Trying to understand how vibrating engines can “shut off” fuel flow..
This has bugged me for years. Why would fuel, turning partially to foam in a vibrating carburetor float bowl, shut off the pressurized flow of fuel through the needle and seat? The carbs’ floats are buoyed by the weight of the liquid that they displace—so it seems natural that lightweight foam bubbles should allow the floats to drop—allowing pressurized fuel to enter the carbs easily.
Sled racers come to DynoTech Research sometimes to diagnose and try to correct erratic high speed engine operation often caused by fuel turning into foam in vibrating carb float bowls. Foaming carbs—especially on stiff mounted, high revving, slightly imbalanced engines—has been the bane of racers since I built this dyno testing facility 28 years ago.
The SuperFlow engine dyno measures fuel flowing from the tank to the pump, and if that flow is erratic it’s impossible to tune to max HP. Erratic fuel flow can result in diminishing top end HP and seizures. Racers look at dry pistons and white plugs and try to fix that with bigger jets and instead make things worse by running bowls dry even more quickly! But we often fix the erratic fuel flow by 1) softer motor mounts, 2) softer carb boots and weighted carbs, 3) larger needle and seats, 4) rebalancing the rotating assembly, and 5) higher fuel pressure—sometimes as much as 15psi is required to overcome the foam that seems to create an invisible barrier, preventing the stock fuel pumps’ 2-5psi of pressure from refilling the bowls.
Back in 1988 Tim Bender switched to solid motor mounts in his mod Yamaha Exciter 570 FIII oval racer a few days before leaving for Eagle River to keep the vibrating twin’s clutches perfectly aligned. But subsequent testing on Silver Lake (the ice fishermen hated seeing the Bender Racing trailer arrive) revealed erratic, surging top end power and dry pistons that couldn’t be cured by rejetting. So back to the dyno, and sure enough, the fuel flow was erratic or worse due to foaming fuel. Stock motor mounts cured the problem and the Exciter went on to win Eagle River three years in a row with stock motor mounts.
Lectron carbs are used by lots of racers, but seem very susceptible to foaming due to their light weight. An the clear plastic float bowls often allow us to see foam appear—coming and going at different revs on vibrating engines. Several years ago I watched then-Pro Stock Bike dragracer Antron Brown being interviewed on TV after losing a round at the Brainard MN NHRA race. He complained bitterly about his Suzuki (they all ran Lectrons) “surging” and “laying down” in high gear, causing the race loss. Vibes!
The next day I called Fast By Gast (supplier of Lectrons to all of the bike racers) owner Paul Gast and explained to him on a three-way conference call with his right hand man Kevin Gilham about vibration-induced foaming and the issues it causes the sled racers, and what we’ve done to try to rectify that. It had been hot in Brainard—close to 100F and several of the bike dragracers had the same problem in high gear.That discussion led to the design of the excellent high volume, high mass billet aluminum floatbowls for the Lectron carbs.Kevin Gilham, who’s a great guy, would eventually go on to become the owner of Lectron in Texas.
RVP testing and trying to relate to what’s happening in the vibrating float bowls…
Now we know that the hotter the fuel, the more front ends boil out of the fuel. But the RVP test is done with 20% fuel and 80% headspace—volume that must be filled to create pressure by the heated, agitated fuel. But a carburetor has a much different fuel/ headspace ratio—more fuel % and less headspace %. That ratio surely varies a bit from manufacturer to manufacturer. To examine the effect of reduced headspace on heated, agitated fuel, I “Carb Vapor Pressure” tested some Sunoco Maximal that was purchased from a bulk tank that was actually 2psi RVP tested properly. I retested it this time with 80% fuel and 20% headspace—maybe more like a carburetor might have. The result was 4psi—two psi higher than RVP or perhaps double the RVP. So that means that maybe real fresh 5 psi RVP fuel might test at 7 to 10 psi “CVP”! But the carbs have vents that should allow the boiling off front ends to escape—but are those vents adequate? And could the agitation from testicle-numbing vibes at 9000 RPM be more severe than the mild agitation of a RVP test? And could the violent high frequency vibrations be boiling off even more than just the front ends creating a rush of expanding gases that the vents can’t keep up with? If a sled’s fuel pump puts out, say, 3psi all it would take is a pressure rise of a couple of psi to slow down the fuel flow into the carbs. Then if it goes to 4 psi then voila fuel flow = zero lb/hr momentarily until the float bowls begin to run dry of front ends and base fuel, float bowl pressure drops and fuel flow commences once more (hopefully before seizure occurs!).
To examine that possibility, the next time we get a sled with erratic fuel flow and foaming float bowls I’ll connect one vent hose to a pressure transducer to see what might be happening there. If that’s the issue, a band-aid fix might be, simply, opening the vent passages or increasing their numbers. Then, all of those boiled-off molecules can be quickly replaced by the fuel pump, and the engine should be happy even if it’s ingesting primarily heavy-end fuel molecules.
There seems to be a difference in sensitivity to fuel delivery problems from manufacturer to manufacturer regardless of mass. Even some heavy weight aftermarket carbs that make good top end HP are plagued with vibe-sensitivity, even though they use standard Mikuni or Keihin floats and needles and seats. Perhaps insufficient venting?