Weber Carb Jetting

Engine, Transmission and related drivetrain.
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abisel
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Weber Carb Jetting

Postby abisel » 07 Jan 2016 12:52

I looked around and didn't find what I was wanting so...

I have an L20B with a Webcam 91, a Cannon manifold and dual Weber 40DCOE carbs. The lower end of the engine is stock. Cam timing and ignition timing is correct. Using NGK BPR6ES plugs. Also, the distributer is the later electronic version and has been disassembled, cleaned, lubed and reassembled.

The head has new SI stainless valves, steel seats, bronze guides, and stock springs. The intake valves are 43mm instead of 42mm. The exhaust valves are stock. The intake runners on the head have been opened up to match the manifold gasket. The Cannon manifold matches the gasket too.

The fuel system is with a Carter electric pump, and Aeromotive filter and pressure regulator with the pressure set at 3psi. All plumbing is -6AN.

The carbs are currently set up as:
33mm venturies
150 main jet with 200 air correction on F16 emul tubes.
50F6 idle jets
45 accel pump jets
55 accel inlet/exhaust jet
200 needle valves.

The engine is running rich, sounds like a full race cam at idle, emits puffs of black smoke and hurts your eyes standing next to it. The transition from idle to full throttle is smooth and it gets up and goes like a stripped ass ape. No hesitation or stumbling. Just the idle is not right. I haven't checked the plugs after driving at highway speed, turn off the engine, pull over and pull the plugs. I need to do that.

So I need to rejet the carbs to lean it out and smooth out the idle.

My thoughts are:
34mm venturies
140 main with 190 air on F16 emul tubes
50F8 or F9 idle jets
40 accel jet
50 accel inlet/exhaust jet

What are your jetting thoughts/experiences?

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abisel
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Location: St. Charles, Missouri

Re: Weber Carb Jetting

Postby abisel » 15 Jan 2016 04:09

Update:

I think I am zeroing in on jetting for the carbs.

From the original jetting of which was running way too rich at idle
    33mm venturies
    150 main jet with 200 air correction on F16 emul tubes.
    50F6 idle jets
    45 accel pump jets
    55 accel inlet/exhaust jet
    200 needle valves.

I tried this jetting:
    34mm venturies
    140 main with 190 air on F16 emul tubes
    50F8 idle jets
    40 accel jet

I increased the venturie size thinking the info I found http://www.classicrallyclub.com.au/docs/Tips_tuning_Weber_carburettors_DVAndrews.pdf would help, but still had hesitation and stumbling between low speed and high speed transitions. The engine did run a bit smoother at idle but still stumbled when transitioning to the main jets.

So I then tried this:
    33mm venturies
    140 main with 190 air on F16 emul tubes
    50F9 idle jets
    40 accel jet

Going back to the 33mm venturies instead of 34mm helped to increase air velocity and the 50F9 idle jets helped to richen the mixture above the 50F8 jets. Stumbling and hesitation is still there but only when you stomp on the gas at around 2000 RPM. Idle is smoother but still a bit rich.

Next I will try 32mm venturies to again increase air velocity and see what that does. Also, I will try a 35 accel pump jets to see if the hesitation is due to overly rich accel pump.

Getting closer to a smoother idling engine with smoother transitions.

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bertvorgon
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Re: Weber Carb Jetting

Postby bertvorgon » 15 Jan 2016 05:16

I have found, and seen, that getting the best velocity of air, so that the signal for the main jet start up is as good as possible. This will make the best, flexible running motor. In the real world, if not full on racing, where you need the top end power, it was way better, to have off idle and mid range smoothness.

Same goes for camshafts, too big and everything falls flat till 5,000 RPM, then whammo.
"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer

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funwithmonkeys
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Re: Weber Carb Jetting

Postby funwithmonkeys » 15 Jan 2016 12:52

You mean like this?
engine report.jpg
engine report.jpg (385.68 KiB) Viewed 5832 times

580 lift cam is fine for toddling around town.....it just means that the hand of God pushes you into the seat above 4500rpm
If no one from the future comes back to stop you from doing it then how bad of a decision can it really be?

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defdes
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Re: Weber Carb Jetting

Postby defdes » 16 Jan 2016 03:36

It seems like you are getting on the right track, but with tuning these duals I generally try not to change more than one thing at a time. It makes singling out the issues that much easier.

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abisel
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Location: St. Charles, Missouri

Re: Weber Carb Jetting

Postby abisel » 24 Jan 2016 12:12

Another update:

I tried the 32mm venturies, but that didn't help the hesitation. So went back to the 33mm venturies.

So with:
    33mm venturies
    140 main with 190 air on F16 emul tubes
    50F9 idle jets
    40 accel jet
It runs pretty good but still a slight hesitation from the idle circuit to the main circuit during acceleration. So I tried a 45 accel pump jet and that seemed to help a little. Then I replaced the accel pump standard light spring with a medium spring and that helped even more. It still idles a bit rich and I have all the idle mixture screws screwed down to the seat. Yet is idles ok. A bit of an engine rocking but not much. So now I will try the 32mm venturies and see what if anything that does.

Still, with the idle mixture screws seated, that tells me the mixture is too rich. Yet if I go to a leaner idle jet, it hesitates during acceleration. But maybe not with the larger accel pump jet and stiffer spring.

Working it.

Chickenman
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Re: Weber Carb Jetting

Postby Chickenman » 26 Jan 2016 14:26

You should be able to kill the idle with the idle screws, regardless of idle jet. I suspect that you have the butterflies open too far at idle and thus uncovering the first idle progression hole much. Edit: At idle setting, the first transition hole should be covered by the butterfly. If any part of the first transition hole is uncovered at idle setting, this will cause a tip in hesitation as mentioned.

This is a common problem with big cams driven on the street. Especially if the vacuum advance is not used and hooked up to manifold vacuum. However, IR manifolds have a problem running vacuum advance because of the strong pulsing in the manifold. It is a PITA to get dizzy vacuum pot tuning correct with an IR manifold. It can be done... but it is a real PITA. Race cars don't need Vacuum advances and most guys don't run them with IR manifolds, so I'm going to assume that you don't run vacuum advance.


Note: If you are running a 123 Dizzy, or some other method of programmable electronic engine timing then let me know.
That alters the WHOLE ball game and you can build special advance curves with extra advance BELOW, 1,500 rpm to help out with idle quality

Big cams like a LOT of ignition timing down low because of charge dilution and low VE caused by overlap at lower rpm's.

With a big camshaft like you are running, you need a idle mechanical ignition timing of at least 15 degrees BTDC. Anything less is not going to be sufficient. If you get too much mechanical advance at high RPM's, you will have to limit the travel with stops or welding the timing slots in the Dizzy. ( This is assuming that you are not running any Vacuum advance ).

Check the progression hole as mentioned. It must be covered. Fuel at idle should only come in by the idle port.

An age old trick to raising idle speed but deceasing actual butterfly opening at idle is to drill a small hole in each butterfly. This will bypass some air, raise the idle speed and allow you to reduce the throttle valve opening. Start off with a small hole. No bigger than 1/32" in diameter and drill it on the Butterfly side that is opposite to the progression holes in the carb body. One hole per butterfly. See what it does to idle quality and butterfly position. If desired idle speed still requires opening butterflies too far into the transition slot or progression hole, open them up a smidge at a time ( smidge = .005" to .010" in Chicken language ) . Number drills are best. I would advise going no bigger than 1/16" drill size maximum.

The idea is to get the idle speed set to approx 1,000 rpm, while having the first progression hole " just " covered. Once that setting is correct your idle quality, idle transition and even light cruise tip in should be Mucho better.


Important note: Depending on the Weber Model number, some newer versions have an additional idle air bypass screws. IE: Weber 40DCOE 151's should have the air bleed bypass screws. Most tuning manuals will tell you to screw these down closed.... but that is incorrect if you have a big Camshaft. If your Weber has the additional idle air bypass screws, you can open these to bypass additional idle air, thus enabling you to re-establish the correct throttle plate to idle transition hole position.... without drilling any holes in the butterflies. Post closeup pictures of Carb idle adjustment screws and supply Carb Model numbers please!!


I'll try and find the specs for transition opening later....
Last edited by Chickenman on 27 Jan 2016 20:31, edited 3 times in total.

Chickenman
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Joined: 06 Sep 2010 15:10
Location: Coquitlam

Re: Weber Carb Jetting

Postby Chickenman » 26 Jan 2016 14:58

Picture of Factory 40DCOE throttle blade ( 40DCOE Model 31's ) with air bypass hole pre-drilled. Used on Lotus Elan Twin Cam engines with Sport Camshafts. Note the small hole to reduce idle butterfly opening amount.

Image

Chickenman
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Location: Coquitlam

Re: Weber Carb Jetting

Postby Chickenman » 26 Jan 2016 15:25

It was really hard to find Weber 40 DCOE articles showing the correct butterfly valve to progression hole setting. Following is as close as I could come. From Pelican parts regarding 45 DCOE idle settings, but principle is the same on the 40 DCOE models. Position of throttle blade to 1st progression hole is critical.

Picture of 45 DCOE showing first progression hole un-covered. This is an incorrect setting caused by the wrong butterflies being installed during an overhaul Note the original witness mark on the Carb body that shows 1st progression hole was completely covered with original throttle butterflies. Edit: Ignore pointer. It is pointing at idle discharge port and needle. Look at the edge of the throttle butterfly.

Image

Picture of same Carb body but with the correct butterflies installed . Note how 1st progression hole is now completely covered at idle setting. Proper centering of butterflies is critical. Clearance when fully closed must be no more than .001" when measured at any point.

Image

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abisel
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Location: St. Charles, Missouri

Re: Weber Carb Jetting

Postby abisel » 26 Jan 2016 18:29

Chickenman, thanks for your inputs.

I took out the plugs to inspect the progression holes and could see the throttle plate through the first progression hole and it was to the rear (away from the engine) and yep, half way uncovering the 1st progression hole. Then I backed off the idle speed screw and the throttle plate moved toward the engine and the first progression hole was then only half way covered in the opposite direction. So it looks like I had the idle speed screws turned too much and will try to adjust the engine idle with the throttle plates completely covering the 1st progression hole and adjust the mixture screws to get idle. 40DCOE do not have air bypass screws like the 45s do.

Close up of progression holes. You can just see the throttle plate starting to uncover the 1st progression hole (hole shadow from flash) and the engine idles at this setting with the mixture screws seated.
DSC_0091-1.JPG
DSC_0091-1.JPG (38.21 KiB) Viewed 5672 times


Idle speed screw. Same on both carbs.
DSC_0086-1.JPG
DSC_0086-1.JPG (114.66 KiB) Viewed 5672 times


The Webers I have are both 40DCOE28 Italian made completely rebuilt with new everything except spindle shaft bushings which were fine.
DSC_0094-1.JPG
DSC_0094-1.JPG (174.7 KiB) Viewed 5672 times


The WebCam 91 isn't really a big cam but it does get up an go above 3K RPM and full throttle. It'll put you back in the seat a bit.

The dizzy is a matchbox dizzy with no vacuum connection. Strictly internal mechanical advance. I could mark the crank pulley with a a few degree marks and see what the mechanical advance does. What max mechanical advance should I look for at what RPM? Timing at idle is currently set to 12 BTDC. I'll advance it to 15.

I searched around the web and found this article about progression holes:
http://www.alfabb.com/bb/forums/carburetors-fuel-injection-air-intake/32856-weber-dcoe45-152-45dcoe-progression-circuit-modification.html

If I still have issues with hesitation, bucking and a spit once and a while, I think I will remove the carbs and make sure the throttle plates are fully closing and fully covering the 1st progression hole (and make sure the new spindle shafts aren't twisted) and go from there before I drill a hole in the throttle plate. The throttle plates are all new 78 degree plates (same as the old original plates) and maybe I need to make sure they are still seating properly. I don't have a 0.001 needle to measure the plate to bore clearance, but maybe a single strand of wire from a piece of stranded 22 gauge wire I use in my model railroad will work. I'll measure one and see.

Still working it and getting closer.

Chickenman
Posts: 461
Joined: 06 Sep 2010 15:10
Location: Coquitlam

Re: Weber Carb Jetting

Postby Chickenman » 26 Jan 2016 19:13

Nice setup!!

Yep, you're getting there. E28 model of 40 DCOE unfortunately does not have the air bypass screws. Not a biggy... you'll just have to work on the idle progression hole deal by other means.

You need a bit more than 12 BTDC on your ignition timing and a Web91 camshaft. 15 degrees BTDC at idle would be a good starting point. That should allow you to back off the idle speed screws a bit more. You need a dial back timing light to set the Maximum mechanical advance. Need some more info on your engine build including engine size, CR ratio, full Cam specs and most importantly, fuel brand and Octane that you normally use or have available.

A " safe " setting for pump gas with 93 Octane and 10 to 1 CR would be around 34 to 35 degrees total mechanical. So 15 degrees static timing, plus an additional 20 degrees in the dizzy. 20 degrees is a fairly normal amount for the Dizzy cam plate to provide. Read Jason Grey's Distributor Guide for more info.

http://newprotest.org/projects/510/jasonGrayDistributor.pl

I like to have around 32 degrees mechanical all in by 3,000 RPM, then the last few degrees tapering in by 3,500 to 3,700. I then rev the engine to 4,000 RPM to make sure that it doesn't keep advancing more. Full mechanical advance should be " all in " between 3,500 to 4,000 RPM. Final tweaking should be done on a Dyno, but stay a bit conservative to prevent detonation. Detonation velly , velly bad!!

Continued below...
Last edited by Chickenman on 26 Jan 2016 19:40, edited 4 times in total.

Chickenman
Posts: 461
Joined: 06 Sep 2010 15:10
Location: Coquitlam

Re: Weber Carb Jetting

Postby Chickenman » 26 Jan 2016 19:17

Part Deux:

Note: Assuming an idle RPM of 1,000 , you want the ignition timing to start about 200 RPM to 300 RPM above that. A common mistake is to make the spring tension too light. The distributor weights must be against the low limit stops at idle RPM. If you have the spring tension too light, then you start getting timing advance at idle. The weights start to " float " off the stops and will bounce. This causes erratic idle and also eats up some of your total timing. One sure sign of this is a timing mark that won't remain stable at idle. It will tend to float or bounce back and forth.

What I like to do is check the timing at idle speed ( 1,000 RPM ) and then reduce the idle speed to about 700 RPM and see if the timing drops. If it does drop, then the spring tension is too light. Disassemble the dizzy and add a touch more tension to the " Light " spring. Leave the heavy spring alone. The light spring controls low RPM starting point. The heavy spring takes over at mid-range and High RPM. Note: you can adjust the spring tension by carefully bending the outside spring " Tab " that the spring hooks on to.

If my memory serves me correct, the light spring goes on the long slot and the heavy spring goes on the shorter slot. The heavy spring may not have any tension on it at all when the weights are fully closed. That is normal.

Chickenman
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Joined: 06 Sep 2010 15:10
Location: Coquitlam

Re: Weber Carb Jetting

Postby Chickenman » 26 Jan 2016 19:31

Just noticed something in the picture. Plug leads should never be Zip tied together like that. Can cause cross fire. Separate them with proper lead separators.

What brand are the lead wires? Are they carbon cores? You want a good spiral core wire like MSD, Magnacore or NGK are all good brands. NGK wires are very high quality and cheap !! Plus they smell like bubble Gum :mrgreen:
Last edited by Chickenman on 26 Jan 2016 19:41, edited 1 time in total.

Chickenman
Posts: 461
Joined: 06 Sep 2010 15:10
Location: Coquitlam

Re: Weber Carb Jetting

Postby Chickenman » 26 Jan 2016 19:35

For aligning throttle plate, remove carbs. Back off idle speed screw all the way so that throttle blades are completely seated. Then view seated throttle plate against a bright light. Ideally you should not be able to see any light at the edges of the blades. Adjust centering as necessary.

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510wizard
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Re: Weber Carb Jetting

Postby 510wizard » 27 Jan 2016 08:53

Chickenman, thanks for all of schooling on this subject, I am sure it is a great help to all of us, at least it is to me. Love this kind of stuff!


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