question on a info in the epic Weber thread

[Mikez31ss]

After "rebuilding" my DCOE40 18 carbs I've learned enough about my carbs now that the epic Weber thread is actually useful to me now.

I've got everything back together now and I'm ready to sync the carbs and set the idle speed and mixture.

On the first page I read this:

The throttle plate should cover the progression port closest to the intake manifold so that the fuel from the idle jet goes through the opening with the tapered, spring-loaded mixture screw only. (The word "only" is used in a relative sense, because there is going to be some fuel going around the butterfly plate along with the air passing through). Your goal is to have that last progression port lined up with the center of the edge of the throttle butterfly. Then fiddle with the mixture crews, then idle jet and idle air-correction (the F# on the idle jet).

Sounds like a simple fool-proof method although I'm sure there may be some room for tweaking. Does it work?

One other question on synchronizing the carbs. On one of my carbs I can only fit the synchrometer in one of the trumpets due to the BMC being in the way. Since there is only one idle speed screw per carb that should be fine...correct?

[bjhines]

There is no idle speed screw PERIOD. You are missing the point of the set up instructions entirely.

[cygnusx1]

 The screws on the back of the barrels are idle mixture "volume" screws. (one per barrel).  They adjust the amount of "emulsion" that flows through the idle circuit.   

Edited August 25, 2010 by cygnusx1

[cygnusx1]

.....The screws that adjust the throttle plate stopping position can be called idle speed screws because they adjust the volume of air allowed past the plates at idle.  They should be adjusted as per your quote above using the progression holes as a guide, and then fine tuned +/- 1/4 turn to get your flow synchs right.  

 

 

 

 

I worked hard to research test and write this.  It works.

 

http://forums.hybridz.org/index.php/topic/28318-weber-jetsall-who-live-for-their-triples-please-read-this/page__view__findpost__p__863257

 

 

 

 

 

 

 

 

Edited August 25, 2010 by cygnusx1

[Mikez31ss]

Thanks cygnusx1 for a helpful answer

The main ***** I had when ordering parts and working on the carbs is the variation in terms for parts. My manual and most documents I've DLed for reference call those screws "idle speed screws". In some docs they're referred to as "throttle stop screws". There was at least one other term for them. I don't really care what we call them here as long as we're talking about the same part.

Just so we're clear here I'm not confusing that part with the idle mixture screws.

About my question on synchronizing the carbs it seems logical that if I can get the tool in one trumpet of each carb that should be fine since there is only one idle speed...oops throttle stop screw per carb.

And thanks for the link to your writeup.

[Lazeum]

The point regarding the "idle speed screw" from Bjhines is because people tend to use this screw to setup the idle speed whereas it is used for sync between carbs. You will see people having their throttle plate opened way too much to get right idle speed because of misunderstanding of the physics of the carsb.

 

Regarding the sync between the carb, I also have a synchrometer but I end up using a fuel or heater hose to listen to the suction of air. All carbs should be even.

Some poeple don't like this method, personnaly I had great results!

 

Go slow and since it is difficult to damage anything at idle, just try stuff and see what happens.

[cygnusx1]

Note this point.  Carb synching is not critical to a good running set of webers.  It mainly effects idle smoothness and a tiny bit of throttle response smoothness ONLY coming off idle.  It's over-talked about. IMHO.

 

 

 

Having said that.  If the carbs are not synched, you will try to cure a rough idle by fussing with the idle-mixture-volume-screws (lets call them IMV screws    ) and that is WRONG.  Those screws effect idle, and much more beyond idle.

Edited August 26, 2010 by cygnusx1

[Mikez31ss]

Note this point.  Carb synching is not critical to a good running set of webers.  It mainly effects idle smoothness and a tiny bit of throttle response smoothness ONLY coming off idle.  It's over-talked about. IMHO.

 

 

 

Having said that.  If the carbs are not synched, you will try to cure a rough idle by fussing with the idle-mixture-volume-screws (lets call them IMV screws    ) and that is WRONG.  Those screws effect idle, and much more beyond idle.

Good to know, thanks for the tip. If you've heard my car, it has a pretty big lope so a purring motor isn't going to happen.

I can't see the throttle plate through the inboard progression hole with a light on any of the carbs. Maybe that's something that works on carbs that haven't been installed. I can tell that it is there by using a straightened paper clip.

So I'm going to fire it up tomorrow and see how well it goes. All I need it to do is run the way it ran before. The jet combination on there now seems to be just about right. Appreciate the help. Here's a short clip of it running before the rebuild ...fter setting for 5 months and w/ 5 valves out of adjustment lol

 

FWIW this is what's in the carbs:

Main Jet 130

Air Correctors 165

Idle Jets 45 F9

Needle valve 175

Main Venturi 30

Aux Venturi 45

Emulsion Tube F11

Accelerator Pump Jet 45

Pump Bleed Back ?

[bjhines]

MikeZ31ss, The design of these carbys assumes everything with the engine is perfectly adjusted and equal between cylinders. The various adjustments will affect more than one aspect of the mixture and response of the system overall.

 

I was short, but not usless in my response. The instructions offered on several sites and books talk about this in some detail. The throttle stop screws are just that. You set the throttle plates and leave them be. No 1/4 turn to equalize. 1/4 turn to equalize is too much. Achieving idle and off idle quality is more of a parts choice than settings choice.

 

For you guys that do like to reset those screws, sometimes it works sometimes it makes things worse.

There were many variations on the transition holes including slots and grooves. Some of them were more finiky than others. Some folks drilled the throttle plates to acheive desired idle and off-idle performance on race engines. This means that the position of transition holes and relative sensitivity to throttle position will vary from model to model.

 

Later DCOE versions had air bypass screws to assist with equalization. These are going to affect idle mixture as well so both adjustments must be performed together. This was primarily a "street" addition to the design.

Edited August 26, 2010 by bjhines

[Mikez31ss]

Not a problem here. Maybe my question wasn't framed quite right. I was a little ticked at your response because you seemed to have missed the emphasis I put on the placement of the throttle plate in relation to the progression holes and that it was a question.

I had typed out a response when mozilla decided to crash so it got lost lol.

I've got DCOE40 18 carbs so I guess mine are a little older. I did use the newer style idle mixture screws that came in the kits. Whether that's a good thing or not considering my inexperience well...we'll see.

My main goal here was to clean the carbs and eliminate some leaks they had. I'm satisfied with the jetting.

It's been a good experience for the most part. I was pretty pleased when I modified the new grub screw aux venturi's to accept the springs from the old aux venturi's lol. I didn't want to drill into my carbs and go out looking for the right sized screws.

I just wish I knew where those extra o-rings were supposed to go

Edited August 26, 2010 by Mikez31ss

[bjhines]

The answer to your ideas on throttle plate setting is; Exactly what the instructions say. Completely covering the first hole to be uncovered.

ie. NO open transition holes.

 

This is where various sources diverge on this exact setting. Some say to set plate toward leading edge of hole and others say trailing edge. In most cases they are still telling you to cover the first hole. The differences in these settings probably represent 1/8th turn of the throttle stop screws.

 

The stop screws are notoriously "rough" on the ends. It could be helpful to lathe the ends of the screws for better profile and consistent settings. One screw may be on the rising side of a bump and another on the falling side, causing very uneven settings from adjustments less than 1/2 turn.

Edited August 26, 2010 by bjhines

[Mikez31ss]

Got a leak on one carb. Only one side leaking though. The float is set and working.

Any ideas on what could cause one barrel to leak?

[Tony D]

Throttle plate angle is irrelevant until you watch a keniscope trace of cylinder pressure on a dyno and can see the difference different cylinder flows make. Consistency is important.

 

As BJ is hinting: you change parts to match what you have. You may make these carbs 'this engine only' by doing that....but then again that is the POINT of tuning. You are optimizing the carburettor for THIS engine.

 

The reason those screws and crap are on there is to accomodate production change and tolerance in manufacturing of the carburettor as much as anything else. And to facilitate the mounting of these on as many aftermarket applications as possible.

 

Filing leading edges of throttle plates to tune transition port coming on off-idle is something that works for THIS engine...that modification will not be applicable to the next one. You will need to reinstall new throttle plates and start over on that one.

 

This goes to the old saying I've used for decades: "Anybody that says 'carburettors are easy' has never spent time actually TUNING one!"

 

As has been stated, the directions are sound, you want it covered. The movement of the throttle plate will uncover it. The screws you talk about will move BOTH plates. If they aren't transitioning at the same time---and this is possible due to production tolerances in drilling of the carb body, it doesn't mean necessarily the throttle shaft is bent---then you have to alter the late-transitioning throttle plate via slight filing to have the ports uncover at the same time.

 

BJ is dead on with the 'true the screw' suggestion as well, a nice 5 degree face with slight rise in the center of the screw, polished with an Arkansas Stone and deburring of the lever in the same area will make everything work more consistently and smoothly during initial setup.

 

Me? "SHIFT CONTROL ARROW UP" for more fuel "ARROW DOWN" for less. Screw this filing and screw polishing crap for me any more, I'm getting too old!

[Mikez31ss]

Thanks Tony. Still wondering why one side is leaking. Sticky valve?

BTW I take it you aren't interested in buying these to help me fund an L28et swap

Was it you that coined the phrase: "a carburetor is nothing but a controlled fuel leak"?

[Mikez31ss]

I'm 99% sure the sticky intake valve can be eliminated. I thought gas was just coming out of one horn but it's actually both. One side is just much worse than the other.

I can get gas to drip out the horns just by manually operating the accelerator. Seems like either something in the carb is stick allowing fuel to squirt too long or somehow fuel is hitting the outside of the throttle plate.

[cygnusx1]

Make sure the fuel level is exact on the leaky one, make sure the floats are set with both high and low limits, make sure the needle valves shut tight, and make sure the screws or springs that retain the choke are snug.  My Webers also leave traces of fuel on the upwind end of the barrels, sometimes noticed after a drive. I found snugging the choke retainer screws improved the weeping a bit.

[Mikez31ss]

Make sure the fuel level is exact on the leaky one, make sure the floats are set with both high and low limits, make sure the needle valves shut tight, and make sure the screws or springs that retain the choke are snug.  My Webers also leave traces of fuel on the upwind end of the barrels, sometimes noticed after a drive. I found snugging the choke retainer screws improved the weeping a bit.

Thanks but I've already eliminated all the easy possibilities. The floats are dead on and the bowl isn't overfilling. I doublechecked that. And the carb body isn't cracked. The chokes are secure. I pulled the carb and rechecked everything but I can't see anything wrong. I pulled the middle carb just to compare the two and I can't see any difference in the operation off the car except that one leaks and the other doesn't

[cygnusx1]

Thanks but I've already eliminated all the easy possibilities. The floats are dead on and the bowl isn't overfilling. I doublechecked that. And the carb body isn't cracked. The chokes are secure. I pulled the carb and rechecked everything but I can't see anything wrong. I pulled the middle carb just to compare the two and I can't see any difference in the operation off the car except that one leaks and the other doesn't

Well then, you are right where I am.  Let me know when you figure it out. 

 

 

 

[Mikez31ss]

Well then, you are right where I am.  Let me know when you figure it out. 

But...but... but...you're one of the kung fu masters and I'm just one of the grasshoppas lol.

I'm going to fire it up again tomorrow and see if I inadvertently fixed the problem while trying to find it.

[cygnusx1]

I am more like fung ku.