Carb/Vacuum Issues - Again

[NJFJ40]

OK - have another thread here but got some more feedback from vacuum readings. I started it up with choke on and it idled fine. Initial cold temp reading on the vacuum gauge was around 16. As it warmed (with choke still on), the engine RPM increased slowly (as normal) and the vacuum reading slowly rose to around 20 as the engine came up to temp. It hovered there for a few mins with choke still on then, as I was watching the gauge, the engine suddenly lowered in RPM and the gauge dropped to around 16 PSI and stayed there.

When I reved the engine, the needle went up to 24 or so as it should but then it settlled back down to 14 (lower then before) and the idle became even more rough and seemed about to die.

I assume there is something in the carb system that adjusts as the engine warms. Trying to figure out why it runs fine at cold idle and as it warms then vacuum drops and idles rough after warm.

Note that there was never any erratic vacuum readings or violent movements on the guage as would indicate other problems.

I guess I have more reading to do.

BTW - Am I the only one who reads about 3 hours for every 15 mins of wrenching? Or is this just the norm for a newbie?

AndrewT

 

[dd113]

No idea where you are with this so....

tighten the in/ex manifolds, look/tighten the carb/insulator nuts, where is the timing? Put a vac gauge on it when it is runing 16 and try to adv the timing and see what it does. It should go higher. What about valve adj??

 

[NJFJ40]

Thx Dave.

In/ex manifold was just completely torn apart and rebuilt with all new gaskets and parts - all new studs and bolts were installed on the various components. Torgued to 33 ft lbs on the in/ex manifold to the block (as per spec). All smog gear was also replaced and new vacuum lines run to the proper location as per emissions FSM (PO had not done it correctly). I just hooked this up and started it the other day and had these 'new' problems. I don't think the manifold bolts loosened so quickly with only idling. I did check the manifold bolt nearest the front (easiest to get to with all smog gear there) and it was still at spec.

I will check and re-tighten the carb mounts tomorrow. Also need to check the timing so that seems like the next step.

AndrewT

 

[IDave]

And like Dave D said, check your valve lashes.

 

[Pighead]

This is on a 1980?
Aren't there a bunch of little plastic valves and a mile or so of vacuum hose on those?
Don't some of those valves do things with engine vacuum at certain temperatures?
I don't know if I would be so quick to assume it's something in the carburator...
Eliminate the basics first, re-torque head and manifold (I torque my manifold closer to 40ft/lb)
& adjust valves with the engine at operating temp. Perform the leaky manifold gasket test of your choice. Try timing with the vacuum gauge instead of a light. Try some creative tuning, the FSM is not always perfect. My '74s liked a few more turns of the idle mixture screw than the FSM recommended. Then I would start testing those plastic valve gizmos, not unheard of for them to fail even new-out-of-the-box.

 

[Pin_Head]

The sudden drop in vacuum is consistent with one of the thermal vacuum switches opening. Make sure that all the vacuum lines are properly routed and are not vented to atmosphere. You could also do the general vacuum leak test by spraying propane gas around all the vacuum lines and connections and looking for when the idle speeds up.

 

[IDave]

You like propane best, Charlie?

 

[Pin_Head]

Anything that burns will work. I always seem to have a propane torch around. You can put a hose on it to get to the tight places.

 

[IDave]

Anything that burns will work

Got it. So Mapp gas would be good, too (but more expensive).

But don't light it before you shove it down around your gaskets, right?

 

[NJFJ40]

OK - A few questions.

What is the method to set timing via vacuum? I assume this means adjust timing until you get optimal vacuum reading?

What adjustments do I need to make/check on the engine (not vacuum) valves and is there somehwere I can read-up on how to do this?

PH - So, if I understood you, one of the thermal valves opening and then getting a vacuum drop should indicate the leak is somewhere downstream in that circuit? I assume that when the vacuum switch opens as the engine warms that means it allows vacuum beyond the valve - so I can start by re-checking vacuum lines downstream. Also - any idea which type of vacuum VSVs, BVSVs, VTVs, and VCVs are thermally activated? I plan to read the emissions FSM more - may shed some light.

Thx to all for helping. I know I will figure this out. Just another learning experience and I appreciate all the help along the way.

AndrewT

 

[Pin_Head]

Got it. So Mapp gas would be good, too (but more expensive).

But don't light it before you shove it down around your gaskets, right?

Yea, don't light it and make sure it doesn't light inadvertently when you are checking.

Also - any idea which type of vacuum VSVs, BVSVs, VTVs, and VCVs are thermally activated? I plan to read the emissions FSM more - may shed some light.

Generally they are the ones screwed in to the water jacket on the head and the thermostat housing. There is another one on the EGR valve, but it does something else.

 

[crewzerdog]

Andrew - I've got basically the same thing going on with my cruiser - starts and runs fine during warm or cold weather with only a little bit of choke needed for short period of time (<1-2 minutes). Runs great with choked pushed in for a few miles (vacuum guage says steady 18 in Hg) then when truck gets a little warm it starts to run a little rough for say about 10 minutes (that time has increased now that weather is much colder) - have to pull choke back out about 1/2 to 1/4 way to keep it running when coming to a stop at intersections, etc. -- then the strangest thing happens - when it has run at normal operating temp for a while the engine suddenly/quickly revs up pretty high and I then immediately push in the choke and it idles down and runs smooth as silk - still pulling 18 in Hg on the vac guage.

Like Pinhead and IDave say - it must be in those VSVs but I've not been able to track the exact one causing the problem down as yet. I need to check the vacuum next time it goes thru this "cycle" to see what happens when the engine starts to run poorly and when it starts to rev back up. I've done the new manifold gasket (twice) and carb insulator plate gaskets, etc. - so don't think I have the "traditional" leak - especially if vac guage is steady at start up and 'final' operating temp. Must be one of those darn VSVs. If you figure it out, let me know - it might just head me in the right direction. Didn't mean to hijack your thread but misery likes company.

 

[65swb45]

Vac switching valves are definitely a possibility. So is the manifold. So is the brake booster. Try coasting or downshifting to stop and avoid using the brake pedal to see if it makes any difference.

I bought a 78 from a gal about 12 years ago who's parents had bought her a mani gasket job at the local stealership and a few hundred miles later the truck had similar issues. When I tore the truck down to re-gasket it myself, I found all the manifold nuts and bolts that are under the insulator [the ones you can't get to right now] were ALL LOOSE!

I've never used factory spec on manifold hardware since!

 

[Pin_Head]

If you don't find a vacuum leak with the traditional methods, it is time to start suspecting the infamous cracked intake manifold. It usually cracks under the carb and sometimes you can see it with a flashlight and a wide open throttle. Also check it your heat riser valve is stuck closed. This is often the proximate cause of a cracked intake.

 

[NJFJ40]

Good info. I am going out now to work on it.

Just had the whole thing apart and no cracks in the intake manifold and also the heat riser valve worked fine. Again, replaced all new gaskets and torqued to spec - though will check again now.

Don't think its the brake booster (read other posts on that issue) as I have the booster disconnected as I am using that line for the vacuum gauge.

I am leaning towards EGR valve as I believe the vac switch kicks it on once the egine is warm. A bad EGR valve could explain why the vac drops after the engine warms. Or maybe the connections to the EGR valve in that vac system.

More to come.........

AndrewT

 

[miletwo]

head bolts get 100lbs, manifolds get 45... factory specs are a bit weak with older "broken in" machines. I highly recommend studs over bolts for the manifold as well if you can. It made things much easier to torque down and get even pressure for me. Although it did take a few days to get them from local dealer since nobody else had them.

Good luck,

Jeremy

 

[NJFJ40]

Just worked on it and redid all vacuum connections and added hose clamps to the larger vacuum lines as they did seem loose to me. It started and came up to temp and idled fine throughout - the prior issue did not recur?? Vacuum reading at 17 throughout as checked via the brake booster.

Took it for a spin and it ran well though I had some hesitation and lack of power at higher RPMs. Not sure if this is related to the high-speed idle or maybe vacuum advance not working properly?? Fuel pump is new and so is the fuel filter.

Ihave yet to do any adjusting to the carb. Plan to do that after setting the timing as per instructions from the FAQ here.

On setting the timing - I am trying to mark the BB but how do you get it in the 'window'? Is there a way to manually advance the engine without starting it?

AndrewT

 

[Robert LaDuke]

Take out a spark plug or two, put the gear shift in 4th, and rock the truck forward or back.

 

[NJFJ40]

What a night!

Took a break from trying to find the fawking BB to set timing and went to re-torque manifold bolts (which are all new and OEM items). I took the advice from this thread and torqued to 40 ft-lbs. On the first bolt closest to the front, I get about 1/4 turn and the bolt breaks!!! FAWK!

Luckily, I realized that this is holding the elbow on the exhaust manifold so I was able to remove the next bolt in and then bend the elbow out. Also luckily, the bolt snapped with about 1/4" of thread protruding from the block so I gently held the manifold gasket away from the block, got the vice grips and was able to extract the broken piece and put in the old bolt I had from before. Ofcourse, this was after I removed the air rail, emissions stuff in the way, vacuum piping hardline assembly , carb (again), etc...

Turned out OK as I wanted/needed to remove that stuff anyway to get to all the in/ex bolts to re-torque them. I was amazed how loose the 2 top bolts were in the middle of the block after only about 10 miles of driving. Did that and powered down to 37 ft-lbs which is the max range on the FSM. All is well now I hope.

Tomorrow I re-install the gear again and back to where I left off before I tried to 'fix' things today.

FYI - I ran it for 20 mins before the fiasco described above and it seems to run better everytime I start it up. I do think Timing is off and look forward to doing the carb tuning once the timing is set properly.

AndrewT

 

[IDave]

This is one reason studs are bettah!