Idle adjustment question, Idles high or slowly stalls Options

[JamesM]

Now I just need to figure out which way to go. I definitely don't want to downgrade to carbs. I also really don't want to have to replace the cam, since the engine was rebuilt only 300-500 miles ago, and it runs very strong over 2000 RPM. @JamesM has told me some enticing details about going MS3, so I'm waiting to hear back more from him about that option.

Just now seeing this thread.

My initial thought as to the high idle that dies when you close my brain immediately jumps to timing. To much advance kicks up the idle speed and by the time you close the bypass screw you have cut off to much air to keep it running.

The vac retard at idle normally pulls down the idle speed so would make sense that if you don't have proper vacuum its not pulling enough timing. Might be able to account for it with a different map on the 123 (or just manually cranking back the timing)

But with so many unknown variables I would say Megasquirt is the "Easy Button" though some custom tuning may be required to accommodate whatever this engine wants.


FYI one of my personal 914s with Megasquirt, big valve 2056 with an EMW "J" cam (307 duration) idles like stock ~900 rpm even with a highly lightened flywheel.

Stock idle RPM anyways, with the cam and headers it has a wonderful lump to it.
https://youtu.be/O5JkdbyhWMc

[BillC]

<sigh> The mystery continues.... (IMG:style_emoticons/default/headbang.gif)

So I jacked the car up, pulled the valve covers and spent waaaaaaay too long trying to measure the cam lift. I finally figured out how to put my dial indicator on the pushrod end of the #3 cylinder exhaust rocker (couldn't get the indicator on any other rockers, it was just too big).

Anyway, I measured .280" lift at the pushrod end of the rocker. If you assume a 1.3 rocker ratio, that gives a valve lift of .364", which is right in line with the specs WebCam publishes for the stock cam. So, it looks like the shop installed the correct stock cam, as best I can tell without dropping and disassembling the engine.

Since several people mentioned timing, so I checked and confirmed the marks on the flywheel and the cooling fan. And, they all line up -- "0" shows on the fan in the little notch inside the fan shroud when the notch in the flywheel is lined up with the case seam.

Next chance I get, I'll check the timing in case it is off.

[Superhawk996]

FWIW - Djet (and general plenum vacuum) isn’t going to care much about lift per se. What really affects it is the duration & overlaps which you can’t measure without a degree wheel and lots of angle data and careful plotting of the lift for both intake and exhaust at those various angles.

A stock cam would be great news though based on lift you did measure.

Have you had the chance to remeasure plenum vacuum with a reliable gauge?

[BillC]

I spent some time playing with the car this afternoon -- I backed it out of the garage and worked on it in the driveway.

I checked the timing, and it was about a degree advanced, but it wasn't off much. Just to confirm my process:

• I removed the vacuum hose from the throttle body and plugged the port.
• Then, I had my wife rev the engine to 3500 RPM and hold it there.
• Then, I set the timing light to 27 degrees and turned the distributor until the "0" on the cooling fan showed in the notch inside the shroud.
• Afterwards, I locked down the distributor, reconnected the vacuum hose and reinstalled the plug on the cooling shroud.

And so, the car was back to idling happily at 1600-ish RPM. I pulled the small vacuum line off the decel valve and hooked up my tested vacuum gauge, and it showed about 10" of vacuum. I slowly started turning in the idle bleed screw to bring down the RPM. I got it down to 1050-1100 and it was pretty stable, but idling at 7"-8" of vacuum, which just seems low.

At this point, I buttoned everything back up and took it for a quick test drive. Got about 1/2 mile down the road to the first stop light, and the car was idling at 1600 RPM again. I didn't change anything! Drove for a couple more miles and went home, but idle stayed at 1600-ish. I had a little bit of light-throttle stumbling at 2100-2300 RPM, but otherwise the engine felt good and ran well.

I don't know. I can make the car stall by closing the idle bleed valve, so there aren't any huge vacuum leaks. And, as the idle slows, the vacuum gets less (i.e., gets closer to atmospheric) whereas I would have expected it to get higher (i.e., closer to perfect vacuum). I'm still wondering if it's the cam, since too much overlap could explain the low vacuum at low RPM.

[emerygt350]

Did you check the timing when it was at 1600? If your weights are sticking you will see it far more advanced than it should be. The 10 is concerning.

I would:
1) check the timing when it sticks at 1600.
2) if the timing it not overly advanced (anyone know what it should be at 1600? I would guess somewhere around 17) than I would put the mighty vac on the mps and see how the engine reacts to 17inhg.
3) if that brings the idle down to where it should be (ish) than I would do a crusty style valve adjustment http://www.914world.com/bbs2/index.php?showtopic=28758

I would then check my vacuum again. If it's still at 10, compression and leak down?

This is all assuming the lift rules out the aggressive cam.

If you think it might be the aggressive cam, just pop the mightyvac on the mps, give it 17inhg or more and start the car. See if it idles where it should be.

[BillC]

Did you check the timing when it was at 1600? If your weights are sticking you will see it far more advanced than it should be. The 10 is concerning.

I would:
1) check the timing when it sticks at 1600.
2) if the timing it not overly advanced (anyone know what it should be at 1600? I would guess somewhere around 17) than I would put the mighty vac on the mps and see how the engine reacts to 17inhg.
3) if that brings the idle down to where it should be (ish) than I would do a crusty style valve adjustment http://www.914world.com/bbs2/index.php?showtopic=28758

I would then check my vacuum again. If it's still at 10, compression and leak down?

This is all assuming the lift rules out the aggressive cam.

If you think it might be the aggressive cam, just pop the mightyvac on the mps, give it 17inhg or more and start the car. See if it idles where it should be.

The distributor is a brand-new 123Ignition electronic distributor. So, no weights to stick.

I did a quick check of the valve adjustment while I was under there trying to measure valve lift. The valves were all fine.

I'll try a compression/leakdown check and then the MityVac on the MPS when I get a chance. But, it'll be next weekend at the earliest.

[Superhawk996]

I did a quick check of the valve adjustment while I was under there trying to measure valve lift. The valves were all fine.

I'll try a compression/leakdown check and then the MityVac on the MPS when I get a chance. But, it'll be next weekend at the earliest.

Don’t take this the wrong way but we recently had someone that was adjusting valves too tight causing starting & running issues due to low vacuum. Adjusting them so that only the slightest of drag is on the feeler gauge? Checking again after the jamb nut is tightened to ensure they didn’t tighten up when the jamb nut was tightened?

Compression and leak down are valid next step and will turn up issues if valves aren’t adjusted properly. Leak down would be preferred so you can narrow down the source of leakage. Leak down is another infrequently used specialty tool that not everyone has and takes a little practice to use properly so understand if that’s not an option.

[BillC]

Don’t take this the wrong way but we recently had someone that was adjusting valves too tight causing starting & running issues due to low vacuum. Adjusting them so that only the slightest of drag is on the feeler gauge? Checking again after the jamb nut is tightened to ensure they didn’t tighten up when the jamb nut was tightened?

Yes, and yes. I understand why you are asking, and it's good to check, but this isn't my first rodeo, and I had no problems with my previous 914 (a 1.7). For this car, I set the valve clearances to .006" intake and .008" exhaust, which is spec for a 2.0 (from what I found).

While I was under the car, I also confirmed that I have stock aluminum pushrods.

Compression and leak down are valid next step and will turn up issues if valves aren’t adjusted properly. Leak down would be preferred so you can narrow down the source of leakage. Leak down is another infrequently used specialty tool that not everyone has and takes a little practice to use properly so understand if that’s not an option.

I have a compression tester at home, but I don't think it does leakdown. I'll check when I get home and order a leakdown tester if I need one. Any excuse for more tools! (IMG:style_emoticons/default/cheer.gif)

One thing I forgot to mention earlier, is that whenever I disconnected a vacuum hose for testing, the idle would jump up until I either plugged the port or connected the gauge. So, that also seems to rule out a vacuum leak.

I also made sure the AAR was closed before I did any testing.

-------------------------------------

Edit:
I wonder if I might have a valve seat coming loose. Leaking around the valve seat might explain why I can get an okay idle speed while sitting in the driveway, but it jumps way up once I drive it, which has happened to me both times this scenario occurred. Yeah, I'm reaching, but running out of ideas....

[Superhawk996]

Edit:
I wonder if I might have a valve seat coming loose. Leaking around the valve seat might explain why I can get an okay idle speed while sitting in the driveway, but it jumps way up once I drive it, which has happened to me both times this scenario occurred. Yeah, I'm reaching, but running out of ideas....

There are still plenty of other things it could be . . . Not many good . . . But do the compression / leak down / MPS vacuum test before we start thinking about the uglier possibilities

[emerygt350]

Ahh, forgot you had the 123. You are not using the advance feature while troubleshooting right?

Any vacuum leak, even a leak added to an already leaking djet car will cause the idle to rise. You have tried plugging everything right? This has been going on so long that I forget what has been done.

I would do the mps vacuum first since it is so easy.