Cadillac Flathead engine performance issue - too lean?

[Stude Light]

Need some advice.

1939 LaSalle with fresh engine rebuild.  Generally runs great and quiet, can't hardly tell it's running at an idle.  I also rebuilt the Carter WDO carb using the Carter step by step instructions along with the correct meter rod height gauges (thanks Jon for the correct tools).  I have an issue....

 

During normal driving, the engine does not deliver smooth acceleration.  There is a slight surging felt through the acceleration.  If I accelerate fairly hard or go to wide open throttle the performance is fine.  During cruise it's fine. It's just the light and moderate acceleration.  Could this be an indication that it is running a bit too lean? I was thinking of lifting the metering rod height just a touch and see if it improves. Not sure what else it would be.

 

Any thoughts?  Thanks.

Scott

[FLYER15015]

Scott,

Where is she "timed" ? A degree or two, or three ahead won't hurt her.

You sure the vacuum advance is working properly ?

Does the plate that holds the points move VERY smoothly ?

 

My '40 Buick LTD had 3 balls that the dist plate rotated on. Come to find out there was a "notch" in the housing.

I replaced the balls with plastic sliders and all is good now. No hesitation on take off.

 

Check these and report back.

 

Mike in Colorado

Edited September 28, 2020 by FLYER15015 (see edit history)

[carbking]

Timing could certainly be the culprit; also a sticking intake valve.

 

One other issue (don't laugh) is gasoline with too HIGH an octane!

 

Many enthusiasts burn the highest octane fuel they can find, when their engine has insufficient compression or timing to make use of the octane. If the carburetor is perfectly tuned, and the ignition and compression are also good, av gas, racing gas, or 93 octane pump gas could cause a surge under high vacuum conditions in a lower compression engine.

 

Just as an example, my shop truck 10.25:1 compression with 450 HP is very happy on 87 octane pump gas except when pulling the 16 foot trailer, or being driven in anger. Under either of these conditions, 93 octane pump fuel is used.

 

Jon.

[Stude Light]

No vacuum advance on the 1939 cars.  The mechanical advance is working and distributor support has new bushing and shaft. I'm running the lowest octane I can find which is 87 ((R+M)/2). I have it timed per the book plus a few degrees further advanced.  I very much doubt a sticking valve (new rebuild) and I can easily make this go away by crowding the throttle a bit more and it goes away but comes back as I go moderate accel It never occurs at high throttle, cruise or idle.  Other than feeling a small amount of surging during moderate acceleration it runs perfect.  It feels worse when the engine is cold vs hot.  It's not just the initial accel but during a steady throttle acceleration (as you are trying to get to speed).

 

Update: I just pulled the plugs (after 1000 miles since rebuild).  #1 and #8 looks like the rest - lighting bad on those. Maybe a bit too clean?

Edited September 28, 2020 by Stude Light (see edit history)

[Grimy]

What does the vacuum gauge read?  Could be a vacuum leak from intake manifold or vac side of fuel pump.

 

Continuing a previous suggestion, the three balls or their channel may be worn.  Try watching timing mark with timing light while rpm is raised and lowered.

[FLYER15015]

Scott,

Where are your 2 mixture screws at ?

1 1/2 from closed ? 2 turns ?

Carbking is the best at this, but I would guess 2 turns is MAX.

 

 

Mike

[Stude Light]

Looked at timing and it moves as expected with rpm.  Without having a vacuum advance, it just follows rpm up the a maximum then holds there. 

 

Unfortunately, I don't have an available vacuum port at this time.  I tested using starting fluid at idle and no rpm changes so probably not leaking.

 

Idle mixtures screws are between 1.5 and 2 turns.  I just set them based on rpm a while back. They probably don't come into play at moderate throttle position, which is where I have the issue.

 

Thanks for the input thus far.

[FLYER15015]

I read up a bit on the WDO carb, and it seems the metering rods being set right and the same is critical.

 

Tell us again what she's doing.

"Bucking" during light acceleration, "starving" for fuel or just a lack of "pulling power" ?

 

Is there any way you can video an acceleration run with sound and post it here ?

It would go a long way in explaining the symptoms for us.

 

Mike in Colorado

[Grimy]

2 hours ago, Stude Light said:

Unfortunately, I don't have an available vacuum port at this time.

Where is the vacuum source for your wipers?

[Stude Light]

5 hours ago, Grimy said:

Where is the vacuum source for your wipers?

Yes, I could remove the line that goes from the intake manifold to the aux vacuum pump (part of fuel pump assembly) and use that. I was referring to an extra port that would make a simple attachment. Since I have to take off the exhaust manifold this winter and get it ceramic coated (correct porcelain coating from Independence Porcelain failed) I need to remove the intake also as I doubt I could reuse the shared gasket. That intake has a plug I’ll remove and create the spare port.

Issue was you could feel a slight surging fore/aft during normal accelerations - like it was starving a bit for fuel but nothing you would be able to see/hear in a video, just a feel thing.

 

I made two changes and the problem is solved. First, timing wasn’t as advanced as I thought so I bumped it up another 5 degrees to make about 10 BTDC at idle and I lifted the metering rods up about 1/64”.

 

I’ll drive it for a couple hundred miles like that and see results. If I get time, I’ll go through the hassle of pulling vacuum wiper pipe off intake, check vacuum and really dial in idle jets to vacuum rather than rpm, but I plan to go to Gilmore Tour and Show this weekend. Thanks again for input. 
Scott

Edited September 30, 2020 by Stude Light (see edit history)

[John_Mereness]

Yes, perhaps try a vacuum gauge.

 

Have you checked the firing order and also if firing on all 8 ?  

Funny, but I drove for a year with two crossed wires and never really noticed it - then a relative for was a tank driver in WWII walked by one day, cussed and swore and - lets say I never heard the end of that.

Also, vacuum leaks ?

Also, distributor needs to be in good condition.

And, it needs a good vacuum advance 

Rich does not hurt too much - by the way, a good chance your porcelain failed via running too lean.

 

 

 

[John_Mereness]

My 41 60 Special really liked high octane - 93 (it ran cooler and had much better feel about it). 

[FLYER15015]

AH, metering rods and timing!

I knew you could get her right !

 

Congratulations !!!!!!!

 

Have a great tour !

 

Mike in Colorado

[Stude Light]

3 hours ago, John_Mereness said:

Rich does not hurt too much - by the way, a good chance your porcelain failed via running too lean.

It only failed in some spots.  The hottest area (closest to the output of all the cylinders) looks great. I think bad processing and they offered to redo it but I'm just going with highest gloss ceramic I can get.  A number of flatty caddy folks tell me the porcelain just doesn't hold up, a few have good success.  I've also read that new cast iron vs aged cast iron can make a big difference. Not sure David Buick (who made his money in plumbing and patented the porcelain over cast iron process for bathtubs) had thought that 45 years later, a car company would be using his process for exhaust manifolds.....