After running two different modified flathead Ford v8’s on the dyno and experiencing a max on one engine of 3500 rpm and the 4in stroker pulling to only 4000 b4 “quitting” or “laying down”, I felt I didn’t get to experience the “full pull” to 5000 or 5200 or so because of ignition problems. Aside from that after examining the fuel curve was noticed it was excssively rich on the lower end but at the 4000 mark was so close to being perfect,just leave the jets(049 versus 045stockers) alone at higher speed,but WHAT controls the lower or bottom end of the fuel curve??? Is it the power jet that can be altered and if so, does the number on the power jet need to get smaller or larger NUMERICALLY. I am very close to proper mix in the upper rpm but WAY fat on the bottom. The engines are extensively modified in the ports(intake—exhaust) but both run the same Edelbrock slingshot with 2–48 carbs(slightly more flow than a 97) and an ultra tall free flow air filter designed for a Porsche.Both engines run fairly strong cams(around 350 lift) with Isky springs—- I have pulled 5200-5400 many times in the past so I don’t think springs are the problem. Car has run 80 mph in the 1/8 when previously pushed hard. Compression readings are all close to 150 psi on all cylinders in both engines. Car runs very strong on the street but in theory should run better once low end RICHNESS is made better.Am I asking for 2 much???
That is just to send your thread back to the top so it’s noticed by someone who can accurately answer your interesting question.
A good article on power valves from Jere Jobe. Although it’s more about holleys, I’d imagine it holds true with the aspect of Power valves. https://www.motortrend.com/how-to/0510sr-holley-94-vintage-carburetor/
You're using a wideband AFR meter? One of the best docs I've read is by Dave Vizard "How to Tune Holley Carbs" or somesuch. A lot of it is excerpted online (which I wouldn't like if I was selling books) and describes tuning each circuit. You're not tuning a Holley, but the exact same principles apply. Carbs that use power valves, the number refers to the manifold vacuum level at which it opens/closes. It is really important to understand you can't determine if the jetting or steady cruise mixture is suitable, if a defective or incorrect power valve is installed. A power valve that is open when it shouldn't be will run rich when it shouldn't and screw up your diagnostics. For all that though the power valve is more about "when" extra fuel comes in, not "how much", there are basically fixed jets underneath the power valve that determines this. A vacuum gauge you can observe driving will help you with tuning a carburetor.
The power valves on a stromberg 97 are not operated by vacuum. they are actuated by the accelerator pump. The power valve is in the bottom of the acc pump bore and the pump pushes the button in the valve opening it . this starts at bout half throttle. the numbers are the size of the power valve, like jets the higher the number the more fuel. It sounds to me like you need to jet your car down and try again as over half throttle you are in the power valve
Car runs 2 lbs fuel pressure thru out the pull. I stood on the sidelines and watched the gauge from start to 4000 rpm with no change. Both pulls measure hp at the wheels. 1st engine was bored 080 stock stroke had 118 hp to rear tires. 2nd engine was bored 020 with 4 in stroke and only netted 10 more hp at 4000 than the bigger bore stock stroke engine. All 6 pulls from 2 different engines 2 weeks apart did not “miss a lick” and all 6 runs ran flawless then instant “shut off” as if power went thru ground—-no hiccups or misses, just like cutting off key at 4000 rpm. Ignition is MSD for 49-53 flat—-nearly new inside,looks gr8 but has me baffled. Since it happens at 4000 rpm someone suggested it may have a rev limiter but I can’t see anything unusual and yes,I have tried a chromed accel super coil and a Petronix 40,000 volt coil and they both do the same thing—-just shut down totally at 4000 rpm. Yes,it has a vacuum advance canister on the dizzy and maybe when plate inside dizzy is advanced, something may direct the spark to ground. As long as you don’t exceed 4000 rpm,car is very quick and responsive thru rpm range.My stock h10 plugs were pure white until jets were at 049 and only then did they have a slight tan or brown color on the porcelain
Hate to disagree with Moriarty but Stromberg power valves are based on drill number sized so larger numbers are smaller size power valves.
Seems to me like the jets are what need to be changed, I am sure at 4000 rpm's that you are in the power valve. Usually with multiple carbs you jet down from stock. (my 40 has 4 97's with number 40 jets) I would at the least go back to 45's and try it again. that is after you have repaired the ignition problem.
Bottom end of the fuel curve is controlled initially by your idle needle valve, and the size of the idle air bleed. Initial fuel is fed through the LOWER idle discharge hole. The throttle plate (should) cover the UPPER idle discharge hole, which thus does not flow. As you start to open the throttle plates, the UPPER idle discharge hole gets uncovered, allowing more fuel in. This acts as a transition until the main metering jets start flowing. Are you sure that the upper idle discharge hole, lower idle discharge hole and idle air bleed are: a) unblocked (gently probe with a piece of bent copper wire strand from some scrap house 110V wiring), b) not flogged out oversized by someone during a previous overhaul (harder to check - need pin gauges)? Either of a) or b) will cause the bottom end to act funny at idle or immediately off-idle. If you have compensated for the a) or b) at idle (normal tuning point) then the off-idle transition may be waaaaaay rich. Cheers, Harv
I know smaller jets are not the answer. I have enuf modifications that for years I ran 046 jets and have built plenty of engines. My plugs are Snow White —-as in no color.went to 047 still white. 048 still white even on a 200 mile round trip. Finally went to 049 got a little bit of light tan brown color and fuel curve show spot on 13.2 on the air fuel ratio. Car runs extremely well—-quite scary even for a Flathead but lays down at exactly 4000 rpm. I am looking for answers in the lower rpm range cause from 1200 to 2500 is overly rich. I have run Muncie 4 speeds and nine in Fords cause my “heavy foot” has tore up many 39 style boxes and banjo rear ends. Early Ford stuff was marginal even when new,now that it is over 60-70 -80 years old, I would not trust it across town. I use my hot rod every day and don’t cut it any slack—-it has to perform or else find another venue. I don’t feel smaller jets are the answer to my problem,but I appreciate any and all suggestions.I have over 60 years of flatheads, high performance and stockers—-every hot rod I’ve had(over 10 over a 50 year period) have had no other power plant other than “strong “ street driven flatheads!! Thanks to all John in s.Texas.
Here are the results of my fuel curve on the dyno/ the top pic is when 080-stock stroke laid over at 3500. The bottom pic is 4 in stroke 020 that laid over at 4000 rpm. The penciled line is perfect—-above the line is lean—below the line is rich or fat. The numbers along the bottom of each chart are RPM—-numbers left and right are fuel/air mix ratio.According to the dyno operator my “mix” is near perfect near 3500 on first engine and near perfect at 4000 rpm second engine. I would have NEVER even suspected being rich or fat until the figures indicated. I run 2 —97 style carbs on Edelbrock slingshot with 049 jets—-ran 046 for years until these last 2 engines—slightly diff cams—both around 350 lift,same heads reused,same intake,carbs,air filter,same headers swapped from engine to engine. My question is what canbe done toget closer to the ideal mix at lower rpm or am I asking for 2 much. I have run 2 diff engines with 2 diff dizzys and both laid down near the same rpm. 1st run was right angle drive with cut down Chevy with advance curve tailored for flats(ran points) 2nd run was late model MSD dizzy with vacuum advance.1st set of run ran a thinner air filter—2nd set of runs ran a taller air filter—showed very little difference
Did you get a chance to check the upper idle discharge hole, lower idle discharge hole and idle air bleed? Cheers, Harv
Please consider a wide band oxygen sensor. You will be amazed how various conditions affect air fuel ratio. Weather, barometric pressure. Most cool little chrome air filters richen up the mixture tremendously. Do you think low fuel level in the carb bowls might be causing problems? Tee in a pressure gauge a carb fuel inlet. Test under load at full throttle. Check float level , set to factory recommended height. I try to go by the old advice "when working on a precision engine never assume anything". speedshifter Greg White
I was having the same problems. I run a high rise 2 x 2 Tattersfield inlet manifold on a 255 french flatty. Two new GB made 97 carbs. I lowered the fuel level a lot and reduced the pressure way below 2,5 psi. Next step was to fit 40 mainjets -now finally no rich smelling anymore.
Will it pull beyond 4000 rpm in first gear with the throttle nearly closed? I'd us an inductive timing light as a first step to check what happens to the spark and timing at 4001 rpm and beyond.
A wideband A/F ratio gauge will tell you a lot more then a dyno can when working on drivability problems. Does the engine die, or just lay down above 4000 rpm? Where is the complete dyno chart, it might have information we need?
I know this is a bit old but I'm interested if it was figured out... Are the choke plates still in the carbs?
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