Technical 350 Wont accelerate unless done slowly

I have a 36 Ford Hot Rod with Chevy 350 Crate engine. Ran great except for small hesitation when first step on gas. Installed bigger jet on Accelerator pump. Ran good, then changed diaphragm in accel pump. Now it wont accelerate if you hit the gas unless you do it gradually. I put everything back the way it originally was but still have the problem. It will backfire and stall for a second, some times quit if I don't catch in time. I have a Mallory Unilite dist. no vacumn advance
I've done the following.
Switched Carbs
New Coil
New Cap
New photo cell in dist.
Checked timing, many times
It ran perfect before I messed with it.. Cant figure out, thought sure coil went bad, found it corroded the end off wire..I replaced it.
Fuel pressure good, filters clean.
Tuned with Vac gauge.
Going to check for Vac leak..

 

"Changed the carb" to wheat , a new one a known good one , something that was on the shelf ? What kind of carb ?

 

What prompted messing with it?

 

Good one.. Newer 650 Holley

 

Just fine tuning the little hesitation...Wish I hadnt touched.

 

Go back over everything you did inch by inch.Don't get in the weeds with the carb. You will most likely find the problem. Repair the repair

 

The backfire most likely blew out the power valve. Take a vacuum reading from the big 3/8 port on the rear of the carb at idle. Install a power valve rated with 1/2 of that reading.

Also with a timing light check to see that you have ignition advance when you rev the engine, what is the timing at idle? What is the timing at around 3000 RPM's?

It is very possible that the balancer slipped giving you a false ignition timing reading, verify true top dead center with the zero mark on the timing tab.

Go thru and check all of the plug wires are correct.

 

5500 stall and 5.38 gears should solve any hesitation

 

Accelerate slowly,
So trouble shooting 102 , when a new problem shows up, go back to the last thing you fixed,
Or the area you were messing around in. 90% of carburetor problems are electrical in nature.

just a hunch
A lazy sticky ignition timing mechanism can do that hesitation / slow acceleration thing. There before you started fixing it and now you changed a few things so it’s presenting a little differently.

Most of the time a hiccup on acceleration is a lack of fuel squirt. But since increasing the fuel squirt magnified your issue and changed it you’ll need to go back to where it was. Now you need to feather in the gas so the engine doesn’t get too much fuel from the extra squirt.

Too many times, I chased my tail with the timing light that was telling me stories. I won’t use one unless I’ve verified the marks with piston stop test.

Are you running pump gas?
Ethanol content?

You’ll get it

 

I put everything back to original.. It seems electrical to me, but changed all the unilite components.. Thought sure it was coil..nope, Running pump high test fuel... Plus the backfire has got me stumped.

 

Where is the back fire ? out the exhaust or through the carb ?

 

Don’t you just love when a sneeze happen’s then with out warning you get a back fire.

 

Plug wire brass end not seated in dist. Cap tower or spark plug tip

 

Guys. Fuel filter.***** in a fuel filter would do exactly this and I’ve seen zero mention of it

 

Checked..I have 2. One at Tank and large at engine.. All clear..

 

Exhaust

 

Certainly sounds like carburetion, but it would sure be nice to get someone with a scope to look at your ignition. A quick scope check with tell you if there is any problem with the ignition.

 

Check voltage at the module with key off, key on, ign on. Confirm good grounds. Confirm ballast resistor is good and wires to it are good. Confirm voltage regulator is good and not spiking or dropping power.

I went through this many years ago, theres a thread on the HAMB about it. Very similar deal. I magically fixed it, but I believe it was a poor connection at the ballast resistor or a bad ground connection.

 

Sure tends to make me think its electric.. Switched carbs, didnt resolve.. I didnt check resister, thought they either work or not at all..

 

Depends on which coil you're running. But yeah I'd check voltage at the coil

 

last time this happened with my my float was set wrong, and i had a warped metering block.

 

As a general rule out the exhaust is usually ign/electrical.

 

Is it getting a pump shot? First thing I’d look at.

 

Been there done that,,Adjusted to spec. good squirt

 

Did you inspect the mechanical advance plates ?

Here’s some info worth looking at I think

https://www.chevytalk.org/fusionbb/showtopic.php?tid/166955

https://www.jalopyjournal.com/forum/threads/mallory-unilight-problems.289776/

I saw that you swapped the carb and same issues. Yep I saw that

 

I cheked vacumn, I get a seady 15 lbs.. Should be a little higher,, I cant imagine timing is off, never had to touch it before

 

Adj. carb...let it really warm up..Sill and dies before taking off. through exhaust..Not as much. Can run at 60 or 70 but if floor it it pops. It also is runing hot now..

 

For something street driven with a mostly "stock-ish" engine, a vacuum advance distributor can make a big improvement in driveability. Have you tried adding a few more degrees of initial timing? Watch what that does for your manifold vacuum at idle, especially when connected to a full time vacuum source as opposed to ported vacuum.

 

Havent touched the timing beause it ran great before the way it is, cant imagine it changed.. Checked it, its a little low .
around 8 to 10.. Hard to say final advance at rpms.. .Hard to in my garage. Nieghbors

 

Sounds like it may be running lean if it's getting hot now. Have you taken spark plugs out and what are they telling you?