The Supercar Registry - Before you buy your next carb, READ THIS!

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Before you buy your next carb, READ THIS!

Hi all;

I'm posting this as somewhat of a "public service announcement" in an attempt to help people learn what to look for before plunking down their hard-earned $$$ on their next carburetor purchase. I know many of you will already know most of this, but I'm sure someone will pick up something they didn't know from this.

Right now, I'm only going to cover some of the more important basics on Holley and Rochester Quadrajet carburetors.

Let's start with some common Holley carburetor issues.

One of the biggies is making sure the carb is equipped with all the correct, sometimes proprietary parts for that particular application. Items such as metering blocks, float bowls, and base plates are often missing or replaced with incorrect components. Even seemingly simple things like choke parts can be VERY difficult to replace on certain carbs.

Let's start with baseplates since these are often missing, damaged, or even incorrect on many of the swapmeet/online specials.

Most of the mid-`60's to 1970 GM Holley 780 carbs used similar baseplate assemblies. (These 780 baseplate assemblies are quickly identified by the "172" stampings on the throttle blades. Some may occasionally have "173" stamped on the rears. The throttle blade diamater is 1 11/16")

However, there are some subtle differences you should be aware of.

The 1966 780 carbs have one notable difference; the accellerator pump arm uses an integral stud that slides into a machined boss on the baseplate instead of the later style threaded stud and clip arrangement.

http://pic15.picturetrail.com/VOL617.../125298546.jpg

1966 780 throttle shafts also differed from 1967-1969 units in appearance. In the picture below, the 1966 style is on the top, and the 1967-1969 style is on the bottom.

http://pic15.picturetrail.com/VOL617.../125298543.jpg

The 1967-1969 4053 carbs and 1969 4346 carbs use the same complete base plate assembly. The 1967 3910 carbs use an assembly that's almost identical, the exception being an additional "kicker" flange on the front of the primary throttle shaft that's not present on the 4053/4346 carbs. Other than that, the 1966-1969 baseplates are functionally interchangeable.

While I'm on the topic of baseplates, I'll show you how to identify an "original" baseplate from a later "service" baseplate.

Starting some time in the early 1970's, Holley re-designed the baseplate castings adding more material on the left (drivers) side rear and right (passenger) side front mounting boss areas. I can only speculate, but I'm assuming this was done in an attempt to help prevent the corners being broken off so easily by ham-fisted mechanics.

Below is a comparison of the drivers side rear corners; the top casting is the later "service" unit, the lower an original:

http://pic15.picturetrail.com/VOL617.../125298530.jpg

Below is the same comparison, but of the right front; again, the "service" is on the top, the original on the bottom:

http://pic15.picturetrail.com/VOL617.../125298541.jpg

Next on the list is metering blocks.

The metering blocks are identified by a code stamped into the top corners. Often a specific metering block was used on a variety of different carburetors, such as the 4519 block which was used on the rear of ~90% of all GM 780 carbs from 1966-1970. The 5583 blocks were used on the primary side of both the 4053 and 4346 carbs.

If you're shopping for a carb and aren't sure of the correct metering block #'s for your carb, don't take chances. Some of these metering blocks are expensive and their absence can quickly turn a "good deal" into a "not-so-good deal."

If you're not sure which ones should be on your carb, ask me and I'll give you the #"s.

Something else to be aware of are the stampings themselves. It's not uncommon to find re-stamped metering blocks. However, IMHO this isn't necessarily a bad thing all the time. Please keep in mind that all those stampings indicate are how that particular metering block is calibrated in regards to idle feeds, emulsion holes, power valve channel restrictions, etc.

There are rare instances when a correct metering block simply can not be located. In these instances, it may be necessary to take a "donor" metering block and re-calibrate it to the correct specs for your carburetor. An example is provided below.

In this picture, the bottom metering block is an OEM unit, and the top one is a re-stamped component. These both came in on a pair of carbs I was working on for a customer. You can see where the numbers have been ground off on the top block so it could be re-stamped.

http://pic15.picturetrail.com/VOL617.../125298200.jpg

I was pleasantly suprised to find that whoever re-stamped this particular block at least took the time to make sure it was calibrated 100% correctly. It was cleaned and put into service and performed flawlessly.

Just something I feel I should mention regarding this subject;

I personally don't have an "issue" with re-stamped metering blocks as long as they are done correctly and there is simply no other alternative than to use/make one. HOWEVER!!!...I will NEVER, EVER do so without FIRST either informing the customer of the situation, or asking their permission to use one.

One last thing to mention regarding metering blocks; sometimes you may find "double-stamped" metering blocks--i.e. they have both a calibration code on them AND a carburetor list number. Using a 4519 again as an example, you may find one that's stamped 4519 and 4053. This simply means that this particular block came off of a later 4053 "service" carburetor, but it is identical in function to it's predecessors.

I've saved the worst for last in regards to Holleys. The dreaded "squirter holes." https://www.yenko.net/ubbthreads/imag...emlins/mad.gif

Some "genius" back in the day thought it'd be a nifty trick to be able to take a vacuum secondary carb and convert it to mechanical secondaries. We've probably all seen the old "screw in the secondary linkage" trick at one time or another. Looked good on paper, but didn't work so hot in real life. The problem was, there was only one small accelerator pump to try to cover the "hole" of all 4 barrels suddenly going WFO. Bog city!

So...what's the solution? "Why don't we make a squirter that will cover all 4 barrels?!"

Again, looks good on paper...but...

In order to install these "long" squirters, you had to either cut slots in the back of the choke horn, remove the choke horn completely, or (and most often) you could simply drill 2 small holes in the back of the choke tower for the rear squirter extensions to pass through.

UGH!

See below:

http://pic15.picturetrail.com/VOL617.../125298199.jpg

"So what's the big deal? It's just 2 holes, right?"

Wrong. Those 2 big holes will likely play hell on you ever getting your choke to function correctly when the engine's cold. Plus, simply put, they don't belong there!

Here's the problem: those holes are a PITA to fix! You can't use putty or epoxy because it may not hold (and you don't want two blobs of some foreign material being ingested by your engine should they fall out some day...do you?), plus it won't take color when the castings are re-colored. The only right way to fix them is to weld them shut, and for those who don't know, zinc is a PAIN to weld.

So, in a nutshell...should you find a carb with these holes drilled in it, rest assured they can be fixed, but take into accound an additional $75.00-$100.00 to do so.

Next on the list is the venerable Quadrajet.

QJets don't usually suffer the same indignities as Holleys do, but there are a few key things to look for.

In no particular order...

Over-tightened castings: Look at the front mounting bosses on the carb--are they deeply gouged and scarred? If so, someone has over-tightened the carb at some point, and you can just about bet that the carb is warped.

http://pic15.picturetrail.com/VOL617.../125298539.jpg

Sometimes they can still be used if they're only slightly warped, but if they're too badly warped, they'll never seal up and aren't safe to use.

Here's another way to spot a warped carb; Look at it dead-on from the front and see if you notice a "crown" in the middle of the casting between the main body and top casting.

http://pic15.picturetrail.com/VOL617.../125298540.jpg

This one looks good.

Last thing regarding QJets for now. One common malady is leaking well plugs. Symptoms of this are difficulty starting the car after it's sat for any length of time, or emitting a cloud of black smoke on warm re-starts after the car has sat for a little while. This is caused by gas weeping past these plugs and draining into the intake manifold. Left long enough, the float bowl can drain completely, hence the hard start after sitting for any length of time.

Two of the well plugs can be seen in the recessed cavity above the throttle blades in the picture below:

http://pic15.picturetrail.com/VOL617.../125298198.jpg

(There are a few more well plugs hidden underneath the baseplate that can't be seen.)

Most any rebuilder will cover these well plugs with heavy duty epoxy during a rebuild, which will eliminate the issues mentioned above.

OK, that's it for now. I have more to share at a later date, but I'm tired of typing for now.

I hope this helps someone out somehow. Just trying to give a little something back since y'all have been so good to me. https://www.yenko.net/ubbthreads/imag...lins/beers.gif

If anyone has any questions, please don't hesitate to ask.

Eric