Engine not running well

[NJ914]

I'm at my wit's end and need some help solving this problem. 76 914 2.0L starts and runs well cold. RPMs up around 1800 and reacts well to throttle. Once the engine warms up, idle comes down to about 1100 and is fairly steady. However throttle now causes immediate hesitation (bogging) followed by backfire and rough running up to 3000rpm. Over 3000rpm is much better. When I bring the rpm up to 2500 and attempt to hold it there, engine stays at 2500 for a second or two then drops down to 1500 and then goes back up to 2500 and continuously cycles up and down. (I believe vacuum leaks cause the engine to surge UP in rpm then down. This is reverse. It stalls downward then back up) This engine was rebuilt about 10 years ago and has roughly 4000 miles on it.

this is what has been done to date:

1: Replaced the CHT sensor.
2: Replaced all vacuum hoses.
3: Replaced all engine fuel lines.
4: Had injectors cleaned and tested.
5: Had CS valve cleaned and tested.
6: New injector seals.
6: New CS valve gasket
7: New intake manifold gaskets
8: New intake manifold boots
9: New throttle body gasket
10: Removed and inspected plenum. All ok
11: New Throttle positioning switch. Also adjusted.
12: Checked MPS. Both resistances and vacuum good. 20" held no change
13: Checked vacuum on decal valve. 20" held. Opens at 15 or 16
14: Checked Aux Air valve. open when cold and closes after 10 min run time.
15: Checked dwel angle. Good
16: Checked timing. right dead on the notch
17: Checked valve clearance. Good
18: New distributor leads
19: Cleaned distributor rotor and cap. (did not change contacts yet)
20: Checked all resistances at computer terminals and all are good.
21: Checked fuel pressure and steady at 29
22: After engine warm attempted to try different Computer knob positions / bleeder screw settings but have no CO meter. It runs best with computer knob turned all the way Clockwise.

After all this, it still runs the same way. #$^@$&%#%&@$%

My only other thoughts are to change the distributor contacts, check the adv/ret vacuum and perhaps play around with the timing.

I've head the throttle body could also cause vacuum leaks but not sure how/what to check.

Any help/suggestions would be greatly appreciated (and desperately needed!!)

[Tobra]

Man, you already did everything I would suggest. Maybe check to be sure the vacuum lines are all hooked up correctly, it sure sounds like a vacuum thing to me. There will be some smart guys along presently to give you the straight of it. Maybe post your query on 914world also

[NJ914]

I'm really frustrated with this. Going to check everything again. Good idea to post on 914world. Thanks!!

[Dave at Pelican Parts]

Have you checked the timing at idle as well as at 3500 RPM? Wonky distributor parts can cause trouble like that. (So can a lot of other things, many of which you have replaced already...)

Have you tried spraying starting fluid at places that could leak while the engine is running to see if the idle changes when you do that?

--DD

[John Rogers]

I read through the list 4 or 5 times and it appears you have the tried and true D-Jet system so........I also did not where see you replaced the trigger points? If I am wrong and I was many years ago but our rule of thumb was "when all else fails to fix the issue, replace the trigger points"! It got so prevalent in the San Diego area many years ago they would get replaced FIRST and any issues would go away!

I could be wrong though and cleaning them, filing them, etc, etc, etc never seems to help. If you do not have D-Jet, forget all I just said!

[NJ914]

I only checked timing at 3500rpm. Going to pull the distributor and completely go over it. Contacts, trigger points, oring, etc...
Also someone recommended using an unlit propane torch cracked open just a bit to check for leaks. Thanks for the help and will advise results.

[John Rogers]

As I noted, you have to replace the trigger points, not clean or file or do anything else to them. I have no idea why that seems to be true but........