Problem:
I've got a 400 with a bad shake and rough idle. I know this could be any number of issues, so I'll try to provide as much information as I can.

Parts:
The engine is freshly rebuilt, it has aluminum Edelbrock 60cc heads, a 270HR 215/215 @.50 comp cams camshaft, 400 Edelbrock Performer intake, hydraulic roller lifters, 1.73 roller rockers, pertronix stock look cast distributor, champion RC12YC spark plugs gapped to .35, a double roller straight up timing set, long tube headers, a 625 Holley Street Demon Carburetor, and a stock mechanical fuel pump.

Compression:
Using a compression calculator, it seems like it has 8.8:1 dynamic compression and 9.91:1 static compression.

Timing:
I've had to settle on 10⁰ initial advance timing, otherwise I get detonation past 3000 rpm. I use 93 octane ethanol free gas. This distributor has an unchangeable mechanical advance of 24⁰. So my total timing is 34⁰. However, the higher I set initial timing, the smoother the idle is.

Observations:
The shake reduces and vacuum signal is stronger when I turn the idle set screw wayyy up. RPM hits about 1000 and the vacuum from the manifold source from the back of the carburetor nears 20inches. At these settings it shakes the least, but I get quite a bit of vacuum from the ported outlet when the idle screw is this high. I've got steady oil pressure at idle at around 37.5lbs. At 10⁰ initial timing, 650 RPM, and around 14.7 AFR, I get a fluctuating vacuum signal around 14-15.5 inches of vacuum. It shakes horribly and sounds rough.

I don't know what needs changing or if it could just be a vacuum leak. Anything sticking out to y'all like a sore thumb? Any advice would be appreciated.

 

Is this rough idle condition with or without the vacuum advance connected, and if connected to which source manifold or ported?

I had a similar issue on a 400 build pages of it documented in my build thread you can read thru..

Long story short the issue was a combination of things, one of which was the curve in the distributor that around 3000 rpm would land slide like 20* timing in a short period and induce my detonation.

Can you map a timing curve and post the numbers? And also what is the AF ratio at the time the knock happens, along with the Vacuum reading.

Also didn't see a mention is this detonation while under WOT, or light/moderate throttle where you may still have vacuum and the vacuum advance adding another 15*

Are you running EFI/carb type?

 

Very interesting. I'll have to work on checking those detonation values and map a timing curve this week. It's detonation at moderate throttle, and my engine is carbureted. Did you have remarkable shaking at idle?

Oh and this is without vacuum advance connected.

 

Has it run like this since the initial start up or did the problem develop ?

 

Well initial startup was with open headers and only a timing light for data. So as far as I can tell it's always run rough. I now have a tach/AFR gauge, but I'm fairly positive it's been shaky since initial start up. Also my spark plugs look slightly carbon fouled. I replaced the ones from start up today with no improvement. Nothing that looks like oil. (I'm very new to all of this, as it's my first engine rebuild ever.)

 

The plug gaps are too small. Increase them to 0.050''.

That dizzy is doing you no favors. 10* - 34* curve is not ideal. 14* - 30* will be more suitable. (The improved combustion with the ally heads require less advance.)

Raising the rpms at idle will increase vacuum and timing, as that's how the engine works, so don't rely on those observations.

I'd say that 650 rpms, assuming in Park, is a bit low.

Try 750rpms, in Park, with the correct plug gaps and full manifold vac advance. Post up the vacuum reading, needle behavior, and timing at that setting, both with and without the vac adv connected.

Ported is a waste of time. Rather connect to full manifold permanently (and adjust the idle down.).

Post up the numbers you used to calculate an SCR of 9.91.

Post up the part number of the cam.

Did you use the exact head gasket suggested by Edelbrock ?

Did you buy an assembled head, and if yes, did the spring pressures match the cam ?

Do you have a correct PCV system installed ?

What altitude are you at ?

Hi there 77 HOS.

 

Well darn, more money to spend! I guess that's all part of the learning process, but man my wallet is hurting on a new distributor! Any recommendations on a cheap option that I can change springs/weights on?

I will gap the plugs out to .050'' as you suggested, and I'm think I'm understanding correctly about the vacuum advance. I'm to hook up vacuum advance for the distributer I currently have to manifold vacuum and tell you what I get with/without?

Here are the exact parts in my engine:

Edelbrock Performer RPM Cylinder Heads 61625 : https://www.summitracing.com/parts/edl-61625/make/ford (From what I read the max lift for these is .580in and the max lift of my camshaft is .566in.)
Comp Cams 32-421-8 Camshaft: https://www.summitracing.com/parts/CCA-32-421-8
Howard Cams Lifters: https://www.summitracing.com/parts/HRS-91168
Comp Cams Rockers: https://www.summitracing.com/parts/CCA-17045-16
Comp Cams Timing Set: https://www.summitracing.com/parts/CCA-2121
Holley 625 Street Demon Carb: https://www.summitracing.com/parts/DEM-1901
My Useless PerTronix Distributer: https://www.summitracing.com/parts/PNX-D134620

Here is the calculator I used to get 9.91:1 SCR: https://www.gofastmath.com/Compressi...tio-Calculator

The data I input is as follows:
Cylinder Bore (inches): 4.030
Stroke (inches): 4
Cylinder Head Chamber Volume (CC's): 60
Piston Dish/Dome (CC's): 15.5 (https://www.rockauto.com/en/moreinfo...&pt=5620&jsn=9)
Head Gasket Thickness (inches): .41 (https://www.summitracing.com/parts/FEL-1013) I believe that this is the head gasket recommended by Edelbrock.
Head Gasket Bore (inches): 4.100 (https://www.summitracing.com/parts/FEL-1013)
Deck Clearance (inches): .087

Connecting Rod Length (inches): 6.58 (stock)
IVC @ .050": 33.5 (I used the valve timing calculator for this on the same website)

While I was going back through this information (thank you for asking me about my CR!) I discovered an error, I put .41 inches in for my gasket bore instead of 4.1inches, so depending on the dish size accuracy and the amount the machine shop actually decked my block my CR looked more like this:

STATIC Compression Ratio:

9.32:1

DYNAMIC Compression Ratio:

8.28:1

This number makes a lot more sense. I don't know if I could even run 93 octane with the cam I have at near 10:1 SCR. But, I'm very new to all this so I could be very wrong.

My PCV system consists of:
This PCV valve: https://www.rockauto.com/en/moreinfo...pt=5052&jsn=11
Connected to this valve cover: https://www.summitracing.com/parts/EDL-4461
Via this grommet: https://www.rockauto.com/en/moreinfo...pt=11785&jsn=8

I have a hose connecting the the PCV valve to a tree fitting on the intake manifold. My altitude is 358ft down here in Mississippi.

Thank you for replying, and I'm sorry for all the excessive info. I've been hard at trying to solve this, but here I am out of my depth again!
(Also this was a 351m when I bought it.)

 

There are so many things it could be. always start with the easy ones first.

Not to be insulting, but we can all make simple mistakes, check those if you haven't already.

Firing order correct ?
Vacuum leak, cover the carb with a rag does it die or speed up ?

Possibilities,
Bent push rod from a sticky new valve.
Screwed up aftermarket ignition
Fouled or bad plug
Carb issue, is it a new carb or a proven used one ?
I don't see anything about your valve springs, did you use the correct springs and were the heads machined for positive seals ? did you check for coil bind ?

And an obvious question, do you know what to expect from a cam like that? do you have a choppy idle or is it more like a miss?

 

Bingo ! Where did you get that number from ?

Thank you for posting up everything in such detail.

I'll check a few things later and post back with comments and questions.

Google ''vacuum gauge readings'' and look at lots of pictures.

Your 14-15.5'' with fluctuating needle is a huge clue. At sea level, which 358ft basically is, your aim will be +/- 21'' with a solid needle, connected to manifold.

Yes to hooking up the dizzy to full manifold vacuum.

440's comment about simple mistakes is spot on.

Yes, you might be out of your depth now, but it won't be long before the depth is a historical memory.

 

I don't feel insulted at all, I make 1,000 mistakes a day if not more!
The firing order is correct, and I ruled out the bent pushed rod about a month ago. I had to reseal the manifold, as oil was leaking into the head intake ports. I haven't tried putting a rag over the carb. Do I just lay it over the top of the choke and primaries? If it speeds up is it a leak?

I changed to fresh spark plugs yesterday, but it didn't make a difference. The heads came fully built with seals and springs from Edelbrock. I didn't recognize any binding when I removed the valve covers to check lash.

This was a brand new carb from Holley.

I expected this camshaft to be a little choppy, but it's not the sound I'm concerned with. It's the shaking at idle, this engine is practically jumping. And the mounts are good, new, and tightened down.

Thank you for your help by the way!

 

You got it, I appreciate the help. I need all I can get!

I calculated deck clearance using this calculator: https://uempistons.com/p-33-deck-clearance-calculator
Stroke
4
Rod Length
6.58 (stock)
Piston Compression Height
1.630
Deck Height (Crank C/L Deck)
10.297
Now unfortunately I am not positive on the deck height. I had a buddy do the machine work (he does good work) but he moved while he was going through my block. He lost the numbers from the machine work... He said they either didn't need to deck it at all or it was an extremely small amount. So I just put in the stock number.

 

I time mine with the OEM distributor, it has a quick advance kit in it and a bushing that limits the mechanical advance to 15 degrees. I use ported vacuum too, but I disconnect it and plug the carb's port for setting my timing. I set for total timing at 34 degrees BTDC at about 2K rpm, as mechanical advance is all in before that., lock down, reattach vacuum hose to port.

In my old notes going back into the late 1980s, I see that I was setting it at base 10 degrees, was getting only a total of 25, but then I hooked vacuum line to manifold vac.
I see where it did better setting base at 16 degrees, I got total of 31, and then I still ran the vac off the manifold ... and it was finally in 2011 that I tried checking it but my mech advance was coming in pretty quick above idle, so I just set it at 34 degrees @ 2K as a trial, started using ported vac too ... was the day I swapped to a Edelbrock 1405 .... timing works well for me.

​​​​​​​I realize mine is different, not a 400, but just 351M, older Crane 204 int./214 ex @ 050 inch / 0.484 int./0.510 exh, Performer 400 intake, E-manifolds. stock heads though I'd use the same on my 400.

I was gonna reuse the manifold vacuum the other day when I put the Holley 1850 back on after rebuilding it, but I forgot to try it, so happy I was with initial start, etc.

I just measured deck clearance on my 400, deck to top of piston next to bore wall ... not the extra 1/16" dish. I was getting 0.087" too?

 

That's how I had seen most people tune for vacuum advance in the videos I've watched on YouTube. The distributor I have is locked to add 24 degrees of mechanical advance, so I can't change any springs or weights etc. I didn't even think this was a thing until it was too late to return it

 

When you mentioned a fluctuating vacuum gage needle, did you mean ... a gentle float like movement almost like drifting, or short quick movements like flickering?

Were there any particular reasons to use a new distributor?

800 worked best for mine, I tried lower.
I used to do that, might try it again today.

 

Yeah, mine too, and it drops to 600rpms in Drive. As smooth as silk.