Flathead dual carb problem

I have a fresh rebuilt stroked 286 flathead, Isky 400j, Msd with no vacuum and with dual 94's on an offy regular dual intake. The carbs are fresh rebuilt with 48 jets and 3.5 power valves and non progressive linkage. Fuel pressure is set at 2.

The engine will start and idle perfect and does not over heat. When I step on it the engine hesitates and then will rev up and run fine but is pumping to much fuel into the engine.

I haven't synchronized the carbs yet and have not checked the timing.

Could this be a power valve issue? Or

 

Most Holley 94's that I have found run #50-#52 jets. 48 sounds a little lean unless you are at a higher elevation.
The two biggest problems with these carburetors that I have found are float valves and power valves. Both of these issues should affect your idle though. Make sure all the check balls are in place and the needle valve in the accelerator pump discharge is in place and the nozzles are properly seated with gaskets.Also....when I suspect power valve leaks, I carefully remove the top and then the bowl from the base and see if the well that the power valve sits in is wet. It shouldn't be if things are ok. If the float valve is leaking you can usually see gas dripping off the nozzles while it is running. Hope that helps a bit. Good luck

 

90% of all carburetor problems are ignition related...

I had similar problems tuning my 286, similar to yours, but with dual Strombergs, and a Schneider 270 cam. After chasing what I thought was a jet problem, it was solved by colder-range spark plugs, more ignition advance (initial is at 8-degrees) and a new coil, as the old one would get really really hot after a few minutes of running at which time the engine would start running rough.

Check everything in your ignition system before swapping jets a million times like I did.

 

I agree with Ol' Blue, 48's in 94's is too lean. Go with 51's.
I would start there .

 

I had a very similuar issue with Locked linckage. Once going to progressive and not opening the Air volume so fast at just off idle issue went away.
The Wizzard

 

Get your Accelerator pump action sorted out. You say it stumbles when you step on it. That's your accelerator pump not "covering" that sudden increase of air with fuel. You need more fuel at the moment you crack the throttle open cause a motor cannot run on air alone.
As far as jetting the other guys may be correct but you air to fuel ratio/fuel economy/and spark plugs will tell you whether to jet up or down.

"90% of all carburetor problems are self inflicted..." That's my take on it.

 

I purchased the adjustable jets from Speedway for both carbs and they worked really well.

 

The carbs should have been rebuilt with stock #51 jets, as previously stated, unless you are in high elevation. Fuel pressure should be closer to 3 psi. But the big problems are not synchronizing the carbs and checking timing.
First, make sure the initial timing is at least "on the bump" or 2 degrees BTDC. (Should be about 25 degrees max around 2500 rpm)
The carbs have to be built the same, and that includes all parts, float and accelerator link settings, jetting, and power valves. The idle mixture screws have to be adjusted repeatedly for the highest vacuum readings, along with keeping the main idle screws adjusted for a low rpm idle. This is done with the carbs dis-connected.
Then a synchronizer is used to adjust each main idle screw to match the air flow, and when that's done the carbs can be linked with "straight" linkage. (No good reason to use progressive on a dual carb settup)
JFYI, I run straight linkage on a built 286" with 3 97s, and on a stock 239" with dual 94s, and have had no problems with either for many years and thousands of miles.
There may be other issues, but getting the carbs set up correctly and initial timing are important first steps.

 

To add a bit...watch advance with a timing light and see if it starts advancing immediately as engine comes off idle... slow advance is a problem.
Are you running air cleaners? If so, do some testing without. The normal small chrome filters are generally junk. Many function as chokes even when brand new, and small size means they will clog quickly anyhow. If engine starts to run better without cleaners, start inventing a way to get bigger and better on there.
If air cleaners are restrictive, it is impossible to make carbs behave in any predictable way.
Jets area touch small, possibly not enough to be a problem, but remember ethanol at 10 or 15% means you are starting out a bit lean from lower energy in fuel.

 

And on synchronization...before you get to that have you flogged the carb throttle bodies to get the pltes to close 100%? You need to back off idle to nothing, hold up each throttle to a strong light, slightly loosen plate screws and jiggle the plates until no light gets around them. This is because with two carbs working at idle, you need very tight control over the very small airflow each will have. Once you get the plates tight, then you can put things back together and start with getting idle adjusted to where you want it as a start to the synch process.