351C 2V Heads Help

[Ron Tanzi]

Did you degree your cam when you installed it? If you didn't did you install it straight up as in not advanced or retarded. It sounds like your cam timing is advanced which will increase cylinder pressure and will cause detonation. Typically advancing the cam a couple of degrees will help increase low end torque.

Ron

 

[basstrix]

If you're running a 2.5 PV, that will have to be replaced and WILL cause a stumble on throttle stab. The original PV for that carb is 6.5. With a cam like yours, I'd run an 8.5. What that means is that the Main Circuit will be enriched via the PVCR's (power valve channel restrictors) when vacuum drops below 8.5" of vacuum. There is A LOT of misunderstanding around power valves..including the power valve video on Holley's website.

What's important about what's stated above is that the power valve enriches the Main Circuit. The main circuit does not come into play until the engine is between 2000 and 2500 RPM (closer to 2500). Until the mains come on, the engine is running through the Idle, aka, Low Speed circuit. The IFR's (idle feed restrictors) are what meter the fuel on this circuit. Another way to say this is that the power valve can be wide open at idle and it won't change a thing because the main circuit is not active....a lot of folks have trouble grasping this concept due, in part, to the misinformation out there. I have a classic example of a PV opening and closing that shows how the PV changes Air Fuel Ratio (AFR).

Here's my suggestion for this carb. Take it back to the config from Holley and start over. It's going to run a little rich on the low speed circuit, but it should eliminate or drastically reduce the stumble. The main circuit will be pretty close to where you want it for wide open throttle AFR.

Make sure the secondary throttle stop is at the original position as Holley set it up. That means to back off the throttle stop screw until there is an air gap, then tighten it until it touches and go an additional 1/4 turn, no more. Holley uses this same throttle plate on carbs up to 750cfm so the transfer slot behaves a little differently. Have the throttle plates been drilled? If yes, measure the holes and let me know the size and which plates are drilled.

Pri Main: 67
Pri PV: 6.5 (I think an 8.5 will work better for you)
PVCR: .039 (check with gauge pin if available or get close using drill bit)
IFR: .035 (also check)
Pri HSAB: .030
Pri IAB: .080

Pri & Sec accel nozzle: .028
Pri & Sec pump cam: Pink

Sec Main: 73
No Sec PV in this carb
No PVCR
Sec IFR: .039
Sec HSAB: .031
Sec IAB: .054

Edit: What is your altitude? The higher the altitude, the richer the original config is going to run.

 

[boilermaster]

pjs2pc,
First the easy part.
It looks like your distributor bushing is going to allow 10 degrees distributor and 20 degrees at the crankshaft mechanical advance.
Does your distributor have vacuum advance as well?.
If so where is it connected. ported or manifold?
A look at your spark plugs suggests some spark plugs are trying to burn clean and some are burning rich (I do not see any evidence of oil fouling) looks like Autolite 25 to me.
I am just going to cut to the chase here and say you need a compression test, check for bent pushrods and make sure all cam lobes are in good shape if all is good there you need to check and possibly re-do lifter preload.
When the cylinder heads were done, was installed spring height checked and adjusted ?.
Stock, non adjustable rockers ?
lifter preload can be adjusted via Crane Cams shims (and others)
1/2 to 3/4 turn from zero lash usually gave me good results with the 5/16 rocker bolts.
Lastly, you are probably going to put your carb back to stock (at least the power valve) and still perhaps some acc. pump tuning.
As Ron stated, cam timing may also be an issue.
It appears to me that you took a wrong turn a long time ago that carb tuning and timing will not help.
Boilermaster

 

[cjstaci]

Sounds like a tuning issue and basstrix’s post should get you going in the right direction.
If your throttle plate is open past the transfer slot you will most likely have a lag or hesitation. Also, 2.5 power valve is too low. Your power valve should be about 1/2 of your idle vacuum in drive, but before you change it you need your idle and timing tuned correctly. What is your total timing advance? With a 280 cam, you should probably be around 12-15 degrees of initial advance but with mechanical and vacuum advance that would most likely put your total advance to high. With a Holley carb, you shouldn’t have to make a lot of changes to its stock configuration to run decent. Changing pump cams and nozzle sizes are for fine tuning. Jet sizes should be pretty close within a couple of sizes. Your engine sounds good in your short videos, leads me to believe it’s a timing and carb adjustment issue. The other thing I think isn’t right is a 3600 rpm stall converter, that seems too high for that cam, I would think a 2500 would be more in the ballpark. Also, in regards to the camshaft, what was the recommended compression ratio and rpm range. I remember someone I knew put a big cam and a high stall converter in a low compression cleveland, it sounded cool but it’s performance wasn’t very good. Just a wrong combination of parts.

 

[AcesArneson]

Did you check your balancer? The rubber dries up and can throw you out of balance.

 

[boilermaster]

pjs2pc,
After watching the video it is a no brainer that you at least need primary accelerator pump tuning, however not knowing where true TDC is
(and therefore curb idle speed) will have an effect again on the pump cam timing and clearance.
you should be able to make it better without, just not perfect.
transfer slot opening is the same deal too, better to remove carb, set the slot and get the mixture via the mixture screw and bleeds.
what I cannot get over is the vast difference in spark plug color and the absence of an ignition timing color line.
In my opinion there is more going on here than pump shot and some
timing.
Boilermaster

 

[pjs2pc]

Thanks again for all the input. Think my next step is to put the Holley back to stock and go from there.

I think the advance stop bushing I have installed is the blue one. So that gives me around 36 degrees all in by around 3250 rpm according to the msd chart and my initial advance.

I really think the carb tuning is the issue. It’s been awhile since I changed that power valve, but I know I did half vacuum in drive at idle. It was low if I remember, but what I don’t remember is how the car performed before I changed the PV besides the junk idle. Time to find out.

 

[will e]

Getting the carb back to 'stock' is a solid plan. The car should run pretty well in that configuration. If it doesn't then you can focus on other aspects first.
I am not super experienced with performance tuning but have spent a fair amount of time under the hood. I also keep spread sheets to track any changes I make and the results.
As a general rule. Mess with the carb (other than the initial bowl, rpm, choke and initial accel pump adjustment) 'last'. Once you've got it idling, cruising, driving and accelerating (without detonation) 'pretty good' then start messing with the carb. There might be further adjustments on the dist advance (initial, mechanical and even vacuum) and maybe mess with hotter/colder plugs while you do carb adjustments but the baseline is set. And if it goes sour on me and I can't figure it out, well I can always go back to baseline.
My second general rule about holly's. Less is more. Less messing with it the better. A lot can be done with just the power valve, jets and secondary springs. For non-radical drag cars this will get you most of the way to perfect. Squirter sizes, accel cams, the larger volume accel pump, different ventries and the like will help mostly getting that little bit of extra out of it, if done properly and in conjunction with the rest of the system.
I am sure other opinions will vary as we all have different knowledge and experience.

 

[Bill Kanode]

pjs2pc,
Just looked up your carb o the Holley web site,
pri. jet 70, .028 pri. squirter 6.5 pv.
sec. jet 80 and .031 sec. squirter.
While I speak holley l prefer vacuum secondary's and don't know double pumpers very often.
both the jetting and power valve seem fine to me as a starting point but I find it alarming that Holley states that you have a .028'' primary and .031'' secondary squirters, this seems backwards to me.
I would suggest that (AFTER) you get your detonation under control,
that you recheck accelerator pump clearance at WOT for .015'' clearance with the pump lever fully depressed.
please check and verify the size and location of the squirters.
Someone may knock me here, but I think you need the .031'' in the front and the .028'' in the REAR !
If I am correct, this will at least get you going in the right direction.
my cleveland , 408 4 v needs .035'' primary with 370 rear gears and 5 speed manual.
you also need both cams in the # 1 position IMHO.
Boilermaster

I do not like using a double pump Holley carb for a street engine but prefer a 3310 with vacuum secondaries that the amount of airflow and fuel can be tailored to your engine needs at your preferred RPM and load. The 780 CFM (3310) has always been my go to carburetor for decades. I will be using one instead of the 600 CFM that I originally bought for my H code engine which has a little more aggressive cam and an Edelbrock dual plane for the 2V engine. I also changed from a FMX to a wide ratio top loader with a 3.00 ratio which came from the factory. I haven't had it on the street yet but am looking forward to doing so. Thx, Bill

 

[351c1971]

pjs2pc,
Agree 100% with what has been recommended here so far.
I would add, don't chase numbers on a timing curve unless you 100% know where TRUE TDC is, Ford balancers can and will slip as well as the pointer can be manipulated keep retarding timing until the detonation is gone (that will save your engine.
check your PCV system for proper operation and make sure that it is not sucking a bunch of oil causing the detonation, a leaking intake manifold gasket can cause the same, pulling in oil from the lifter valley.
You did not state vacuum or mechanical on the Holley, if vacuum, I like to know exactly what rpm the secondary's are just starting to open, too soon and you will get a lean bog, and if you are (all in on timing) possibly lean detonation.
I have found a hitch pin clip works well, placed between the stem of the diaphragm rod and the housing, just snug up the clip and start making passes, might be helpful to know what color spring you have .in there too.
You might be surprised what opening too soon adversely effects performance, if you hear the secondary's open, that is too soon.
Boilermaster

Had the same problem and used the Holley vacuum secondary quick change kit on my 600 CFM Holley. I believe that there are also 2 settings on the secondary linkage to aid tuning.

 

[pjs2pc]

Been awhile with not much progress. Busy with work and kids. Long story short, got the carb back to stock. 6.5 power valve, correct jets, accelerator pump nozzles, etc. leaning towards the accelerator pump circuit. Any semi-quick throttle off idle or anywhere under 2000rpm and engine falls on its face. Hold at 2000rpm and give her a good throttle hit and she’s fine. Attached video should demonstrate pretty well.
View attachment IMG_7590.mov

 

[Bill Kanode]

Been awhile with not much progress. Busy with work and kids. Long story short, got the carb back to stock. 6.5 power valve, correct jets, accelerator pump nozzles, etc. leaning towards the accelerator pump circuit. Any semi-quick throttle off idle or anywhere under 2000rpm and engine falls on its face. Hold at 2000rpm and give her a good throttle hit and she’s fine. Attached video should demonstrate pretty well.
View attachment 91264

I think that either you need a new accelerator pump, adjust the clearance from the pump lever or check the plastic pump cam for correct placement. The point is you are not sending extra fuel when you accelerate which is the purpose of that circuit in all carburetors. With computer control FI we call that "pump shot" to overcome a engine bogging down during that phase of driving. Look at Holley informational website for information on adjusting all parts of your carburetor.

 

[pjs2pc]

I think that either you need a new accelerator pump, adjust the clearance from the pump lever or check the plastic pump cam for correct placement. The point is you are not sending extra fuel when you accelerate which is the purpose of that circuit in all carburetors. With computer control FI we call that "pump shot" to overcome an engine bogging down during that phase of driving. Look at Holley informational website for information on adjusting all parts of your carburetor.

Definitely getting fuel out of the accelerator pump. Considering replacing it just to ensure that it’s working correctly. I’ve got the pink cam in position 2 for a 900-1000rpm range. Adjusting the pump lever never seems to match Holleys advice. If I limit lash as throttle just moves, at wide open throttle there is always a small gap between the lever and the nut. That’s always concerned me some. Seems like I have to pre-load the spring more than what Holley does to get close to no lash with a closed throttle ie I have more threads of the adjustment bolt showing above the lever.

 

[Bill Kanode]

I hope that you don't have a dual pump carburetor as the extra fuel can also cause an engine to bog down a street car. Dual pump carburetors are OK for race car but a poor choice for street cars. With a vacuum secondary carb you can tailor when the secondary butterflies open by using different springs in the secondary pod and some have a fine adjustment screw on top of the pod. Most of my experiences come from a 1969 Z28 that had a 302 with 4:11 gears and the factory used a Holley 3310 with vacuum secondary carb. I changed to a dual pump which was a mistake and went back to the factory 3310 carburetor which performed better.

 

[boilermaster]

pjs2pc,
Try adjusting your pump like this,
choke fully open, wide open throttle engine off.
place a flat blade screwdriver on the pump body lever and fully depress it.
Now measure between pump body lever and bolt head, shoot for ..015 inch clearance.
when you remove the feeler gauge the pumps spring should take up that slack.
Now go back and make sure you see fuel spray the instant that the throttle lever moves off of idle.
If you still have hesitation go to the next larger shooter size.
Boilermaster

 

[pjs2pc]

pjs2pc,
Try adjusting your pump like this,
choke fully open, wide open throttle engine off.
place a flat blade screwdriver on the pump body lever and fully depress it.
Now measure between pump body lever and bolt head, shoot for ..015 inch clearance.
when you remove the feeler gauge the pumps spring should take up that slack.
Now go back and make sure you see fuel spray the instant that the throttle lever moves off of idle.
If you still have hesitation go to the next larger shooter size.
Boilermaster

I’ll work on adjusting and see if it helps any. I had the next size accelerator pump nozzle in and still had the issue, that’s why I went back to stock to start over.

And yes, it’s a double pumper. But I do have a high stall and 3.70 gears. I have a mechanical secondaries 600cfm edelbrock that doesn’t have the hesitation (if it does, it’s at least minimal). Problem is 600cfm isn’t enough and you can hear/feel it in the higher rpm’s. Holley seems to make a little more power or at least it feels that way. Just seems to be bear to tune.

 

[TommyK]

As previously suggested by others, verify 0 degrees on the balancer at TDC and verify t slot exposure.

I think the plugs are fouled because it is idling on the t-slot. This would also account for the off idle stumble.

If those things check out try a blue pump cam.