Supernaut72

'67 425 2-bbl low comp

Recently got a dial-back timing light to replace my standard light that burned out, so I've been attempting to really nail down my engine's tuning.

The whole timing/tuning process went very well until it came to setting idle speed in Drive, not the first time this has been an issue. I set it to 600 RPM like the Good Book tells me, but when I shift into neutral it soars to 1000 or maybe 1100 RPM, completely unacceptable. So, I have to lower it to 900 RPM in neutral and settle for the resulting 525-550 RPM in Drive, though unfortunately that won't be enough as we get deeper into AC season...

To keep it short I won't go into the whole troubleshooting process, but after ruling out every other component I believe I've narrowed it down to an issue with the vacuum advance system. Here's what I've got:

-I'm using a B1 vacuum advance can, rated for 16* advance at 16" or greater of vacuum (confirmed).
-In current tune, engine pulls a bit over 14" vacuum in Drive.
-Interestingly, with the same a/f mixture it peaks at just a hair under 20" vacuum in Park or Neutral, not too shabby for 2600' elevation I guess.


Now, a B20/VC1765 vacuum advance can offers the same 16* advance, all-in by 12" of vacuum. It should work out so that even in gear the engine would be able to pull it to its full advance stop with a 2" buffer zone, meaning it wouldn't be able to throw in extra advance and affect the idle speed so drastically when I shift the car from Drive to Neutral. That should be perfect, but I want to be sure I'm not missing something here before I order one from NAPA.

Thicker oil to help seal the rings better while under load is another possibility I've considered. The engine does have 126,000 miles on it and is running 10w30, but it doesn't consume or dilute this weight of oil excessively, foul spark plugs, or emit smoke. Regardless, 10w40 (or even 20w50!) is on the checklist for summertime since I figure 10w30 + hot weather + lots of idling in our terrible, uncoordinated traffic system = bad news.

Thanks!

Gary M

Mine does that too. I'm thinking it's normal because when you put it in gear your engaging the transmission (a load). When you turn on the A/C it drops the rpm's even more (another load). Others will confirm or dispute.

Fun71

If you have your vacuum advance canister connected to ported vacuum it won't matter which one you use as there should not be any vacuum at idle (and therefore no advance). If you have it connected to manifold vacuum, then it may help your issue.

It sounds to me as if you may not have the carb adjusted correctly. The only things I can think of that would cause that issue are carb not adjusted correctly or timing not set correctly, and it sounds as if the timing is correct.

My engine had a similar large RPM drop from neutral to drive, but after dialing in the timing there is maybe a 100-200 RPM drop. What I now have is the HEI set up with ~18° initial, factory weights/center bar, one light and one medium spring.

Gary M

Fun71, was your RPM drop problem solved by replacing those weights/springs?

MDchanic

Alright, if I may, let's go through the basics first, to be sure we're all on the same page.

When you checked and set the timing, the vacuum to the diaphragm was disconnected and plugged, right?

Is the diaphragm connected to ported or manifold vacuum?

Have you confirmed the action of both your mechanical and vacuum advances with your new timing light?
Have you checked mechanical advance from very very low RPMs all the way up until it stops advancing?
Is there mechanical advance very low, say between 500 and 1,000 RPM?
When is the mechanical advance "all-in"?
When does your actual vacuum advance begin and finish advancing, and how many degrees does it advance maximum?
How do these readings compare to the original specs?

Do you have an original or aftermarket cam?

Is your carburetor idle running mostly on the idle jets, or mostly on the throttle butterflies?

Once we've got all of that information, it will be easier to make recommendations.

- Eric

Fun71

Yep. I had installed the lightest springs from a Moroso HEI advance curve kit and it was giving advance at idle RPM. Going from neutral/park to drive made the idle RPM drop, which then caused the advance to decrease, and the idle speed decreased even more. Once I got the advance out of the idle RPM range things got better. Having the initial at 18° helped a lot as well.

Supernaut72

Gary, I agree there will certainly be an RPM drop when shifting into gear and putting a load on the engine, but 1100 to 600 seems a tad extreme to me. I'd even be happy with 900 -> 600 with AC off, but as it is, idling at 900 RPM in Park it likes to idle at 550-575 tops when in gear, which means it drops down near 500 RPM with the AC on... Good recipe for poor cooling and engine overheating when the going gets hot. CSM specifies 550 RPM in Drive for non AC cars, but specifically says to add 50 RPM to an AC-equipped car, which is a good number since that's about how much of an idle drop the A6 compressor incurs.


Kenneth, I thought so six months ago, but these days I'm not sure that it's a carburetor adjustment issue... It sure sounded like a lean RPM drop, and so my "solution" was to enrich the living hell out of the carb (4 turns of the mixture screws or so, I think it was). It did work! ...But for all the wrong reasons.
Observing that super-fat mixture with a vacuum gauge, all I was doing was choking the engine out to the point that the vacuum signal dropped when in Park... Which was either canceling out the vacuum advance or else just loading the engine up with excess fuel so it idled slower. There are no vacuum leaks causing this. If I leave vac advance disconnected, the RPM drop is totally normal, only 150-200 RPM or so IIRC (a little bit more with the AC compressor engaged).

Currently it's tuned about 1/8 rich from the point of highest manifold vacuum (highest RPM was found there with vac advance disconnected).

Interesting that the Moroso kit was causing that issue for you. I'm using stock weights and springs, but I've run into that issue before on another car with a weight/spring kit. Initially I thought that was happening here, but that's what several hours of troubleshooting ruled out yesterday.


Heh. Funny enough, I included just about all of this information in the original draft of the post, but decided to omit it for brevity. Basically, yes to everything.

Yes, vacuum advance was plugged while setting timing.
Vacuum advance is hooked to the port on the carb that is specified in the CSM, which provides full manifold vacuum.

At first I suspected the mechanical advance, and it was indeed a little bit sticky. However, I cleaned and lubricated it, and verified that it's now coming in when it's supposed to, and at the rate outlined on OldsFAQ for a '66 425 (the nearest data I could find to my specific distributor). Set timing to 7.5* @ 600 RPM and again at 450 RPM to be as sure as possible that the mech advance was not interfering, timing didn't change between those RPMs. Problem still persisted practically unchanged.

Can't recall when mech advance is all-in, I knew I should have been writing this down but didn't have a pen handy. It was somewhere over 2,000 RPM, I know that much, so it's not going all-in too early. In any event I've tuned and tuned and tuned with vacuum advance disconnected and the engine is happy before I hook it up, though I would really like to retain vacuum advance for overall drivability.

The vac advance's specs only provide info for 8" and 16" of vac (0* and 16*, respectively). I've confirmed with the light that it does advance right around 16* when fully engaged in Park. I'm pretty sure I also measured advance with it hooked up while idling in gear, but if I threw out a number without re-checking it would just be a guess. It's been a long couple of days including a lengthy hunt for an advance spring that decided to jump overboard into the dirt, so my memory isn't as sharp as I'd like.
No idea on which vac advance unit was originally specified for this car, I've been trying to hunt that information down but haven't found anything specific to my car.

Original cam, engine is completely stock. It's tuned thoroughly, I've gone through and adjusted everything to factory specs right down to the linkage adjustments on the carb. Runs like a top, generally. It's just this idle drop issue I keep running into.


I guess it all comes down to whether or not the vacuum advance is intended to be fully deployed while idling in gear. My ASSumption is "yes", but that's why I'm asking here.

Supernaut72

Update: Bit the bullet and bought a B26 (identical specs to B20), part #DV1808, from Autozone and installed it. With the carburetor settings left untouched, it only dropped to 590 RPM when put in gear. Leaned it out a little further to 3 full turns on each screw, set to 900 RPM, and the idle peaks at just a hair more than 900 after the air cleaner's installed, so it seems happy with that.

The exhaust is much less acrid now, and I definitely detect stronger throttle response when lightly goosing the throttle to get the car rolling. The idle seems stronger and smoother with AC on or off.

I re-checked timing before the swap and found that putting the car in gear would drop it all the way to 17* and it would dither around there. Now the advance in Drive is right about 23.5*, so there's a major difference with the new vacuum advance unit. Advance in Park is of course still around 27*, with 3.5* or so of mechanical advance coming in at 900 RPM, fitting neatly within the spectrum of the specification I found on OldsFAQ.

I could probably even reduce the idle RPM gap a little bit more by going to tighter centrifugal advance springs to knock out that 3.5* in Park, but if 900 RPM was supposed to provide 3* to 5.5* centrifugal advance in a HC 1966 425, I don't see why it would be different for a LC '67 and I'll probably just leave well enough alone. It makes enough sense that they designed it that way, the engine would probably like a tiny bit more advance while idling with minimal load as opposed to idling in gear with the full load of the torque converter.

For that matter, I'm not sure where I'd actually find tighter springs so that's just one more reason not to bother. The Mr. Gasket kit I had in my Chevy's glove box only had one set of springs that was of roughly equivalent weight to the stock springs, the other two pairs were lighter.


Still only seeing a little more than 14" vacuum in gear, so that's probably about as good as I'm going to get with this oil/engine combo at this altitude. Going up to 10w40 may help, it may not. The oil is still looking good after 1500 miles and we won't be hitting 100 degrees for another couple of months, so I guess I'll hold off for now and report results when the time comes for that.

Fun71

Lucky you. We're supposed to hit 100 this weekend.

oldcutlass

See if you can drop your idle down so the engine idles in gear to 550-575, closer to 550 if you can. Your engine calls for 5* at 850 with vac disconnected, how ever I doubt the 2.5* will matter much. More importantly is what is the total timing with and without vac advance and at what rpm does it stop advancing. Should be 34*ish without vacuum and with your 16* vacuum can 50* with. I don't know how your 2400ish altitude combined with temp will effect your tuning.

Supernaut72

Yikes. Start tearing up some of the concrete and asphalt as a public service, that might keep things a little cooler. We have a more passive policy down here, we just let our asphalt biodegrade instead of maintaining it, hence our lower temps. It has nothing to do with elevation or anything.

So you think 550-575 would be sufficient to keep everything happy with the AC running? If so, fantastic, I like tame idles but without an idle kicker solenoid like later cars had, I've been worried about AC head pressures and engine cooling if I put it closer to the 550 RPM non-AC specification (which = 500-ish with compressor on). It's only an issue because almost invariably, I'll be idling at a dead stop 50% of the time when I drive anywhere. I do have an HD fan clutch that moves a helluva lot of air, so I guess it could work out.

As for the timing, I went with 7.5* only because my Chassis Service Manual actually specifies 7.5* for a low-comp 2-bbl 425 and 5* for the hi-comp 2-bbl. Even if The General made a typo, yeah, it's only 2.5* so probably not affecting much in and of itself, particularly in a low-compression engine. As for total timing, I'll have to spend more time giving it a good run-up with the timing light next time I run the engine, I believe I measured 32* or so in total advance, which would put me around 48* with the vacuum in at cruise. If I'm not mistaken about that, this would indicate that the engine might even like a couple more degrees of base timing to hit that 50* sweet spot for cruising, but we'll see what I find.

oldcutlass

Tinker with it, remember the factory settings are a recommendation on the conservative side. There is always room for improvement.

Supernaut72

Yeah, good point. I recall you telling me a while back that as long as the total is where it should be, wherever the initial falls is "correct", so I guess we'll see what it likes best now that I've figured out my idle drop problem. Regarding the idle, should I be setting the idle speed with the AC on or off, in this case? With it on, I think I'm right around 550 in gear currently.

Only other thing to figure out before summer hits is whether I've got a bunk hot idle compensator. Old one was so fatigued that it opened whenever the engine got to regular operating temperature, meanwhile I've never observed the new one open up and can't find info on what temperature it's supposed to open up at. Willing to bet I've just never had the engine hot enough that it needs to open up, but perhaps that's an issue for another time.

Fun71

Unfortunately, the EPA has designated "dust" as airborne particulate matter (pollution) that must be controlled/reduced, and we are held to the same air quality standards as other areas of the country, so every road and parking area in the county has been paved. I guess the rationale is if you pave over the entire desert, then there won't be any airborne dust.
EPA = buncha morons in this case.

Kennybill

(I set it to 600 RPM like the Good Book tells me, but when I shift into neutral it soars to 1000 or maybe 1100 RPM, completely unacceptable.) Is the switch for the "Switch-Pitch" torque converter working properly?

Supernaut72

Haha you've gotta admire the EPA's commitment to their cause. They will not be satisfied until everyone and everything, including nature itself, stops polluting our air.


Hey Ken, yep, switch pitch linkage is properly adjusted and is definitely working correctly. Even at 600 RPM there's very little idle creep, and when I roll into the accelerator lightly there's a definite shift as it goes into low-stall mode. If I floor it, it remains in high-stall as intended and the engine is allowed to wind up a lot more. Remarkable system for its time.

The issue is resolved, thankfully. Idles at 900/600 in Park/Drive, respectively, though on Eric's suggestion I might still reduce it to ~850/550 and see how it does with the AC on. I've driven the Olds to work the past couple of days and there is so much more "pep" off idle that it feels like a totally different car! Plus no more raw fuel odor lingering all around the car when it runs.