Hi All, experiencing a tricky problem and I’m worried I’ll blow the engine if I don’t figure out.

73 1.7, fly cut heads, SSI/aftermarket exhaust, mostly stock otherwise.

Core Symptoms:

- Idle rpm inconsistent. Steady, but it seems to keep changing its mind on whether it’s 400, 1000, 1500. Decides again after you’ve been driving and slowing down for a stoplight or something. Gets a bit better when warm.

- If I floor it, when I change gears the RPMs continue to zoom to redline levels. Can’t say for sure it’s an air leak type of surge, might be sensor or linkage (investigating next) based. Does not seem “floaty” like air if that makes sense, more hard.

I’ve owned the car a long time and it general it runs great. But this has been an issue all summer. Just replaced exhaust and shored up SSI. Pretty Thorough review of vacuum lines, points, sensors, timing, etc. Swapped in another MPS.

Looking for ideas here, tonight was scary with the RPM revs.

Will update as a evaluate more. Thank you are any help!

Drew

Sounds like a vacuum leak. Are we to assume it is still FI?

Yup, stock D-jet. Have put in a good amount of time looking for leak spots, have not come up with anything thus far. Propane and eyeballs/wiggles. I’ll take suggestions on locations for sure!

What would fly cut heads and ssi exhaust have to do with any of this?

Idle hunting with djet is almost always a vacuum leak or lean condition. Have you checked valves and adjusted timing, and was that all good? If not, do it first. Gotta start with a foundation.

Throttle cable sticking... return spring weak/broken?

double post

No, sticking cables don't cause hunting, they cause sticking.

Just brain dumping config to expose all potential variables and try to give you guys all info upfront (as a courtesy).


Return spring looks okay but the whole linkage setup looks worn/old enough investigate and rule out.

Timing and other foundational stuff done but will check again.

Sounds like the MPS is going south.

I would agree to first check for a sticking throttle cable. THEN

Check the vacuum retard/ advance, and the advance weights

check the decel valve

check the aux air valve

check in general for vacuum leaks, most probably a split in a vacuum elbow, or a hose directly connected to the intake manifold

Hey Geo, how does a cable cause hunting idle? Sticking for sure, but hunting is not mechanical which would stick. Guess what, we wasted that minute and are back to vacuum. And bonus points for LEAN.

He said the idle was steady, not hunting, but that it idles at varrying rpms.
That could be a weak throttle body return spring or sticking throttle cable, among other things. When it idles high, can you bring down the rpms by manually pulling on the throttle cable in the engine bay to ensure the butterfly is closed?

Also, what is meant by ‘if I floor it when I change gears the rpms continue to zoom to redline levels’?
I would expect the rpms to go up when you floor it.

Good catch Robert. I was thinking hunting. Dang he edited! LOL
If it doesn't hunt then is is likely mechanical.

Thanks guys this is helpful. Looking to get back into it today.

I meant when I accelerate hard and release gas to shift it was like I still had the pedal down. And yeah the way the idle changes it does seem like it could be cause the throttle plate not closing all the way.

I’ll report back. Very much appreciated!

If you disconnect the throttle cable at the throttle body, does the cable move freely without any binding?

I've done another round of investigation, here is what I've found. In summary, it appears to be a combination of both the cable issue and an air related leak:

- Went through cable from pedal to throttle body. After doing so, cannot reproduce the "redline RPM with pedal released" condition. So that's good.

- Timing is fine. New points and condenser.

- I am still experiencing an issue with having two idle different idle speeds. Link to a video showing the issue is below. Car will have a very low (400 rpm) idle. If I drive the car OR sit in neutral and hold the car steady at 3500 RPM, when you release the pedal the idle will settle at 2100 rpm. If I put the car in gear and drive at low RPMs usually the idle will reset back to the 400pm level.

https://www.dropbox.com/s/1dahe0hdhb3hr1d/IMG_1002.MOV?dl=0

- If the idle is at the 2100 pm level and I yank/wiggle the thottle cable, no effect.


- Went through all the air hoses again and checked for leaks. Used propane and starting fluid plus visual check. AAR is working fine. Next step is to try and bypass the decel valve.


So getting closer and the car is drivable again but something is clearly wrong and elusive. Thanks again for all input!