Running GREAT and then rough…..hhhhmmmm, engine runs rough at one place in the power band Options

[ctc911ctc]

-Update -

I removed the TPS, calibrated it as per instructions on the Bird. Here are some pictures.
I would like to be able to "RING" this out, not sure what I would be looking for. Though we know that one course is to install a new board and then calibrate. There is little else I could find to assure operation. Any experience with asserting these things are good.

Some pics



Any ideas would be great!

-End of Update-


I am a bit befuddled as to which way to go so I decided that the 914W team would like to hear about this puzzle.

Background
'74 2.0, garaged from '84 until 2018, 22K miles
Running since June.
Repaired or replaced:
Entire Fuel System - tank, lines, injectors, pump, pressure reg
Adjusted the valves
Removed and rang-out the engine harness,
Rebuilt the MPS
Rebuilt the Plenum and tested it for leaks - none observed
New gaskets at the plenum and manifold at the engine
Rebuilt Throttle Body, very minor leaks around the actuator (throttle), New circuit card
New Plugs, points, cap and inspected the distributor for all of the regular ailments - super tight.

Engine idles great, pulls like a monster - sounds like a German symphony

HOWEVER
Last night I drove it back from the paint shop on 93 heading south from NH to Boston (lots of beeps from newer Porsches (IMG:style_emoticons/default/smile.gif) (IMG:style_emoticons/default/wink.gif) ) and found that the car started to buck and the engine runs rough when just a small amount of throttle is applied.

More Detail

If you are doing 55 and want to maintain that speed, it is necessary to apply a small amount of gas to sustain the speed. This is when the cars starts to exhibit this "miss/run-rough" puzzle. If more throttle is applied the bucking stops, the engine is smooth and all is well. Conversely, while de-accelerating there is no bucking.

This problem is NOT velocity-dependent, sustaining at 30/40/50/60 will exhibit the same reaction.


WHERE WOULD YOU START TO LOOK IF YOU HAD THIS CHALLENGE?

[ctc911ctc]

Good point Jerry, that CHT caused me a lot of headaches when I first started the engine - swapped it out (it was stone dead) and my richness trouble subsided.

Will check the AFR once I get the bucking sorted.

On to the bucking gremlin!

Thank You!

Will check next:
grounds
Fuel Pressure
Wiring to the ECU from the TPS (have done this before)
Trigger points and operation
Ignition system (complete)

Also check the CHT sender.

For overall operation of DJET, you should check the AFR although this requires installing an oxygen sensor in the exhaust. I did this on my 1.8L DJET when I first got it running. Turns out it was running very lean and I had to adjust the MPS.

[ChrisFoley]

I've experienced similar symptoms when the MPS was adjusted a bit too lean. At some areas in the fuel map it gets lean enough to cause slight misfire.

I wrote this precisely because of the bucking you are experiencing.
It is my contention that all you need to do is enrich the MPS setting a small amount to eliminate the misfire which results in bucking during light throttle cruise.
I bet you would also notice an increase in power at WOT.
Easy peasy.

If your MPS is not set up for in situ adjustability you can still open it up to make a change, and reassemble it in half an hour.

[ctc911ctc]

Chris!

The muffler you sold me works and sounds great!

Great advice, I rebuilt the MPS as per your instructions and developed a method of 'relative relationships' to put it back together. Using the H method I was within a few percents of calibration.

I am going to check all of the basics and then PRIOR to going deeper will make this adjustment.

Question - if the ECU adjustment is turned to the RIGHT a click or two would this accomplish the same thing?

Thank You, Tangerine Chris!

I've experienced similar symptoms when the MPS was adjusted a bit too lean. At some areas in the fuel map it gets lean enough to cause slight misfire.

I wrote this precisely because of the bucking you are experiencing.
It is my contention that all you need to do is enrich the MPS setting a small amount to eliminate the misfire which results in bucking during light throttle cruise.
I bet you would also notice an increase in power at WOT.
Easy peasy.

If your MPS is not set up for in situ adjustability you can still open it up to make a change, and reassemble it in half an hour.

[brant]

The ECU knob only affects the mixture at idle and not the mixture of the motor under load or at other rpm

[ChrisFoley]

Question - if the ECU adjustment is turned to the RIGHT a click or two would this accomplish the same thing?

Brant is correct. And because the injection pulse is so short at idle the range of the ECU knob is quite narrow. In my experience when the MPS setting is near optimal the effect of the ECU knob will be more noticeable, and idle will be clearly better away from either extreme position.
I suggest that you back out the center screw in the MPS diaphragm by a turn and a half. Record the inductance values for comparison then install it in the car for a test drive.

[ctc911ctc]

Great advice Chris - Will DO!!!!!

Question - if the ECU adjustment is turned to the RIGHT a click or two would this accomplish the same thing?

Brant is correct. And because the injection pulse is so short at idle the range of the ECU knob is quite narrow. In my experience when the MPS setting is near optimal the effect of the ECU knob will be more noticeable, and idle will be clearly better away from either extreme position.
I suggest that you back out the center screw in the MPS diaphragm by a turn and a half. Record the inductance values for comparison then install it in the car for a test drive.

[ctc911ctc]

*************Update**********

The bucking has Stopped! And the car runs great!

Chris,

You were correct, the MPS that I rebuilt must not be calibrated correctly (I used the H version of measurement and adjustment) since I swapped the one I rebuilt and cleaned and looks like new with the one I bought through eBay which is old and ugly but holds vacuum.

The old one installed stops the bucking, so as soon as I get my bench rebuilt I will put the one that I repaired back on the bench and get it tuned correctly.

THANK YOU CHRIS!

Great advice Chris - Will DO!!!!!

Question - if the ECU adjustment is turned to the RIGHT a click or two would this accomplish the same thing?

Brant is correct. And because the injection pulse is so short at idle the range of the ECU knob is quite narrow. In my experience when the MPS setting is near optimal the effect of the ECU knob will be more noticeable, and idle will be clearly better away from either extreme position.
I suggest that you back out the center screw in the MPS diaphragm by a turn and a half. Record the inductance values for comparison then install it in the car for a test drive.

[rjames]

Glad you got it running well again!

And no disrespect to Chris, but I suggested that the MPS needed calibration several posts earlier. (IMG:style_emoticons/default/bootyshake.gif)
(IMG:style_emoticons/default/beer3.gif)

[ctc911ctc]

OOOooooopppppsss,

Sorry!

My mistake (IMG:style_emoticons/default/pray.gif) (IMG:style_emoticons/default/pray.gif) (IMG:style_emoticons/default/pray.gif)

Glad you got it running well again!

And no disrespect to Chris, but I suggested that the MPS needed calibration several posts earlier. (IMG:style_emoticons/default/bootyshake.gif)
(IMG:style_emoticons/default/beer3.gif)

[ctc911ctc]

UPDATE!


Bucking is back - very slight - I am going to adjust the MPS again as I had within this thread.