1974 L18 "POP" then Loud Vacuum Leak & Carb Leak Found

[Cardinal Grammeter]

EDIT:  I'm sort of hijacking my own thread here.  I definitely have found a leak that looks like carb.  GOTO reply #11 for this carb problem.  (This original discussion about the loud leak.  I never could observe it again, so it will probably remain a mystery.)

 

So I'm looking at the electric choke for movement - its getting its 12v which means the bullshit choke relay off the alternator is working (another bonus consider all the rust destroyed electrical) and there is the loud

 

Pssssshhhhhhhhhhhhhh  (note the "P" because the loud hiss started with a "pop")

 

And the fast idle of 15-2000 jumps at least another 1000-1500 higher.  This is the high idle I thought I would have to adjust with the fast idle screw.  I knew it could very well be a vacuum leak and now I know it is true.

 

It can't be a hose coming off because this thing is massive.  The only big things I can think of are

• Power Brake unit diaphragm ripped, or

• Ingested a chunk of intake manifold gasket.

But whatever it is, it is humongous.  (PCV is drawing a little with no effect if plugged, and EGR is drawing nothing as it should with the vacuum line bypassed.)

 

To find the leak I guess I'll spray whatever around and see if the idle changes.  With the fan blowing, propane might not work so good.

[datzenmike]

Every vacuum leak I have ever seen resulted in an RPM drop. A vacuum leak leans the mixture... why would it rev up? Do you have a tach? because a 3,500 rpm idle is not possible with the idle set properly.

 

 

If carb set properly to idle there is no EGR ported vacuum at idle, thus no EGR at idle. There is no vacuum advance at idle either. If the throttle is not fully closed to idle there probably will be some vacuum advance which will bump the idle up very slightly.

 

 

Unexplained high idle could be over sensitive setting of the BCDD, or carb secondary is part open, stuck or something jammed in it..

[Cardinal Grammeter]

While a vacuum leak leans mixture, it also reduced manifold vacuum which is what opening the throttle plate does.  But keep in mind that the electric choke is not moving much at all and the choke is only partially open - in fact, I manually set it open "one step on the fast idle cam"  and don't touch the throttle (otherwise it would close completely.)

 

So there is a very rich condition, so add some more air and rpm should increase.

___________________________________

 

This last running of the engine was the first time it idled properly - fast idle adjustment was good.

 

During previous runs with the crazy high idle, I got the throttle on the idle screw by holding the choke open so the fast idle cam was out of the way.  I then began to set the idle lower where it should have been - I ended up with the throttle plates totally closed (idle screw backed out completely) and was bringing the idle down with the mixture screw.  So air was coming from somewhere because the engine was running.  Vacuum leak is most logical.

 

What is interesting about the Pop-hiss... event was that when it happened, it took the engine a few seconds to stabilize at this much higher idle point.  And also keep in mind the hiss was continuous and so loud it was clearly heard over the engine which makes quite a bit of noise at such a high idle - I'm guessing 2500-3000 rpm.

 

I should hook a vacuum gauge up to monitor subsequent runs.

 

And get the electric choke sorted out:

 

And I'm going to have to compare the two choke thermostatic springs to see which one is faster.  I checked the heater by putting 12v on it, held it near my cheek, and slowly felt a little warmth.  Frankly, I was surprised at how slow this element was heating up, but then slow might be what its supposed to be.

 

Then hunt the leak with spray I suppose.

 

Oh, and better verify oil in the trans too NEXT THING. 

[datsunaholic]

I've had plenty of vacuum leaks raise the idle. Usually because the idle mix has been enriched to compensate, usually with the choke closed.  In every case, I end up with an idle that CANNOT drop below 1500 RPM.  Mix was turned up until it would actually "idle", but at an elevated level.   Turning down the idle speed screw shuts the engine off- 1500 or nothing.  I had a cracked manifold do that.  Since so little air was making it through the carb with the throttle (mostly) closed,  the mix had to be really rich to make it run at all.

[Cardinal Grammeter]

I can get at least a 1500 rpm idle with the throttle plates completely closed when it's leaking.

 

So riddle me this:

 

• Initial engine runs had severe high idle most likely due to massive vacuum leak.
• My last run exhibits normal idle in the beginning
• Then, the Pop-shhh... back to high idle.

If this is a vacuum leak,

• Its a Big One, and
• It is INTERMITTENT!

An intermittent big vacuum leak is pretty much an impossibility.

 

I hope to get some testing done this evening.

[Doctor510]

Disconnect the vacuum line from the Vacc adv unit and plug the line.  Many times you can get ambushed by a leaking vacc unit diaphragm 

[Cardinal Grammeter]

Yes about the vacuum advance.  Timing can sometimes be a culprit for idle rpm issues.

 

However, while I'll check it and everything else, it is a small vacuum hose which cannot flow the air necessary for a massive rpm increase.  And it also wouldn't make the loud hiss.

 

My luck, the asshat that had this before me didn't tighten the intake manifold to the head and I lost a gasket.

 

I'm also having second thoughts that it is an Intermittent problem.  The early high idle could have been something else.

 

I should have answers this evening.

[banzai510(hainz)]

i have had the vacuum advance off and not plugged and it doesnt do anything in my circustance to the carburator

[datzenmike]

 Read...

 

I've had plenty of vacuum leaks raise the idle. Usually because the idle mix has been enriched to compensate, usually with the choke closed.  In every case, I end up with an idle that CANNOT drop below 1500 RPM.  Mix was turned up until it would actually "idle", but at an elevated level.   Turning down the idle speed screw shuts the engine off- 1500 or nothing.  I had a cracked manifold do that.  Since so little air was making it through the carb with the throttle (mostly) closed,  the mix had to be really rich to make it run at all.

 

Engine won't rev higher from more air (vacuum leak) unless it's too rich to begin with.  On a perfectly running engine a vacuum leak lowers the idle.

 

So if you really do have a vacuum leak,.... either your choke is on or it's flooding to make it richer.

 

Did you check the BCDD???? The BCDD is like a carb inside the carb. When activated it allows gas and air into the intake regardless if the throttle plate is closed or not. 

 

 

 

The ported vacuum advance has nothing to do with the intake vacuum. Ported vacuum is taken from just above the throttle plate. Zero vacuum at idle, (when the plate is below the port) and increasing vacuum as the throttle is opened and the port is exposed to intake vacuum. Plugged or open, this port has no affect on the intake vacuum. EGR is the same.

 

 

Do you have a tach because engines are very hard to guess the RPM just by the sound. This may be a perfectly normal 2,300 fast idle warm up. Sounds worse than it is.

[Cardinal Grammeter]

I'm not sure what happened.  But I may have been to blame:  I had two vacuum ports to block off:  EGR (the aft carb nipple near the valve cover) and the fitting in the EGR intake manifold "annex."  

 

So I ran a vacuum line connecting them.

 

If the carb EGR was simply manifold vacuum, then nothing should have happened (this is what I was thinking when I did it.)  But it if was some weird throttle timed port who knows what would happen.

 

So far, I'm not having any issues, but I was setting idle with tach meter and mixture like olden times - turn it in until engine runs rough then turn out 1/2 turn.

[Cardinal Grammeter]

HiJacking my own thread:  Another related problem found:

 

CARB VACUUM LEAK FOUND:

 

I've been driving around a lot the past few days since inspected and the idle is all over the place. 

 

The other day I had to adjust it up.  Today it is idling maybe 1500 rpm at least.

 

So I started spring starting ether all over the place and found one specific area which caused a dramatic drop in rpm:

 

Spray on top of the stud/nut holding the carb to manifold - the driver's side (L), front one - between the fuel bowl and the BCDD.

 

I know people say the throttle base comes loose, but I was crazy thorough with this carb. 

 

I was thinking it could be the EGR manifold extension but if I spray beside the carb base, nothing happens.

 

Just checked the plugs and they look uniform, but coated with white - almost greenish.

[datzenmike]

Not following. Idle warm should be 750-850. Is it fixed or not?

 

'1,500 at least' isn't fixed.

[Cardinal Grammeter]

I've set the idle a couple times when hot.

 

It will be too low, I set it up.

Then it will be too high and I set it down.

 

So I started spraying ether and car cleaner all over the place.

 

I'm concerned that using ether is problematic because the fumes could be drawn in far from where I'm spraying.

 

I'm going to repeat the searching with engine degreaser which is basically kerosene which won't have the potential fume issue.

 

Since I've had it running, the carb just hasn't been right.  When you rev the engine, you don't get crisp throttle response - its like there isn't enough accel pump shot.  The pump is working just fine.  The timing is set at idle which I don't like to do.  I haven't checked total timing.

 

But the fact that the idle is not stable points to a vacuum leak.

 

I don't want to pull the carb until I am sure where it is since removal is so difficult.

 

ANOTHER METHOD:  I've actually done this with a small tube, one end in the ear and then move the other end around.  In many cases I can hear the leak. 

[datzenmike]

The idle is to high to set properly. Mechanical advance will be added in for one.

 

High and lower random idle can also be the BCDD set to sensitive. Turn adjustment counter clockwise.

[Cardinal Grammeter]

Just remembered, I don't have the PCV to crankcase hose on.

[Cardinal Grammeter]

I did the disable BCDD by hotwiring its solenoid - I found that one on another Datsun site (datsun1200 maybe) but it made no difference when the idle was too high.

 

I'm not sure but if you blip the throttle, if the BCDD is adjusted properly, won't it cause an idle hang of a second or two and then rpm comes down to idle speed.  If the BCDD is not working, when you blip the throttle, there is no hang and rpm drops smoothly back to idle speed.

 

I got a BIG DROP in idle spray the ether on that LF carb base stud.

 

This is funny but I could block off the tailpipe, put some compressed air in the intake and shoot bubble solution over everything.  Of course, the throttle plate shafts would leak and the area I'm homing in on is where the secondary shaft opposite the linkage end comes out of the base.  Of course a leaky shaft has the same orifice area no matter how the shaft is positioned in the sloppy hole, so it should be a uniform, constant leak.

 

I'm on youtube now, looking for tricks...

[datzenmike]

Just remembered, I don't have the PCV to crankcase hose on.

 

Won't change anything. The PCV valve is still sucking air, hose on or off. Well unless the hose is plugged.

 

If you gotan idle change then replace the gasket and/or look for a crack. Is the bolt tight?

[Cardinal Grammeter]

Carb is tight (checked it again) and the gasket was new.  I may have used a thin (1/8") phenolic OE insulator, will have to check but I don't think that is the problem.

 

If I spray the ether on the intake at the carb base, the idle does not drop.  But when I spray on top of the stud (on top of the carb base mounting flange), the idle drops big time and notice it drops which seems to be contrary to expectations.

 

I sprayed above the flange, underneath the fuel bowl with carb cleaner through its red straw and got idle drop.

 

I know the PCV valve has little slots in it so it flows just a little at high vacuum.  I don't know what little is, but with mine, if I put my finger over it, there is a considerable idle drop.  Mine looks new, but I wonder if it has crusty deposits that is keeping it off its seat and leaking excessively?  I think I may remove it and plug the hole and see how things behave.  I can ultrasonically clean it too.

 

A TEST:  spray whatever I'm spraying into the PCV valve and see what the idle does.  This is a known vacuum leak.

[datzenmike]

Vacuum holds it open against a weak spring. Idle and deceleration vacuum is the highest. Hardly any under heavy loads. This small leak is accounted for with the idle mixture. When you plug the hole the idle goes too rich.

[Cardinal Grammeter]

I did check the vacuum secondaries, they open properly with WOT in neutral.

 

Highly unlikely possibilities:

• Phenolic spacer cracked
• I had to open up a hole in the base to body gasket in the carb.  It was a port between the throttle plates where rather than just being a hole in both parts, one part had a brass tube that protruded into the other part.  I had to file larger the gasket hole which would not clear this brass tube.  The hole made what remained of the gasket very narrow.  If there was vacuum in that passage and a narrow chunk of the gasket came out, there would be a vacuum leak.

Frankly, I'd like buy a $35-40 fog machine and smoke the damn thing.  For that kind of money, I'd like to have that capability.  I still like the crazy idea of taping up the tailpipe, pressurizing the intake and see where it blows bubbles.

 

I watched a youtube where the tech said when you spray the vacuum leak, the idle could go up or down "depending."

 

I'm done with this today.  Will tear into it Sunday.

 

EDIT:  (next day, Sunday)  More Thinking Before Doing:

 

THROTTLE SHAFTS:

 

There could very well be a vacuum leak at the secondary throttle shaft to base which seems to be the pin point of the spray effect.

 

However, that should be a constant value leak which would be trimmed out by carb idle mixture adjustment - it would NOT explain why one day the hot idle is set too low and the next it is set way to high.  Further, I sprayed all over the throttle shafts on the other end where the linkages are, and NOTHING.  I would expect more wear on the linkage end of the shafts.  So that leaves a conundrum.

 

HOT IDLE COMPENSATOR:

 

Just thinking, the Hot Idle Compensator in the air cleaner is hooked up but I didn't test it.

 

 

PCV VALVE:

 

Thinking more about the PCV valve.  It looks new, but if it is all coked up so that it doesn't seat properly at idle vacuum, there could be an erratic flow.  But (just thinking now), every time I would WOT, the PCV poppet valve would cycle and if unstable, the idle would change often while driving.  I'd say that probably isn't happening, but it is still possible that it's doing that to some extent.

 

 

I am now very hesitant to remove the carb (difficult job) for an exploratory inspection.

 

 

EDIT-2:  Adjusting Idle Mixture Screw Procedure:

 

I found this one in another thread here.  Is this the correct way to do it:

 

"Adjust idle until it is at speck rpm, 800. Screw mixture in until its about to die and out until its about to die, then find the smoothest and fastest happy medium. Idle screw again to 800... repeat. "

 

 

EDIT-3:  (Sunday's results)

 

Mixture Adjustment Weirdness:

 

NOTE:  Linkage:

 

First, there is some kind of problem with the linkage, it binds or something so that it does not close down completely when idle screw is backed totally out.  In fact, sometimes, after manually WOTing the throttle, you can see it hang, and then slowly move the remaining 1/8" at the OD of the throttle cable linkage.  It is impossible to diagnose on the car due to inaccessibility, but after working it a number of times, it was free and OK.  I suspect it is associated with the choke and fast idle links.  Don't know if its going to return or not.

 

With the Idle Screw totally backed out, I can make the engine idle anywhere I want with the Mixture Screw.  I'll bet I can get to 1500 rpm and on the other side, lower the idle until it simply stalls.

 

At no point, can I make the engine miss or run rough with the Mixture Screw. with the Idle Screw Totally backed out - so it is impossible to adjust the Mixture by finding any rough points.

 

So I turned the Idle Screw about 1 turn from back out and used the Mixture Screw to set the idle. 

 

How the Mixture Screw affected engine idle was interesting.  When trying to turn in the Mixture Screw, I would get to a threshold where if I turned it 1/4 turn more, the engine would "lose its fire" and rpm would "collapse" and start dropping as the engine was heading for stall.  So I would quickly turn out the Mixture Screw maybe 1/2 turn to "catch it."  Here is where it got wonky:  the rpm would stabilize at maybe 600 rpm for a few seconds and while at this speed, the carb quit making that hissing noise it always makes - it was totally silent and running.  Then as the rpm started to climb back, the carb resumed making the hissing noise that is typical at idle.

 

The Mixture Screw Rotation -v- Rpm is highly non-linear:  At higher rpm, there is pretty much a linear relationship, however, when getting to the low end, there is this "cliff" threshold where a small turn will cause the engine to die.  But note that at no time, does the engine miss or run rough.

 

I have a theory that the hissing noise is vacuum in the intake leaking past the throttle plates which would make a hiss.

 

When the engine dropped to 600 rpm, there was enough vacuum leak coming from where ever, there wasn't enough vacuum across the plates to make that noise.  This could be from throttle shafts and PCV, the only other leakage points besides the plates.

 

So for now it's set probably around 900.  800 is just too close to this Threshold Point where Mixture Screw is highly sensitive.

 

So how Pathological does this sound?

 

B.C.D.D.:

 

I was going to back the screw out 1/2 turn and as luck would have it, I didn't check this when rebuilding the carb - the screw is stuck/frozen.  Since the "nut" is support by the diaphragm, I didn't force it.  I tried a spare carb and it moved just fine.  I think it is easy to swap out the BCDD's - something like 3 or 4 bolts?  Maybe I should change them.  But then I'd probably lose the adjustment assuming it was correct.  I was thinking it could touch the screw with a fraction of a drop of penetrating oil hoping to keep it off the diaphragm - I don't know what chemicals the diaphragms can tolerate - probably gasoline, huh?  Carb cleaner?  Oil?