Idle adjustment question, take two, Idles high or slowly stalls Options

[BillC]

The old thread asking this question got too long and disjointed, so I figured it would be best to start again with all the data stated concisely and in one post.

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The problem: I can't get my car to idle reliably under 1500-1600 RPM. If I adjust the idle bleed screw to lower the RPM, the engine will slow down and stall.

The car:

• 1973 2.0
• Engine was rebuilt a few hundred miles before I bought it, and I've put less than 100 miles on it since then.
• Cam is unknown. I called the shop that did the rebuild, but they don't remember what cam they put in it. They thought it might be a WebCam, but I called WebCam and they don't have any records for this shop buying a cam around that time.
• Factory D-Jet fuel injection. Correct ECU, MPS, CHT and ballast resistor. New FJ67 fuel injectors. New fuel pump and fresh gas. Fuel pressure is 29 psi.
• MPS holds vacuum all day, no problem.
• All vacuum hoses and fittings have been replaced with new ones.
• Freshly rebuilt throttle body, and TPS adjusted properly. New circuit board installed in TPS.
• New 123Ignition distributor, currently set to setting "1", which is listed in the manual as being the correct position for this car without vacuum retard.
• Valve lash adjusted to .006" intake and .008" exhaust while sitting on engine stand, shortly before the engine was reinstalled in the car (so less than 100 miles ago).
• The car drives well at all RPMs and throttle positions, except idle. However, drivability is a little better on setting "A" with vacuum retard connected than it is on "1", but leaving it on "1" for testing (without vacuum involved in timing, it's one less variable).

The symptoms:

• At 1500 RPM, the car idles about 11-12" vacuum and 7 degrees advance.
• At 1100 RPM, car idles at 7" vacuum and 4 degrees advance.
• At 850 RPM, car idles at 3-5" vacuum and 1 degree advance, shortly before it stalls.
• Car idles noticeably rich below 1200 RPM and is very rich at 900 (by smell).
• If the car has been running less than 20 minutes, it will idle (poorly, but stays running) under 1100 RPM. However, after being driven/running more than 25 minutes, it just wants to slow down and die unless idle speed is kept above 1500-1600 RPM.

Diagnosis work:

• I hooked up a smoke machine and verified there are no vacuum leaks.
• I can make the engine stall by turning the idle bleed screw in.
• Performed a leakdown test with the engine completely warmed up by a 30 minute drive. Leakdown was less than 5% on all cylinders.
• If I connect a MityVac to the MPS and set it to 17" vacuum, the idle smooths out and the exhaust cleans up (doesn't smell rich).
• If I create a vacuum leak by pulling a vacuum hose off the intake manifold, RPM goes up several hundred RPM. Replacing the vacuum hose makes the idle go back to where it was beforehand.
• Timing advance is being measured by adjusting the timing light to put the "0" back in the notch in the fan housing.
• I have a new vacuum gauge, and confirmed that it matches the gauge on the MityVac.
• I put a dial indicator on the #3 cylinder exhaust rocker, and confirmed that this rocker has approximately the same lift as stock (.364" at the valve). Couldn't check any other rockers, since there is no room to put the dial indicator on any of them without dropping the engine.

Current diagnoses/hypotheses:

• Cam not compatible with D-Jet?
• Hidden vacuum leak that only shows up at low RPM?
• Others?

Suggestions?

• Convert to modern fuel injection?
• Rebuild engine with correct cam?
• Convert to carbs? <-- No, not gonna happen
• Go back to a bicycle?

[Superhawk996]

OK - I’ll copy and paste unanswered questions from the old thread.


If I were you, I’d be going back and questioning all my assumptions.

As I re-read this thread there are lots of assumptions being made about fuel mixture (no photos of plugs - are they rich or lean?).

Have you verified the orientation of the dizzy drive? There are 12 teeth on the dizzy drive - each amounting to 30 degrees of advance or retard depending on how it’s installed. Have you verified that the “0” TDC mark on the fan is actually TDC cylinder #1 compression stroke?

Likewise - as the vehicle is spiraling toward stalling have you tired zapping it with starting fluid and what happens?

Has it been confirmed that when the engine is hot that the valves still have some clearance and valves aren’t being held open forcing the loss of vacuum at low rpms?

Even silly things like making sure the coil has a solid 12v even as the vehicle is losing vacuum and trending towards stalling. Are you 1000% sure you don’t have an ignition problem?

These things really shouldn’t be in question. Perhaps I’m just not caught up or missing details or things that weren’t documented here?

If all else checked out after re-verification of assumptions I might even pull the oil pump just to see if I could see any ID on the camshaft nose (not all do).

Again so sorry that we haven’t helped you get this resolved by now. I wish I had the magic answer in my pocket but simply don’t have enough info at this point. It’s gotta be very frustrating I’m sure. Keep goin, be data driven, and logical in the methodology (as you have been) and eventually this will get solved.

[BillC]

If I were you, I’d be going back and questioning all my assumptions.

I tried to make the initial post of this thread with no assumptions at all, and tried to state only facts.

As I re-read this thread there are lots of assumptions being made about fuel mixture (no photos of plugs - are they rich or lean?).

I pulled the plugs when I did the leakdown test. The plugs were black but not gunked up. So, it confirms the idle mixture is rich. Car drives too well for the driving mixture to be too far off.

Have you verified the orientation of the dizzy drive? There are 12 teeth on the dizzy drive - each amounting to 30 degrees of advance or retard depending on how it’s installed. Have you verified that the “0” TDC mark on the fan is actually TDC cylinder #1 compression stroke?

Yes, I did. I confirmed the mark on the fan using the chopstick-in-the-spark-plug-hole method, and it is right on. And, when I installed the dizzy, I adjusted the drive to match the 12 degree angle shown in the diagram (it was 1 tooth off from the previous owner).

Likewise - as the vehicle is spiraling toward stalling have you tired zapping it with starting fluid and what happens?

Have not tried that, mostly because I don't have any starting fluid. However, the engine responds immediately and revs up when the throttle is cracked open.

Has it been confirmed that when the engine is hot that the valves still have some clearance and valves aren’t being held open forcing the loss of vacuum at low rpms?

The leakdown test seems like a good confirmation that the valves are closing completely. There's no way I'd get under 5% leakdown on all cylinders if any of the valves were held open.

Even silly things like making sure the coil has a solid 12v even as the vehicle is losing vacuum and trending towards stalling. Are you 1000% sure you don’t have an ignition problem?

Everything in the ignition system is new. New dizzy, new coil, new plugs, new high tension wires. And, a failing ignition system usually manifests first at high RPMS and/or load, not idle. Even the battery is new, and I keep it on a maintenance charger when I'm not driving it.

If all else checked out after re-verification of assumptions I might even pull the oil pump just to see if I could see any ID on the camshaft nose (not all do).

Can't do that without pulling the engine, and I'm really trying to avoid that. Especially since I won't have time to do all that work before NEG6.

Again so sorry that we haven’t helped you get this resolved by now. I wish I had the magic answer in my pocket but simply don’t have enough info at this point. It’s gotta be very frustrating I’m sure. Keep goin, be data driven, and logical in the methodology (as you have been) and eventually this will get solved.

Yes, it has been extremely frustrating. This engine runs well, except for the idle. I really don't want to have to pull the engine and tear it apart -- I finally got it back together and running after sitting in my garage for nearly 5 years.

Gotta figure out why the engine isn’t pulling vacuum at idle.

Yes, and I have no idea why. I wouldn't have suspected the cam, except I keep running into comments about cams not being compatible with D-Jet. Unfortunately, none of these comments ever specify how that incompatibility manifests, so there's nothing I can point to that says "yes, this is it" or "no, it's something else".

At this point, I've pretty much run out of ideas. That's why I keep asking for help. The engine probably has less than 500 miles on it since the rebuild (odometer is broken, and the PO did not keep great records), so maybe it just hasn't finished breaking in yet. I don't know. The car barely ran when I received it from the shipping company, and I had assumed that was because they had messed up the installation of the D-Jet system (mismatched components, incorrect components, an open vacuum hose, etc. Details in the un-DAPO thread).

[Superhawk996]

Yes, and I have no idea why. I wouldn't have suspected the cam, except I keep running into comments about cams not being compatible with D-Jet. Unfortunately, none of these comments ever specify how that incompatibility manifests, so there's nothing I can point to that says "yes, this is it" or "no, it's something else".

At this point, I've pretty much run out of ideas. That's why I keep asking for help. The engine probably has less than 500 miles on it since the rebuild (odometer is broken, and the PO did not keep great records), so maybe it just hasn't finished breaking in yet. I don't know. The car barely ran when I received it from the shipping company, and I had assumed that was because they had messed up the installation of the D-Jet system (mismatched components, incorrect components, an open vacuum hose, etc. Details in the un-DAPO thread).

With all due respect I don’t think I can help.

You responded to the first post with a TLDR and then created this second post. Having to jump between two posts only makes it harder to help. When asked some additional questions to get you thinking about challenging your own assumptions what you posted were excuses and thought experiments justifying your assumptions.

Example:

Symptom: If the car has been running less than 20 minutes, it will idle (poorly, but stays running) under 1100 RPM. However, after being driven/running more than 25 minutes, it just wants to slow down and die unless idle speed is kept above 1500-1600 RPM.

This is basically indicating the engine runs better when cold and before reaching operating temp. Often indicative of improperly adjusted valves. Which will cause low vacuum. If the car is running as rich as you’ve implied, it aligns to the symptom that it runs better cold when the engine wants a richer fuel mix right at a cold start and would be less susceptible to valves that may be tight.


Your response:
“The leakdown test seems like a good confirmation that the valves are closing completely. There's no way I'd get under 5% leakdown on all cylinders if any of the valves were held open.”

I do realize you posted that you did leak down when warm after a 30 minute drive.

However, you could have a great leak down but have it go south very quickly when hot if the valves are misadjusted and lacking clearance when it’s really hot. You have never mentioned what temps you’re running after these short drives. You pushed back hard on this when asked about valve clearances in the first post as if there was no way the valves could be misadjusted. I want to take all that at face value - but even if had I done the valve adjust and leak down test myself I’d still double check for some clearance when hot and challenge myself given what you’re struggling with (low vacuum).

A leak down is not a be all end all test. A hot compression test would be likely be useful as a 2nd means to prove (to yourself) you still have good vacuum into the cylinder and good valve sealing when hot and it’s easier to perform hot than a leak down test when the engine is hot and uncomfortable to work on.

Leak down testing is also subject to variation depending on what your air source pressure is. A leak down at 25psi will often show much better results than a leak down done at 90-100 psi.

No mention of whether leak down was done on all cylinders or only one.

You also state:
“If I connect a MityVac to the MPS and set it to 17" vacuum, the idle smooths out and the exhaust cleans up (doesn't smell rich).”

This MPS response is another sign that a lack of vacuum is probably a big part of the problem and that you should be challenging your own assumptions that a 5% leak down is be all end all proof that you don’t have a hot valve clearance problem.

I also don’t think you can rule out a miscalibrated MPS or even a bad ECU. Likewise I don’t think you’ve ever posted your CHT resistances cold and hot. Maybe you’ve already done this and convinced yourself those aren’t part of the rich mixture issue?

You seem to be reluctant to post photos, full data, etc., that would prove your assumptions (plug condition, dizzy drive position, leak down test pressure, gauge pictures, etc.). I know posting pictures is a bit of a PITA but it’s what makes this site so useful for helping each other vs text only. Quite often when folks post photos that is when someone spots something that is amiss that everyone else has overlooked.

I don’t mean any ill will towards you and I can appreciate how incredibly frustrated you must be. I’m not posting this because I’m mad or because I think that I have all the answers. Honestly, I’m a little stumped and very curious about what is going on to cause the low vacuum (assuming it’s not the Cam alone). I would like to see you solve this and I gain zero benefit from it!

The reality is the vacuum is crazy low, you say the mixture at idle is rich. Thought experiment responses justifying your assumptions aren’t going to get you any closer to solving this.

Hopefully when you get to NEG6 you can hook up with some guys that can put eyes and ears on it to help.

I truly do wish you well and hope you find a easy(ish) solution. (IMG:style_emoticons/default/beerchug.gif) the only advice I can provide is to question the assumptions you’re making.

PS - Sorry this response is stupid long and may be another TLDR. I followed a great deal of your un-DAPO’d thread and you’ve done a great job with this car!! Keep going!!! (IMG:style_emoticons/default/smilie_pokal.gif)