Seems that the MyRideisMe.com Bonneville experience never runs out of steam. Hanging out at the Nugget one evening, we bumped into Clive from Stromberg Carburetors. After a lengthy BS session, the conversation turned to carb tech. And to cut a long story short, we asked him to contribute to our ongoing “5 Things” series. Alright, so 5 turned out to be 6 – or as the English say, ‘half a dozen’. Here’s what he had to say:
1. Stromberg Carbs Run Better With The Chokes Left In
We’ve all seen those pics at Bonneville with 97 chokes removed and the kicker linkages brazed onto the base casting. It should make sense. No choke means more air space means more cfm. And you’d be quite correct, too.
Extensive 97 flow tests carried out this year by acknowledged race carb expert Norm Schenck showed that the carb did indeed pick up a little cfm without the choke plate installed. So all those Bonneville racers were right, after all? Well, yes and no. Salt Racers are only interested in WOT. On the street it’s a different matter.
Stromberg authority Jere Jobe told that 97s run better with the chokes in, so we suspected what Norm’s tests would show. Only we forgot to tell him the full story. Here’s what he said:
“I retested the signal curve with the choke butterfly and shaft removed, with somewhat disappointing results. The signal was unstable at most of the test CFM’s, and taking the average signal at each CFM to figure the signal curve showed a much less manageable curve than with the choke parts installed. My conclusion is that the choke butterfly serves as an airflow straightening “vane” that directs the airflow to the area of the boosters with reduced turbulence. Even though the choke parts cause a reduction in flow, it is not a good trade to lose good fuel metering for that CFM gain.”
So there you have it. The same story from two very qualified horse’s mouths.
By the way, if you want to keep your choke plates fixed open, try our Choke Lock Detent kit (Stromberg Part 9537K-L), which replaces the usual round-tipped detent pin in the airhorn to lock the choke plate open.
2. Bigger Stromberg Power Valves Have Smaller Numbers
Stromberg main jets are easy. What you see is what you get. Stock Genuine 97s come with 45s which means 0.045inch. Power by-pass valves (PV) – the ones underneath the accelerator pump — use the old engineering Number and Letter Drill system, devised as a way to fill in the gaps between the 1/64th sizes. And to complicate matters, the bigger the number, the smaller the drill!
And to complicate things even further, changing your PV by one number does not always mean the same change in jet size! We offer everything from #72 up to #60 (note that I said ‘up to’). The #72 is 0.025inch, #71 is 0.026, but #70 is 0.028 (a two thou’ jump), then #69 is 0.292 (WTF!) . The gap between #66 (0.033) and #65 is also 0.002inch before it returns to 1 thou’ per size right up to #57. We didn’t make the rules! But it pays to remember this when you’re trying to rejet.
And while we’re on the subject, remember that the PV only starts to affect the fuel ratio at just after 50% throttle. And when you swap them, cut a slot in the centre of a wide blade screwdriver so you don’t
3. Set The Stromberg Float Dry
The float in a Stromberg 97 (and 48, 81, etc.) is supposed to be set so the fuel level (not the float itself) is 15/32 inch (plus or minus 1/32) below the top edge of the casting without a gasket. But to be honest, that’s easier said than done, especially with the engine running and a cigarette on the go.
So for increased customer safety, our Premium Service Kits (9590K-97 and 9590k-81) now recommend that the float is set ‘dry’, as we do at the factory. Rather than a float gauge, our kits now include an extra leaflet about setting the float level. To download a copy, click here. Basically, just get the float so it sits level in the bowl when the inlet valve is shut.
All new Genuine 97 floats are pre-set at the factory, but if you’re rebuilding, you adjust the front ‘tang’ of the hinge to push it nearer or further from the inlet valve. If (and only if !) the carb is empty of gas, hold the bowl section upside down so it closes the valve by its own weight, then eyeball it through.