if the new pump is supplying a higher pressure of fuel to the SPICA pump than the loyal but tired original pump, the injection pump doesn't really care, right? It's going to ramp up the pressure going to the injectors independent of how much the supply side is providing...I think.

That said, I am definitely a believer in the "Strongly consider the last thing that changed prior to the problem" school of troubleshooting and the fuel pump is it.

Do I need to adjust the mixture or should I be looking elsewhere?
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Greg East
Loveland, CO USA
'74 Alfa Romeo Spider
'87 Porsche 944 turbo
'93 BMW 740i
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post #2 of 10 (permalink) Old 05-06-2018, 08:13 PM
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AFAIK, it shouldn't make any difference. The pump pressure can vary slightly with rpm due to alternator output. The supply pump is very sensitive to voltage.

Have you confirmed the rich mixture by examining the spark plugs?

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74 Spider
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post #3 of 10 (permalink) Old 05-07-2018, 07:04 PM
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Have you confirmed the rich mixture by examining the spark plugs?
Or better yet, put a wideband o2 sensor on it and really see where the mixture is...

It is possible that the mixture was adjusted too rich to compensate for the slowly failing original pump.

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post #4 of 10 (permalink) Old 05-07-2018, 07:22 PM
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It is possible that the mixture was adjusted too rich to compensate for the slowly failing original pump.
That's my thinking. Best to go thru the SPICA setup routine and verify everything's in spec. You may find it runs better than ever...

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post #5 of 10 (permalink) Old 05-07-2018, 07:56 PM Thread Starter
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Ok, so I'm not crazy in thinking a nice strong new supply pump could affect the mixture. I had the car really dialed in via Wes's setup procedure (with an assist from Roadtrip's loaner bell crank alignment tool, thanks again!) but with the old pump. I just didn't want to start futzing with the mixture again if the supply pump had no influence on it.


Thanks guys, I'll tweak the mixture and see if my lopey idle and stumble go away and report back.

Cheers.

Greg East
Loveland, CO USA
'74 Alfa Romeo Spider
'87 Porsche 944 turbo
'93 BMW 740i
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post #6 of 10 (permalink) Old 05-08-2018, 12:49 PM
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Ok, so I'm not crazy ...
Well I'm no psychiatrist but you may still be crazy

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post #7 of 10 (permalink) Old 05-09-2018, 10:08 AM
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"Theoretically" it shouldn't. As long as the pressure is above the threshold of 7 psi, Alfa engineers say it's enough to feed the pump reliably . . . .theoretically.

Go ahead and lean the running mixture, but mark the FCS position so you have a reference point at the start.

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post #8 of 10 (permalink) Old 05-09-2018, 10:30 AM Thread Starter
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Pulled the plugs last night and they were all black with carbon. Leaned out the mixture and drove it around the neighborhood, seems to be improved but need a longer trip to prove all is well.

Greg East
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'74 Alfa Romeo Spider
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'93 BMW 740i
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post #9 of 10 (permalink) Old 05-09-2018, 01:09 PM
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What percentage of a turn did you lean the mixture on the FCS?

John Stewart
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91 164S
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post #10 of 10 (permalink) Old 05-09-2018, 01:18 PM Thread Starter
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I lost a little registration with the original placement but I think a 1/4 turn.

Greg East
Loveland, CO USA
'74 Alfa Romeo Spider
'87 Porsche 944 turbo
'93 BMW 740i
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