Drilling A Progression Hole

[Roady]

Hi everyone,

 

I have had triple webers set up on the car for a while now, very happy with them but now looking to resolve a long awaited issue - the progression circuit!!!

 

These are 45mm DCOE (152). From what I gather the 152g came out after these with an extra 1-2 holes drilled into them.

 

Basically the issue is as per the picture - the throttle plate should sit directly under the first progression hole. This way as you just crack the throttle from idle (driving off at a set of lights) you won't enter a lean spot until you reach the first hole which then allows more fuel in (the transition between idle circuit and main circuit). If I adjust my idle screw to position the throttle plate under that first hole the idle is approximately 3200rpm.

 

After some fiddling around I got relief from changing emulsion tubes to an F7 which provides a "richer" bottom end throttle response. This combined with a very rich idle Jet (65F9) has relieved the issue some what and I have been happy with it up until this point. The downside with this set up is that fuel mileage is rubbish as during normal driving you are generally sitting around the progression holes when cruising.

 

The time is now drawing near to put together the new engine in which case it will give me time to address this issue with the carbies off the car. My question to you all is - has anyone here done it (as I have seen some alpha posts about it) and how were you able to accurately measure the spot to drill the hole..??

 

Cheers

Edited November 3, 2016 by Roady

[Cozza]

Yeah I have read different info on the net about drilling new holes but I don't know how you can be perfectly accurate.

The progression holes are one of the only things you can't adjust or change on a Weber.

Have you tried adjusting your accelerator pump length or spring. I figure that would provide some change.

Good luck and keep us posted with how you go.

 

Mick

[Jason89]

I plan to do this with my 40mm webers, start off with a .8mm hole and go bigger if you need. Interested to hear how it works out.

[dat2kman]

Unless you know EXACTLY where to start, and what angle, you can kss the 152's goodbye

These Spanish made ones, then changed to the 152G, due to the "feedback" they got

Experienced tuner shops will tell you to get the better ones!

Jason89, your 40's, are they the 152's? If nt, they should have progression holes in right place, standard!

[Agno]

I was thinking about this the other day actually, good timing!

 

If I was doing this myself I would be drawing a plug in CAD and then getting a few 3D printed to use as guides. I would then mount the carb in a drill press and measure about 500 times to make sure everything comes out straight.

[Cozza]

Jason89, your 40's, are they the 152's? If nt, they should have progression holes in right place, standard!

Jason what do you mean by progression holes in the right place, standard?

I have at least 3 different tipo 40mm webers and the progression holes are all in different locations, some have 2 holes others have 3 holes, all are different diameters.

I have seen images on the net of wenets with 5 progression holes.

I assume the differences were for the original setup of carbs to suit particular engines from factory but not sure.

[Roady]

Cozza- haven't tried length of accelerator pump, but changed pump sizes and spring to see if it made a difference.... minimal as it's such a small thing.

 

As I said best thing I did was change to the F7 emulsion tube and richen the idle Jet right up, so for those who don't want this hassle you will get satisfactory results this way.

 

As this stage actually planning on using the picture above and marking things to scale, using pins pushed through something (plastic) into the current holes to align it all and ensure the hole is drilled in the same spot every time .... maybe

[Roady]

Latest brain storming

[CBR Jeff]

Roady better you than me.

We did something like this with a set of twins on a Datsun 1600 rally car way back when everyone was running them (1600). We picked a set up that might have been for an Alfa (long time ago) and they didn't work properly and had a bad flat spot at low revs. So eventually we carried out this modification and It sort of worked. The flat spot was gone but it was impossible to get to idle below 1500 rpm and it just drank fuel at idle. It would not run properly at low revs and on transport stages it would fowl the plugs if you drove along in 5th at about 2500 rpm.

You need to be very precise and you need to make two identical but opposite jigs. As the progressions holes are on opposite or mirror image on the left to the right side of the carburettor. Where you are talking about drilling your new hole is very close to the butterfly closed position and if you are not very carful you might end up with fuel flowing at idle (very undesirable). Start off small and work up. Don't start with a 1mm hole as once it is drilled you cannot go back (not easily anyways).

The holes in the picture make me think these carburettors were for a specific car? are they all the same? From memory there was a smaller hole in the 152 where you are thinking about drilling your new hole and hole number 3 in your picture is the extra hole that was in the 152g? not 100% on that. I do know from experience that it important to have a matched set of two or thee of these..

Jeff

[Roady]

Hi Jeff,

 

Thanks for the tips. I actually think the hole needs to be 1.2mm which was why I was starting with 1.0mm, but I will heed your advice on this one - maybe start with 0.8mm and go from there.

 

I agree it should on top of the throttle plate - the key will be to make sure that it's not forward if the of the throttle plate else it will leak fuel past the plate at idle as you say, then the only way to fix that is to drill a hole in the bottom section of the throttle plate (problem after problem). The plate itself is about 1.2mm wide. I have included a diagram (courtesy of Weber) outlining where the throttle plate is meant to sit relative to the progression holes.

 

As far as I know they are 152, but open to any other suggestions.

[CBR Jeff]

Good luck. I'll be interested to here how it goes.

Jeff

Edited November 4, 2016 by CBR Jeff

[Jason89]

I just had a play around with the carbs to see if i could get it a bit better. Mine are DCOE 18, 40mm with 3 progression holes. Roady from what i understand you belive your lean spot is before the first port is uncovered yeah?

To start with i made a throttle stop to limit the the throttle opening, hooked up the wideband AFR meter, then drove the car and adjusted the throttle stop to the leanest AFR which is where it missfires and spits back. Once i knew the throttle angle of the lean spot i could look down the progression ports and see what had to happen. I ended up drilling one of my existing ports out from 1mm to 1.2mm which brought the afr from 16s back to 15:1, Its still to lean but it drives alot better. Before i drill anything else im going to go up a idle jet size. 

Just make sure you measure the throttle angle to know where your problem is before you drill anything. The throttle stop worked well for me.   

[Roady]

Hey Jason89,

 

Thanks for the tip. When I have set the idle, which at present is about 1100rpm if I open the Progression cover the throttle plate is not even visible. If I sit it under the first Progression Hole as Weber suggest it give me an idle of approx. 3200 (obviously by this stage the main circuit is becoming involved).

 

During "cruising" with an o2 sensor the real issue is around 2200-2500 where it leans out (again prior to the first Progression Hole even being exposed). I rectified it a little during acceleration with the emulsion tube swap and richer idle jet however it's still an issue with hwy driving.

 

The first hole is already at 1.2mm and I'm not sure I want to widen it further. Could you explain the throttle stop a little more?

[Jason89]

Roady, with that progression hole layout you can see why it would be idling at 3200 rpm with it adjusted so its visible, the throttle would be open quite a bit but i still dont think it would be drawing from the main circuit. You want to confirm your lean spot is between idle and the first progression port, not the last progression port and the main circuit. The throttle stop i made was just a bit of 15x3mm flat bolted onto one of the rocker cover bolts, i bent it to limit the movement of the linkages allowing maximum throttle to be right on the problem area. In your case adjust/ bend it so max throttle is just before the first progression hole is uncovered. Drive it, ( if there is enough throttle) if its lean when the your pushing against the stop then you know you need another hole. But looking back at your original post i think your going to be pretty close. 

[Roady]

Edited and re-wrote post in a bit more detail

Edited July 8, 2019 by Roady

[Roady]

Edited and re-wrote post in a bit more detail

Edited July 8, 2019 by Roady

[Roady]

UPDATE:

 

Right so I finally got around to doing this.  At the same time did a full service, gasket change, checked everything over.

 

Result:

Originally I had a 65F9 idle jet in the car.  Since drilling a 0.8mm hole I’ve managed to reduce the size down to a 60F8.  This is about 3 sizes smaller, which on the surface may not seem worth it, but I can assure you that it was.  The reason my idle jet is still reasonably rich is because of how I have the main circuit set up.  Ive tried going leaner to 55F8 and it opens a hole if you “aggressively” apply power.  To some degree this can be filled with a change in accelerator pump jet, but in all my experimenting (including going to quite large accelerator pump jets) and I’ve actually swapped the accelerator pump spring over to a medium set up, I’ve found this set up works well for my driving.

 

The main idea behind this project was to make it more drivable when cruising and not have the throttle plate sitting before a progression hole and making it lean, causing the engine to stumble but be ok with the throttle plate either further open or closed.  This has cured the problem completely and I would do it again!!!

 

Idle is still normal and can be adjusted as low as about 700rpm but it’s more comfortable (with the 72 degree cam) at around 1000rpm.

 

THE PROCEDURE:

To do this, I used a piece of laminated plastic, to stick pinholes through into the current progression holes.  Then pulled this piece of plastic out and laid it onto some 1mm x 1mm graph paper.  This took me 4 attempts at marking it correctly on the graph paper before I was happy. I then glued the graph paper to a thin piece of metal and used a dremel tool with drill press to accurately drill these holes.

 

Once the 4th (new hole) was drilled in the metal, I installed the tiny piece of metal into the progression hole case, used needles through the old holes to line everything up then used a pin punch to mark the new hole to be drilled.  Then opened the throttle plate, and used some wire to hold it open (you could probably wedge a rag in but you need it out of the way to drill.  Then again used the dremel and 0.8mm drill bit (and a lot of patience to make sure it’s lined up perfectly) and finally drilled the hole.  The piece of metal will work both sides of the carburettor you just need to flip it over when doing the other barrel of the carburettor.

 

Any questions or additions please feel free to comment and I can try and answer any questions as best I can.

 

As a side note, if you’re throttle plates aren’t closing all the way I also discovered that my accelerator pump rod had a small nick in it which was catching on the retainer preventing it from allowing the cam (tied into the throttle shaft) to return, therefore keeping the throttle plate slightly open.

Edited July 8, 2019 by Roady