SPICA fluctuating idle speed... help!!

[sp949]

Just when I thought I finally had the SPICA correctly set up according to the files that John Stewart sent me, a new mystery appears.
My '71 1750 Spider, once warm, at idle flutuates rhytmically between 750 and 1200 rpm. One second up, the next second down.
At 1500 Rpm and above, the fluctuation almost disappears. I still experience some chugging/jerking while driving around 2000-2200 RPM.
I also get a single loud exhaust explosion when decelerating, or a series of quieter backfiring noises in the exhaust.
I don't have a thermostatic actuator, but a manual control, which seems to work fine in staying at 27 mm when fully extended, the dimension for my T237/2 Injection pump. My ignition is the Crane XR 700 electronic.
The Idle stop screw is properly adjusted.
I suspected vacuum leaks, but blocking the brake booster and oil separator hoses, as well as the idle manifold hose has no effect in reducing the problem.
I have actually removed the brake booster hose, and inspected it; putting my finger over the intake manifold vacuum check valve I can feel a pulsating vacuum; no idea if this is normal or not; it has always done this.
I am suspecting fuel pressure fluctuation. Can it do this?; however, my fuel pressure low light operates fine when starting and never comes on again.
I am now befuddled; any ideas?
TIA,
Val Barone

[papajam]

A hunting idle is almost always caused by a lean mixture. Backfiring on decel is a rich mixture. I would suspect;

insufficient free play in the throttle cable
short throttle rod adjusted too long (throttles not fully closing)
long throttle rod adjusted too long (pump lever gap greater than .019")
fuel cutoff solenoid out of adjustment
too much static ignition advance

[sp949]

Quote:

Originally Posted by papajam A hunting idle is almost always caused by a lean mixture. Backfiring on decel is a rich mixture. I would suspect; insufficient free play in the throttle cable short throttle rod adjusted too long (throttles not fully closing) long throttle rod adjusted too long (pump lever gap greater than .019") fuel cutoff solenoid out of adjustment too much static ignition advance

[Roadtrip]

I too would suspect something interfering with the throttle butterflies being allowed to fully close. With the engine running disconnect the short rod and fully seat the throttle butterflies and see if that makes any difference. If no change, then disconnect the long rod and allow the throttle arm on the back of the injection pump to sit on it's closed stop (as determined by the TA adjustment). Did you set the idle stop and wide-open throttle stop with the factory tool? If not, and you're not confident of the calibration, I can loan you the tool.

Did you sync the forward and aft throttle bodies? Check for seepage/leakage in the fuel pipes? Injection pump timed correctly? Is the plug color consistent with all four spark plugs? If the throttle butterfly shaft seals in the throttle bodies are badly worn and leaking, they might cause an uneven idle, but I would suspect they would have to be very badly leaking. With the engine in idle, does the engine choke off and stop if you block off the idle air at the distribution block? If you put your palm over the intake of each cylinder at idle, do you feel a noticeable suction? (If you do, the butterflies aren't fully closed.)

The rapid pulsation of the brake vacuum fitting is normal and consistent with the intake stroke of #4 cylinder.

If after double checking (in lock step order) the setup again, try riching up the mixture a bit. The stumble at 2000-2200 rpm is interesting. Take a timing light and check that the plugs are firing normally.

As long as the fuel pressure light is off, I doubt that fluctuating fuel pressure is the problem. As long as there's 7 psi, that's sufficient to fill the barrel in the injection pump section.

[conedriver]

VB,

Why not richen the mixture, a half turn or so, just to see what happens. You can always revert to the old setting.

Here's what Wes Ingram said about the plug color in the picture below: "Based on the dyno graph and the white plugs my guess is to add fuel but you will have to make the final call. If the 3/4 turn richer makes it better then keep adding slightly until there's no improvement then back off a bit. The plugs should normally show some gray or tan color because of some low speed running. If however you do a "plug cut" which entails going all out at say 7000 rpm on a long straight and cutting the power and coasting (in nuetral) to the paddock then the plugs may be very clean with little color on the porcelain".

Who says you can't fine tune an Alfa engine by email?

Quote:

Originally Posted by sp949 Thanks PJ! Aside from checking the ignition static advance (no pinging or detonation at any load leading me to doubt it) all your other suggestions I have checked more than once and there are no problem. Also, the plug color is grayish, which is not ideal, but neither too rich or lean, I think. VB

[sp949]

Thanks Jim, John, and George! I finally got it!
I used the "Alfa Tune-up Day Instructions" by Robert Perry, which, I think is one of the files that Roadtrip e-mailed me.
I found the following problems:

1. Long and short rods needed adjustment

2. The Sure start device had a washer on the end tip that prevented the TA adjustment screw to fully go in, which made the 0.19" pump gap impossible to set

3. The distributor was loose and the timing was too advanced (about 15 degrees BTDC.

4. The distributor had been installed 180 degrees out, and the rotor was pointed to #4 cylinder when it should have been pointing to #1.

5. The FCS soleinoid needed a minor tweak.

Took me about 2 weeks to go through all the threads about the subject, before discovering all these problems, which I had been putting up with it since I bought my Spider on April 1st, last year.
In the process, I learned a wealth of info about the SPICA, thanks to this board. After fixing all these issues, the engine is as smooth as silk, idle speed is perfect, starting is just turn-key from cold, with good fast idle speed, and backfiring is a thing of the past.
The only thing I am a little puzzled is that the owner's manual says that ignition timing at idle is 1-3 degrees AFTER TDC, not before, as it is for most engines. I have set it to spec, but the engine feels a little lazy to rev up, but otherwise very smooth. I have been using regular gas, as suggested by the previous owner, but I suspect that if I use premium, I can advance the timing safely to about 3 degrees BEFORE TDC, with maybe a little more go while accelerating.
Any advice about this from you consummated gurus?
Again, a world of gratitude to you all.

Val Barone
Florida

[papajam]

That's great news, Val! Thanks for the update.

The 1-3 ATDC is the spec timing. However, if the engine does not ping with the timing advanced a few degrees, your good to go (and your mileage should increase a bit too).

[john.harrill]

The problems you described are so typical of Spica cars "fixed" by well-meaning but mis-guided previous owners. Even professional mechanics are prone to not follow the rules when tuning these cars, due to either a lack of time, or a lack of knowledge.

The daughter's car ('78) was an example of such a car. The local Euro-shop had tuned the car, but only after it had warmed up. Then, after finding out it was "hard-starting" they simply enriched the mixture by adjusting the fuel cut-off solenoid. The other adjustments were way off.

[emr5503]

Val,
Time it at 5,000 RPM then see what the timing is at idle and forget it.
Ed

[Roadtrip]

Val -
Your car is a poster-child why new owners of Spica cars should go through the COMPLETE fuel and ignition tune-up check after getting a newly bought car home. There are A LOT of really badly tuned Spica cars running around out there, tuned by guys who are TOTALLY clueless on what they were doing. It's a credit to the Spica system that it can be so completely out-of-wack and still actually run.

[sp949]

Quote:

Originally Posted by emr5503 Val, Time it at 5,000 RPM then see what the timing is at idle and forget it. Ed

[sp949]

Quote:

Originally Posted by Roadtrip Val - Your car is a poster-child why new owners of Spica cars should go through the COMPLETE fuel and ignition tune-up check after getting a newly bought car home. There are A LOT of really badly tuned Spica cars running around out there, tuned by guys who are TOTALLY clueless on what they were doing. It's a credit to the Spica system that it can be so completely out-of-wack and still actually run.

[emr5503]

Val,
Just pulled Joe Benson's articles from the mid '70's and he said to set the fixed advance at TDC or 1 degree retarded from that (it's been so long since I set mine I couldn't remember how I set it).
Sounds like you're good to go.
Ed