Engine Stumbling Issue Help Request

Hello Group, I have a 1970 CJ5 with a Buick 231
Headers, some type of upgraded or non stock cam, electronic ignition , holley 4 barrel and edlebrock manifold.
I have taken it to a couple of mechanics trying to get it to run smoother .
The issue has been a stumble or rough spot while accelerating doing 2200 to 3000 rpms.
My latest mechanic who is extremely capable . Over a few days off and on replaced jets a few times, replaced the ignition module, disconnected the vaccine advance and set the timing a few times.
His summery is that the Holley is the issue and he suggests getting a proform carburetor with better adjustments.
The plugs are clean and tan, good compression,
Starts great, and before and after the rough spots runs and idles fine.
Fuel pump is functioning right too, and the tanks are vented.
My question is has anyone else had this issue? Or any suggestions before I buy another carburetor. ?

 

on my 304 V8 the holey 600 ran just fine, when the truck avenger 1st came out it was only available in 750cfm version. that turned out to be too large for my Jeep, and it had off idle stumble and when increasing power stumble.

I put on an edelbrock 500 carb and it ran fine.

 

If that's a ta470 btdt the stumble is well documented on several forums. It's way over jetted for a 231. At a minimum bring the primaries to 51( what's your altitude?) and power valve to 6.5. From there I determined mine would need squirter work too and just swapped it for a 390 which I put spring loaded seats in and fabbed my own bowl vent( tuned right up and life is good). My 231 likes 16° of initial timing ( Isky 256 cam) and 32° total all in by 3000. I run ported vacuum.

 

We are at 4K feet and I appreciate your help. I think he used 52 jets if I’m correct and he said the timing was too high with the vacuum advance connected it was at 41 ? I think he said it was set to 31-32 degrees I’m not familiar with the power valve ?
So is the 390 a different Holley carb? Sorry but this is beyond my skill set,

 

I don't have experience at altitude but 52 seems in the ballpark. Mine drove fine on the primaries but had the same stumble when you got on it. I could never determine if it was rich ( secondaries opening too fast?) or lean when it stumbled. The 390 is a similar carb, lower cfm. It doesn't have the off-road mods but you can easily/ inexpensively add them. Those timing #'s are odd. Is your engine an even or odd fire?

 

Here's a link to follow to the owners manual for your carburetor
Holley 0-90470 470 CFM Holley Off-Road Truck Avenger Carburetor

 

I don’t know about the odd fire or even fire ?and I could be wrong on the timing numbers . But I know he said it was 41 with the vacuum advance connected.

 

Thanks!

 

Update, I spoke to the Holley tech and he said this carb is for extreme off road use and not intended for street use. He said my power valve is most likely a 2.5 and that’s too small for street use. He told me to check the idle vacuum and divide that in half for the correct power valve. Then he will direct me where to buy the correct one. I will post as things progress .
Thanks for the help.

 

Today we checked the vacuum at idle it was 19.
The power valve was actually a 6.5 too small for my vacuum. So I ordered the correct 9.5 power valve and
Will report back if it helped. The Holley tech was optimistic this was the cause of the stumbling.

 

I read and re-read this post. Have you determined what Holley carb you have? Typically, a 390 (cfm) has "9007" as part of the list number ahead of the choke butterfly. This is the best Holley, in my opinion, for the 225/231 Buick V6. Its the smallest 4 bbl which helps keep velocity up to to atomize the fuel better. Many here like the 470, but I've never been happy with that on anything smaller than a 4.3 Chevy. The same 470 was great on a 4.3, but I could never get it "right" on a 231.
Have you determined whether you have an even or odd-fire 231? The '75 -'77 versions are odd-fire, and later were even-fire. Send a photo of the distributor cap for us to determine what you have.
Before you go crazy swapping jets and power valves, determine what you have.
-Donny

 

From this posting, it looks like an even fire distributor cap: Engine And Transmission And Transfer Case Identity Help Requested

 

Yes it’s a 470 and the distributor cap image is here. Thanks!

 

So it’s too late on wasted $ and throwing parts at it.
I thought it would be a simple fix and only need timing and arb adjustments.
I’m hoping the new 9.5 power valve will help when it gets out in. I can avoid the stumbling by keeping the rpms down . It’s super fun to drive so it’s hard not to keep the rpms down.

 

Power valves and jets are relatively inexpensive and easy to change. If you are careful the bowl gaskets can be reused several times. Also check your bowl levels after. Easily done plenty of utube vids or in the manual. Ask your mechanic if that 41 is total advance ( likely, that's alot Ime) initial timing for your engine should be 15° btdc ( could go a bit either way with aftermarket cam etc) and total advance around 30-34 in by 3000 rpm. The balancer can be marked up in 10° increments past the factory notch to help with this. Hopefully you'll be squared away shortly.

 

Great info! He said with the vacuum advance hose attached it would top out at 41 or something like that . Maybe he did not have the time to correct the distributor? It is electronic do they have balancers ?
This has been a great project and a learning exercise .
Is this forum funded by members?

 

Yes it is..

 

How do we donate?

 

Via Paypal. See this thread: Keeping The Lights On Around Here

 

Not sure what you mean by balancers. The GM HEI ("high energy ignition") distributor for these engines has an electronic trigger but no electronic control of advance. You have mechanical advance weights in the distributor (centrifugal advance), as well as the vacuum diaphragm.

In my experience, Holleys are easy to tune and all use the same "dials" you use to dial them in. Get a Holley book and read up. Should be cheap used on Alibris or Amazon. https://www.amazon.com/s?k=holley+carburetor+book

I would look more to the accelerator pump shot cam if it stumbles. The power valve is more about adjusting the mixture for low speed and heavy throttle (constant low vacuum). Or it could be a vacuum leak. I would set the timing 100% by the book and see what happens.