Weber carburetors

In all the years I’ve been turning wrenches on cars as both a professional and a hobbyist, I’ve had plenty of successes and more than a few botched repairs, probably about the same as anyone else with my experience. While I’m reluctant to refer to myself as a skilled technician, I will acknowledge that I enjoy being the student, and the desire to learn has only grown stronger as I’ve gotten older. Before diving into a project, I’ll research it as much as possible in order to approach the task in a more educated way.

This willingness to go ‘back to school’ as it were paid off when working on the Weber carburetors on my Alfa. In my first blog post on this topic back in early October of this year I said the Webers seemed “unnecessarily complex”. In fact, they really aren’t. The perceived complexity exists in the myriad combinations of different sized jets and deciding whether to replace them. Changing jet sizes up or down can be a trial-and-error process, involving test drives, spark plug examinations, and yet more jet swaps. In my case, there was no need to replace jets, as I was having no related driveability issues. That kind of experimentation becomes necessary when installing Webers on a modified engine, or on a vehicle not originally fitted with them. (One of the best online articles I found re: Weber side draft operation was on a Datsun 240Z website, as these side drafts are a very popular upgrade on that sports car.)

The sole reason for the ‘rebuild’ was to replace leaky seals and gaskets, after which I would need to reset the basic settings. I was using no fewer than four different Alfa Romeo / Weber service manuals, and while there were some slightly different approaches among the four, they all agreed on the basics. Those basics were: Remove the top, side, and bottom covers (note that the float is attached to the top cover). Do NOT remove the jets, throttle plates, or throttle shaft. Clean out the carburetors. Set the float height. Reassemble the carbs with new seals and gaskets. Reset the idle air/fuel mix, the carb synchronization, and the engine idle.

**Throttle plates (butterflies) and shaft were sprayed clean but left in place**

One service tip I learned is that good access to the carbs is available without unbolting them from the engine. The float, the internal mesh filter and all the jets are all accessible once the top cover is removed, held on with 5 screws plus the fuel line banjo bolts. On my car, one does not even need to touch the plenum cover. However, I wanted to get to the bottom cover, and also wanted to perform a thorough ‘off the car’ cleaning.

**All screws are slotted; this right-angle bit on a ratchet provided good leverage and avoided stripping screw heads**

The float setting is a bit fussy. There are actually two settings: the top and the bottom. The top cover gasket must be in place. The two setting measurements are 8mm and 14.5mm, and the books claim that a deviation over 0.5mm can be troublesome. I fashioned my own measuring tools, one using a hex key wrapped with masking tape, and the other made from a piece of rubber hose. Both floats were off enough that they needed both their top and bottom settings adjusted.

**Float is attached to top cover. Two homemade measuring tools are shown.**

The new rubber carb mounts arrived from Classic Alfa, and I was a little disappointed that the studs were not anchored in place. It’s a good thing I discovered this before putting it all together, and the problem was easily resolved with some threadlocker. While I can’t prove that my old rubber mounts were part of any problem, the rubber was clearly past its prime.

**Loose studs fixed with threadlocking liquid**

**Old rubber mounts are probably 20 years old. One cannot see this while mount is on car.**

It was news to me that there was a mesh screen filter inside each of the carbs. Thankfully, a replacement was part of the carb gasket kit. I went through about 5 cans of Gumout before I was reasonably satisfied with the cleanliness, and even then, there was some dirt on the outside (but none on the inside).

**Mesh screen filter at top. Old metal screen on L, new plastic one on R.**

The corner had been turned, and it was time for reassembly. First, the rubber mounts were bolted to the intake manifold (remember that half the bolts are hidden on the cylinder head side of the manifold). Adding yet another step to this litany of work, I decided at the last minute to replace the coolant, which was more than several years old. This meant hunting down some old-fashioned green antifreeze and distilled water (Walmart was down to its last 5 gallon bottles of distilled, but I only needed 2). The manifold was bolted back to the cylinder head, the coolant hoses were reattached (it’s a wet manifold), and the radiator was filled.

The cleaned carbs were bolted to the rubber mounts, and the choke and throttle linkages were reattached. I decided for now to leave the intake plenum off the car. The engine would run without it, and I didn’t want to do any more work than necessary in the very unlikely event that the carbs needed to be removed again.

**Cleaned and resealed carbs back home (for now)**

It was time to try to start the car.

Many minutes were spent pumping the throttle and cranking the engine in short bursts so as not to overheat the starter motor. The engine wasn’t even trying to start. After about five minutes of this, I knew that something else needed to change. Each two-barrel carb has two idle mixture screws, so there are four, one for each barrel (obviously). My repair books differ in describing the basic setting of these idle mixture screws. While all the service literature agrees that each screw should be turned down to ‘bottom’, then turned ‘out’ (counterclockwise), the exact number of recommended turns differs. I had settled on one full turn out, while some books recommended two, and others didn’t even give an exact number. When the car wouldn’t start, I turned each idle air mix screw one full turn further out. Climbing back into the car, the engine started on the next try.

Next, the books advised on how to continue with the basic settings, including carb synchronizing and idle setting. Before either of these can be set, however, the engine needs to reach full operating temperature. I watched both the temperature gauge and the coolant level, and both were fine. But as the engine warmed up, the idle kept increasing. Nothing I did at the carbs seemed to have any effect on my ability to lower the idle. After a few minutes, the car was ‘idling’ at 2500 rpm, and it should be between 800-1000 rpm. While I was happy that the car was running, and running well to my ears, something else was up with the idle control. It was time to shut it down and hit the books again.

It’s long been a tenet in the old car hobby that cars like to be driven; they don’t do well when they sit; and as long as you’re on top of maintenance, there’s no reason not to expect some reliability from an older car.

I’ve been lucky with the Alfa Romeo (although frankly, luck has little to do with it): having purchased the car in March of 2013 with 54,000 miles on the odometer, by July of 2019, the car had just shy of sixty-six thousand on the clock. In a little over six years, I managed to drive a 1967 Italian car almost 12,000 miles, which neatly works out to 2,000 miles a year. That changed, though, when the brakes seized, resulting in a complete teardown and rebuild of the braking system that took a year to complete. The car had not sat silent under my ownership for that long before, and when I did restart it, it ran poorly. Suspecting the fuel had gone sour, I drained the tank, added fresh premium, and swapped out the plugs. Success! Except … now I had a fuel leak under one of the carbs.

So, much of September was spent reading up on Weber carburetors. At first blush, they seem unnecessarily complex. Add to that complexity the words of the late Pat Braden, as he wrote in The Alfa Romeo Owner’s Bible, a copy of which I own (and I’m paraphrasing here): “if your car is running fine, don’t touch the carbs. Everyone wants to fiddle with the carbs. If it’s running ok, leave the carbs alone”.

Well, Pat, the car does ‘run’ fine, but liquid fuel dripping onto my starter motor does not get me very excited, at least not in a good way. I did some more research, including reading some very helpful posts on the Alfa BB (Bulletin Board), and concluded that the gaskets and seals were probably old, and the float height should be measured and adjusted, but other than that, I am going to leave the carbs alone!

**From bottom: plenum; carbs; intake manifold**

I’ve never removed the Webers from my car before. Removal didn’t look complicated, but would certainly prove to be time-consuming. First, the upper plenum is removed (two hose clamps, one bolt, and two nuts, all easily accessible). Next, the lower plenum comes off (ten nuts and washers, four of which are totally blind). Now one has access to the carbs themselves (eight nuts, four of them blind). Once the banjo bolts for the fuel connections are undone, the carbs can be removed from the car. The carburetors bolt to 4 rubberized mounts, each of which has 4 studs. To remove the mounts, one first must remove the intake manifold, as 8 of the nuts for the mounts are only accessed if the intake manifold is unbolted from the cylinder head (7 nuts, some of them only reachable with an open-ended wrench, which only allows 1/6 of a turn at a time).

**With carbs gone, rubber mounts are obvious. Note starter motor.**

**Intake manifold half off cylinder head**

Back to these rubberized carb mounts: it was eye-opening to learn from the Alfa BB that side draft carbs, hanging off the right side of the cylinder head, are prone to enough vibration to cause fuel delivery issues. To combat that, Alfa employed a bracket extending upward from the right motor mount to the lower plenum, and mounted the carbs on rubber mounts which absorb vibration. But a number of the Alfa owners on the BB stated that these mounts should be considered service items: eventually, the rubber hardens and develops hairline cracks which allow air to enter the intake stream, throwing off the fuel-air mixture.

**Manifold on bench, giving better view of carb mounts**

Former owner Pete must have replaced them at one point, because I had an old set among the spares he had given me. Checking the website of my favorite (really only) parts supplier Classic Alfa, I saw that they had a carburetor gasket kit for $35, and new carb mounts for $25 each. That all seemed reasonable enough, and as usual, my order arrived from the UK 48 hours after I placed it. I was ready to get to work.

**Removal of top cover exposes all jets**

**Service books clearly state to leave throttle plates and shaft in place unless obviously defective**

It was important for me to stay focused on the goal: I wanted to clean out the carburetors, inspect them for any obvious faults, then reassemble them using all new gaskets. Perhaps it’s from a lifetime of dealing with old cars, but I do have the habit of over-repairing my vehicles. The issue with Webers is that other than setting the idle mixture, idle speed, and float height, any other adjustments involve a lengthy trial-and-error game of swapping jets. One more time: aside from the fuel leak, the car ran fine. I selected the rear-most carb (the leaker) and removed all the covers.

**Screens under choke control were black**

Having several service manuals with exploded diagrams at my side, things didn’t look too bad. There was clearly some dirt built up, but no obvious faults or defects as far as I could see. Numerous cans of Gumout were emptied to clean things up, and I’ve been pleased with the progress. The float needs to be carefully measured and adjusted, and once that’s done, reassembly will commence, which is where I will pick up next time.

richardscarblog

All automotive, all the time

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