Jump to content


A word about progression holes on a 151 Weber


Recommended Posts

Well today in order to get my carbs even better (this is where the issues come in as they were running fine, and I cannot seem to leave well enough alone).

I opened up the screw covering the progression holes. Now all manner of weber tuning manuals state that you need to have the butteflies covering the first progression hole. And when you first tip into the throttle you start to uncover it. Well I noticed that mine were not this way. Instead they were almost a full turn more closed that than. Meaning I could move the butterflies a few mm before I would even start to uncover the first holes. Well I figured this had to be wrong (even though I had a reliable running, idling, and starting car) So I loosened up the throttle levers, and then made my fine adjustments with a magnifying glass so that the slightest touch of the throttle would start to uncover the first hole! I was very proud of my overkill on how perfect I got it. I hooked up the throttle levers and started her up expecting a rock solid smooth idle.

What happened?

Well She started up quickly and then revved to 2300 rpm and stayed there. I then proceeded to muck with my air/fuel mixture screws that were previously perfect to try to drop the idle. Well I got her around 1500 rpm but with an afr of around 19:1!!!! barely struggling to go. So I turned the throttle plate screws back to where they were and she is idling smoothly again and I got the afr back to 12.5:1.

Just goes to show you, that general settings are not the same for all cars or even all model weber carbs. Maybe this is a progression hole thing with the 151's. Who knows, but I certainly cannot set mine as factory recommended or else I rev to the moon.

Very interesting things these webers!

Link to comment
Share on other sites

yeah, those two are attached at the hip. I think the best practice method of setting the throttle plates is just to ensure you do not uncover a progression hole too soon. Mine are not uncovered at all, and in fact the throttle plates have to move bit before I uncover them at all.

Now one could keep changing jets out I suppose, but I am not sure you could ever get there. I am only 1.5 turns out from closed on my mixture screws. I cannot go too much more closed.

Again, I think the rule of thumb is to not uncover the first progression hole at idle, If you have to close it more than that, no harm no foul.

by the way, this new L28 I am running has absolutely NO flat spot anywhere. I think that is mostly due to my 30mm chokes than anything else.

I wonder if going to 32mm chokes would allow me to open up the plates a bit since the velocity of air coming in at 1000 rpm would be slower.

I just have to buy some 32mm chokes and try.


Link to comment
Share on other sites

I found that you can also "cheat" by playing with base timing a little, to get that idle where you want it. A couple degrees back and the idle drops quite a bit. I also found that increasing the idle jet size, and then turning down the idle mixture screws to compensate, helped my transition, response, and torque. Effectively increases the amount of fuel passing through the progression holes. mainly because my idles were too small to begin with. I run 32mm in my 151's.

Edited by cygnusx1
Link to comment
Share on other sites

I soon hope to plot a/f vs throttle plate turns vs enrichment screw turns. I just made a "sniffer pipe" out of copper tubing for my LM1. I'll also follow your lead at steady cruise A/F at 2500rpm on a flat section of road.

I also have 1.75mm needle valves, 55F9, 60F9, and a fuel pressure regulator jets coming from pierce manifolds to plot.

Just need a good day of testing.

Link to comment
Share on other sites

Some useful info (source:http://www.poustusa.com/weber_carb_pages/weber_carb_page.htm):

Idle Mixture and Fuel Mixture:

There are alot of theories about setting idle mixture adjustment on a Weber carb to determine Rich or Lean jetting....A Weber will run correctly with the mixture screws from 1/2 turn to 3 turns out...The late style DCOE carbs with the extra Air Bleed Screws on the top take 2.5 to 3 turns to operate correctly...So you cannot say that because it is 1/2 turn out that you need smaller jets....You need to find what is right for that particular engine...An engine with a strong vacuum signal will draw more fuel in with less turns of the screw than an engine with a weak vacuum signal will...This is the kind of thing that throws the "certain number of turns" theory out the window..

The Mixture screw and idle circuit is CRITICAL to the overall drive ability of the car. It does not just control idle but the entire low speed running and part throttle transition. The mixture screw lets in an ALREADY MIXED volume of Fuel and Air to the engine. This is not an Air Screw. The more you open it the more mixed fuel and air enters the engine. Clockwise is Leaner and Counter-Clockwise is Richer.

The mixture screw is very easy to set whether it be a downdraft, side draft or multiple side drafts. Start with the screw or screws out 1.5 turns....Start the car and let it warm up. Set the Idle SPEED to approx 900-1000 RPM.. Make sure multiple carbs are synchronized...Turn the mixture screws in until the idle starts to stumble and get rough...On a side draft with 2 screws do them each a little at a time....Then back them out until the best idle quality is achieved. This is a very simple operation...The car should idle well and small adjustments leaner (Turning them in) should make the idle drop off. Opening the screws more should make it a bit richer but it should still idle. Assuming you have no vacuum leaks this is a very simple process. If you cannot get a good adjustment on these screws and you have to open the idle speed screws quite a bit to get the engine to idle there is a good chance you have a Vacuum Leak and you need to fix it.

DCOE Series

On a DCOE or multiple DCOE's you should be between 3/4 turn and 1.5 turns out for all older model DCOE's (DCOE 2, 9, 18, etc.) and 2 1/4 to 3 turns out for late style DCOES (151 and 152 with air bleed screws under the white caps.)

Idle Jets and tuning the idle circuit:

Very Simply.. The bigger the number the richer the jet. A 50 Idle jet is a .5mm fuel hole. On the DCOE series carbs they have 2 numbers on them like 50F8. The 50 refers to the .5mm fuel hole and the F8 Refers to the Air Bleed hole in the side.

In a DCOE you are actually tuning the Air and Fuel for the idle circuit with the idle jets. This is a little more tricky but not too bad...Basically you can tune just about any car in the world with an F8 or an F9 idle jet. The F8 is Leaner (Has a larger Air hole) than the F9...Soooooo. You can have a 50F8 and a 50F9 and both have the same amount of fuel but the 50F9 has a richer MIXTURE (Less Air to the same volume of fuel.) Generally speaking you should start with an F8 and play with the fuel size until you get close then experiment with an F9 to see if that works any better...

Choke Tubes and the Main Circuit:

Okay...So now your car should idle correctly, the carbs are synched, the timing is set, everything is good....Time to get the main circuit right. Drive the car. It should come off of idle good and transition to the main circuit smoothly...If it falls on its face and will not take any throttle or runs better if you back off the then you could have a few different problems depending the type of carb. Before you blame the carb make sure you have the Fuel delivery right and the Spark Timing. If you do not have enough advance you will have this problem and it will have nothing to do with the carb tuning.


Here we go....This is the real tricky part. I feel that Sidedraft DCOE carbs are easy to tune but you have to have a real feel for Rich and Lean and understand how the carb works. To get a good understanding of this I cannot recommend too highly to get the Weber Factory Tuning manual and really understand what each component does. That said this is the real Cliff Notes version of tuning the power circuit of these carbs.

Choke Tube or Main Venturi size is the basis for everything in tuning DCOE carbs. If you get the Venturis wrong you will never get it running right. Too big and you will always have a flat spot that you cannot tune...Too small and it will always run rich and not make any power. If you have poor throttle response at low RPM

This is a guideline only to get you started or to make sure you are not totally out of range making it very difficult to tune. There are many of you running cars successfully with choke sizes outside this range. The difficulty in tuning these types of cars is that there are many combinations that work well depending on the engine and the state of tune. That is why everything is tunable. You can tailor the carbs to suit your needs, driving style, engine, location, weather and altitude conditions, etc.....

Here is a very basic chart of what venturi sizes you need to popular vehicles and engine sizes for STREET USE...All out race engines are a totally different story. Use the engine size and HP rating to estimate what you need for your car or engine if it is not listed.

This is a guideline only to get you started or to make sure you are not totally out of range making it very difficult to tune. There are many of you running cars successfully with choke sizes outside this range so don't e-mail me about it. The difficulty in tuning these types of cars is that there are many combinations that work well depending on the engine and the state of tune. That is why everything is tunable. You can tailor the carbs to suit your needs, driving style, engine, location, weather and altitude conditions, etc.....

Engine / Car Approximate HP Carb or Carbs Starting Choke Tube Size

Datsun Z 2.8 - 3.0 250-280 Triple 45 DCOE 36mm

Datsun Z 2.4 - 2.8 200-250 Triple 40 DCOE 34mm

Do not try to choke down a DCOE carb that is too large to try and cure a problem...you will just create more problems....A 45 DCOE should NEVER need less than a 34mm choke to run properly. If it does then the carbs are too large or you have another problem...A 40 DCOE can use as small as a 28mm choke tube but chances are the performance will not be good with anything smaller than a 30mm choke....I have found that a correctly sized 40 DCOE application should always stat with a 30mm choke and do up to a 34mm max....Bigger or smaller than that and you probably have either the wrong size carbs or another problem that is causing you to tune outside of this range...This same info applies to IDF carbs as well...

Link to comment
Share on other sites

What an amazingly good post Blue.. but reading this makes me want to fiddle that much more! :) DANG IT.

I have ordered 32mm choke tubes for my 151's. I just need to find out how to install them. I would love to install them without having to remove the carbs, but not sure if that is impossible.

I have searched for a how to video, but have found nothing. Guess I can just pull one off and experiment! I have never had a flat spot with my 30mm chokes, but I really do think it would make more mid range and top end with 32mm. I think I have an engine that is healthy enough and pulls enough vacuum at low rpm that it can take a bigger choke without losing too much throttle response.

Link to comment
Share on other sites

I believe that the installation of the chokes changed from the older -18's to the newer -151's. I have both and they are definitely different.

On the -18's Dave is absolutely correct. You just remove the air horns and the aux venturis and the chokes slip right out. Very easy to do on the car.

The 151's however are different as there are screws which hold these in place. The aux venturis use a screw in the bottom of the carb (with a lock nut attached) and this needs to be removed in order to remove the venturi. The chokes themselves are held in place with a screw (and lock nut) on top of the carb. The screw actually looks like a needle jet adjustment and I believe that this is new to the -151's. The weber docs I have read suggest that these are set at the factory and not made to be adjusted by the user but I have also read of people using these to get better idle balance between carbs as well. You will need to remove this before the choke can be removed. I just completed this on my -151's so I know it to be correct. I did this on the bench but believe it is possible to reach both of the screws while the carbs are on the car. Just not as easy as the -18's.

Hope that helps.


Link to comment
Share on other sites

I removed the aux venturi just now trying to play with it after a rejet (yes another one). More on that later.

The Aux Venturi are held in place by the set screw that is about 45 degrees from horizontal toward the bottom of the carb. It was a snap to slide out by hand. I think the screw holding in the chokes are in place are just beside the pump jet cover screw.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...

Important Information

By using this site, you agree to our Privacy Policy and Guidelines. We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.