351C where to start?

[351mach1]

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.

 

[Q1SVT]

 

[basstrix]

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.

 

[Q1SVT]

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.

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

 

[Stanglover]

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.

 

[boilermaster]

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

 

[Q1SVT]

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.

 

[351mach1]

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!

 

[Tnfastbk]

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

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

 

[Q1SVT]

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

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

 

[351mach1]

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:

Trial	Air / fuel ratio (lambda)	vacuum (inches)	observations
idle - park	.9	19
idle - drive	.91	17
50 mph / light throttle	1.08	17
wide open throttle trial 1	.85 to .95	No vacuum	lambda all over the place
wide open throttle trial 2	.78 to .80	No vacuum	Hit gas less hard
cruising 50 mph	1	14
cruising 50 mph take 2	1	15
cruising 60 mph	.96	16
cruising 60 mph trial 2	.92 - .95	14
cruising 70 mph	.95 - .96	11 - 15
wide open throttle	.9	no vacuum
cruising 40 mph	1.05	14
cruising 45 mph	1.06 - 1.08	18

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

 

[Q1SVT]

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?

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

 

[351mach1]

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

Duh!

 

[boilermaster]

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

 

[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

 

[351mach1]

Got some time to play around. I replaced the intake gaskets which looked horrible. I am about 99% sure I have no vacuum leaks. I am at the point where the Mach is drivable and does not reek of fuel. It responds mostly well in most situations and honestly I think my limiting factors at the moment are the 3.00 gears in the rear and my transmission beginning to slip under too much load. I will keep tweaking the tune. I am planning to give Holley a call to see what they say about some of this but I know I am not going to be 100% happy with everything until I swap the rear end and transmission but I will try to make it as good as I can get it for now.

The biggest issue I am having at the moment is that when I *think* I have everything tuned as perfect as I can get it, I am idling extremely low. With the transfer slots on the primaries set as a square, I am idling at 550 RPMs in park and 500 in gear. I am actually surprised it idles that low, doesn't stall out, and is actually fairly smooth. I believe the misfire I am having is from being too lean. So what I am juggling with right now is how to get the idle higher and rich enough that its not stumbling at idle and making sure that is not contributing to my off idle stumble. I read that you can adjust the secondaries to raise the idle. My secondaries are basically closed at the moment and very far from secondary transfer slots. I can only get about 50 RPMS out of the secondaries before the idle becomes extremely rough and the motor stalls out. I went back to the factory position on the secondaries. I am considering drilling the butterflies but I wont do that until I talk to holley. I also read that you can install a petcock to essentially introduce a controlled vacuum leak to raise RPMS. Most of these "tricks" seem to be for heavily modified motors with very low vacuum which mine is definitely not.

Something I don't understand is that why can you adjust the curb idle screw if the transfer slots are meant to be set in a very specific position [a square]? I can definitely just screw that in and get the idle where I want but then I know I have too much transfer slot exposed and potentially causing a rich condition and possibly contributing to my off idle stumble.

I played with the pump cams more and no matter what I do, the pink cam basically does nothing. I barely even get a trickle out of the nozzle (and yes I am adjusting the spring tension each time). I am back to using the stock orange cam and still playing with which nozzle seems to perform the best. Currently I found that using the .035 I do not lean out at WOT off the line but it still leans out on WOT from a cruise. Each nozzle seems to perform better in certain situations but none of them are perfect across the board which I think is due to other adjustments in the carb. I am also finding it difficult to get consistent numbers when running tests. I can get a general idea but then the numbers seem to be all over the place and not consistent.

here's what I am running:
Stock 1974 351 Cleveland (8.5:1 compression), 4v heads, cam advanced 4 degrees on the timing chain, Edelbrock performer intake.
-C6 transmission
Holley 4160 600 CFM Street warrior with vacuum secondaries, electric choke
-66 main jets
-8.5 power valve
-secondaries are on a metering block
-orange pump cam
-duraspark 2 ignition. 15* initial, 35* total, 20* on the vacuum can
-Autolite platinum 26 spark plugs (might switch back to the 25s)
-91 octane costco fuel; 10% ethanol

I am averaging about 10mpg but I have been doing a lot of WOT runs trying to figure out the tune.

The following numbers were with the .031 nozzle and this was the best I got the engine to feel. Since than I have mucked around some more and not feeling as good...

Condition	Vacuum (inches)	Air/Fuel Ratio	RPM	Observations
Idle - park	19	12.9	665	~100 rpm above square transfer slot
Idle - drive	17.5	13	580	smooth idle
Cruising - 50 mph trial 1	15	14.0	2260
WOT - starting at 50 mph cruise	0	13	—	AFR went to 16 then settled in at 13
Cruising - 60 mph trial 1	15	13.5	2580
Downhill - zero throttle	22	12.8	—
WOT from stop	0	11	—	initial lean spike on AFR, going down to 11
WOT from stop trial 2	0	12	—	Initial lean spike, going down to 12
Downhill light throttle	18	14.8	—
Cruising - 50 mph trial 2	15	15	2080	slight downhill
Cruising - 60 mph trial 2	17	14	2540

RANDOM NOTES:

• Power from stop feels good
• Power from WOT started at a cruise feels like it is lacking

Returned to garage for additional adjustments

• Adding .035 accelerator pump nozzle appeared to address initial lean spike on AFR.
• It did not minimize lean consistently however during subsequent informal trials and it still does not feel like it is pulling well.
  • It does not lean out from a stop with .035 nozzle.
  • From 45 cruising to WOT, it still leans out.
• Feels like the amount of power is now disrupting the function of the transmission, which is maybe a good problem to have!

 

[boilermaster]

351mach1,
it does look like you are gaining some ground here, IMHO you need to go back to the 6.5'' hg.
power valve and now do a little main jet tuning, I always tune the primary main jet before the power valve, you are kind of putting the apple cart ahead of your horse,
pretty sure Holley will tell you the same, enjoy the wait on the phone.
Boilermaster

 

[basstrix]

Don't get hung up on having the transfer slot (TS) perfectly "square". That's a starting point and it's more important for engines having relatively low vacuum at idle. When vacuum is low, the throttle plates have to be opened more to let in the same amount air vs at higher vacuum. The problem associated with the TS is that the low vac engines use up too much of the available range where the TS can effectively enrich the AF mixture. The vacuum signal (to the boosters, aka high speed circuit) is weaker on these engines thus you end up with a sort of dead zone between where the idle circuit and HS circuit transitions. Going back to the square TS opening, maximizes the available range that the TS can enrich and reduces that dead zone. To get there, you have to let air in somewhere else...like drilling holes or cracking open the secondaries. You should not have that problem with the amount of vacuum your engine has.

WRT your idle condition, if it's lean, open the mixture screws a little at a time. You should be adjusting for max vacuum, max idle speed with both screws out the same number of turns. When you're close to the right number, you'll be making 1/8th turn adjustments. I mark the metering blocks where the screws are when lightly seated so it's easy to see how much I've changed them. Do all of this with vacuum advance disconnected. Don't try to hit a specific AFR with the mixture screws, rather pay attention to what AFR you end up with when it's idling at max speed, max vacuum.

WRT the 8.5 vs the 6.5 PV. The vacuum at cruise is noted at 15". Both a 6.5 and an 8.5 PV will be closed under those conditions. The only reason to try the 8.5 was to bring in the rich charge slightly sooner when the throttle is stabbed. The 8.5 will have a negative impact on economy....if the engine is operating on the HS circuit, every time the vacuum drops to 8.5 or less, that PV circuit opens and more fuel dumps in. It's not much, but it does add up.

Strange behavior with the pink cam.....that's the std cam holley uses on a lot of their DP carbs. Gotta give the engine what I needs....so if it's not the pink cam, so be it.

Have you checked the flatness of your carb's main body and the metering blocks? I had a brand new 650 DP body from Holley that gave a horrible stumble. All I did was swap out the main body...the old body had the choke horn cut off, and I wanted the option of having a choke. I spent an hour with a rasp and got the body flat and eliminated the stumble...that was after going through bunch of other checks that "should" have fixed it.

Sounds like you're making great progress. Thanks for the update.

Edit: 11:1 is too rich on WOT. 12 is closer to where you want to be, so I agree with Boilermaster, on the main jetting. You could stand to drop 2-3 numbers on the mains...but beware, that might make the stumble worse. To get that 11:1 increased, you'll have to change the PVCR jets...I doubt they're replaceable.

My engine with a 650DP likes a #62 main and my 600 likes #63....BUT....I'm running at 15:1 at cruise and that might be too lean for your liking...I'd hate to see you hurt your engine.

 

[351mach1]

Just ordered 68-72 jets. Ill swap back to the 6.5 PV when I change the jets. Any thoughts on which jets to try first? 68 then keep going up?

Edit: While waiting for the jets I think I will swap back to the 6.5 PV and follow @basstrix tuning advice. Basically not get hung up on the transfer slots and retune. I will shoot for 700-750 idle RPM and retune the idle mixture from there. I think I will also go back to the autolite 25s sparkplugs just to try to keep things consistently stock.

 

[basstrix]

I added some notes to your AFR vs speed/vac input. Your mains are really not too far off....maybe just a tad rich....you're getting into tuning of the metering block to get the WOT AFR you're looking for. You may just want to live with it being a bit rich and not go down that long and crazy rabbit hole.