VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc Question for Junkyard Dog

Hey Dog,

I just noticed in a Summit Racing online catalog that a Holley carb main jet looks just like a Carter W-1 main jet. I'm wondering if you've noticed that too? If the threads are the same it would be an excellent substitution because of the price and availability.

Nope.....I don't get the Summit Racing Catalog....but thanks for the heads-up! I appreciate it!

Dog,

The Holley jets are threaded 1/4"-32. I don't have a W-1 jet in front of me but that Holley thread configuration doesn't sound the same.

Oh well, some of these swaps from other applications work and some don't.

When I restored my '36 PU in the 1970s I found a Ford Econoline van door hinge pin that was a perfect fit in the '36 hinges. Also Porsche 356 door window inner and outer rubber seals were a perfect fit.

Most thread pitch in Carter carburetors is 24 tpi. That includes both small and large diameter pieces. So the Holley jets won't fit the Carter W-1.

Chipper,

There are only a couple of exposed threads on the W-1 jet below the gasket but checking with a thread gauge it looks like 1/4-28. I'll check the threads in the casting with a tap to verify that.

I'm planning to make a home made copy of the updated 1932-36 W-1 that was offered as a replacement for the clunky, leaky carb with the 2-screw float bowl cover and 2-screw air horn. Towards that end I've picked up some of the improved W-1 carbs for parts. On one of them I put bushings in the throttle shaft and accelerator pump pivot shaft bores to restore a proper fit. In my carb parts collection I have a NOS throttle shaft with a flat arm that will be used. It's been around here so long that I have no idea where or when I got it.

My stock '36 engine runs best at my near sea level location with the original '36 metering rod and a 0.094 main jet (stock is 0.0955)in one of those 2-screw carbs.

To run it up Pikes Peak in 1976 I swapped in a .089 main jet at the bottom of the hill in Woodland Park and it ran cleanly all the way to the top, 14,110 feet.

If hot rod engines can be tuned to optimize jetting, why not antiques also?

1/4-28 seems to be the jet thread size. Be careful checking with a tap. A little too much pressure can cut new threads and damage or destroy the original ones.

As I wrote Carter generally used 24 tpi but there are exceptions. Zenith used some really odd thread pitches. Guess that was so you had to buy the parts from them instead of from another manufacturer. I have had to have several taps specially made to work on carburetors.

Use your fingers trying the tap...Couple of turns you'll know it's wrong...1/4x24 should be, 1/4x28 is getting odd for reverse flow jets...I only carry one in my tap set, you won't find it in a standard tap set...1/4x32 and I'd have to look through the drawer...I used to drill my race bikes to get what I wanted...Depending on the pipe configuration...Can't check your jets with the thread gauge...?

Sorry, my jet sets are at the other house or I'd check them for ya...I know...Big help I am...Halloween...Gotta get ready for the kids...Then it's off to the store to pick up the left over straw bales in the morn...The fun never ends...

Kevin47 & Chipper.

I checked with a bolt and it's definitely 1/4-28. Most of the spare jets I have are 95 so that size must have been used in many applications.

Kevin, if the smaller jets were abundant I'd feel free to experiment by drilling to various sizes. I also raced bikes (AMA District 37 desert racing) and have drilled jets successfully.

A conservative approach to jetting that doesn't require parts alteration would be to calculate the effective open area of various jet and rod combinations and mix and match to get the desired open area.

That's the approach I used when I converted the carburation on my '71 Dodge van (I bought it new and still have it) from 2-barrel to Thermoquad. The Thermoquad, like the Quadrajet, is really complicated but works great when properly matched to the engine. That van surprises people, especially when pulling my travel trailer up steep grades. The paint is peeling and rusty but without any emissions contols it makes really good power.

Kevin, if you find an extra 94 jet amonmg your spares I'd like to hear from you. Or I can make one using a 3/32" drill (0.0938").

To drill all you need is a small set of numbered drill bits.

Chev Nut,

I'm a street rod builder at heart and took on a restoration project in the 1970s because I had a '36 Chevy rat rod that was saying loudly "Please restore me". It seemed like a good motivator to learn some new skills and it turned out very well and my wife and I still really enjoy it.

But back to fine tuning carburetors for a moment. In my experience of optimizing carburetor jetting I have discovered that changes in metering dimensions of fractions of one thousandth of an inch can mean the difference between "close" and "just right". The gasoline molecule must be tiny. Obviously drills or even off the shelf reamers don't allow that kind of precision. What does allow it is making a collet to hold metering rods in a lathe, polishing them with crocus cloth, and continually measuring with a "tenth reading" micrometer until the desired dimension is reached.

If you have a number drill set and look at it you will see that some of the drills are within 0.001" of each other and many have much larger gaps.

In any event, I've had much better luck making precision holes with reamers than with drills, but for truly optimizing carburation the metering rod polishing method has worked best for me.

Some drag racers have told me they get the same result smearing polishing compound on pipe cleaners (remember smoking?) and sliding the pipe cleaners through the jet orifices?

Does any of this matter? It depends on how well a restorer wants his/her engine to run.

Some of the jets used in Rochestter carburetors had the opening slightly tapered on the top of the opening and Chevrolet claimed this was equal to going one step richer.
The jet is always a compromise as there are so many factors as to what is best. All the way from altitude to power delivery or economy.

Chev Nut,

There is a reference I've found very useful on this subject: Rochester Carburetors, Quadrajet, Two Barrels, Monojet, 4CG by Doug Roe & Bill Fisher.

Even the mysteries of the Quadrajet are explained and it is a very complicated piece with infinite variables that can be fine tuned. It's a great read.

Chipper,

I just checked a Quadrajet main jet and it is also 1/4-28 and similarly shaped to a W-1 jet. This may be as simple as canibalizing some junk but more modern Carter carbs.

Decades ago when I had a '64 VW Bug I discovered that round Mikuni (vs. hex) motorcycle main jets were identical to Solex main jets with much better price and availability.

Good info. Will try to remember that.

I have soldered jets shut and then drilled them to the desired size. I know that the lead would eventually erode (faster than brass) but probably not in my lifetime given the little these cars are driven.

I have also soldered jets closed and then re-drilled to the desired orifice size. Works great!