Weber carburettor air corrector jet

[Vanden Perre]

Hello, does someone have something interesting to tell about the following question?

I have a standard 105 h.p twin cam and checked this afternoon the jets of the webers. They are as per workshop manual. This is: Main jet: 115, Pump jet: 40, Idling jet: 45F9, emulsioning tube: F11 except for the air corrector jet which is 140 instead of 200 as per the manual.
Does it make sense? Could it be a problem? Do I better replace the 140 by 200?
Thanks.

Olivier.

[65 Lotus]

I've just spent the past several weeks getting the air corrector jets dialed in for my Elan.

After the engine is warmed up, get on a stretch of road where you can hold the speed steady at 100 kph. Does the engine bump or stumble, even a little bit? If so, your air corrector is too small (provided everything else is operating properly of course, ignition, etc.).

As you get closer to the optimum size, the "bump" will become less frequent and much more subtle; you must pay close attention to notice it. Also as you get closer to optimum, the engine must be fully warmed up, at least a solid hour of operation.

I started with 115 main jets and the stock 200 air correctors and got a lot of high-speed, rich stumbling. The first drill-out to 210 made a huge difference, but it did not solve the issue. I continued to drill the air correctors progressively all the way to 250, but it was still running a hair rich even with this very large size. I then decided to decrease the size of the main jet to 110 and started with 135 air correctors. With the 110 jets and the air correctors drilled out to 145, the car now runs superb and is a joy to drive.

Of course I don't know your particular engine, but mine would be slobbery rich with a 115 main jet and 140 air correctors at sustained open throttle conditions. Checking operation at the above condition will tell the tale. Did you check the actual diameter of the jets to make sure they haven't been drilled out?

Thanks to Keith Franck (again) for helping me figure this one out.

[denicholls2]

The exact same symptoms have been attributed to "lean stumble" and I'm struggling a bit with the suggestion offered, though it seems to have worked for you.

Here's my understanding:

In the region in question, the engine is typically runnning at the top of the service range of the idle jet, not the main jet. The idle jet does not have sufficient throughput to feed the engine in this region, and the lean condition that results causes stumbling. The solution is either an air corrector to enrich the mixture in this region, or a larger idle jet.

Once above the transition point and fully on the mains, the solution proposed by 65 Lotus would be correct. But unless I misunderstand the Twink's fuel demands, you aren't yet on mains steady-state at 100kph.

In my case, the fact that it was a lean condition causing the stumble was readily diagnosed by either goosing or gently increasing throttle, in which case either the accelerator pump or the actual transition to mains (well above 100kph) provided the added fuel required and caused the engine to smooth out.

Can someone in the know correct me if I'm wrong here?

To Olivier's question, it appears based on my understanding that your carb has been tweaked for exactly the condition I mention above. The smaller air corrector enriches the mixture slightly. If I've got it right, the jets pass fuel and enrich as they increase in number and the air correctors pass air and lean as they increase in number. If I'm correct, replacing your air correctors with the standard 200 values will introduce lean stumble, so unless your engine is unhappily rich at highway speeds, leave it be.

[Vanden Perre]

Hello,
Thanks to both of you. You mention lean and rich mixture. How can I say it is too rich or too lean? What to check?
Thanks.
Olivier

[RotoFlexible]

Hello,
Thanks to both of you. You mention lean and rich mixture. How can I say it is too rich or too lean? What to check?
Thanks.
Olivier

How is your fuel consumption? You can also check the spark plugs to see if they are dark or sooty.

Another approach is to return the air correctors to the original specification. The symptom that prompted the previous owner to change them may reappear.

[65 Lotus]

I diagnosed my stumble/bumping problem initially by changing the standard 115 main jets to 120's (which I happened to have). I went with the assumption it was in a lean state, as that seemed to be the case most of the time from what I had read about other folks problems. When I did this this though, I was surprised to find the problem got significantly worse, so ok I was too rich, not too lean. Hmmm.

Rather than put in 110 jets (which would require money), I decided to drill out the air correctors as it was easy to do on the lathe and I had some spares I purchased off ebay. I didn't know at the time how much drilling would be required to make what kind of change, so I just slowly increased the size and noted the effect of each iteration.

As I mentioned, the first drill out from 200 to 210 made the problem substantially better, so I knew I was on the right track. When I ended up at 250 though (the largest air corrector Weber offers) and the bump was still there occasionally, I knew something was fundamentally wrong so I spent the money and went with the 110 main jets. I selected the 135 air correctors as an educated guess based on Keith Franck's correspondence.

When I installed the 110/135 setup, I still had a slight bump, but it generally ran fine. Just to make sure, I installed the original 200 air correctors to see the effect. It was instantly obvious I was lean, verified using the slight increase in throttle test noted by denicholls2 above, so I knew somewhere between the 135 and 200 air corrector size was where I needed to be. I'm now at 145 on the air correctors and have not yet detected any bumping, but I remain vigilant with drills in hand just waiting for it to happen!

BTW, this process was over a month and a half of working with it, not just an afternoon's adventure. When you get really close to the right mixture, you have to be carefully attuned to the sensations with a fully warmed-up car. You need to be in the zen state with still air on smooth roads and no one in front of you to detect the very slightest abberation.

While the idle jets are certainly contributing to the mixture at 100 kph in 4th gear, the mains are most definitely solidly online doing their thing with my engine. The identical problem condition was noted at higher speeds as well, but I settled on 100 kph as the condition felt the same both there and at higher speeds, and I didn't risk a citation running around the back roads of Ohio any faster than necessary. The condition was just barely evident at 90 kph (56 mph), but was most definitely there at 100 kph (62 mph) and above, so I knew I was on the mains.

Actually, I don't think I have the idle jets quite right yet which may influence where I finally end up with my main jet/air corrector combination, but for now it runs pretty darn good and I'm satisfied with it for this season. Going to do it again next year starting with new throttle plates as the ones I have in the carbs now are poorly matched and don't allow for a very good idle. It's been a fun, satisfying process though, as I do enjoy tinkering with the machinery as much as driving.