Dual 97s Too Rich!

burly

burly

Senior Member
Got the dual Stromberg's mounted on stock 85 hp. Early thickstun intake. Starts okay, 2.5 fuel pressure, .043 mains, 69 power valves and air fuel adjustment backed way out. Revs up smooth but at idle almost like the choke is pulled out but both chokes wide open. Loping and black smoke out tail pipes, this is without air cleaners. Really starts to load up. Think .042 mains would solve the problem or would you go even smaller? No drips or leaks so far. Appreciate any recommendations you guys might share.
 
The main jets will Not help your idle problem. Check floats for proper level and not sinking in the gas. Look down the car bore when it is idling and look to see were the gas is running out of.
 
burly said:
Got the dual Stromberg's mounted on stock 85 hp. Early thickstun intake. Starts okay, 2.5 fuel pressure, .043 mains, 69 power valves and air fuel adjustment backed way out. Revs up smooth but at idle almost like the choke is pulled out but both chokes wide open. Loping and black smoke out tail pipes, this is without air cleaners. Really starts to load up. Think .042 mains would solve the problem or would you go even smaller? No drips or leaks so far. Appreciate any recommendations you guys might share.
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When you say "air fuel adjustment backed way out", do you mean the two idle mixture screws ? If so, that's probably the problem. Those are supposed to be adjusted at a warm idle to get the smoothest idle (highest manifold vacuum). I don't know what else you might mean by air fuel adjustment ?

Sal
 
Nice catch Sal, missed the mixture screws. run them all the way in and back out 1/2-3/4 turn good starting point.
 
I set my floats [dry] just a tad below level, try just under one turn out on the idle screws. disconnect the link between them , balance them with a uni-syn also this is where a vacuum gauge is your friend.. once you are happy with your highest vacuum and a nice idle connect the link between and away you go. From there you just need to keep an eye on it and maybe a fine adjustment if needed, like changing to the 'S' setting etc..

Ohh and all this may not fix your problems if your carbs are old and flogged out..
 
Check for bad or weak spark at the plugs.
And go smaller on the power valves. You don't need that much gas with basically a stock engine. Also have a look down the carbs with the engine running. If you see raw gas leaking into the venturis it is coming from the bowl overflow circuits in the carbs indicating the floats are too high.
 
Yes sir, I meant the 2 (in this case 4) idle mixture screws (I call them air fuel adjustment screws because some folks consider the one screw (in this case 2) that adjust the idle rpm the idle screws) and at warm idle they ran the most smooth backed way out then both carbs were synchronized with uni-sync. Manual fuel pump only. Can see and unfortunately have felt strong spark at plugs on a few occasions. I will re-adjust floats and if problem continues what size smaller power valves would you recommend with a stock dual carbureted engine? Appreciate all the replies fellas!
 
Well, something is not right if you have to back the idle screws way out to get a smooth idle. That is not the normal. I would start with figuring that issue out.
 
burly said:
Yes sir, I meant the 2 (in this case 4) idle mixture screws (I call them air fuel adjustment screws because some folks consider the one screw (in this case 2) that adjust the idle rpm the idle screws) and at warm idle they ran the most smooth backed way out then both carbs were synchronized with uni-sync. Manual fuel pump only. Can see and unfortunately have felt strong spark at plugs on a few occasions. I will re-adjust floats and if problem continues what size smaller power valves would you recommend with a stock dual carbureted engine? Appreciate all the replies fellas!
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I would ask "Uncle Max" on the Ford Barn about main jet and power valve sizes for your carbs. He's as good as it gets with Stromberg carbs on flathead V8's.

Sal
 
With one 97 the idle mixture is usually about 3/4 turn out but with duals expect higher about 1+. I am running the same jets as you are now. I would also suggest looking at the float level. You may have jets that are not seated correctly??? By the way the S & W taps on the throttle can make a big difference.
 
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I would suggest blocking off one carb at a time and testing each one.It very well could be only one carb bad. I did this and quickly found the problem. Would only work if manifold has common chamber. I do not know about the Thickstun.

John
 
Since a stock engine has just one power valve,
Around a 65, go to 2 power valves for 2 carbs about half as big. Say 2 35's.
Some guys block one carb with a power valve plug.
But to get an even atmosphere in the manifold plenum
I don't recommend it.
If your tail pipe is real sooty and stinks , it's way too rich.
Keep looking...you will find the problem.
 
Init1 said:
Since a stock engine has just one power valve,
Around a 65, go to 2 power valves for 2 carbs about half as big. Say 2 35's.
Some guys block one carb with a power valve plug.
But to get an even atmosphere in the manifold plenum
I don't recommend it.
If your tail pipe is real sooty and stinks , it's way too rich.
Keep looking...you will find the problem.
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What planet are you on?
 
burly,
Your jets sizes and PV sizes are correct, generally speaking. Did you personally
check the size of the jets or go by the # stamped on them ? Stromberg PV's , even
new ones out of the box more than occasionally leak. I check every PV for leak down
with a home made fixture. You gotta check everything.......the devils are in the seemingly obscure details. 3/4 - 1 turn out for the idle mixture screws will get you running if all else is proper. Little things mean a lot...for instance ....did you remove the emulsion tubes and clean them....how about the dinky little air bleed holes, ABOVE the idle mixture screw orifices, they gotta be open. A basic rule is 2 sizes up or down
from stock jet size is the limit....look elsewhere.
Finally if per chance I built these carbs I want them back pronto for a pro bono
re do and test and setup.
Charlie ny
 
"CharlieNY" makes a good point above. I chased a carburetion problem on a 327 in a Corvette for 6 months before I finally got serious and checked everything. I found that it still had the stock jets (marked #61, I believe), but they had been drilled out to at least 5 sizes larger. Going back to stock jets solved the problem.
 
Before removing the original intake I mounted, ran and pre adjusted both 97's individually. After adjustment both seemed to run perfectly. Power valves and jets came from Uncle Max so I'm sure they're what their stamp indicates? Hopefully after adjusting (lowering) the floats this weekend I'll find the sweet spot I'm looking for. I'll try to remember to post my results next week. Thanks again guys!
 
Have dual Strombergs on a 221 running stock main jets and #71 pvs at about 600 ft elevation. Had run 43 mains with 65 pvs. Engine stumbled on acceleration from dead stop.

I would remove carbs, clean with carb cleaner and compressed air in all passages. Something is plugged as Uncle Max suggests. You may have to pull the tubes.. Set float levels to spec and set idle mixtiure adjusment 3/4 turn out.
Try it.
 
running two 97's on pretty much stock 221, using 35 jets, too lean, engine stumbles, pull out choke a little and she runs like a dream--maybe need slightly bigger jets maybe 40's?
 
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