'77 Spider (SPICA) - Rough Idle, misfire on #4 cylinder - Alfa Romeo Bulletin Board & Forums
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post #1 of 8 (permalink) Old 08-12-2017, 06:40 PM Thread Starter
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'77 Spider (SPICA) - Rough Idle, misfire on #4 cylinder

I have a '77 SPICA spider that I've been trying to get running after a couple of years sitting. Entire fuel system has been replaced (tank, pump, rear filter, front filter) and fuel pressure is good (solid 15 psi on battery, 18+ on alternator). Timing is spot on at the 'F' mark. I've adjusted the SPICA pump the best I can (at least the TA appears to be working, and the .019" reference gap seems correct when the car is warmed up). Short rod appears to be adjusted so that the throttle plates are closed at idle(can kill engine by plugging idle air inlet). Not really able to adjust the FCS mixture control because the engine is running poorly.

The problem is rough idling and what appears to be misfire on the 4th cylinder. I can pull the plug to that cylinder and the idle quality doesn't change, whereas repeating the test for cylinders 1-3 causes obvious stumbling - see video:
.


I've tried a bunch of different plugs with the same effect. Pulling the ignition wire for #4 and placing it by the engine block results in a spark as it should, and I can feel the high voltage pulse in my hand, so it seems that the distributor point is working. The plug for #4 pulls out black and sooty, so it appears that it's getting fuel as well. I also noticed that I can see the valve one time when I pulled the #4 plug out (see picture)...is that normal?
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Last edited by Zuofu; 08-12-2017 at 06:45 PM.
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post #2 of 8 (permalink) Old 08-12-2017, 07:38 PM
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OMG you have balls pulling off plug wires with you're bare hand. I did that 45 years ago on my Spider and ended up flying 5 feet onto the lawn. Have never done that since ...took me a half hour to get the feeling back

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post #3 of 8 (permalink) Old 08-12-2017, 08:25 PM Thread Starter
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OMG you have balls pulling off plug wires with you're bare hand. I did that 45 years ago on my Spider and ended up flying 5 feet onto the lawn. Have never done that since ...took me a half hour to get the feeling back
I definitely sparked myself right after taking the video, gonna use some more insulation from here on out.
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post #4 of 8 (permalink) Old 08-13-2017, 04:48 AM
Did you run a compression test? If you aren't building good compression in #4 than you will see incomplete burn.

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post #5 of 8 (permalink) Old 08-13-2017, 04:50 AM
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I definitely sparked myself right after taking the video, gonna use some more insulation from here on out.
No need for insulation.
Pulling a wire off a spark plug with the engine running will ensure maximum coil output voltage in the wire. Instead, pull the wire out the distributor cap. This will result in zero volts in the wire.

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post #6 of 8 (permalink) Old 08-13-2017, 01:47 PM Thread Starter
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Ok, did some more tests, found the problem. Compression was good, 175 psi on cylinder 4. Problem goes away when I cap the vacuum line going to the brake booster. However, the brake booster is brand new (from Vick Autosport). I replaced it because the old one was leaking and full of fluid. I pulled out the check valve to make sure it was indeed checking, and it is. For now I have capped the booster on both sides, but any ideas for a long term fix?

Edit, I unbolted the master cylinder from the booster, the large port of the booster seems to leak some vacuum (I can feel it pulsing with the misfiring engine), maybe my booster is DOA?

Last edited by Zuofu; 08-13-2017 at 02:16 PM.
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post #7 of 8 (permalink) Old 06-04-2019, 03:53 PM Thread Starter
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Sorry to bump an old thread, but the definitive fix for this was to rebuild the brake master cylinder completely. The seals of the MC were leaking brake fluid into the booster and causing it to leak vacuum.
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post #8 of 8 (permalink) Old 06-05-2019, 01:07 AM
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Very kind of you to come back the thread with a conclusion. So many threads are left hanging, with no resolution ! Glad you got it sorted in the end.
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