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Discussion Starter #1
Have emission type dcoe's on 1974 spider, converted from spica ages ago by po. Was balancing them using air flow meter over the weekend. Found that the two throats on the front carb are about 20 % at variance with each other. Does anyone have a view on how important this is and if it needs to be addressed how best to do so? The car has a flat spot around 2000 rpm and there is a sporadic carb cough at idle. Otherwise seems to run great above 3000 rpm.

Thanks for any ideas

Mike
 

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Discussion Starter #3
Hi ed
Thanks for the response. I had synchronized the front and rear carbs as you kindly suggested in previous posts but my observation of a persisting disparity is between the two throats of the front carb, not between the two carbs. I haven't seen a discussion of this issue anywhere here.
Just wondering if this makes a big difference to engine running and if so, how to correct this

Thanks for any thoughts

Mike
 

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There are air-bleed screws on emission Webers in addition to the idle screws. These need to be set equal across all throats with an air flow gauge and then you do the fine adjustment on the idle screws.
 

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Discussion Starter #5
Got it
Thanks for the info. I learn so much from the bb and appreciate your generosity in sharing wisdom. Will have a crack at this over the weekend.
Thanks so much
Mike
 

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BTW you want the air bleed screws as little open as is possible to equalize the throats. If they are too open the throttle will not drop nicely ....
 

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There are air-bleed screws on emission Webers in addition to the idle screws. These need to be set equal across all throats with an air flow gauge and then you do the fine adjustment on the idle screws.
I have always run mine fully closed.
 

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From what I read in a Weber-published tuning manual (I don't have any dcoe's with these), the air bleed screws are specifically there to balance between throats of the same carb. Once each carb has its own throats balanced, then you balance the two carbs (or more!) to each other, using whichever of the two throats to compare.
All of this is much easier than a triple SU healey six with wonky linkages! :laugh:
/Neil
 

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From what I read in a Weber-published tuning manual (I don't have any dcoe's with these), the air bleed screws are specifically there to balance between throats of the same carb. Once each carb has its own throats balanced, then you balance the two carbs (or more!) to each other, using whichever of the two throats to compare.
That makes sense. But I think that this fine balance between throats is only a significant advantage if you are tuning for low emissions.
 

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Ed, absolutely. In my mind it would also be helpful for rebuilding carbs, where you may have some wear in the throats, or shafts which are not quite right, etc - situations where otherwise you're trying to figure out what you can possibly do to get rid of that light gap here and there around the throttle plates.
But I've only got a set of "classic" DCOEs so no hands-on experience with the bleeds/bypass screws.
/Neil
 

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Discussion Starter #11
Back to the emission type carbs this morning. Started with balancing again. Rear 3 throats drawing 5 (?units) on the synchrometer. Front most throat about 6. Couldn't alter air flow with air bleed adjustments so just left as is. Close enough?
Used a colortune to assess mix. I will try to append a video, but I found that possibly tending to lean at idle, and no amount of unscrewing of the idle mix screws could make the colortune signal turn yellow. I could make the mix leaner by winding them in. Engine seems to idle best with these screws out at least 4 turns.
At about 3000 rpm the mix looked like it was getting too rich with yellow signal from the colortune.
Do I have to get richer idle jets and leaner main jets?

Mike
 

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I could make the mix leaner by winding them in. Engine seems to idle best with these screws out at least 4 turns.
That is normal.
At about 3000 rpm the mix looked like it was getting too rich with yellow signal from the colortune.
Do I have to get richer idle jets and leaner main jets?
What idle jets, main jets and emulsion tubes are you using?
I have no experience with flowmeters or colortune but I do have a wideband AFR on my Spider with Webers. I have about 13.5 at idle with a warm engine. It will increase into the 15's and the idle speed will drop if I turn in the mixture screws by 1/2 turn. I had 12.5 at idle when I was running richer idle jets.
I presume that you are reading the colortune with the engine unloaded. I doubt that the readings at above idle speed are relevant unless you can find a way of seeing them while you are driving the car as the mixture will change with the load on the motor.
 

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Discussion Starter #15
The colortune reading was taken under no-load. Curious about the afr meter. Which one do you use and where is the sensor installed?
Thanks
Mike
 

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I have a Zeitronix ZT-2 in my Spider and a ZT-3 in my GTV6. The ZT-3 is bare bones and it only reads AFR. The ZT-2 reads RPM and has inputs for other things.

The picture shows the sensor. It is upstream of where the cat converter would be if I had one. There is a thread about tuning Webers with a wideband AFR.
 

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Discussion Starter #17
Happy new year Ed!
well I went ahead and bought myself a Christmas present, I have ordered the ZT2.
Looking forward to getting the objective data on the carbs
Mike
 

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Discussion Starter #18
Looking at my exhaust system there isn't really a place to put the sensor where it can read all 4 cylinders merged together. The down pipe from the manifold stays separate practically till the front muffler. The small merged section just in front of the muffler allows only horizontal placement of the sensor which, I read, may lead to sensor damage from condensation. To place the sensor between "9 and 3 oclock", I would have to place it on the pipe from 1/4 or 2/3.
Does the orientation of the sensor matter in this application?
Would I get usable data sampling 2 of the cylinders (at least would be sampling one throat from each carb)?

Thoughts or suggestions?

Thanks

Mike
 

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My sensor is horizontal. I had a narrow band sensor in the same location for years with no problems.
 
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