Intro 1972 Irish Green 1.7L; help needed with jerking engine Options

[andreic]

Hello,

some of you may remember me. After owning for several years a red 1975 1.8L 914, on which I reinstalled the L-Jet, got it to work well, and have sold this summer, I am now working on a 1972 1.7L D-Jet which I got this spring. Photos attached.

So far I got the brakes to work well (new front calipers from PMB were installed, and all four flex lines were replaced), replaced all fuel lines and the fuel pump, replaced all vacuum lines, new spark plugs, points, cap, wires and rotor. Also adjusted all valve clearances. I did a compression check and all was well, I got around 120lbs on all cylinders.

What still plagues me are two engine issues. One is that when the engine is cold, if I rev it up it will make a loud noise that sounds like a bad rattle. It is only there on hard acceleration, and it goes away almost completely as the engine gets warm, say, after 10-15 minutes. I have come up with several ideas what it could be: exhaust noise from badly sealing exhaust ports; valve noise; or something else in the exhaust (the muffler is an aftermarket one, and I am not sure if there isn't something in it rattling).

Another issue I have is that the car is not riding smoothly. It is hard to explain, but even if I am driving at constant speed, not moving my foot on the gas, it will jerk ever so slightly forward every couple of seconds, as if getting some extra power. The issue again is much more visible with the engine cold, but even properly warmed up you can feel it. It is stronger when accelerating than at constant speed; on acceleration the car really stumbles some times. When the engine is cold it can be pretty bad, enough that some times I worried if I would be able to climb a hill. I first thought it was ignition related, but after going through everything electrical I doubt it is this. (Replacing the spark plug, points, cap, wires and rotor changed nothing.) The car also seems a bit underpowered compared to my other one.

I am starting to wonder if the two things (the rattle and the jerking) are not related, and if it could be a valve issue. By the way, once or twice in the span of an hour's worth of driving I thought I heard a mild backfire.

Any suggestions on what could be the problem? Any help will be greatly appreciated.

Andrei.

[andreic]

Ok, things are weirder than I thought. After going for a drive yesterday, this morning I looked at the exhaust pipe that coils around going to the muffler in the back of the car (I have an aftermarket muffler, possibly a Bursch). It was originally painted black, but now all the paint appears to have flaked/burned off, and there is some chalky residue on the pipe.

Also today I tried starting it again, to warm it up in order to check compressions. It ran very badly until it warmed up, barely idling, and occasionally misfiring, even with my foot on the throttle; it had almost no power even going down hill. After it warmed up it was slightly better, but still running uneven and almost dying at idle. After I shut it down it did a very loud backfire (it being dark, I even saw the burning gas coming out of the exhaust!).

I did not try to run it much, but did the compression test, which was fine - 120-125 psi on all cylinders, very even.

I am puzzled, because to me this sounds like it could be a timing issue. I thought I had adjusted the timing right. I will double check it again tomorrow, after I get a new timing light (my old one died right after I had got the timing to what I thought it should be). Or could it be running way too rich/lean?

One other odd thing. As the engine is running, especially at idle, I hear a loud hissing noise, as if air is being aspirated through a tight channel. My current guess is that it comes from the throttle body, and I am hearing it because I am running the engine without the air cleaner (all I have is the rubber piece coming off of the throttle body, and that is not connected to anything). Could this be right? The noise seems to come right from the intake manifold (the common black metal cavity from where the 4 intake tubes go to the cylinders).

Thanks.