Engine and Transmission Tech

I'm trying to tune my new engine with a vacuum guage. Bill D. built me a clone of his engine.... then went to Key West on vacation so I'm on my own!
Here's what I got....

Engine is 3.4 GM 60 degree V-6. bored 30 over, Edelbrock Performer and Guzman top. Holley 390 with a mild cam and roller rockers. I'm really happy with the overall performance but still trying to get smoother acceleration.

Think I've got the floats figured out, and Billl and I changed jets a couple of times and think we're close (altitude in KC is 1040 ft). Starts easy but under hard accel. is stumbles as I go into 2nd. Also running 5.5 PSI on fuel pressure at the carb.

Vacuum reading at 2000 RPM is 17.5 inches, after twisting the dist. to max reading and backing off 1.0 Anyone know what I should or...can use as a baseline? Is 17.5 good /bad or not an issue?.

After reading Bill G's write-up and watching U-tube videos till 2 AM... I started messing with the idle mixture adjustment screws... saw little change in vacuum readings and came to the conclusion (right or wrong), what ever I do there, it's probably not what's causing the stumble. Checked gap in accelerator pump arm at 15 thou.

Never had to mess with this stuff on the 35 PA!!!

Ideas?
Thanks
Don B.

I had the same issue with a 600 Holley. Experimented with accelerator pump cams. Found one that worked. Stumble gone.

Set your accelerator pump arm snug (no spacing) at idlle/engine off. The 15 thou. is actually a WOT setting I believe.

[www.holley.com]

[www.crankshaftcoalition.com]



Edited 2 time(s). Last edit at 04/21/2014 08:25PM by kerbau53.

If it is when you accel then it is your accel pump. If it is when you let off to shift it could be a vac leak that you have tuned for at idle. When you tune with a vac gauge you need to plug as much vac accessories as possible including the brakes. Also on these engines the manifold needs retorqued after a good warm up and cool off. They are known to settle and have vac leak after first running.

Geoff and James...

Thanks for the info! I'll reset the pump arm gap. I set it cold engine and off. I'll also check all the lines for vacuum leaks... my cheap vacuum guage hose seemed to go on the vacuum outlet too easy... should have been a signal to me?

Info on pump cams was first time I really seemed to get what all that was about (told you I'm a rookie) and I'll study harder.

To the question of "when" the stumble occures... seems to be when I pick up the throttle hard after releasing the clutch.. into 2nd. Does not occure if I feather it. (no fun there).

Thought it sounded like fuel starvation, so I checked and reset the rear float level (it was low to the fill hole), and it clearly helped alot. However, after 15 minutes of terrorizing the neighborhood, the stumble is still there, but in a much more muted degree.

Give me a couple of days to get back with results.... packing for a long awaited vacation.

Again, what kind of vacuum do you pull? All the U-tubes were tuning SB V-8s at around 20.5 to 21 inches so I assume no point of comparison there.

Cheers and thanks again
Don B.

I have about 15.5 in/hg at idle. I too tune the carb using a vacum gauge. Good luck. Sounds like you're almost there.

It sounds like you're too lean.
But we really need to start at the beginning and make sure that nothing is missed.
First things first is going through your tune up.
Pull all of the plugs and inspect them. They should all be fairly clean and the same color. A light tan color is perfect but if you have "oxygenated " fuel then they will be snow white.
If they are black get them cleaned, or better, just replace them. As they will never fire properly in that condition.
Gap them @ 0.030" and put them back in. That gap might seem a little small, but for tuning purposes it removes a ton of ignition variables.
Besides a 0.030" gap will fire a carbed engine well even if the mixture is a little off.
No need for a compression check at this point as the engine is running smooth at idle and is pulling well when fed gradually.
I assume that you have a distributor.
So pull the cap and rotor and make sure that they are surgically clean and mark free.
Try the mechanical advance and make sure it moves freely.
Inspect the plug wires and reassemble the whole mess.
Disconnect the vacuum advance and plug the hose. This will stay off for the rest of this exercise.
Now re-torque your intake as they do loosen off as James said.
Pull your carb and tighten the six Phillips screws that hold the throttle body on as they loosen over time as well.
Set the accel pump cam clearance at WOT, blow out the air bleeds as they tend to plug up easily. And reassemble everything.
Set the idle mixture screws exactly 2 turns out.
Fire up the engine and warm it up.
Set your float levels at the lower edge of the weep holes or just slightly lower (within a 1/8") Higher is not better!
From now on you will always set them to the same setting.
Now set the total timing to 36' btdc. IE rev the engine until the timing stops advancing (within reason of course) and set your 36' there.
That should leave you with something around 12-14' btdc at idle. As long as it's somewhere between 10 and 20' at idle it will serve our purposes at the moment.
Set the idle speed to 900 rpm.
Screw the idle screws in 1/8 turn each equally at a time until the engine starts to slow. Then back them out equally until it reaches it's highest rpm.
Reset the idle speed to 900 and connect your vacuum gauge.
Readjust the mixture screws and the idle speed until you have the highest possible vacuum reading.
Note that number and divide it in 1/2.
Now pull the front float bowl off the carb and check the power valve. It's rating needs to be 1/2 of your idle vacuum reading.
The factory power valve is likely not correct for your engine and can easily cause the stumble.
Put it all back together with the correct valve and a new set of bowl gaskets. This will be coming apart lots so reusable gaskets are a good choice.
Now go for a drive. Pick a stretch of road that you can drive comfortably on at 40-50 mph without being a hazzard.
Pick a gear that lets you cruise at around 3000 rpm hold it there and see how the car responds.
The idea is to only have the primary throttles open not the secondary's.
If its smooth and even then maybe your lucky and the primary jetting is spot on. But probably not. If it's jerking and feels like it's missing then it's likely lean and you need to jet it up until it smooths out.
Either way, get it to a point that the primary jets are rich enough to run smoothly at speed.
Then start leaning it out a couple of jet sizes at a time until the engine just starts to get rough again. now go rich one size and you should be good for the primary side.
Taking the time to get this right will make your engine happy and get the best fuel mileage that is available to your combo.
Next we'll play with the secondary jetting.
Pick a gear that you can comfortably accelerate from 2000 to 5000 rpm in at full throttle.
Now go do that!
Time it with a stop watch and make a note of the time.
Now you're going to jet the secondary's up or down a couple of sizes at a time until you reach the fastest time possible.
Once there then do the same thing again only leave the jetting alone and adjust the ignition timing to get the fastest acceleration possible. Lock the distributor down and leave it there forever.
Go back to the secondary jets. Probably they will need an adjustment up or down a size to be perfect.
Now you can hook up the vacuum advance again. Use the ported fitting and not the manifold vacuum port.
If the vacuum advance can is adjustable you can tune it as well for fuel mileage. But that's another day.
Now you can set the idle speed an fuel mixture screws to what is comfortable to you.
Rule of thumb is leaner is better. If the engine stalls coming back to idle after snapping the throttle then it's too rich and needs to be leaned out. It should idle back down cleanly with no drama.
If you still have a stumble after all that, then likely we'll need to play with the secondary diaphragm spring.
But I'm betting that it's running well now.
Sorry that's a little long winded.
And it's not the perfect way to set it up.
But it gets the job done with a minimum of tools and experience.

Cheers
Fred

Here's a cool vacuum gauge page - has scenarios to explain what's going on be demonstrating what the gauge us doing.

[www.secondchancegarage.com]

Thanks to all who sent info and suggestions...

The issue has seemed to be resolved. Minor tweeking to rear float level and accelerator pump arm helped. However, I had also adjusted the choke setting the incorrect direction. Bill D. figured it out. Warm idle is now 950 RPM and no stumble on hard shifting to 2nd. Vacuum tuning is a learning process!

Time to call it good and move on to other items on the list. Just put a new issue on the Forum listing on hot re-starts.

Thanks again
Don B.

Glad you got it sorted. I copied Fred's instructions for my files. Good stuff if you're not a 4 barrel guy. Which I'm not.