Rough idle when getting hot

[delco1946]

That was a good point about the vacuum lines. Finally got around to testing yesterday. I crimped a bunch of vacuum lines, include the master Cylinder which is rebuilt so wasn’t expecting that to be a problem anyway. Finally pinched down the line where manifold vacuum comes from the engine to the carb. At no point did I ever notice a difference. So I’ll deduce my vacuum lines are the problem.

I’ll check my plugs today. I’ll check the pcv again. I get slight oil mist up from the valve cover breather. Wondering if the valve covers could be leaking vacuum too?

[viven44]

The valve covers are not under vacuum i.e. cause a vacuum leak.... You should be able to run the engine with the valve covers off (theoretically speaking because oil will go everywhere)....

Well technically... to elaborate... if you had asked that question with the PCV in mind... the PCV carb inlet pulls a decent vacuum on the valve cover... but again not enough to put the crankcase under vacuum... the breather on the other side will provide fresh air to balance any vacuum created there (or at least it should unless you stick a PCV valve on the other side... don't ask me how I know it.... )...

At any rate, your problem is probably isolated to the vacuum lines that are coming off the intake manifold, then running into the control valves into the thermostat and from there on to other various places.

[ArdWrknTrk]

Technically, the breather (L, driver's side) should be sucking -at slight amount of -wind.
This is your PCV at the rear of the RH valve cover being a metered vacuum leak, and clearing blow-by and oil mist/smoke from inside the engine.

I'd imagine if your PCV still rattles and the hose to the flange is clear, you have enough blow-by that the PCV can't keep up.

[delco1946]

Well,  it shouldn’t be vacuum related ( to carb/ emissions devices) since I pinched off the entire source of vacuum to all emissions related devices, and it made no change, (including thermostat controlled switches).

I checked all my spark plugs, and some didn’t look fabulous, however, I think they generally fit within a normal tolerance. I tightened down the valve covers, put a new gasket under the breather filter that mounts to the driver side valve cover.

However, I did just check my PCV at idle. It’s pulling a lot of suction which makes me think it’s no longer functioning? It’s not that old 1yr?), but it’s got full manifold vacuum going through to the valve cover, which, if I understand how the system works, it should be closed with that high of suction and open up as the vacuum gets weaker.

The engine ran noticeably smoother when I put my thumb over the valve and plugged it.

[ArdWrknTrk]

There's a check weight in there, so pretty much the opposite.
Engine has to be pulling enough vacuum to unseat it in order to draw from the valve cover.

Does the PCV rattle if you shake it?

[delco1946]

It did rattle when I shook it, yes.

[viven44]

Your PCV valve's behavior is normal....

This is starting to sound like Jeremy's... timing...timing..timing being off... read from pg.3
https://forum.garysgaragemahal.com/1986-460-Smog-Pump-Delete-td157977.html

Could you tell us what happens to idle quality if you hook up the distributor vacuum advance to manifold vacuum port instead of the ported vacuum port ? You can hook up either to the intake manifold directly or the manifold port on the carburetor.

If the idle improves drastically, then your initial timing needs adjustment

[mat in tn]

timing is a great thing to check. I think chasing pcv is going down the wrong rabbit hole. if you were to put a vacuum gauge to the pcv valve draw it should pull full manifold vacuum. that's why you can hold it to your thumb and it hold strong. its potential not flow. if you were to cap off the driver side, fresh air intake as a test then yes, the crankcase vacuum will increase unless there is sufficient blow by supply it. I still think it can be a lean situation. is there any pinging when under a load? this is not exclusively a timing issue. by hot are we saying operating temp or is it getting overly hot? a cold engine with enough choke will mask this

[ArdWrknTrk]

100% in agreement with Matt!  👍

Check, not only static timing, but full mechanical advance (3,500 or so on a V-8)

[delco1946]

1. I replaced the pcv valve since it’s only $3.

2. Never could find a video that illustrated what pinging sounds like so I don’t know.

3. When I say hot, I don’t mean over heated but just fully warmed, parked on a hot day ( still well within the normal range - let’s say over the “r”. )

4. i can assure you it is running lean since that’s the only way I can get it to pass emissions. I could enrichen the a/f screws on the carb to see if that helps?

5. I’ll try connecting vacuum directly to the distributer and see what happens.

I’ll add that I know squat about timing so I can’t really offer much in that department :/

[mat in tn]

since you have this tuned for the emissions test, It is a great time to verify vacuum leaks. they should be easier to sort out. one simple test, find the smallest direct vacuum port preferably manifold vacuum. and uncap it while running and warmed up. if it is truly lean then the idle will reduce. if it is rich it will speed up. in the best case you will have a timing light/ tach hooked up to watch. at this time you could adjust the idle mixture screws. And watch the tach. only adjust enough for it to smooth out then recap the port. if capped or uncapped no longer makes a difference, stop there.

[delco1946]

Matt - if I introduce an air leak the idle definitely stumbles. Are you saying to adjust the mixture screw while artificially inducing a leak? Why do that? ( just trying to understand what that accomplishes or diagnoses?)

[delco1946]

I hooked up manifold vacuum to the distributor. This results in immediate increase in rpm’s. I wouldn’t say the engine idles smoother tho, just faster with the same slight lopsided-ness.

Interestingly, with the new pcv valve I can now hear it rattling very quickly with the engine running. Don’t recall hearing that before.

Also, I do have a timing gun, but I don’t really understand how to use it. Yet.

Vacuum pressure vid:

https://youtube.com/shorts/xzuh5YOWxI0?si=BGTWw-K0Q8SQSQGh

[mat in tn]

I'm assuming (and we all know better) that you unplugged a very small port. like 1/8".   if you were to back out the idle mixture screws say 1/2 round each, does it smooth out? my concern is that it is leaned out too much in the attempt to pass the test. ironically this can lower one reading yet raise another.
are you certain that the egr valve is fully closed. at idle this will result in a vacuum leak. if this truck sat for an extended period before being resurrected there are many items like this to prove.

[viven44]

Video is not available

[delco1946]

I’ve tested the egr and actually installed it circa 2020.

I definitely did not induce a vacuum leak when I leaned the screws to pass deq. I basically turned it down so that it just barely ran in Feb.

[viven44]

Brand new spark plugs ?

When was the last time you compression tested all cylinders ? I wonder if one of them is falling off the lower cliff when things get hot.

When I had Big Red, looking at my notes when things got hot all the cylinders lost some compression when hot (as might be typical).. cylinders 2 and 6 had barely over 80 PSI when things warmed up. I just don't think the valves seated all that well... maybe a over-torqued rocker arm?... I never got to the bottom of it before I sold Big Red. I actually put Big Red up for sale and someone wanted me to show compression values so thats when I realized things were close to cliff.

Big Red was still a torque monster... but was running real close to the cliff...

         Rear
120              119
120              120
91                 99
120              122
         Front

[delco1946]

https://youtube.com/shorts/6lVTt9A3B1M?si=p-KuEG0llYOnEADe

3.5 turns but higher pressure and better idle….

[85lebaront2]

That is an excellent idle for a 351M or 400.

[viven44]

Yes that looks great. Maybe I jumped to conclusions. I had already assumed the idle screws were adjusted.