1968 Idle Going Lean

[p51]

OK.

Here's the setup: I've got an Edelbrock 1406 carb with a 1" insulator below it (to keep it cool) and a Holley 4.5-9psi fuel pressure regulator feeding it with a Mr Gasket fuel pressure (dry) gauge right after the regulator. After the engine is warmed up (but not overly hot) and the thermostat is open I set the idle to ~825RPM with an AFR ~14.0 and a steady fuel pressure of ~5.5psi. The max manifold vacuum is at a steady ~18" (so I assume there are no vacuum leaks).

Here's the problem: I drive the car for a while, engine heats up (~195F), under-hood temps go up and all of a sudden my idle mixture goes lean to ~22AFR+ and stays there (and, of course, idle becomes rough). So, either too much air or too little fuel. There is apparently still some gas in carb fuel bowls because I can give it a bit of throttle and the AFR richens back ~14. The car is still drive-able at this point but now there is hesitation on acceleration. When I get back home I check the vacuum and it's ~17" and steady (so again, I'm inferring no "heat related" vacuum leaks) and the idle RPM is now ~700RPM (probably why the manifold vacuum is slightly lower). Fuel pressure gauge right after the regulator is now reading a steady ~4psi (below the regulator spec'ed range). So it seems like there is sufficient pressure to keep fuel in the bowls, albeit, probably at a lower level than with the normal 5.5psi.

So my thoughts are that problem could be:

(1) Dirt in the idle circuit
(2) Partial vapor lock in the fuel line
(3) Heat related issue with the mechanical fuel pump

Q: If the pressure to carb fuel bowls drops (5.5psi to 4psi) would that cause a lean condition? Why? Shouldn't there still be sufficient fuel in the bowls @4psi?

Thoughts?

Thanks
James

 

[CougarCJ]

Are you using the thermal vacuum switch on your water neck?

If you are, and it is working properly, your vacuum advance on the distributor has changed.

 

[robert campbell]

+1 on Scott's thought. Check your timing when just warmed up and check it again when it is very hot. Are you using the dual inlet diaphragm and the tree switch on the thermo housing? They were designed to retard timing.

but as I remember you and I talked about constant vacuum verses ported vacuum to the dizzy. One factor of using constant vacuum on the dizzy is the fact as you open the throttle the engine actually retards itself do to loss of vacuum. Instant flat spot. With ported vacuum and more initial advance the vacuum becomes present just as you open the throttle and advances the engine. That was the way all engines in this era were designed regardless of make.

Also removing the idle mixture screws and gently using some compressed air will clear the idle circuit.

Rob

 

[p51]

Scott, Rob,

Thanks for the suggestions.

I have a single-ported vacuum advance. I had already checked idle timing when it was hot... I thought maybe the distribution lockdown wasn't tight enough and that I might have spun the dizzy. The advance was exactly the same as it was when the engine was cold vs warm vs hot. So, I don't think that's the problem.

One tidbit I did come across from Wikipedia is the following:
"Even temporary disruption of fuel supply into the float chamber is not ideal; most carburetors are designed to run at a fixed level of fuel in the float bowl and reducing the level will reduce the fuel to air mixture delivered to the engine."

Apparently even when there is fuel in the bowl, the fuel *level* can cause a change in AFR. I guess even the small amount of change of weight of the fuel determines how much flow there is going through the jet.

With that idea, the lower pressure indicated by the gauge when hot indicates (I think) that there was fuel in the bowl but at a lower level. So, my current theory is that I was getting air in the fuel line when the engine was hot. Unfortunately, I never just let the engine cool down to see if the AFR and fuel pressure went back to its normal/"cool" value... I mucked around with the idle screws and regulator. That probably would have been a good clue if it was air in the fuel lines causing the problem.

 

[p51]

Scott, Rob... you da men!!... so far... I think... maybe :wink:

(Long story version)... I re-routed the fuel lines in engine compartment (the previous owner routed the fuel line under the thermostat housing), made sure that the tank was venting correctly (so as not to create a vacuum for the mechanical pump), cleaned the idle circuit with carb cleaner/compressed air, cleaned the venturi boosters. Then I warmed up the car, checked timing, made sure that the fuel to the carb was 5.5psi, set the idle ~825RPM with AFR ~14. Running fine. Took it for a drive and about 10min out idle AFR jumps to ~25 and acceleration is jerky. Got back home (feathering idle all the way). Put a vac gauge on the engine and the manifold vac is now down to ~14" from about ~18" normally (yes, yes, with manifold vac to the disti...). First thought in my mind was "oh sh**, a temperature related vacuum leak somewhere in the engine (eg manifold)"... and then...

(Short story version)... I put my mighty vac on the vacuum advance canister and, lo and behold, the canister is not holding vacuum (it was when I checked it the other day, but with the engine only warm). My guess is that the diaphragm was starting to fail and has now gotten worse.

So, question... have either of you guys ever seen a temperature dependent failure of a vacuum advance canister. Working when cold (which it was) but failing when hot?

I'm going to let the engine cool and see if it starts working again. Regardless, a new vac canister is going to be on order. Any particular brand that is recommended?

Thanks guys
James

 

[robert campbell]

Is it a dual inlet vacuum advance?

Rob

 

[p51]

Is it a dual inlet vacuum advance?

Rob

Nope. Single-port vacuum advance. I've ordered a Standard Products VC31 from Amazon. Arrives tomorrow.

 

[robert campbell]

That vacuum is adjustable with a small Allyn wrench inserted into the tube. We can talk about that further, later.

Rob

 

[p51]

That vacuum is adjustable with a small Allyn wrench inserted into the tube. We can talk about that further, later.

Rob

Rob

Thanks. Yep. Knew that.

So, I got the vac advance canister in and its working. But it now appears that this is not the main problem. Although my old vac canister had a slow leak, I think the constant vac produced from RPM of the engine kept it from leaking down too much. So, in effect, it was working. Anyway, with the new vac canister in I still have the same problem, very lean (25+ AFR)/rough idle which smooths out with throttle (although it still sounds like there is a miss in one of the cylinders). I'm starting to think I have a vac leak somewhere (I have always thought that there was some small leak somewhere because of the low vac and flutter of the vac gauge when the vac advance is not hooked up. 15" +/-1.5"). I now think the leak has gotten a lot worse. And I am suspecting it is a leak from the manifold down into the lifter valley. So, in the next couple of days I'm going to try a couple of techniques to see if I'm getting a vac leak below the intake manifold.
(1) The "cigar" smoke test - look for smoke esp coming out of the valve covers
(2) Vac gauge on the oil dip stick to see if vac from the running engine is being transferred into the lifter valley and, then, into the oil pan

Aside: I think this is either a vac leak from the manifold into the lifter valley or possibly worn valve guides. But I don't think its valve guides because I've never gotten blue smoke on startup and because this got a whole lot worse all of a sudden.

Aside2: I did a compression check and all the cylinders are a healthy 172-180psi so I dont think is a valve seating or head gasket problem.

Aside3: When doing the compression check I looked at the spark plugs. All looked about the same brown color. A little darker than what would be ideal (but definitely brown, not black) and with a little layer of dry soot (?)

Any other suggestions on how to find a manifold leak that is internal to the engine?

Thanks
James

 

[franklinair]

Just in case its an intake manifold gasket leak:
With the engine idling (you say is rough) squirt some aerosol lubricant along the perimeter of the intake. If the gasket is leaking, it will momentarily smooth out. That will locate the leaking area. The same test can be done at the base of the carburetor in case the vacuum leak is there.

Neil

 

[robert campbell]

My GNS was an "intake gasket eater" for awhile. It was the result of Exhaust gas reversion exacerbated by a cam with higher than stock overlap. I have aluminum heads also. I used Felpro top of the line gaskets, but after a year or so the gaskets would misform around the end intake ports (1,4,5,8). They would cause an intake leak and the car would run like crap. and always at operating temp, due to the rich mixture during the choke sequence masking it.

I encourage an intake removal, and then you and I will discuss the ported vice unported vacuum to the dizzy once again to the chagrin of the forum!!!

Rob

http://www.hotrod.com/articles/ctrp-1011-race-engine-reversion/

 

[p51]

Just in case its an intake manifold gasket leak:
With the engine idling (you say is rough) squirt some aerosol lubricant along the perimeter of the intake. If the gasket is leaking, it will momentarily smooth out. That will locate the leaking area. The same test can be done at the base of the carburetor in case the vacuum leak is there.

Neil

Neil

Thanks. I will do that. However, if the leak is from the intake manifold into the lifter valley this method may not work. That's why I'm planning on trying the "smoke test" - blowing smoke into the manifold to see where it comes out.

Thanks
James

 

[p51]

My GNS was an "intake gasket eater" for awhile. It was the result of Exhaust gas reversion exacerbated by a cam with higher than stock overlap. I have aluminum heads also. I used Felpro top of the line gaskets, but after a year or so the gaskets would misform around the end intake ports (1,4,5,8). They would cause an intake leak and the car would run like crap. and always at operating temp, due to the rich mixture during the choke sequence masking it.

I encourage an intake removal, and then you and I will discuss the ported vice unported vacuum to the dizzy once again to the chagrin of the forum!!!

Rob

http://www.hotrod.com/articles/ctrp-1011-race-engine-reversion/

Rob

Thanks

Assuming an intake leak, I am planning on using FelPro gaskets. Here is what I was thinking of...
https://www.summitracing.com/parts/fel-1250/applications/year/1968/make/ford?prefilter=1

These "Printoseal" gaskets seem to have good reviews. I am also thinking about using RTV along the "China wall" rather than a gasket. What would you suggest?

BTW: What is a "GNS"?


*****
Re: "Ported vs manifold"

Sounds like you are bound and determined to get me into a discussion about how to hook up vac advance. :wink:

Here is a e-zine article on the subject... (read the section on "Where should the vacuum canister be routed?")
http://www.superchevy.com/how-to/ad...now-about-vacuum-advance-and-ignition-timing/

There is also a detailed paper by a "GM engineer" (yes, a "Cheby" guy) on the subject:
http://www.camaros.org/pdf/timing101.pdf

Note: Once you open the throttle even a little bit both manifold and ported vac look exactly the same. The only difference is with manifold you have the advantage of advance during idle. Yes, with ported vac the advance "kicks in" when you open the throttle. But with manifold vac it is already kicked in before you open the throttle. After the throttle is open they are advanced the same and drop off the same as you open the throttle more.

Here is a guy that did two videos on vac advance. I think he pretty much nailed it... roughly, if you're close to stock, then manifold. If a bit radical, then ported.

Stock cam:
https://www.youtube.com/watch?v=UInmE9hg8Rk

Radical cam:
https://www.youtube.com/watch?v=O0OIVchIRT0

The takeaway (to me) is to use the port that best fits your engine.

Thanks again,
James

 

[robert campbell]

Gold Nugget Special. My car was always not "Martied" as a GT/CS, but the jury is still out and that is another story.

You provided the links before and all of the major manufacturers in the 1960's used ported vacuum. In fact the more you hop up an engine the more you "may" consider a constant source. But at the same time the more you hop up a motor the more you will drop vacuum advance totally. Just like the Hipo 289, the 427 Ford motors and the later Boss 302, Boss 429 and every high horsepower motors from any brand in the 1960's. I run non-vacuum advance dizzys on all my hot engines and hotted up customer cars.

The problem with constant vacuum is that it requires you to reduce initial advance. You advance to say 12 degrees initial advance and hook up to constant vacuum you have way to much advance at idle and early acceleration. As much as 25 degrees.

I feel with your motor you should eliminate the vacuum advance like most performance motors do.

Contrary to what your links provide, ported vacuum is the way to go with a vacuum advance dizzy. Just like all the engineers required their stock style motors to run.

Dial your motor into about 12 to 16 degrees initial and use ported vacuum and then adjust your vacuum advance to slow its start and to reduce its total. Trust me it will make way more power and be smooth through the transition from idle to low speed to high speed circuits. I do this every day and would do it to yours.

Those engineers at Chevy, Ford, and Mopar knew what they were doing!!

Try it and quit reading links to people that think they know more than the engineers that prepared these engines. You may be surprised!! Or drop it and go to a non-vacuum advance dizzy.

Rob

Listen to this bad boy with an MSD no-vacuum dizzy reduced to 19 degrees total centrifugal advance and 16 degrees initial!! 35 total!! Turn up your speakers!! Yaaaaa Babeeee!

https://www.youtube.com/watch?v=WYu05pNnz9U&t=21s

 

[p51]

...Those engineers at Chevy, Ford, and Mopar knew what they were doing!!...

Rob

I will try ported vacuum once I get this "lean out" problem figured out. I think one of the reasons that ported never worked for me was because of this (assumed) vacuum leak that has always been there. BTW: My motor is just stock 302.

But just to let you know that last paper that I linked to was **actually written by a Retired GM engineer**... "John Hinckley, Retired GM/Chrysler Engineer". So this is not me just picking up random postings on the net.
Here is some info about John Hinckley.
https://www.corvettemuseum.org/learn/about-corvette/corvette-hall-of-fame/john-hinckley/

https://www.linkedin.com/in/john-hinckley-313353b3


Anyway, the more important issue right now is this lean condition at idle. What I did so far is check for vacuum leaks:


(1) First I did the "Swisher Sweet" test :smile:... I bought a bunch of cheap cigars, capped off the throttle intake, and blew smoke in through the manifold vacuum port. No smoke from around the manifold. I saw some smoke from around the throttle, possibly the throttle shafts. But I thought it might also be the carb gaskets so I replaced them - manifold, gasket, spacer, gasket, carb. Afterwards still had some small amount of smoke (shafts?) but less. However this did not solve the problem... the idle was still rough. Better, but still "no cigar" :wink:

(2) Second, I started the car, let it idle, and put a vacuum gauge on the oil dip stick to see if the vacuum signal was leaking into the crank case (eg from the manifold into the lifter valley) . Gauge showed a very slight positive pressure (no vacuum) which is what one would expect from blow-by. This seemed to indicate no vacuum leak from manifold to lifter valley.

(3) Then I got an idea... cap off the valve cover pcv and breather cap, blow smoke into the oil dip stick, and see if it comes out the carb! And it does! (I had to blow pretty hard, it took a while, and the hyperventilation along with the nicotine gave me a pretty good buzz :wink: )

So, some questions for you...
Q: Is there any path that smoke could follow from the crank case to the carb except via (1) a leak in the manifold gasket to the lifter valley or (2) through the valve stems? (I can't think of any, but I may be missing something).
Q: Would the valve stem seals (assuming they're good) stop smoke from going down the stems?

Also, per my previous post do you have any suggestions on particular FelPro manifold gaskets to use and do you use RTV for sealing the manifold at the China Wall?

Thanks much... aways appreciate your insights...
James

 

[robert campbell]

James,
I love our banter!!! I love your old "farm style" test. I like it!!! You are correct that there should be no other path than by the intake gaskets. The valve stem seals would not be part of this test you did.

I also use RTV only on the valley seals or china wall. I also use 4 long studs to guide the manifold down so there is no movement upon contact with the RTV.


My favorite small block gasket is this one. I use it exclusively.

https://www.summitracing.com/parts/mrg-5832/overview/make/ford

They seem to be a bit stronger than the FelPro blueline, but the Felpro is a good choice.

Rob

 

[p51]

James,
I love our banter!!! I love your old "farm style" test. I like it!!! You are correct that there should be no other path than by the intake gaskets. The valve stem seals would not be part of this test you did.

I also use RTV only on the valley seals or china wall. I also use 4 long studs to guide the manifold down so there is no movement upon contact with the RTV.


My favorite small block gasket is this one. I use it exclusively.

https://www.summitracing.com/parts/mrg-5832/overview/make/ford

They seem to be a bit stronger than the FelPro blueline, but the Felpro is a good choice.

Rob

"Guide" studs... great idea. Thanks.

 

[Mosesatm]

Rob

(3) Then I got an idea... cap off the valve cover pcv and breather cap, blow smoke into the oil dip stick, and see if it comes out the carb! And it does! (I had to blow pretty hard.....

Where is someone with a camera when we need one!!!!

 

[p51]

Where is someone with a camera when we need one!!!!

The things we do for our classic cars... jeez... :wink:

This is where I got the idea... Scotty Kilmer...
https://www.youtube.com/watch?v=zMok2y05jNE

 

[p51]

So, adding to the mystery, today I thought I would check to see if I was getting any "sneak" vacuum signal at either the pcv connector or the oil cap on the valve covers when the engine was idling. I capped off all other connections to the crankcase and checked vacuum first at the pcv opening and then the oil cap opening on the valve covers. Both had slight positive pressure with no apparent sneak path from the manifold vacuum (why?). This is all consistent with the slight blowby that the good compression check indicated. But...

...before hooking up the pcv valve back to the passenger side valve cover I noticed a slight amount of smoke (oil?) coming out the oil fill hole on the driver valve cover (normally not seen because the pcv sucks it up). There was no detectable smoke coming from the pcv opening. Could this be worn valve guides? Is there any way to easily check for worn valve guides with the valve cover off (eg wiggle them)?

And finally, amazingly I found video on youtube of Rob and me arguing over manifold vs ported vacuum :smile:
https://www.youtube.com/embed/XNkjDuSVXiE?start=79&end=158

We can argue later about who's who in the video :wink: