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.