What size jets on 97 Stomberg?

[Roger/Sacramento]

I have recently been running my new build in a '41 Pickup on the road. A friend said that the exhaust smell was very strong when he followed me. What jet size should I use? Because they are not exactly cheap to replace maybe someone can get me close. I am at sea level.

276 cubic inches (1/4" overbore with Mercury crank)
Offenhauser 400 heads
3 Stromberg carburetors
Crab ignition
L100 cam

By the way I am using an unregulated new mechanical fuel pump and have not experienced any excessive pressure causing leaks.

[ken ct]

Call Charlie NYor email Uncle Max for info. ken ct.

[ctaulbert]

I have a 284 cubic inch motor, with three 97s on straight linkage. I'm running .042 jets, and #67 power valves. I've put about 5,700 miles with this setup so far, and it seems to work well.

[Charlie ny]

Roger,
It's been my experience that in cases such as yours the power valve/s
could be leaking. It's imperative that the power valves be leak checked before
use whether in a single or multiple setup. You can quite easily chase an over rich
condition to distraction that usually is a leaky PV. Trust me even brand new PV's
can leak.
Charlie ny
[email protected]

[G.M.]

Go by the color of the plugs not smell. If the plugs are black and sooty it's usually the jets are to big but could be as simple as a dirty air cleaner.G.M.

[ken ct]

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Go by plug readings ,though as Charlie said 1 or more could be leaking.In any instance i think #67 are not very much smaller than stock PV's I would check for leakage and or change them to #71 which is about as small as you can go [strom 81 size] ken ct. A DIY jet change would run you about $40 and a PV change would run around $75 OMO. ken ct.

[Walt Dupont--Me.]

I'm running four 97's with straight linkage on my 276 in my 40 coupe. I had Uncle Max build the carbs. He did a beautifull job. .042 jets and I believe #67 PV. On my first 100 mile trip I noticed it was lean on a hill, T5 trans and 350 gears. When I got home I pulled a couple of plugs and they were white. I went one step higher on the jets .043 in all 4 carbs. Perfect, no end to the rpm. and the plugs still run white, 18MPG regular driveing, not on pike. I'm sure I'd get better mileage on on the pike. Walt

[Pete]

Buy an .043 drill for 3 bucks.
Soft solder the holes in the jets.
Redrill on a drill press or lathe with new drill.
Total cost for jet change:
$3 for drill.
$1 for solder.
20 cents for electricity to run drill and solder iron.
Your labor at $75 an hour.

[flatmotor40]

I have 2 97's on mine and now it seems to take a while to start when it sits for a few days like the bowls don't have gas but I let my electric pump run for a little before hitting starter and have to really pump pedal and choke it still has to crank a while.This has just started mine also smells rich but plugs are fine my wife say it is running rich Thanks for help Flatmotor40

[Robert Dip]

There always seems to be frustration on getting that engine combo just right. I just finally finished going thru mine on the ’39 coupe. It was running much too rich…black sooty plugs and about 5 MPG. I change so may parts on the engine to find out the culprit….distributor, coil, wiring, plugs, and yes, carbs. Ordered smaller jets ( from the real source, not the new phoney ones for parts or complete carbs ). Jet sizes were originally .045…went to .043, then to .042….still no real progress. The stupid culprit was the air filters….the old helmet style with the new paper filters….so restrictive that the engine was always on ‘choke’ mode. Did this story before, but probably worth repeating. It’s the simple things that can get you. I see you have another style air filter….so first things first….remove them and proceed from there. Another matter that got my attention is in Tex Smith’s book on flatheads where there are many pages dedicated to the carbs and the 97s. It clearly states that with multiple carbs, having less vacuum pressure in the carbs, you may have to increase the jet size and not decrease. Wow, this was something I had never heard of before. For my dual carb set-up, this became correct. Once I pulled off the problematic air cleaners, and had to re-adjust timing, I now ran too lean. Plug color are the true source of the right mixture, and now they were white instead of the proper light brown. ( I also noticed a little increase of engine temp ) I then increased the jet size to .047 and the formula is now perfect. A 250 mile trip to a car show proved the combination to be as good as it gets, with engine temp running between 160 and 180, depending on stop or go….now at 15 MPG. Be patient, and you will get the combo needed. By the way, new jets are not expensive and easy to change. Oh yes, an electric fuel pump is definitely the way to go…fills the bowls and avoids excessive engine cranking.

[don-wi]

I've found that it is harder to read plugs with ethanol gas.

[Bill OH]

I have a 1938 ford 221 with Isky 3/4 cam and dual 97 carbs - have had .042 main jets and 65 pvs and would not run unless throttle was to the floor to bring in pvs. I installed .045 main jets and #71 pvs and all is well. I am at 700 ft above sea level. I have run .045 mains with 65 pvs and motor ran well until wot and motor would fall back on performance, the reason that I went to .042, but that was no good. I have settled on .045 mains and #71 pv(Model 81) and I am pleased with the performance. You being at sea level, you may want to go to .046 mains and #71 pvs. Hotrod Company has the parts. As a side note, I have blocked heat risers and about 5 or 6 gaskets under each carb - no percolation problems.

[Walt Dupont--Me.]

I'm useing K-N filters on my 4 carb set up 4in. tall. $35.00 bucks a piece but you'll naver have to bye another filter. Just wash with soap and water and spray that stuff on them. Walt

[Roger/Sacramento]

The picture shown on my original post is actually an early photo and the air cleaners are now just with a screen and no other filter (see new photo). Checked the plugs and they are black. My speedometer is not calibrated yet so I can't check the mileage but, I know it is real bad. The gauge moves like a second hand on a clock.

Thanks for all the comments from the members......

[Bruce Lancaster]

Continue to worry about the lids...screen or small holes does not necessarily flow well. Try feeling the breeze coming through a window with a screen, then open the screen...
I would give them a quick trial: First, remove them while engine is idling and see if there is a change at idle. Then pop in a set of new plugs and go for a ride and see if black returns or not.
A crab is generally good on the curve, but be sure static timing is at spec or more advanced and that vac brake is nearly fully backed off, as retarded spark can mimic a rich condition on the plugs.

[uncle max]

Another tip or two...
Turn your scoops around! Strombergs do their best when gathering air from a non-turbulent dead-air space. Forward facing scoops contribute to siphoning, especially at higher speeds.
Also, it's hard to tell from the picture, but it looks like you need to be using chokeplates. The plates are necessary to provide a smooth airstream over the open ends of the discharge jets, for proper venturi signal and atomization. Contrary to the myth that plates should be removed for "more flow". End of sermonette.