Technical Dual Stromberg Carb issues

Pulled my Holley 94's off my AV8 build and put on two newly rebuilt Stromberg 97's. Running 43 jets and 69 power valves in both carbs. Car wasn't running all that bad with the Holley 94's but thought I'd improve performance. Intake is an Edmunds atop a rebuilt 42 flathead. Have the carb linkage synchronized, idles fine, starts instantly but have a serious throttle lag from idle to acceleration. In fact it has stalled at times attempting to increase rpm's from idle. Curious part is if I close the chokes the engine revs well from idle. I have other 97 carbs I can install but obviously I am doing something stupid. Help, advice is appreciated. Happy New Year

 

Are the accelerator pumps working? Have you verified that they are squirting fuel as they should? It is obvious you have a lean off-idle condition. That's what the accelerator pumps are supposed to compensate for.

 

From what I can see both seem to be working. Both acc. pumps are brand new.

 

Try some .045 main jets and a 65 power valve.

 

Sounds like an intake leak of some sort. How does the engine react when you adjust the idle air screws?

 

Is the vacuum brake hooked up? Where?

 

Idles smoothly at about 800 rpm and responds well to idle adjustment. Gaskets are all new

 

Vacuum line hooked to intake

 

Where on the intake? Original was a small port between two ports near the gasket surface at the block.

 

Do you have the accelerator pump rods set on the winter position, I have had the best performance there.

 

May order new accel. pumps, change jetting and power valves. Just don't want to waste time chasing my tail. Maybe install new parts in other carbs and swap them out. This shouldn't be this hard frustrating to say the least.

 

Newly rebuilt means the emulsion tubes were removed to clean, and the J tubes blown clear too?

 

Thats where it is

 

Completely disassembled all parts cleaned, blown out and mating surfaces cleaned and surfaced. That's the frustrating part because I'm really meticulous to a fault. Probably qualify as having OCD

 

Yes set at winter position

 

Try Running an “unlit” propane torch on all your gasket surfaces.
Check to be sure your choke plates aren’t falling partially shut when reving off of idle.
Exactly what fuel pressure are you running?
I’ve surely never seen any performance differences between 94’s and 97’s.
“Carbs are synced” I’m going to ask because I’ve seen many who make mistakes when using a sycronizer. .

 

when You are try to find optimal tune up to Your carbs, dont use any vacuum outlest. Plug all. You can open those later if possible.

If You have basic engine, its better to use progressive linkage.
Jets = when You drive on road, higher RPM
Power valve = bethween Idle and Jets. I my engines (free flow engines) different size power valves. Another soft spring, another harder.
Idle = You can try to adjust carbs so that only another carb (if You have plenum in Your intake) is used idle and another carb is tuned so that it comes delay.

I have note that for example Thickstun PM-7 intake plenum is so far from carbs, not easy to use progressive linkage or tune so that use Idle only one carb and another comes with later when more trottle.... Normally PM-7 works best when both carbs are open same time. Powervalves, jets can be different size...

I think Your engine is lost vacuum while when You speed up. Many carbs + basic engine --> vacuum is weak.
Vacuum is need to keep as good as possible because carbs are working like milker...
When You have hi RPM problem is gone
When You have supercharger, hi-compression, long stroke, big bore.... then engine easier tune because engine has more vacuum

Carb tune is interesting because all engine have different psyche (like woman.... never know what happens next... ) and many times engines are work different ways.

Of course first You need to find out that there is not any leaks. Starter pilot or brakleen ....


Aulis

 

With the engine not running, open the throttle slowly to full throttle. You should see a steady stream come out of both sides of the squirter. Keep watching until the stream quits. It needs to be steady, not dribs and drabs, and a good stream. If both carbs check okay, I would probably be looking for a vacuum leak. However, you stated that it idles well and it really wouldn't with a vacuum leak.

With the engine running, look down the carbs at idle and see if any fuel is coming out the main venturi. There shouldn't be any, it should be dry in the carb throat. If fuel is coming out at idle, it could be that you have the throttle plates open far enough to start pulling fuel out the venturi. This could point to a vacuum leak.

 

I

Idle speed is too high, the carb is designed to supply fuel for about 450rpm, the high idle speed means the throttle blades are tipped too far open and the 'transfer slot' (a small hole in this case) is overly exposed and cannot supply fuel when you accelerate, that little circuit operates BEFORE the accelerator pump to get you up and running, when you disable the transfer slot you have a horrific stumble.

 

Get some power valves with synthetic plungers.

 

Thanks for the continued flathead education all. Looks like my plate will be full the first of the year.

 

My experience is that 97's have a cold engine stumble and need some choke until the engine gets to operating temperature. Does your stumble go away after the engine reaches operating temperature
Then, I seem to go the wrong way on jetting unless I use an AFR to set things up and it seems that flatheads like and run a little better rich....
good luck

 

Pulled the carbs today. Ordered new accelerator pumps, emulsion tubes and gaskets. See nothing wrong with either but eliminating the variables. Also ordered an electric fuel pump in spite of my stock pump holding 2-1/2 lbs. pressure, again no more variables. Figure it will at least prime the engine after sitting a while. Staying with the 63 power valves and the 43 main jets for now.
Another question...... Can I leave the existing diaphragm in the stock fuel pump and still use the electric as a supplemental fuel source. Perhaps it's best removed?

 

Turn the carbs over and have a look for that hole I spoke of, if it is exposed you'll have a stumble no matter what else you do or change, that hole is drilled to eliminate the stumble, that is its' purpose. take a photo of it and post it.
The electric fuel pump will push thru the mechanical, no worries.

 

Finally got the new internals for the carbs. Pulled the emulsion tubes and they were coated in crud. New emulsion tube replacements installed. Even though they are new I also replaced the accelerator pumps and check valves. Checked float levels. Staying with the 43 main jets and 69 power valves. Have an assortment of jets coming this week so we shall see. .

 

Yikes. I thought you said they were removed and cleaned before?

Also important is to clean the cavity around where the emulsion tubes fit. A brush is required.

 

This shows exactly why you have to remove the emulsion tubes to rebuild a Stromberg 97. Unless they are clean and all the small holes are open, the main venturis will never flow correctly. This is a very common problem with folks who are doing their own at-home rebuilds . . . they neglect to perform this vital step.

 

Carbs were rebuilt but not by me

 

Sorry I assumed it was you since you described the work and said you are meticulous. Maybe you should “out “ the rebuilder so nobody else gets taken.