I recently purchased a pair of new 97s from Summit Racing to replace a set of 9super7 leak-o-matics. Cannot get rid of the off idle stumble, will not make a smooth transition from off throttle to acceleration. However when it is cold it runs great with no hesitation. Plugs look great when I checked them during a steady mild uphill cruise, once I get above the transition point it runs really good. Still running stock jets and power valves. Engine is an 8ba flathead that is .030 over, Isky 400 jr. cam, Bubba's Ignition Chevy distributor with Pertronix HEI, Gear Drive laker headers packed with stainless steel wool. It's not anything exotic but it actually ran better with the Speedway carbs but I could not ever get them to seal up, intake manifold was constantly covered with gas stains and always smelled like gas. Looking for suggestions. Forgot to mention this is a 31 Model A coupe so its a pretty lightweight car.
With the engine not running, look down the carb throat and slowly open the throttle. You should see a steady squirt out of the accelerator pump. It should start as soon as you begin to move the throttle. If it doesn't you may be able to adjust the linkage to get a good squirt. You may have to take the top off and examine the pump.
@Kim Strobeck is going through this same stuff right now on his 31 with an 8BA and two holly's. You can't expect them to perform out of the box. He is using 4.5 PVs and 47 jets, smells g***y but runs pretty good sitting in my drive two days ago. 97s are a different animal and my favorite. Every engine is different...different compression, cam (vacuum), timing, progressive vs straight linkage, al***ude, etc. There is no one recipe to set them up. I'd be buying a box of jets from 35-50 and a few PVs and start experimenting. Science is about to happen. Buy an O2 sniffer and shove it those headers, keep notes.
I've done my fair share of motorcycle trips through Weaverville. If I recall correctly, you're a bit up there in elevation, 2,000 feet or so? Like Bandit Billy said, buy some jets and power valves and tinker away.
Comments below are for genuine USA-produced Strombergs from the 1930's; they may work with the clones. Before changing jets / power valves, try playing with the settings of the idle mixture control screws and the throttle positioner screw. If the idle mixture control screws are out too far, you can experience this condition. MAXIMUM setting of the idle mixture control screws is 1 1/2 turns. Try the idle mixture control screws at 1 turn, then set the idle RPM with the positioner screw.. It won't cost you anything to try, and it might work. Jon
You are correct on the al***ude. I feel like the jets are pretty close judging by the spark plug color when I read them. Not so sure regarding the power valve. I have tried resetting the idle mixture screws several times but no changes for the better.
Might not help you but I had too rich of a mixture when I ran two new 97 Strombergs on my 255 Flathead. As there are no different idle jets available, I soldered them closed and redrilled smaller - this solved all my problems. No black plugs anymore, no sooty smell behind the car.
For true. I fettled carbs for fifty years and didn't know how much I didn't know until I got a direct reading O2 meter. One reason it's a necessity today is the pump gas they're forcing upon us. Yes, lead is toxic, but those of us who grew up reading plug insulator and tailpipe colors were lost when it went away. So today, I won't even touch a carb which doesn't have an O2 bung in the pipe. jack vines
Okay, just got done rejetting down to 43 jets in both carbs and it definitely improved the stumble issue. Gotta drive it some more and do a plug reading and go from there. I have a set of 41s that I may end up trying if I think I need to. Still have a slight hesitation but nowhere near as bad as before. Naturally I had to do one carb twice because I had a leak on one of the main jet caps, new gasket lightly oiled and this time it sealed.
Every Stromberg thread should start with WHAT Stromberg we're talking about. While there are some basic things to do to all carbs there are many tricks developed over the years for each of the different iterations of the Cl***ic 97. Orig - early ChiCom - new "real" ones - late ChiCom. Stromberg 97 has a good manual and their tech page has a lot of tips and tuning info - https://stromberg-97.com/cms/uploads/2024/02/Stromberg_Owners_Manual_v23_WEB.pdf
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