440 Six Pack Tuning

[Hellcat]

Hello Everyone,

I'm working on a 440 in a '70 Cuda. I'm having some trouble tuning it so I wanted to get some of your thoughts. It runs decent, but at idle my vacuum is bouncing between 14" and 12". It used to be bouncing between 15" and 10" so I guess it is better, but not yet right. I upgraded the ignition and found a plug wire that wasn't crimped tight enough. I assembled the wires myself using MSD 8.5mm wire. I checked the wires and found the one bad so I plan on double checking them just to make sure I didn't miss one. I believe I should be getting 15" of vacuum at idle since it is a fairly smooth idle cam and talking with Don at FBO they agreed it should be 15". Any ideas of what might be causing the low and bouncy idle vacuum? Bad wire, leaky valve, timing, mixture/carbs not set right, etc..... Here is a summary of what is in the motor......

440 bored 0.030"
10.5:1 compression ratio
stealth heads from 440 source, bought them assembled so I didn't check the valves....maybe that was a mistake
Comp Cams XR274HR-10, Hydraulic Roller 274/282 duration, 224/230 @0.050", valve lift 0.538/0.534
Comp Cams roller rockers set at about 1/2 to 3/4 turn pre-load (took a couple tries to quiet them down)
Carbs rebuilt by Promax, 6.5 power valve, 64 jets in center, 84 jets in outboards
mixture screws are set at about 1.5 turns out on the center and 3/8 turn out on the outboards
idle speed is just above 800 rpm
Ignition: FBO HRR688 Ignition Module, Mopar Performance distributor recurved by FBO
Timing: 16 degrees at idle, 34 degrees total, didn't mess with hooking up vacuum advance this time
Hooker headers 1-7/8" primary
Flowmaster 3" dual exhaust

Compression:
1: 201
2: 195
3: 208
4: 200
5: 205
6: 201
7: 195
8: 206

Thanks!

 

[Adam]

Mis-adjusted valve? especially cylinders #2 & # 7
or Vacuum leak
or Mis-fire

 

[jamesman]

I have your solution.
I too rebuilt a 440 six pack & had the same problem.
Same similiar camshaft you are using
After hours of no success, I called my former crew chief when I
raced in NHRA drag racing & he told me the solution.
This solution that worked for me is:
1) Remove the carburetors

2) Turn upside down to view the butterflys

3) Note holes on each side of the butterflys

4) Drill these out 2-drill sizes larger than existing size.

It worked,evidently the newer camshafts create less vacuum.

Please advise if this works

 

[Hellcat]

jamesman, I haven't had a chance to run it again since last Thursday, but I will some night this week. I am planning to first check for a vacuum leak. If I don't find anything there then I'm going to check my plug wires again just to make sure I got them all crimped properly and they are making good connection. From there I will use a leak down tester and see if I have a leaky valve somewhere.

What did drilling the holes through the butterflies do for your engine? I don't think that would resolve my issue. The vacuum needle is bouncing, and from what I've read that is indicating something is wrong with a single cylinder. So, There could be a vacuum leak at one of the intake runners, leaky valve (or mis-adjusted), bad plug wire causing a misfire, etc.... The holes in the butterflies would affect all of the cylinders in the same manner so I wouldn't think that would cause the vacuum to bounce. Also, I would think making those holes larger would decrease the vacuum. It seems that with more air being able to get through the butterflies it would actually help bring the pressure in the intake more towards ambient air pressure, just like opening the throttle would do.

 

[moper]

I would try adding some fuel to the outboards. A jumping needle can be late cam timing, ignition timing, cylinder sealing issues, manifold/carb sealing issues, or lean. Rich could too - but not to the same extent. The two lower reading holes may be cylinder sealing because with the others where they are I'd think the cam timing is fine. You probably should have had the heads gone through if you got them from 'Source. But rather than tear into it now - try adding some fuel to the outboards.

 

[magnum440]

hello try drilling out idle mixture plugs on front and rear carbs so you can adjust them with vaccum guage installed for highest reading,,helped on my setup which is close to your build..hth

 

[jamesman]

Hello, It may be a vaccuum leak at the manifold.
Easy test by spraying starting fluid around each intake runner.
If engine raises RPM than yes you have a leak.
If the heads were shaved,then Manifold should have also.
The problem I had with similiar engine was it did not create enough vaccum.
When the engine went to idle, it would run for about 10-seconds & then shut off.
Enlarging the butterfly holes enabled more vacuum & fixed my problem.

 

[Hellcat]

Thanks everyone for the replies. This last time I ran it, I got the vacuum up to approximately 15" at idle. The bounce was still there, but it was only fluctuating about 0.5". You can also feel the pulse if you remove a vacuum hose and put your finger over it. This is the first time I ran it with the recurved distributor and ignition module from FBO, so perhaps that helped. I used a propane cylinder and hose to check the intake/carbs for and leaks and found nothing. It is pretty hard to get the hose under the intake so I guess it could possibly miss a leak under there. It definitely is running rich at idle, and I'm having trouble finding the sweet spot with the mixture screws. I have access to the outboard idle mixture screws and I used a procedure of covering the bleed ports at the top of the carb barrels to zero those in. I followed a procedure outlined in an older article (dont' have the reference on me at the moment) and it seemed to work good. I can make the engine stumble by closing off the center carb mixture screws, but it doesn't seem to gain any RPM as I increase the mixture....obviously it does once I bring it off the lean condition that makes it stumble but it is very subtle. That run was a couple weeks ago. Last night I did a leak down test on every cylinder, but I didn't run the engine before hand so it was cold. I put 90 psi in and the lowest reading I got was 86 psi, and all the leakage was clearly coming past the rings. So it would seem everything in the engine is sealing well. I'm thinking I might take it to a dyno guy near me and have them finish tuning it, but I want to make sure I've done everything I can first.

 

[moparleo]

One thing. On the checking for intake leak with a flammable substance. DO not DO IT that way. All you are trying to do is get a change in rpm. It doesn't matter if the rpm's go up or down. So the safe way is to just use a spray bottle with water in it. It will accomplish the same thing with out putting flammables on the top of a hot, running engine. By the way it is always advisable to have a fire extinguisher handy whenever you work on your car.