Oh great, another carb thread

[Honolulu]

Just one, really, and it goes something like this...

By now our (carburetted) coupes have gone many tens of thousands of miles, and in doing to consumed a goshawful lot of gas. To wit: 100,000 miles at 14 mpg is more than 7,000 gallons, flowing mostly through two teeny-tiny main jets. That's a lot of flow through those little holes.

Given that, has anyone replaced main jets due to wear? I once worked next door to a competent USA car mechanic, who was of the conviction that carbs could simply wear out, and the jets wore out of spec.

What say you?

 

[Mike Goble]

14 mpg at 60 mph uses one gallon every 14 minutes. If your carbs are balanced, that's one gallon every 28 minutes through a 117.5 jet. That's a relatively minor flow and not enough to wear any jet. You could probably gravity fill a gallon jug through the jet in 28 minutes.

 

[decoupe]

Fills in or wears out?

Based on the difference in flow rates on my injectors after cleaning I would think the real problem is the hole gets smaller with age/mileage from gumming up with fuel quality issues and long periods of inactivity that results in deposits accumulating.

Just a thought.

 

[61porsche]

I've never seen a worn jet of our type.

SU's; yes. Mucked up by a previous owner, yes. But that's what a jet reamer/ guage is for. Last I checked the tools were still cheap and worthwile if you're blueprinting a carb.( 7 bucks from a VW shop/ supplier)

But generally, old zeniths once rebuilt need a bit of tweeking of diaphrams, o-rings, etc. where small air leaks effect idle. Not to mention the accelerator/ pump jet.

Given all the technological changes up to now, overboring the throats/ venturi's, needlebearing shaft seals, flow bench, etc. I bet some enterprising owner could make a pair of Zeniths work for a large displacement motor with relative ease. Seen it done on a Porsche where originality/ stealth/ big bores meant something to an owner/racer.

 

[Honolulu]

DeCoupe: an injector is a "normally closed" orifice, and the tolerances there are so much smaller than the opening in a carb jet, it's easy (to me, anyway) to envision that an injector would tend to clog from particulates. Injectors are supposedly protected from this by the (5 micron IIRC) filters incorporated in them. Their tolerances also make them susceptible to gumming from evaporation of residual fuel on shutdown. But thanks for your thoughts.

Ever the engineer, and being unemployed, let's do some math. Not knowing the actual ID of a main jet, let's assume that it is, say, .05 inch and its area is 0.00196 square inch. A gallon of fuel is 231 cubic inches, so a gallon makes a "string" of fuel 117,857 inches, or 9,821 feet long.

A carb that has been around for 100,000 miles at 14 mpg has seen 3,571 gallons go through (assuming all through one main jet) or a "string" of fuel that was 35,070,079 feet or 6,640 miles long. Can it be imagined that length of fuel can be so clean as to flow through without some wear?

Simply put, I should find out the correct diameter of the 117.5 main jet, get a gage of correct size, and check what has happened to mine. But that would require effort. Still, having little better to do...

 

[decoupe]

But you are in Hawaii. And it's winter here.

 

[Honolulu]

Winter? Huh? wazzat?

Well, okay, weather can play a difference, but here on Oahu you can't get above 2500 feet, the temp is always low 70s to high 80s, humidity is mostly in the 70-80 range and it's almost always breezy and mostly sunny. This is part of why some folks get rock fever and leave after being here maybe six months - there are no "seasons". Not having a job in a state where everything is on average 38% more expensive is also a contributor, but I was born here, won't leave, nowhere to go. 87 octane is $4 a gallon.

And I have to ask: What is the correct (metric or SAE) diameter for our jets? I can use my number drill set to check jet diameters on the spare pair of carbs that are just laying about. Brief check shows jet gages run about $38.

 

[jmackro]

I once worked next door to a competent USA car mechanic, who was of the conviction that carbs could simply wear out, and the jets wore out of spec.

Carbs that don't have ball bearings on their throttle shafts wear where the shaft passes through the body. As the hole through the carb body wears, it allows extra air to be drawn into the carb, leaning out the mixture.

Since some grit will be fine enough to get past any filter, after 3,571 gallons has passed through the main jet let's assume that its id does get bored out.

Therefore, if the throttle shaft bearing was wearing at the same time that the jet diameter was increasing, the two would offset. And I'm sure the clever engineers at Zenith calculated the hardness of materials, porosity of fuel filters, and diameter of the jets & shafts so that the fuel-air mixture is precisely maintained as this wear takes place. :smile:

What say you to that theory?

 

[pamp]

Jets

Never seen jets wear, only plug. Sometimes reamed by folks wanting more fuel, usually because they were plugged and they got over zealous. My 2 cents.

 

[Honolulu]

Jay: Yes, throttle shaft wear drove me nuts while I was caretaker for a friend's 2002a. It would idle just great in the garage, then crap out as soon as I shifted to either forward or reverse. I tried and tried to get the dang thing right, only to have it stumble as soon as I closed the hood. Finally I really stuck my head in there and heard the telltale hiss of a significant air leak, and remembered what I'd read about Weber 32/36 steel throttle shafts wearing in aluminum bodies. Sure enough. The kid came back, reclaimed his car, and ran a red light, only to be massively t-boned by a Benz. Even the opposite side of the car was creased. He was never clear about the extent of hurt suffered by the impact-side passenger.

All in all, okay, there's no sympathy for the idea of jets being worn larger, other than by owner action. And I've seen corrosion and fine "dust" in the float bowls of some carbs. When I get the inclination, I'll take a number drill set to the jets on the spare Zeniths and see what may be discovered.