After a whole summer I'm still trying to get to the bottom of a throttle body mystery. My 1971 1.7 liter engine is quite clean, and I have installed all new vacuum lines, installed a Pertronix electronic ignition unit, cleaned up the distributor, replaced the spark plugs plugs, cleaned the K&N air filter, etc. I'm having some irritating issues that I'll list here:

* Its hard to maintain a constant speed. If I let off the gas a tiny bit while trying to maintain, say 30mph (or really, most any speed) the car sputters and stumbles and jerks. If I quickly let off the gas all once, its much smoother.

* At idle, the engine hunts and lopes. It just bounces from 500-1000 rpms over and over.

* While accelerating or driving at constant speed I can feel the car lightly stumble and hesitate.

* While accelerating its not totally smooth - almost like there is a little more power that it is hesitant to give.

I suspect that at least some of these issues are throttle body (throttle valve) related. I've pulled mine out, cleaned up and re-adjusted the throttle position switch (twice, just to be certain). Running my finger through the inner bore of the throttle body feels smooth, and after a good cleaning I ran some really fine sandpaper around just to make sure.

After doing this it made no difference at all.

Here's my last clue. I have a heavy return spring on the throttle body. With this spring the gas pedal is a bit too stiff. But if I use a lighter, more comfortable spring, the idle surges and won't return to normal unless I force the valve to close tightly by hand. This is what makes me think something is up with the throttle body.

Interesting side note: I found that my softball-sized vacuum sensor (MPS) could not hold a vacuum. I bought another one from Camp 914 (thanks Craig!) that does hold a vacuum. Strangely, this made no difference either.

I'm confused and irritated. Help?

What condition is your throttle body seal?

Could be a vacuum leak, poor setting on the ECU richness wheel - but I am not sure of the 1.7 ECU having one, a bad head temperature sensor, bad fuel pressure at the FPR 29-31 lbs works, dirty fuel lines, rust in the tank, or possibly a shorted out Pertronix, but I have no experience with the last item, I have only read that they can go bad on install due to shorting them out. You could also have a possible bad ground, but I doubt it.

I have had ALL of the other problems outside of the Pertronix and no bad grounds.

Every one of them can cause acceleration issues, idle issues, and generally bust your chops big time. Go through each one. It is the only way, then get someone to tune it for you if you need to, but I am not sure about tuning with Pertronix. Does the dizzy sit on the TDC 1 hash mark on the metal where it meets the dizzy cap?

I had 2 months of headaches, and they just went away and something new and unrelated is a problem that I need to figure out.

Good luck!

John

Thanks for the responses. I don't believe the Pertronix unit is at fault because the old points behaved the same way. Also, the Pertronix units either work or don't work - there's no middle ground. Also, the early 1.7s don't have a mixture screw or knob on the ECU.

It's possible that the fuel lines are imperfect or that there's junk in the tank. I took off the gas cap and looked in with a flashlight and it seems okay from the surface. What isn't okay is that recently the boot smells of gasoline near the tank, although not in the cabin.

I changed out the fuel filter a couple of weeks ago. Where would I spike into the fuel system to measure fuel flow?

Also, the throttle body gasket isn't very good. It's mostly a mish-mash of thin gasket material and silicone. It seems to seal well enough, but who really knows? I'd love to buy another but nobody produces them.

It doesn't sound like a throttle body problem to me.

In my experience (which is limited), problems with tuning show up most often at idle and light throttle. You seem to be having problems with both of these things so I think there is something else that needs to be fixed (although a bad TB seal might be partially responsible).

I would start with basics - plugs, wires, cap, rotor, ignition timing.

If that all checks out ok, look for vacuum leaks. Adjust TPS.

Check fuel pressure. Have injectors checked out.

I always start with ignition.

Good news is that Pat got me on making the throttle body gaskets, these will be available in about a week. I'm starting a thread as a prebuy.

lean hunt at idle is caused by vacuum leaks so check for those first...new injector seals, replace vac hoses, intake manifolds to plenum seals etc...

Same things were happeing to my car , turn out to be bad throttle body leak, put spare throttlle body and new trigger points. car runs better than it ever did.

If it is not a throttle body or vacuum leak, there is a way to check fuel pressure on the drivers side.

Pinch off the hose going to the 5th injector. Put a fuel pressure gauge in that tap.

Adjust the FPS at the driver side rear of the engine. 13MM and 10MM nut and bolt.

Go to 30 psig. May clear up the problem.

I was running 50 and had to replace the FPR with a working one and got right to 30.

Lots of issues went away.

John

Fifth injector? I don't know what that is.

fifth injector is also called the cold start injector. located on the intake plenum two wires go to it, it only activates near freezing temps via a thermal switch that is located near the distributor. if you dont live in a cold climate , it should not be active. it really has to be cold to activate.


have you done a complete test for vacuum leaks yet? sniff for leask with starting fluid or propane, check fromt eh manifold seal at teh heads up to the throotle body.


good luck