Stutter at 3000rpm

[mkinne]

My '73 1.7L (stock FI) is developing a stutter specifically at 3000rpm.

Heres the scenario:
Accelerate away from a stop in 1st gear, shift to 2nd, and the car stutters as the engine passes 3000rpm. Used to happen about 1 in 10 times, now more like 1 in 3. Sometimes it will stutter again after shifting to 3rd, also always at 3000rpm. On very hot days it may occur at about 2700rpm.

This is definitely associated with engine rpm, not throttle position. Happens under hard or easy throttle, up or down hill, rain or shine, etc. Plugs, wires, cap rotor, fuel filter, all recent. (within 6 mo.) Fuel pressure good and steady.

Could this be the "flat spot" mentioned in the PP Catalog description of temp sensor II?

TIA

[JP Noonan]

Try unplugging the throttle position sensor. The car won't accelerate as well but if the problem goes away you may need to clean the internal contacts, or replace it. Look in the tech ref section I think there is a tech article.

See if the dizzy is rusting inside, might be the advance weights are sticking. Mack sure the vac hose to the dizzy is in good shape.


Not sure what you mean by the "flat spot" for temp II. How does that happen?

[Germain]

No, this doesn't remind me of the "flat spot" created by a faulty temp II sensor.

It makes me think it is one of the following;

1) Advance not working
2) Advance grounding (as it moves it pulls that wire)
3) Timing issue

That would be my first 3 guesses. If none of those fix it, find out if you have a gas, air or spark problem. This is easy. Drive the car at night with a timing light on any sparkplug wire. Now when its cutting out, make sure you are getting a spark. If not, you have a gas/air problem.

Lets just take it from this point. Let us know if you fixed it, or if its a spark problem.

Good luck

[mkinne]

Thanx for the feedback.

Here is some more info:
Forgot to mention that I have a Compufire ignition module in the dizzy. Been in there 4+ trs with no problems.

JP -
TPS is in great shape. I've tried unplugging it with mixed results. Maybe I felt a stutter, maybe I didn't. Hard to say for sure with the poor accel. I have tried different driving styles to try to "force" the problem. No luck.
Dizzy is also great shape and advance moves smooth and easy. Vac hose is good.
The PP catalog mentions a "flat spot" associated with a faulty temp sensor II. I also do not know what this means. Has anyone had experience?

Germain -
Can you describe the "flat spot"? Have you experienced it?
Advance seems to work perfectly.
Is there a way to check if advance wire is grounding?
Timing is spot on.
I will try the timing light trick but not sure if it will work. The stutter lasts only half a second. "Feels" like a quick, sharp stab at the brakes.

Any more thoughts?

Matt

[Germain]

The engine doesn't cut off, it just seams to put out 50hp instead of 90hp for 100 rpm range. At least thats what I experianced. Unplug yours and drive it.

To check the advance, make sure no wires are bare, other then that you should be fine. You can always find a ground by checking for conectivity between something and a ground point.

Have you checked your dwell?

Take the car out on a night drive, hook up a timing light to cyl 1 and watch it as you go. Make sure you have a continuous spark.

Other then that, it would have to be some type of fuel delivery problem.

Keep us posted.

[mikey]

my 1.7 has the same problem at around 3000rpm. I have standard points ignition. I haven't been able to figure it out either. I tried the TPS trick with no luck. It almost feels like the ignition key is turned off for a split second...right?? hmmmmmmmm.....

Mike

[John Rogers]

Check the distributor, especially the weight area for rust, grease, lint, etc that can cause a sticking advance. Pull the Compufire rotor (the round black thing) to see if it is cracked from excessive axial play on the distributor shaft and the weights are loose? There is no mention of vertical motion in their docs, but it is supposed to be only 3 to 5 thousands which is hard to get even in a new distributor. Good luck.

[mkinne]

mikey... Yes, that is just what it feels like.

Germain... Have not checked dwell. Was thinking about fuel delivery problem, but how could that be associated with a specific rpm?

John... Dizzy is clean in and out. Compufire parts are all good and spacing is perfect.

Anyone... Are there axial and radial wear specs for the dizzy? Could a worn out dizzy be the culprit? I'm leaning towards something in this area because of the rpm clue.

BTW, car ran perfect today (except for flat tire) to and from work. Didn't notice a single stutter. I hate ghosts!

[Germain]

Fuel delivery at a given RPM is determined by,

1) Position of throttle sensor and phsyical position of the cable
2) Amount of gas it can suck up (is the fuel pump generating enough pressure)
3) Temperature outside
4) Air/Fuel mixture in the manifold (or like area)
5) Temperature of the engine
6) The computer calculating it correctly
7) Condition of the pistons/internals (remember we need to put fuel in an engine and compress it, then burn it. If the engine is off on it's own internal timing then it would be in theory able to be off at certin RPMs)

Basically this is kind of a start up list. The 3000 RPM isn't just a magical number that everyone see this at. It is the point where the advance starts, the engine is moving from a relaxed timing to a more agressive one.

Next you have the cold vs. warm engine. When a engine is cold, it uses very little sensoring, it just dumps a bunch of fuel/air in and burns it. Once its warm, the sensors kick in. Ever notice problems only when the engine is warm?

So in conclusion, lets check the spark before going any further. We can make sure what were looking at.

Good luck
(p.s. I'm not proof reading this one, so I hereby declare all spelling errors and grammer issues null and void )

[mkinne]

Thanx for the continued effort Germain!

So, advance starts at 3000rpm? Hmmmmm... Could it be sticking and then jump up? All was clean and smooth last time I was in there. I'll check again this weekend.

Speaking of temperature... I would say that the problem is only slightly more evident on particularly hot days. (lately, every day here in So. Cal) The car also suffers from poor throttle response on very hot days. It will run sharp, quick, and responsive in the cool mornings and then become a slug in the afternoon. Not every day though. Yesterday it ran great. Any help?

Matt

[Dave at Pelican Parts]

The advance *ends* at 3000 RPM on a 1.7 or a 2.0. It continues until about 3200 RPM on a 1.8.

Check the color of the deposits on the plugs. That will tell you the mixture when the car was shut off. Try driving at 3000 RPM (with half power) for a minute or so, immediately shut the motor off, coast to the side of the road and pull the plugs to look at them.

Black and sooty is too rich, white/grey is too lean, tan/brown is good.

--DD

[Germain]

DD, my 1973 2.0L. Where does the advance start on these? I thought it was at 2800 - 3200. Is this correct?

[Dave at Pelican Parts]

Check here:
http://www.dgi.net/914/tech/distrib.html

The advance kicks in just off idle, and goe up until about 3000 RPM. That's why the timing is set at 3500, to make sure all of the centrifugal advance is in.

--DD

[mkinne]

Car has run perfect since last thursday.
I hate ghosts!