Stutters at 2500 rpm

[78cordoba]

I'm at 15 intial now and 36 total, but mixture screws still aren't working properly. The idle actually increase when screws are fully seated. Is 750 cfm too big for a 400 and should VA be hooked up to ported or manifold? It seems to run better on manifold.

 

[Wietse]

In case closing the mixture screws increase idle rpm your throttle blades are likely to far open as it is getting a lot of fuel (too much) from the transfer slots and the added fuel from the mixture screws fattens it up even more.
You can pull a spark plug and check, will likely be black.
In that case your engine is running with too much fueling (closed mixture screws increase rpm) and not enough air. (throttle blades (too) far open to keep it running)
A PCV valve is an additional air source you could use, do you have 1 installed and tied onto the vacuum port on the carb base? (if have, make sure it is working)

Giving it 18-20 deg of initial advance and have a pcv valve in place should increase the rpm and should give some space to close the throttle blades more by adjusting the idle screw, this will bring the carbs transfer slot back to a more suitable sizing and reduce the fueling also and gives you the chance to do fine tuning with the mixture screws.

Drilling holes in the throttle blades is another way of adding air but i would stay clear of that unless you have no other means of achieving more air supplying the engine at idle.

 

[Geoff 2]

I should have added in post #40. You connect the vac adv to a manifold vac source. NOT ported vac adv, which is USELESS. You can Tee into any man vac source except for the PCV hose.
If you then follow the instructions in post #40, your problems will be fixed or at least much improved. You need to do that & report back with the progress.

Your t/blades are open too far at idle. I have been trying to tell you THAT all through this thread. Post #40 is how you fix it. Because the blades are open too far at idle, you are getting too much fuel from the over-exposed transfer slots. It is too rich. That is why screwing the mixture screws IN increases idle rpm because it is correcting the over-rich mixture.

 

[78cordoba]

I should have added in post #40. You connect the vac adv to a manifold vac source. NOT ported vac adv, which is USELESS. You can Tee into any man vac source except for the PCV hose.
If you then follow the instructions in post #40, your problems will be fixed or at least much improved. You need to do that & report back with the progress.

Your t/blades are open too far at idle. I have been trying to tell you THAT all through this thread. Post #40 is how you fix it. Because the blades are open too far at idle, you are getting too much fuel from the over-exposed transfer slots. It is too rich. That is why screwing the mixture screws IN increases idle rpm because it is correcting the over-rich mixture.

i did i am at 15 initial and 36 total still having the same issue.

 

[78cordoba]

If i get to the point where I feel like I may need to drill the holes, and I have an extra carb for parts whats size holes in which throttle plate would you suggest

In case closing the mixture screws increase idle rpm your throttle blades are likely to far open as it is getting a lot of fuel (too much) from the transfer slots and the added fuel from the mixture screws fattens it up even more.
You can pull a spark plug and check, will likely be black.
In that case your engine is running with too much fueling (closed mixture screws increase rpm) and not enough air. (throttle blades (too) far open to keep it running)
A PCV valve is an additional air source you could use, do you have 1 installed and tied onto the vacuum port on the carb base? (if have, make sure it is working)

Giving it 18-20 deg of initial advance and have a pcv valve in place should increase the rpm and should give some space to close the throttle blades more by adjusting the idle screw, this will bring the carbs transfer slot back to a more suitable sizing and reduce the fueling also and gives you the chance to do fine tuning with the mixture screws.

Drilling holes in the throttle blades is another way of adding air but i would stay clear of that unless you have no other means of achieving more air supplying the engine

 

[Wietse]

IF holes need to be drilled you start on the primary throttle blades, obviously starting small and increase as required.
Start with 0.040 - 0.050" and increase in small steps as it can make a big difference.
When replacing throttle plates you have to pay close attention on their positioning in the bores as if not centered they can bind and get stuck open.

 

[Don Frelier]

IF holes need to be drilled you start on the primary throttle blades, obviously starting small and increase as required.
Start with 0.040 - 0.050" and increase in small steps as it can make a big difference.
When replacing throttle plates you have to pay close attention on their positioning in the bores as if not centered they can bind and get stuck open.

I doubt holes are necessary with a mild edelbrock cam.
Something or more than one thing else is likely wrong.

 

[78cordoba]

I doubt holes are necessary with a mild edelbrock cam.
Something or more than one thing else is likely wrong.

Could I possibly have to much fuel pressure im not running a regulator, but I only have a mechanical fuel pump. Another problem I have is if I step on it from a stop it dies when I let off. If I ease in it won't die. If I'm in park I can give it throttle no problem seems pretty snappy. Not sure if that's related to the idle issue or not but is something I also need to solve.

 

[78cordoba]

car starts and runs perfect at neutral, very responsive to throttle,
-revs up good at neutral
- does start cold perfetly

when put in gear....
- when hitting the gas pedal too fast it stalls out
- when hitting gas pedal slowly it drives okay
- while driving you can floor it and it won´t stall
- when you stop at a light it idles perfect
- when sarting of a light and hitting gas to fast it stalls
- it starts right up after stalling

 

[Geoff 2]

You only drill holes WHEN & IF THE TIMING AT IDLE HAS BEEN OPTIMISED.
I have just about worn out my typing finger explaining to you how to do that.

You can do this simple experiment to see what the correct idle timing will do. Then later/afterwards make changes to make it permanent. Leave vac adv disconnected & plugged. Simply adjust initial timing to 32-36*, not critical. Idle rpm will increase. Re-adjust idle speed & mixture screws. Take for a short test drive. Keep rpm below 3000. See if flat spots etc are improved or fixed.

 

[Wietse]

car starts and runs perfect at neutral, very responsive to throttle,
-revs up good at neutral
- does start cold perfetly

when put in gear....
- when hitting the gas pedal too fast it stalls out
- when hitting gas pedal slowly it drives okay
- while driving you can floor it and it won´t stall
- when you stop at a light it idles perfect
- when sarting of a light and hitting gas to fast it stalls
- it starts right up after stalling

So when a load is applied things change for the worse, most things you mention are related to manifold vacuum, which relates to a change in timing due to the vacuum advance is changing things.

You need to check your idle timing with vac. advance disconnected, once you know that, connect the vac. advance again and check timing again so we can estimate better what timing your engine runs with at what conditions.
Also your total advance need to check (vac. advance disconnected), total advance and at how many rpm is it "all in".

Your initial is 15*, so when you floor it you only have a small advance in timing (without vac. advance) and, depending on the AFR, a lean mixture does not work together with that timing. Peak cylinder pressure will occur very late and the piston is already very far in the hole to give any punch on the crankshaft.
As Geoff already mentioned, up the initial timing and adjust your carb idle screw to bring down the rpm again.
With some cam aim for 900-950 rpm idle in Park/neutral.
Also let us know what rpm you get if you drop it in Drive.
The observed rpm drop indicates how your torque converter is behaving, either being loose or tight at idle.

 

[78cordoba]

I have tested everything you ask. I just now set the initial timing to 36 like you asked but I still can adjust the mixture screws

 

[78cordoba]

also at 36 initial I'm bucking the starter. If I set it around 900rpm for idle it drops to 600rpm in drive

 

[78cordoba]

Regardless of where I am for initial timing I cannot get my idle mixture screws to work even when I'm at my highest rpm by adjusting the distributor.

 

[Wietse]

If I set it around 900rpm for idle it drops to 600rpm in drive

That is a problem.
Your converter puts too much load on the engine which is the reason your throttle blades are too far open and the carb does not react on mixture screw changes.
You will need to source an additional way of adding air to the engine, either an adjustable PCV or make a fixed orifice yourself which can be done quite simple by installing a plug with a drilled hole (orifice) and install this in a hose that connects from the crankcase PCV point to the carb base.
Start small and increase with little steps, at some point you are able to close the throttle far enough so the engine reacts on the mixture screws.

 

[78cordoba]

That is a problem.
Your converter puts too much load on the engine which is the reason your throttle blades are too far open and the carb does not react on mixture screw changes.
You will need to source an additional way of adding air to the engine, either an adjustable PCV or make a fixed orifice yourself which can be done quite simple by installing a plug with a drilled hole (orifice) and install this in a hose that connects from the crankcase PCV point to the carb base.
Start small and increase with little steps, at some point you are able to close the throttle far enough so the engine reacts on the mixture screws.

What would cause a stock convertor to put to much load on the engine

 

[Wietse]

It depends on the make/model/type converter and how it is designed.
A stock converter will apply "drive force" at low rpm, for example a race type (high stall) converter will barely move the car at idle whereas a stock type will start pushing the car forward, or reverse at idle rpm's.
This is all related to the vanes inside the converter and their angle, converters with a high vane pitch (tight converter) will displace a lot of fluid, even at low rpm.
Race type converter have less vane pitch which requires more rpm to displace sufficient fluid to apply a force.

But basically your converter and camshaft do not match, a stock converter with a stock(ish) cam will work well.
The reason is the stock cam will provide decent vacuum and allows the engine to have strong idle that can deal with the load applied by the converter.
By increasing camshaft duration this tolerance will increase to drop, to compensate you need to idle at higher rpm and provide more air to the engine, which results in messing up the transition slot on the carb.
This will cause an issue when the carb does the fuel feed transition from the transition slot to the main jets, there is a point where the transition slot stops feeding fuel and the main jets supposed to take over.
You are running out of transition slot and the main jets have not started to flow fuel yet.

If I am not mistaken your camshaft has a 258* duration @ .050"?
That is quite some camshaft for a stock converter.