351C where to start?

I bought just about everything possible to solve this issue, just haven't had time to install it all. I still need to install:
-Stainless fuel lines
-Fuel Pump
-AFR gauge
-8.5 power valve (and carb rebuild kit)

The good news is that I think I found what was fighting me and I have the engine tuned really, really close. So turns out the "brand new" cardone dizzy I bought had a leaking vacuum advance can. With a non leaking vacuum advance, it runs much better. After much reading of forums, I also decided to switch to manifold vacuum for the vacuum advance.

I still have the very slightest of small stumbles but I will install the AFR gauge to help dial in the carb tuning and hopefully get it 100% solved.

I ended up using the blue pump cam and .035 nozzle for now. The pink cam is actually significantly less aggressive than the orange and the orange and red appear to be identical.

I am not exactly sure what detonation/pinging sounds/feels like but I definitely did something that my engine was NOT happy with. With the defective vacuum advance can installed, I had 2-3 instances where at WOT I would have a severe hesitation, with some smoke from the engine, and I immediately let off the gas and everything went back to normal. After looking around the engine bay it appears that it was some kind of backfire as the intake gaskets got pushed out which also is causing a very very minor leak at the engine block as well. Ill add reinstalling the intake manifold on the list of things to do. With the new vacuum can on, the engine is running great and this issue does not happen at WOT anymore.

I did a compression test and the engine seems to be in good working order. Cylinder one is actually closer to 140. I checked it a second time and got a higher number.

I think after reinstalling the intake, replacing the steel lines and fuel pump, installing the AFR gauge and fine tuning the carb, I am happy to report that this engine appears to be healthy and will be I'll be confident in saying that it's in good working condition. Oh, and the engine is pulling close to 18" of vacuum so I will try swapping out the 8.5 power valve and reclean the carb when I install the fuel lines and fuel pump.

Sounds like you're almost there with getting the tune dialed in. Great work!

Were you able to find a definitive cause to your stalling problem? The filter on the suction side of fuel pump was suspicious IMO. In general, fuel filters on the suction side of a pump will have a negative impact on fuel flow and will promote vapor lock...although that's much more of a concern with a filter back at the tank. If you're going to put a filter on the suction side, close to the pump is the best option.

Q1SVT beat me to posting up the pump cam chart. I'll go ahead and post a color version that might help discern each curve.


With regard to the pink cam, I encourage you to try it. The pink cam spreads the pumps shot over a much broader range of throttle application. It might help, it might not, but you already have it, so why not take an hour and test it. I suggest testing in both screw positions.

Like the pink cam, I encourage you to try the 8.5 power valve if you're still experiencing a stumble.

Glad you found the defective vacuum canister. Proper base timing and the addition of vacuum advance can certainly alleviate or reduce a stumble. You can adjust the contribution of advance from the canister using an allen wrench accessed from the hose nipple on the canister. Below is a chart I created by measuring advance vs vacuum at different adjustment positions.

You described a concerning engine behavior under full throttle. I suggest you prioritize knowing the AFR and the ignition timing advance curve to understand why this happened. Ideally, you let the engine tell you what AFR it wants using a dyno or making timed runs at the drag strip. I use a device called Dragy in lieu of a drag strip. As a safe starting point, I suggest you shoot for a range of 12 to 13:1. My engine likes 12.8:1, but you should be safe in the above range. DO NOT shoot for 14.7:1 under wide open throttle! That is too lean. 14-15:1 is good for cruise speed at light throttle, but richer is ok too...just won't be as fuel efficient.

Where is your PCV connected? It should be at the base of carb or some port common to the intake manifold plenum. Also, make sure your brake vacuum booster is not leaking. That will lean out the runner the line connects to....your #8 looks like it could be lean.....#4 looks like it's been cooked.

The advance curve consists of the sum of base timing, mechanical advance, and vacuum advance. 34-36 degrees total under WOT is a fairly typical number that a cleveland likes. Here's a quick rundown of determining the advance curve: Disconnect vacuum advance, and set the base timing with engine idling. Then rev the engine and hold stead at various RPM and measure/record timing at each RPM. You shouldn't need to go beyond 3000-3500 RPM...and if you're like me, it's unnerving to rev to those levels without any load on the engine. Subtract the base timing from the values at each RPM step to determine contribution from mechanical advance. Next, you want to measure the RPM and manifold vacuum aka Manifold Absolute Pressure (MAP) at idle, cruise speed in high gear, and WOT. This must be done while driving, so you'll need a vacuum gauge and long hose. Use the chart I posted to add in the contribution from vacuum advance. Vacuum should not be more that 2-3 in-hg at WOT, so it usually has no contribution under that condition. Be sure you're in the 34-36 degree total advance under WOT. Even 36 might detonate....so this is important step.

I suspect the engine condition you described was detonation. The pressure inside the crankcase must have been pretty high to push the end seals out from under the intake. Was the dip stick pushed up or PVC blown out? Your distributor's advance plate has 2 positions. You need to determine which position it is in. A "10L" advance plate is a good one to have. It will let you set base timing at 14 deg and add 20 more via mechanical for a total of 34. BTW, 10L stands for 10 degrees of distributor advance (equals 20 degrees at crankshaft) and a left hand rotation distributor. You have to pull the reluctor and advance plate to check what you have. Set it to use the 10L if your distributor has that. Not a bad idea to install the Mr Gasket advance springs at the same time.

Again, great progress and please let us know what was causing the stalling and blown out intake end seals.

basstrix said:

As a safe starting point, I suggest you shoot for a range of 12 to 13:1. My engine likes 12.8:1, but you should be safe in the above range. DO NOT shoot for 14.7:1 under wide open throttle! That is too lean. 14-15:1 is good for cruise speed at light throttle, but richer is ok too...just won't be as fuel efficient.

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351mach1 ​

... the numbers are good, but you need to be running Pure Gasoline...
14.7 under Pure Gasoline equals 14.08 with just 10% Oxygenated

A couple of awesome information post guys, thank you.
I need to save this thread.

For my own engine, I'm pretty close to the settings given, but could be improved slightly.......... in spring.
One thing I will mention is my engine, 351C M code CC heads and 13cc dished pistons running at 17in/hg, did not like an 8.5 power valve at all. I went back to the 6.5 the Holley 670 came with.

351mach1,
You are getting some priceless tips from some of our top of the line contributors.
T would like to add, if you change out fuel pump, spend the time to re check and adjust float levels
and idle mixture, as any change in fuel pressure will have an effect on float levels and therefore air /fuel ratio.
Please be advised that all intake manifolds (especially dual planes) will have cylinders that are either richer or leaner than others, that is why they don't lock down a specific air fuel ratio and suggest a range instead.
Lastly, I am not a fan of the print o seal intake gaskets for our 4V cylinder heads, you will never find a perfect match with that Edelbrock manifold.
I might get some flack here, but I have been making my own for quite some time and have had better results by doing that.
I end up glueing them to the intake manifold and not the cylinder head.
Boilermaster

Boilermaker wtf glueing gaskets, didn’t you know that’s a ….

Bin doing forever. lol
I only glue them to the heads with the bottom of intake runner and gasket alignment.

WOW! Some really phenomenal information here! Really, thank you all for taking the time with such in depth responses. I will have limited time to work on the car over the next 2 weeks but will do what I can and report back here.

I have driven the Mach about 200 miles now (since getting it running right) all around Vegas; stop and go and highway (even up to 110 mph) and its drivable. Not perfect but I am getting around. My transmission is on its way out so that is preventing me from really beating on it but I have been having some fun. It has not stalled since I have installed the new vacuum can but I also have only been taking it to about 80% on the throttle when doing a pull. My gut is telling me that the stalling had something to do with detonation as I am not having any stalling or hot starting issues when driving like a normal human being.

I should get a full day of work on it on Wednesday and we will see where I get with it.

Thanks everyone and I'll keep ya posted!

Q1SVT said:

Boilermaker wtf glueing gaskets, didn’t you know that’s a ….

Bin doing forever. lol
I only glue them to the heads with the bottom of intake runner and gasket alignment.

View attachment 97564 View attachment 97565

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Well at least according to the can it only causes birth defects in California, so the rest of us should be safe.

Tnfastbk said:

Well at least according to the can it only causes birth defects in California, so the rest of us should be safe.

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lol... i live in Kalifornia and got it shipped here somehow

BIG UPDATE!

I was able to put a lot of work in on the tuning and had some eye opening realizations.

First up: I replaced the steel lines, mechanical fuel pump, recleaned the carb, and swapped to the 8.5" PV.

I purchased: https://www.npdlink.com/product/fuel-line-tank-to-pump/106974/203251 What should have been a simple 30 minute job turned into a 2 hour struggle fest. The replacement stainless steel hose is a great simple idea in theory but the one that I received was not bent correctly. It was about 2 inches too long in the long straight section under the floor board and the rear by the gas tank was completely wrong. After a long, ugly, fight, I made it work. I am not totally happy with it but it works and it was definitely a better idea than making a line from scratch. If it was a true exact swap, it would have been very easy and amazing. Disappointingly, the original lines appeared to be in good condition with no rust on the inside of the tubes on either end.

After getting everything back together, I was struggling to set the float levels on the carb; either they were too low and I would run out of gas or they would overflow and shoot gas out the vents. After scratching my head for awhile, I figured out that the new mechanical pump was putting out over 10PSI of pressure. Down to the auto part store we go and find a pressure regulator. I cannot believe how many terrible reviews almost everything gets now adays; like is everything just straight garbage from china? Anywho, I crossed my fingers, got the pressure down to 6psi and that solved the float adjustment issue. The car was running pretty well after this but I wasn't done yet.

Next I installed the AFR gauge. This is what was truly eye opening. As I have mentioned in previous posts, I am a handy wrench but this is the first time I have ever worked on a carb. The first thing I learned was that my idle was running way too rich. Up until this point, I set the idle screws at 1 and a half turns out and tried to find the highest vacuum from there but apparently I didn't do a great job. Using the AFR gauge, I think I landed on less than a turn out to get close to 1 lambda (I ended up using lambda readings because that seemed to make more sense to me with the E10 gas). Next thing I learned was that the pesky stumble I have been fighting was 100% a lean stumble. It was quite obvious with the AFR gauge and wow that thing is priceless. Now here is where I am super confused: I was getting the lean stumble with the .035 nozzle but it went away when I went down to the .033 nozzle. Can someone please make that make sense? Regardless of whatever is going on in there, I was easily able to see the lean stumble on the gauge to figure out what was going on.

Some other observations: I tried various pump cams and the issue I had with the pink cam is that the pump arm wouldn't even touch the cam. I ended up removing the arm and bending it to try and force some more pump shot. I still have room for improvement but with the limited time to work on it, I ended up sticking with the blue cam. IIRC the blue cam is supposed to be used in conjunction with the 50cc shot diaphragm. I think I can still dial this in further but I am looking to figure out why I am getting better results with the .033 nozzle and blue cam. From what I have read, a stock 351c should not need the 50cc pump.

Some answers to previous replies:
-My PCV is set up correctly and plumbed into manifold vacuum at the front of the carb
-When I experienced the detonation pushing out the intake gaskets, the PCV valve and oil dipstick did not get pushed out.
-I am pretty sure I have the timing dialed and my dizzy is recurved properly. I logged my experience with the distributor in this thread: https://7173mustangs.com/threads/duraspark-ii-with-ram-air-debacle.45930/
-I still need to dial in the vacuum can. I stole the current one from an autopart store dizzy and it seems to be non-adjustable and might already be leaking. I will order a Standard VC31
and see how that does
-I am not having any stalling problems anymore. I am still confused as to what was going on there? I have a spark checker on board now so I can check for spark if it happens again.

So next up on the to do list is to replace the intake manifold gaskets. I am concerned that I am getting a misfire at idle that is being caused by a vacuum leak and/or being too lean. I wont be able to accurately gain any more useful information until I replace those gaskets. I am also wondering if I should swap to autolite platinum 26 plugs? I know the 25s are the recommended plug for the Cleveland but does using the duraspark ignition (with open chamber heads and dished pistons)
change which plugs to use?

To do list:
-Replace intake manifold gaskets
-Check brake booster for leaks
-Check vacuum can for leaks and adjust
-possibly replace spark plugs with AP26 if still getting a mis at idle
-continue to tune using AFR readings.

So I was in a hurry trying to get some preliminary AFR readings. I haven't had a chance to digest this information yet but I wanted to post it on here to see if any of the experts have any opinions on fine tuning. The gauge seems to jump around quite a bit so it was quite difficult to dial in exact numbers. I am not sure how normal this is. It seemed that more often than not while cruising that the gauge would be reading lean. I would like to focus on my to do list then re-run my tests. Also this is with 91 octane E10 gas:


Lastly, I believe this is a miss at idle. It wasn't doing this before leaning everything out. I do know that I need to replace my intake gaskets so not worried about it too much yet.
View attachment PXL_20250215_022330326.mp4

351mach1 said:

I was getting the lean stumble with the .035 nozzle but it went away when I went down to the .033 nozzle. Can someone please make that make sense?

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Because fuel droplets do not burn... fuel vapor does.

You might find over time you can go to a .031 and a different smaller Carb pump cam... tuning is a cyclical process until all is dialed in :^)

Love your effort, stick wit it

Q1SVT said:

Because fuel droplets do not burn... fuel vapor does.

Click to expand...

Duh!

351mach1,
Make 100 % sure you dont have any vacuum leaks or these numbers are invalid.
you are pretty lean except for idle (marginally lean) and WOT (power valve open).
You might want to post your carb # and or main jet size, thinking most 600 cfm vacuum carbs have a 65 or 66 main jet you may see better numbers with a 68 or 70 main jet.
once cruise is better 13.5 -13.8 af, (thats only my personal opinion) you will be able to go back to a more normal pump cam.
most here tend to speak air fuel ratio and not lambda, so you will get more response with a/f cause most are not going to take the time to convert it into what we understand.
Boilermaster

351mach1,
I finally looked up your #330 pink cam, holy crap, 30cc on #1 position and 37.5 cc in #2 position,
me KNOWS you are masking something here.
218 white #1 17cc #2 19.5cc
427 blue #1 18cc #2 20 cc
240 red #1 18.5 2 20
466 orange 18.5 20
234 black 19 18
290 green 24 30
330 pink 30 37.5
336 brown 36
Boilermaster