Because I can't leave things alone, and maybe because I'm always trying to figure out the last 3% of why my 1971 1.7l engine doesn't purr exactly right, I spent some time with the Haynes manual to look things over.

I thought I had checked out the fuel pressure valve before, but I never really have. The book says (as do several knowledgeable D-Jet wizards) to set the pressure at 30psi. Mine is set to 35psi. Five pounds too much. Huh.

So I adjusted it back to 30psi and the car ran terrible. I thought there might be a change in timing and I was right, but even by re-timing the engine it still ran horrible. So I pushed it back to 35psi. Now it works.

Mostly.

But this raises a question. Why is it set so high? I nudged it up a little higher to 36 or so and it actually ran a tiny bit better. I don't like setting it so high, so I'm confused. The fuel pump isn't stock as far as I can tell. Or least it doesn't look at all like the one in the Haynes manual.

Any thoughts?

What is your actual air-fuel ratio? or idle CO?

What first somes to mind is that you may have crud in the injectors and the increased pressure is compensating for restrictions and crappy spray pattern.

If you are actually running no richer than spec, it's probably something like that. If you're actually running richer than spec, you may have to adjust to a new definition of 'normal.' Or find whatever else is keeping it from running right that someone decided to mask with richness across-the-band. TPS "accel pump" not working right is something that this could compensate for.

But define "ran better"... Idled smoother? accellerated better? improved transient response? More power? Lower emissions?

I'm not sure about the fuel ratio or idle CO levels. I don't have one of those nifty gadgets that show such things. Maybe I should buy one, but I'm not sure which to buy. Any suggestions?

The imperfections I have are as follows:

(1) At constant speed, usually constant high speed, I can feel it miss a little bit. Not enough to freak me out, but enough to know that something isn't right.

(2) The idle is steady enough, but it seems to have three different places it like to idle at: 1100rpm sometimes, other times a perfect 850, and sometimes 500. It seems to change if I drive the car around all day, stop for a while, and then jump back into the car. This doesn't bother me too much but its worth noting.

(3) Once in a while, I drive the car in the late afternoon and stop to eat dinner somewhere. When I go back to the car its dark and much cooler, and the car runs like its running on three cylinders. If I leave it alone overnight it runs perfect. I've noticed this only twice - once on a date (embarrassing) and once on a drive from Portland to Seaside, a drive up and over some small mountains to the Pacific shore. It was 100 degrees when I left Portland, but at Seaside the fog rolled in and the temperature was about 55 degrees. I stopped and ate, and the car barely wanted to start. It made it home but I white knuckled it the whole way back. The next morning it cranked up and ran perfectly. I haven't been on a long drive since.

The thing I'm trying to nail down is problem #1. Light missing, seemingly at random, at constant speed. With the increased fuel pressure it seems to act a little better. Timing was most of it, but there remains a little blip in the system someplace. I can't just adjust the lean/rich knob on the ECU because the 1.7 has no such adjustment.

Hmm.

i wonder also, if the fuel pump isn't stock and doesn't produce the same volume then the higher fuel pressure may bandage fuel needs? that was a good sugestion to check at least c.o. & h.c. levels. if it ran good until the fuel was messed with, that should rule out mechanical and electrical aspects of the drivability. in other words a fuel related issue.

i've also wondered if todays fuel quality effects drivability. by raising the fuel pressure (additional fuel) it may alter drivability also?

i'm running 32 p.s.i. if for no other reason to help cooling. air cooled motors like porsche vw run cooler and better with a richer (raised c.o. level).

My 2.0 had the same problem for two summers. The car ran great at a high PSI but when I brought it down to spec it would run like crap. It ended up being partially clogged injectors. Well actually just the two new injectors I had in the car were having "spray issues".

It turns out the E-10 gas we have in the northeast was the culprit. Now I use a fuel stabilizer with ethanol treatment and the problem has not returned this summer.