post tune up problem

[tmctguer]

I just finished installing new plugs, plug wires, rotor, points, cap, and ignition coil. I checked dwell and timing and adjusted everything to spec. I also adjusted my valves. Man, was I proud of myself !!

Things ran fine for about 50 - 75 miles. Now, although I have a smooth idle at about 900 RPM, I am getting hesitation and what sounds like slight missing between 1,000 and 2,500 RPM’s, especially as i accelarate from a standing stop. once I get to approx. 3,000 RPM’s, the motor seems to be running strong.

I have not made any adjustments to my MFI, or changed any items in the fuel system (filter, pump, injectors, etc.). all the fuel connections and heat exchanger connections on the MFI system are good. I also have not changed anything on the CDI system.

I ran a bottle of Techron through the car after i did the tune-up. I haven’t pulled my new plugs to see how they are looking. The car was running rich prior to doing the ignition tune-up mentioned above.

Any suggestions on what I should look at next ?

[Early_S_Man]

Well, I hope the coil you installed was the Bosch 0.221.121.001, or maybe the 'new' Perma-Tune coil, as those are the only two I am aware of that are matched to the ouput impedance of the Bosch CDI and Perma-Tune boxes, and the use of conventional coils can destroy either unit!

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Warren Hall
1973 911S Targa

[This message has been edited by Early_S_Man (edited 07-05-2001).]

[lsolon]

What happens if the cdi unit goes bad. How can you tell? Mines a 72 911

[tmctguer]

to answer your question about which coil i installed, it was a bosch unit. i don't have the part number handy, but it is one sold by pelican for the '73 car.

[Greg-slant/cab]

Sometimes things are not related. I did a recent tuneup and started having a rough idle and a hesitation like a miss. I checked and rechecked all the tune up items and decided it must have been in the fuel delivery side. I went ahead and changed the fuel filter and changed the injectors and that solved the problem.

[tmctguer]

did you try cleaning the injectors before replacing them? i was thinking about replacing the fuel filter next.

i purchased the tools to adjust the MFI, but wanted to eliminate all other possibilities prior to goofing around with that beast. i've read some horror stories from folks who have messed theirs up pretty badly.

[Greg-slant/cab]

I did not clean them, I just replaced them. I figured the cost factor was low enough that I went ahead and replaced them all. I still do no know if it was the injectors or the fuel filter, but either way the car runs much smoother and better after the replacement.

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Greg-slant/cab
http://www.geocities.com/gregslantcab/

[bfranklin]

Check out your spark plug wires and the ground straps (primarily the one from the transmission). If they look old or suspect, replace them.

Brad

[911pcars]

As in any troubleshooting mode; start with visual inspection first. Spark plug connections tight? No loose primary wires?

Remove the spark plugs and check the gap; some too wide? Check wire resistance; dist. cap. to spark plug wire end.

Run the engine at night or when it's dark and look for any stray sparks jumping to ground.

It's difficult to troubleshoot without seeing things.

Sherwood Lee

[tmctguer]

some good news on trying to solve my hesitation problem. i changed the fuel filter and planned on doing a compression test.

when i pulled the spark plugs, all of them showed a good "burn pattern" showing i've got a good air/fuel mixture. i was really expecting to see fuel fouled plugs as they were before i started my work. so, i appear to be on the right track.

tomorrow, i'm going to get a tank full of 87 octane gas per the "Check, Measure, Adjust" recommendation (i've been using 92) and finish the compression check. i'm thinking about removing and cleaning my fuel injectors (ultrasonically, as suggested by previous posts).

after that, i'm going to start on my MFI.

[tmctguer]

update on this saga......i reset my timing (it had "drifted to 10 degrees ATD) and that helped things a bit. while is was in there, i checked for manifold leaks and found none. i also disassembled my MFI thermostat and cleaned it. it wasn't in bad shape.

unfortunately, i still get the hesitation in the 1000 - 2000 RPM range.

here is one for the experts on the board:

i did a compression test and here are my results:

cyl. 1: 142, cyl. 2: 155, cyl. 3: 150, cyl. 4: 155, cyl. 5: 153, cyl. 6: 155.

cylinder 1 is definitely low. how worried should i be about this? is there a follow-up test that should be done (e.g., leakdown) that might tell me more?

please advise.

[tmctguer]

sorry to keep this thread running (with so little interest.....), but i've moved onto cleaning my injectors.

when removing the flare nuts from the injectors, i noticed that the velocity stacks on cylinders 5 & 6 have cracks in them where they are bolted to the engine housing. i suspect the PO over-torqued the nuts.

when i get the injectors back together, i will test the area for a vacuum leak by spraying carb cleaner. hopefully, there is none as i would hate to have to replace that part !

[Matt Smith]

Keep going through the check stuff before you hit the MFI- that's how it's supposed to be done. But I suspect your mixtures or 'basic adjustment' may be out a little bit. This can certainly lead to the symptoms you describe. The hesitation can probably be covered just by changing the part load adjustment- maybe a little richer as this phases out at around 2500-3000 and the main mixture takes over. It's very easy to do anyway; just keep a record of how many clicks and in what direction you changed it in case you make it worse. Don't be too afraid of MFI. It is your friend.

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'72 911 TE

[tmctguer]

replaced my injectors this evening after a multi-day soak in techron. after installation, the car ran smooth, but after it warms up, i still have the hesitation under 2000 RPM.

i hooked up a vacuum tester and the needle fluctuated like crazy. however, it settles down after i revved the motor up. i then watched the tachometer and the vacuum gauge, and sure enough, the vacuum gauge fluctuates from idle speed until 2000 RPM.

so, i'm still on the hunt for my problem. the last person to respond to my thread suggests that i might solve my problem by synchronizing the stacks. i'll probably go there next. the GOOD news is that i've looked at the plugs and they suggest that i have a good mixture, which is better than when i started.

any other suggestions would be appreciated.

[Early_S_Man]

The only two vacuum ports being monitored are from cylinders #1 and #4 going into a 'tee' and then to the vacuum retard module onm the distributor. Inconclusive at best, I would think!

By all means, get a Uni-Syn or Synchrometer and test the flow and get it balanced!

Also, read the thread below on distributor maintenance, and check your advance curve per the '72-'73 spec book for compliance with specs through the entire rev range:
http://www.pelicanparts.com/ultimate/Forum3/HTML/009677.html

Your problem sounds ignition, not fuel related, to me! At some point, you probably need to put the engine on an ignition scope (Sun Analyzer or equivalent) for evaluation.


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Warren Hall
1973 911S Targa

[gblumenstock]

I had a similar experience the other day when I replaced my distributor cap. The car idled fine but lost power and sputtered a bit in the 2000 to 3000 rpm band. I recalled that I had tugged a bit on the ignition wires to get enough slack to swap the connections. Sure enough one of the Beru connectors popped off. Since you just did all this maintenance, are you sure all of the ignition wires are seated properly? Seems much more likely than a change to the MFI. I usually try to change one thing at a time so I can back out of any missteps. Something I learned in my Mad Research Scientist days.

GB
79SC Targa (168,000 miles :-))

[tmctguer]

yes, i have gone back and double checked all of my work, including spark plug wire seating at the plug and the cap. i've corrected anything that i missed the first time.

i'm going to test the advance on my distributor at various RPM's. then i'll probably pull it this weekend, and see what it looks like on the inside.

after that, i'm going to set the mixture on the stacks, and if it still is hesitating, i'll take it to a shop to see what THEY think it is.

although it has been frustrating trying to solve this problem, i have been learning along the way. plus, the car is still very driveable, and i like turning a wrench.

i think i'm getting close, and i know things i've done have helped me get a good baseline on the car, plus have improved performance, certainly as evidenced by the plugs that are no longer gas fouled.

for everyone who has chipped in, thanks for the tips. i've taken them all seriously !

[tmctguer]

next step in my saga.........today, i removed the distributor, cleaned it, and re-installed it. i didn't find anything TOO horrible inside, although i removed a lot of grease that had accumulated over the years.

the hesitation is still present. my dwell is 35% with timing at 5 degrees after TDC, at approx. 900 RPM.

i am on the low end of the dwell spec (38 +/- 3 degrees) on a new set of points. i will try to regap them to get as close to 38 as possible.

also, i am going by the tach in the car because i can't quite figure out how to get my tach/dwell meter to measure RPM's (works fine for dwell).

that's where i'm at so far. still looking, but getting my car sorted out while i'm in the process.

[tmctguer]

next step in my saga.........today, i removed the distributor, cleaned it, and re-installed it. i didn't find anything TOO horrible inside, although i removed a lot of grease that had accumulated over the years.

the hesitation is still present. my dwell is 35% with timing at 5 degrees after TDC, at approx. 900 RPM.

i am on the low end of the dwell spec (38 +/- 3 degrees) on a new set of points. i will try to regap them to get as close to 38 as possible.

also, i am going by the tach in the car because i can't quite figure out how to get my tach/dwell meter to measure RPM's (works fine for dwell).

that's where i'm at so far. still looking, but getting my car sorted out while i'm in the process.

[tmctguer]

tonight, i did a quick synchronization of my velocity stacks. i didn't do it by the book (get all mixture screws turned out 5 half turns, etc.). i started out just wanting to see how close all 6 were to each other. they were relatively close, but i fined tuned them all to be close to exactly the same.

i leaned out the ones that were "underflowing" compared to the others. i immediately saw an increase in idle speed to approx. 1100 RPM. i did a test drive and the hesitation still remains, but less prevalent.

when i got back home, i noticed the idle speed had settled back down to approx. 900 RPM. i checked timing and it was still dead on. i was putting on the plastic cover that protects the fuse panel in the engine and the idle jumps back up a bit. i tapped on my CDI box and it jumped a little again. it never settled back down to 900 RPM, so i don't know if if really found anything. however, i'm going to keep looking for a possible loose connection at the CDI unit (???).

in the meantime, i am going to return to the "check, measure,adjust" instructions over the weekend, and try to set the throttle valve push rods, check the micro-switch setting, then re-synch the stacks.

i'm trying not to get too far out of order.