Hi My name is Terry Lucas from Oakdale California. I have a few questions regarding the Big97 tuning. I purchased 2 primary Big97s for use on a 286 inch Flathead ford. I’m not using progressive linkage. The engine is modified enough and dynos at 180 hp. Cam is Iskenderian and fairly mild and idles relatively smooth at 800 rpm. My tune questions are based on mixture at medium cruising speed at around 1800 rpm steady at sea level.They are richer than they should be and nearly feel like the choke is on but it’s not. Wondering if it’s possible the accelerator pumps discharge in the venturi while cruising can be a problem. Fuel mileage is not good.Fuel pressure is regulated carefully at 2 psi. I have rechecked the factory float level settings and found it ok. All this leads to the question - have you got a jet size suggestion from experience or customer experience that makes sense? I’d rather not buy 2 hundred dollars worth of jets if you have helped someone with the similar issue. I have not removed the jets to check their size out of the box but vague info suggests they are 56’s. Next question is somewhat related is - Will the power valves be close to correct? Or is there a size that will work better in my combination you would recommend. I’ve owned a European import shop for 40 years and built and raced Flathead fords at Bonneville salt flats to explain my experience level. Thanks for any help. Terry Lucas
It sounds like it is just plain rich - as it is very fat even at cruise. If it was mine, I'd put an O2 bung in both headers and get an AFR meter. It can tell you a lot - such that you're not just wild-ass guessing on jet sizes. There are not many folks with BIG 97 experience, so I would not trust tribal knowledge in this case. Once you have an AFR meter setup (and ensure you have NO exhaust leaks), then step your jet sizes down - maybe 4 to start with and see what the AFR meter tells you? Truth be told, I've wanted to try two of those on my 284 cube engine (big roller cam, big valves, heavily ported, etc). Maybe two are too much? My instinct says that two of these should run well with your engine - just need to first get the main jets figured out, then potentially work on heavy-load situations with the power-valves.
Nice looking car and Flatty. You may be able to jet those down but you put 500 CFM on a 180 HP engine. That, as you already know form your experience, is a lotto'carb. At the very least I would run them progressive if the intake will handle it efficiently. Clive may be able to assist you in re-jetting to see if can tone those things down.
The manifold you're running is not really conducive for progressing linkage - so you'll need to see if you can tune these carbs to work in your situation and if you just can't get them to work, then maybe go with Stromberg 48's.
Wide band O2 is a great suggestion (Bored and Stroked). Any thoughts on the accelerator discharge nozzles pulling fuel?
Jimmy Six The stromberg BIG97 has I think 56’s in it which are bigger than original 97’s (46’s I think) Stromberg jets are pricey compared to Holleys and don’t think they interchange. I was hoping to get someone that may have done this type of build and combination. I’m still workin it out. Thanks
I doubt it - as everybody would be having issues if that was the case. If you really wanted to find out, just temporarily disconnect the pushrod and gently take off and get to the cruising speed where you felt it was really rich. Yes, it may stumble a bit without the accel pumps, but you should be able to ease into it and answer your question. If any trouble getting going, just start on a downhill run. Once you're rolling, you don't need the pumps for this test. You could also do this with and without an AFR meter.
I've never worked on a Big 97, but if they take the same jets as their "little brothers", then they are unique to Stromberg carbs. You'll get it sorted out and having O2 bungs and an AFR meter (even if only temporarily installed) sure helps tuning!
Terry I just spent some time working with a pair big 97 carbs since they are 250 CFM each overall that's not to much carb for what your doing. The accelerator circuit for the Stromberg by design will not allow fuel to be pulled in under load like the Holley design as it pertains to their four barrel carbs. The main jets are fairly large if it was me I would go down two sizes that should help a bunch. Ronnieroadster
Thanks Bandit Billy Cars a work in progress so far. Just got it rolling and working it out. I should have spent more time on the dyno but the pulls I made were at full throttle which didn’t give any low to mid throttle stuff so until I got it in the chassis and on the road had no idea it was going to act rich at low throttle. Progressive linkage is not easily possible on this old square script Edelbrock .Bored and Stroked recommended wide band O2 and that’s best starting point I think. I do hope Clive has some advice on some of my concerns ( jetting power valve size or any other tune tricks.) I have to compliment the manufacturer here - they are high quality stuff from what I have used. I know a side float valve arrangement is easy to overcome with pressure but 2 psi regulated keeps that in check. Terry
Thanks for your response and advice Ronnie In working with it did you find any issues that led you to different power valves or were they good original. As to CFM they pulled on the engine dyno easily from 2500 to 5000 so size may be ok in this combination.
Thanks Bored&Stroked My headers are from one of our Bonneville projects and I realized they have a bung in them.if I have room to get the sensor in it it will make tuning easier Terry
I am running two Big 97's on a blown 276 flathead. The Big 97's come equipped with .056 jets from the factory. I don't want to discuss my tuning as it is a different combo then yours. I tuned it by reading the plugs but the best way would be to tune it with an AFR meter as B&S suggested. I will add that I know you stated you confirmed 2 PSI fuel pressure but I cannot over emphasis how important it is not to run too much fuel psi with these carbs. Also make sure your ignition system is up to snuff and you are running correct heat range plugs for your application. I also have a 286 flathead with three original 97's on it that are run all together with no progressive linkage. Confirm fuel psi again, confirm good ignition system and plugs, and then check with O2 sensor and AFR meter if possible. If you are still too rich after checking all that, I have two pair of .051 jets I made with a pin drill out of original Stromberg jets that you can try if you need to.
sshep Thanks for your response I have a pressure regulator that is not one of the cheap ones and is adjusted to 2 psi. But I’ll recheck pressure at top side The ignition is a MSD well hidden using an original ford crab cap distributor for switch circuit set at 26 degrees total. I’ll go with what you all suggest ( wide band with a meter) and post my results and modifications. thanks Terry
Sounds good, let us know how you make out. Not sure which fuel gauge you are running. Just my own personal experience, I don't like the liquid filled psi gauges. I am running the Holley non-liquid filled gauge on mine.
My last 255 cui engine is nowhere near your motor but I did run two 97 carbs - but not the big ones. Direct linkage connected both carbs. They were way too rich, black soot and eye watering fumes. I reduced the fuel pressure well below 2 psi, lowered the float level a lot and used smaller main jets and valves. All of it helped a bit but still too rich when cruising through town. So in the end I had to solder the idle jets and drill them smaller to achieve proper runing engine.
Terry to answer you question the stock power valves were left alone as well as the 56 jets however I did experience an issue at idle so I modified the idle dump tube diameter with a pin vise to correct the problem. However the combination I was working with is completely different than yours its a supercharged ARDUN. Anther important detail I tune both carbs to run as one thus the throttle shafts are not progressive there tied together. In all my years of tuning flatheads I have found two Deuces run best both tied together. Ronnieroadster
In my opinion, and it’s only my opinion… You would be much better off with a pair of 81’s. What you’re looking for is driveability & throttle response. It’s hard to get that if the carbs too big. I agree that the rich issue could probably be helped with smaller jets but you still won’t have the velocity thru the Venturi’s for optimum performance. If you only ever drive on the street (1) Big 97 would be all you ever could possibly use. If you take it to the track or the flats and you are willing to put up with some poor performance on the street then that’s the way to go. Tell me what you expect your max rpm to be?
At 5,000 RPM you would be using 400 CFM so although pride and ego might play into it, for best driveability I would stay 400 CFM at the most.
Just to add some more "fuel to this fire" . . . the standard practice for rating all older 2 barrel carbs was 3" of mercury. In the mid-50's or so (when 4 barrels came out) they found that under WOT, many higher performance 4-barrel cars would not pull 3" of mercury, so they changed the rating standard to 1.5" of mercury. So, comparing the CFM ratings of a 4 barrel to a vintage 2 barrel requires a bit of math - simple stuff. Take the CFM ratings of a 2 barrel and divide by 1.414 - this will give you a comparable rating for a 4 barrel. Now this is also assuming that both were rated with a "wet" versus a "dry" test. (I'm not going to get into that!) So, you take your 165 cfm Stromberg 97 - divide it by 1.414 and you get 116.7 - that is the 4-barrel rated CFMs. For the Big 97's (assuming they're rated at 3"), you take 250 / 1.414 and you get about 177 4-barrel CFMs. So, two Big 97's should be about the same as 3 standard 97's. (Generally speaking!) Given this, you can see how having 3 Stromberg 97's is equal to about 350 cfm (4 barrel, wet rating). That should give you all something to ponder now! B&S PS: I have no idea as to how Clive tests/rates the Big 97 . . . anybody know?
Thank you 28RPU I appreciate your opinion and I may end up looking for 81s like you suggest. There is a certain amount of challenge to getting these Big’s to a better drivability. You’re right that ego may play a role in the challenge. If I can get it to run acceptable I’ll post what it took so it might help someone. I was hoping by my original post that someone had experienced these carbs drivability as 2 primary’s and had jet size ideas and power valve suggestions. The type of Edelbrock manifold I have doesn’t lend itself to primary and secondary progressive linkage. This afternoon I brought home an Innovate wide band from the dyno shop and will install it temporary and start test driving fuel curves. May take a while to get some results. Thanks again 28RPU! Terry
Thank everyone that took time to respond to my questions. I was hoping to get a simple answer like put this size jet and that size power valve. Here’s what I’ve done so far Installed an Innovate wide band with a screen that has a bar graph and digital display.Test drove it and found these numbers. Idles at 700 @12.8 AFR Cruise at 10.7 AFR @ 1800 rpm 3rd gear light throttle steady. Cruise at 11.2 @ 2200 3rd Cruise at 2600 AFR leans to 12.5. Transfer starting to hide and main jets in play. Full throttle 1800 to 4000 stays above 14.7 AFR ( main jets and power valve in play).A little too Lean for this load and rpm. Accelerator pumps were not discharging at all up to 1/3 throttle (leather sticking in the bore it seems) I installed a stronger spring set in pump plunger. Fixed this. Pumps are new. Checked main jet sizes ( should’ve been 56 and were 52’s.) Clive suggested 52 so I left them for now and started working the transfer over rich problem by removing the idle jet tubes. A .030 drill slides in the metering hole. Solder it closed and drill back open with .025 drill Retest 02 readings Idle set 12.8 1800 cruise 3rd gear 12.4 afr 2200 rpm 3rd gear 12.5 2600 still around 12.5 Full throttle no change Could go a little leaner on idle tubes ( maybe use a .022 or .023 drill size)and up the mains to 53 or 54’s It’s fairly acceptable and actually takes all the throttle you want to dish out. If you wonder whether BIG’s are too large it’s starting be very drivable. Still some work to do but they all need somethin. I was our tuner on a salt flats lakester called Spirit of Modesto in early 2000’s and I ran a Brazwell ( a roundy round)Holley 4 barrel. We set a 197.5 mph on race gas Don Monaco driving and 205.3 mph with a 50 shot of nitrous for a fuel record and a red hat for Chuck Billington. Don’t know the cfm rating on the 4 barrel but it wasn’t too small. Just got the mixture right Terry Lucas
Hi Group For those who might be interested in using 2BIG97 strombergs (primary models linked to open together) on a modified 286 inch or so Flathead it works very well with some minor mods. Mains approximately 54 (reamed 52s with Hasset jet reamers). 63 power valves Idle tubes soldered closed and drilled back to .022 ( I did have them at .025). Original .030 Cruise AFR at 2300 rpm light throttle steady 13.9 to 14.4 on the screen of the INOVATE 02 wide band. Full throttle it will take it all from 1500 in third to 4000+ at 13.9 AFR approximate with the reamed jets As Clive cautioned me these are very sensitive to air filter restrictions. I removed the inner filter elements and made inner screens from stainless screen material. All my tuning was with just screens. Car is very drivable. I appreciate every body’s input. It made me serious about getting it close. These are very basic carbs and I believe they are quality. I also appreciate Clive’s suggestions. Terry Lucas
The Jalopy Journal
