A co-worker who has done his share of old/classic vehicle rebuilds has been having some issues with one of his carbs. Well he has come across a resource for a rebuild and one of the statements from this source was simply that with new fuels, all of these old carbs should be rejetted (enriched essentially).
It that a reasonable assessment?

 

Unless he is using E-85 I would say the changes to fuel enrichment are inconsequential and there is no need to do anything further than adjusting the idle mixture screw(s). I'd look for dirt or sludge in some of the tiny passages.

 

Hrmm.... I wouldn't think so since the ratio to air and fuel will still need to be the same. It'll just have less energy content than it used to due to ethanol. Nothing can change that aspect except getting non-ethanol fuel. It won't change how the carb meters the fuel.

Most of the work that needs to be done to an older carb is rebuilding it with gaskets and seals that are safe with modern gasoline. Otherwise, newer fuels can cause issues with it.

 

I have limited knowledge here (most likely an understatement), but I guess I am wondering about it from this perspective. Modern, regular fuel has less "energy" because of the addition of ethanol, correct? I guess this rebuilder was maintaining to run properly, running a richer fuel mix would be required to off-set the lower energy generated by e10.
I have definitely heard about using modern "rubber" seals, etc. to offset the damaging impact of alcohol on those components.

 

Stoichiometric ratio for gasoline is about 14.7:1
Ethanol stoichiometric ratio is about 9:1

So a mixture of 90% gas and 10% ethanol will still put you around 14:1 (I haven't done the math) and with enrichment circuits set to around 11 - 12.5:1 I think no further jetting will be needed.

If it will make him sleep easier, tell him to go up two jet sizes. But unless he's planning a 200 mph record run across Bonneville it won't be necessary.

 

Interesting, I didn't know that ethanol had a different stoich than gasoline.

 

Bronc, Yup. And do you want to guess what Nitromethane is?

1.7:1!

 

"...running a richer fuel mix would be required to off-set the lower energy generated by e10."

Generally speaking, yes. But as FTF explained, there might not be enough difference to make a difference...and that is why, JMO, that you run what the carburetor brung and any jet changes would actually be one of the last adjustments to make based on what your engine 'likes', your manner of driving, elevation, vehicle usage, etc. There is no other valid formula for jetting outside of 'drive and try'. It is not something that can be pre-determined.

 

I saw video about NHRA engines - a see-through cylinder running at racing RPM and fuel loads and the injectors looked like a garden hose. Unbelievable how much nitro passes through a top fuel engine in a 1/4 mile run.

 

I appreciate the feedback and education. I was actually asking for my own benefit, not my co-worker. He's an executive. I don't think he would pay much attention to my input on the matter (:

 

At R&D there was a "wet fuel flow bench". It was a million dollar piece of equipment that allowed you to plumb in the fuel system to the flow bench and using a fluid we called "Stoddard" (some kind of petrochemical that had the same specific gravity as gasoline but wasn't as volatile) you could measure fuel/air content under all conditions. Downstream of the flow bench element there was some kind of liquid separator magic that extracted the Stoddard back out of the airstream.

Usually a 4V carb would flow about 20 - 40 cfm less with wet flow than dry flowed - since the fuel passages like the boosters could pull in a little extra air. That is why you may see two different flow numbers quoted for a single carb part number. So the next time an aftermarket carb salesman quotes "enhanced" flow numbers for his product or modifications you can ask "Is that 'wet' flow or 'dry' flow'?" .

Even so, we had huge fleets of test cars running around different extreme sites in North America doing final 'real world' tuning on the vehicles.

 

"Real world" is where it's at for us out here in the pucker brush...'try n drive' is the lab.

 

Holley 390, I know that Holley's are set up from the factory @ sea level and 70°. Increasing altitude = lower jet sizes. Lower temps = larger jets required. If I remember its for every 20° decrease you increase the jets by 2. I'm running a 54 now in 20 - 40°average daily temperature. 52, which was factory in the spring when its 40° -70° and a 50 in the dead of summer when its 70°-100°. A lot of times i dont even have to re adjust the idle mix screws. Just changed the jets for the increase in pressure required to pull the same amount of fuel at the colder temperature. I can almost tell the temperature when it starts to get close to the edge of the temperature ranges. Not to the degree but its consistent enough that i can make the assumption that the weather has changed past the point of optimal performance. Our carburetors do not operate in a vacumm. Air has different mass at different temperatures and reacts differently but proportionately across the temperature ranges to the pressures in the carb.

 

Readjusting for winter is something that I continually forget to do every year. It gets into the 100° here in the summer and can get down to 0° in the winter on special occasions. I should be rejetting.

 

"Our carburetors do not operate in a vacuum." Nor do our engines. Each one is different and there is no explicable reason for some of the differences...they just are...

The jetting recommendations for altitude, temperature, etc., are a guide and not a rule. I'm at 2000 ft. elevation BUT I actually ended up a jet size over stock rather than two sizes smaller which were too lean in my engine.